Our solar system hosts a cornucopia of worlds, from the hellfire of Venus to the frozen plains of Mars to the mighty winds of Uranus. In that range, the Earth stands alone, with no planet coming close to its life-friendly position near the Sun.
Outside our solar system, however, it’s a different story. Observations using space-based and ground-based telescopes have indicated that a new class of objects dubbed super-Earths – worlds that are about two to 10 times our planet’s mass and up to two times its radius – could be among the most common type of planets orbiting other stars.
That’s because during the past few years, astronomers have found plenty of these super-sized rocky bodies orbiting different types of stars. Among these planetary systems, those around M-class stars, which are cooler and fainter than our Sun, are particularly important. Because of the low surface temperatures of these stars, the regions around them where an Earth-like planet can maintain liquid water on its surface (also known as the Habitable Zone) are closer to them — making such potentially habitable super-Earths in those regions more detectable. [6 Most Likely Places for Alien Life in the Solar System (Countdown)]
Scientists also believe that these smaller stars are the most abundant in the Sun’s corner of the universe, implying super-Earths would be plentiful in our solar neighbourhood, as well.
Nader Haghighipour is a member of the NASA Astrobiology Institute and the University of Hawaii-Manoa’s Institute for Astronomy. Among his research interests is figuring out how these worlds form, and most importantly, how they arrive in their current orbits.
Some of his work hints that migrating giant planets could be responsible for the close-in orbits of smaller bodies. Their massive gravity could excite the rocks and protoplanetary debris on their paths and cause them to be scattered out of the system or coalesce into smaller planets such as super-Earths.
“When giant planets approach the central star, especially around an M-dwarf, I’m interested in how they affect accretion of small planetesimals in a disc in front of them and how that will result in the formation of super-Earths, particularly in the habitable zone,” Haghighipour said.
Faster discovery pace for super-Earths
Haghighipour recently surveyed the state of super-Earth research in a paper that appeared in the Annual Review of Earth and Planetary Sciences. The first super-Earths were discovered in 1992 around pulsar star PSR B1257+12, but it’s only in the past five years that the pace of discovery picked up.
This was in large part due to the arrival of the NASA Kepler space telescope, which spent close to four years hunting planets in a small region of the sky in the constellation Cygnus. Kepler ended its primary mission in 2013 after the telescope exceeded its design lifetime. During this time, it provided a treasure trove of extremely high quality data that has revolutionized the field of exoplanetary science.
Short period super-Earths are easier to detect around smaller stars than those that are the Sun’s size or larger. This is because smaller stars show larger reactions to the tug of the planet as the planet orbits the star. If the planet happens to go across the face of the star from Earth’s perspective, a super-Earth blocks out more of a small star’s light, making it easier to detect.
“That super-Earths in short-period orbits around cooler and smaller stars are easier to detect has set the ground for this becoming fashionable, and now there’s a great deal of attention in using radial velocity and transit photometry techniques to find such planets in the habitable zones of M stars,” Haghighipour said.
These planets are both detectable by the Kepler telescope and also ground-based ones. Most commonly, discoveries from the ground take place with two instruments. One of them is the High Accuracy Radial Velocity Planet Searcher (HARPS) on a European Southern Observatory 3.6m telescope at La Silla, Chile. The other is the W. M. Keck Observatory’s High Resolution Echelle Spectrograph (HiRES) in Mauna Kea, Hawaii.
While NASA scientists re-examine Kepler’s mission – its science work is on hold after two of its four reaction wheels failed – they are hard at work planning its successor mission, the Transiting Exoplanet Survey Satellite (TESS). TESS will have both advantages and disadvantages while searching for super-Earths, Haghighipour said.
“Because TESS is going to cover the entire sky, as opposed to Kepler that focused on only one portion of the sky, it may be able to find more [exoplanets],” he said. “As far as accuracy and precision, because it’s not going to stay on one region of the sky for as long as the Kepler did, the accuracy may not be as high as that of the Kepler.”
One particular star system of interest to Haghighipour is Gliese 667, a triple star system which lies about 22 light-years from Earth. Haghighipour was part of a team that identified at least one super-Earth in the habitable zone of GJ 667C in 2012.
This year, another group led by the University of Göttingen in Germany revealed that where there was one super-Earth, there may actually be many. The new analysis found that the M-star in the GJ 677 system (known as GJ 677c) has about six or seven planets, including anywhere from three to five “super-Earths” in the habitable zone. [The Strangest Alien Planets (Gallery)]
Because the star is so faint and dim, to be in its habitable zone these planets must crowd in close. The researchers estimated that the planets have very short years, between 20 and 50 days, and may even have one side perpetually facing their host star. Even in this state, however, the astronomers believe it is possible that life could survive there.
“It’s the most reliable detection [of potentially habitable exoplanets] that we’ve had,” Haghighipour said. The challenge, he added, is to understand the planets’ habitable environments from a distance.
While calculating the location of the habitable zone of a star is relatively straightforward, modelling the planets’ dynamics and climate is far trickier. It is unknown if these worlds have plate tectonics, for example – a geophysical processes that regulates the abundance of CO2 and H2O in Earth’s atmosphere. Their interiors remain masked to astronomers, and understanding exoplanet atmosphere composition is something that some teams are only starting to accomplish.
Identification efforts continue, however. Haghighipour has been working on detecting super-Earths in the habitable zones of M-stars since 2009 along with observers at the University of California, Santa Cruz and the Carnegie Institution of Washington. Gliese 667Cc is the most cited discovery from this collaboration, but there are others.
On the theoretical side, Haghighipour has two papers published in the Astrophysical Journalabout habitability in binary star systems. He also has been trying to figure out how super-Earths form at different distances from their stars.
“It’s possible each system has had its own history, and its own way of formation. There is no reason to believe that one way of formation for planets in a system, or for super-Earths in habitable zones, can be applied to all systems,” he said.
Perhaps this research could shed some light on the formation of our own solar system. Both super-Earths and “hot Jupiters” – gas giant planets that closely orbit their parent stars – appear to be common in other systems, so why not ours?
“Honestly, we have no definite answer for that. There are many different models that present different ideas for why there are no super-Earths and hot Jupiters in our solar system. But in order for these models to be successful, they have to explain other properties of the solar system as well,” he said.
For example, a giant gas planet close to our Sun would likely have disturbed any rocky planets wanting to orbit nearby. It will be an interesting theoretical puzzle for astronomers to figure out as they continue classifying worlds outside of the solar system.
With the second spacecraft this month now on its way to Mars, you could be forgiven for thinking we’ve forgotten that there is a number of other planets in our solar system.
Due to arrive in orbit about the red planet in September 2014, MAVEN will be the first probe to explore the upper reaches of the Martian atmosphere. It will do this by taking a number of dives into the upper atmosphere, dipping to only 125 km about the Martian surface from its home orbit of 6,000 km.
The hope is that to find clues to a possible warmer and wetter past.
But with Opportunity still trundling along, Curiosity, the Mars Orbiter Mission, MAVEN, the Mars Reconnaissance Orbiter, Mars Express, 2001 Mars Odyssey and the planned InSight, ExoMars and Mars 2020 rover missions, are we forgetting that there’s more to the solar system than Mars?
Sure it is the most viable planet that we, the human race, could go and walk on but it’s probably not the best hope for the discovery of biological activity.
Don’t get me wrong, I’m a massive fan of any space mission, and every step we make in space is a “giant” leap for us down here on Earth. Every endeavour we have undertaken on Mars has thrown up yet more intrigue, and we’ve barely scratched the surface.
But let’s not kid ourselves; it looks pretty dead up there. If we do find any biology on Mars, it going to be most interesting working out how it has hung on for billions of years (and try and get some survival tips).
I admit “The Mars Overload” is a bit of misrepresentation, as we are currently exploring (or travelling to) pretty much every other planet in our solar system right now (with two notable exceptions). So what are they all up to?
The Messenger craft is currently 3,400 days into its mission in orbit about Mercury, and has now imaged the whole surface of the Sun’s closest neighbour. It’s currently in a bit of a limbo, with it’s extended mission finishing in March this year.
Venus currently has the European Space Agency’s Venus Express spacecraft in orbit and the Japanese mission Akatsuki hopefully en route. Venus Express has returned the strongest indications yet that Venus is geologically active, and if confirmed would be the first planets (other than our own Earth) to be discovered so.
Akatsuki, which was planning to study Venus’ extreme climate, unfortunately failed to insert into Venutian orbit in 2010. But hope is not lost, and it is currently held in an elliptical orbit with plans to make another attempt into a closer orbit in 2015.
The asteroid belt
The asteroid belt is the museum of the solar system, and the Dawn mission has been the first to traverse it and focus on some of its biggest exhibits. Dawn’s first stop was orbiting about the asteroid Vesta, and has now left to journey to the largest body in the belt – Ceres – due to arrive in 2015.
At Vesta, Dawn discovered this body’s large metallic core revealing it to be the “last of its kind” as a failed planet.
Any mission to Jupiter has a lot to live up to, with the enduring data set that the Galileo spacecraft collected, coupled with its dramatic ending.
The Juno mission is currently on its way, arriving in 2016 will concentrate on the gas giant’s poles and magnetic and gravity field. The hope that such a detailed mission will reveal more about our largest neighbours interior.
On the cards is more of a successor of Galileo, the European Space Agency’s JUICE mission. But we’re playing the waiting game on this one – with arrival at Jupiter not anticipated to be before 2030.
The most longstanding of current planetary missions is Cassini, launched in 1997. It’s currently in the second phase of its mission and has performed a Galileo-like job on Saturn, returning data on the planet, its rings and moons that will be mulled over for decades.
Like Messenger, is waiting confirmation of its next extended mission, which will keep it running until 2017.
There has been worrying news that cuts to NASA’s budget will force them to choose between extending Cassini or the Mars-roving Curiosity. A terrible choice by all accounts, but given the massive effort taken to get Cassini out there, I really hope that there is some way of keeping them both going.
New Horizons is going to be a science highlight of 2015 when it arrives at the far reaches of our solar system to study Pluto. Since it was launched in 2006 it has seen it’s primary target kicked out of the planet club, but promoted to be the “king” of the dwarf planets.
New Horizons will pass Pluto and it’s companion Charon before heading deeper into the Kuiper belt. Being the first probe to explore this new class of planets in detail, it’s almost guaranteed to return some very exciting stuff.
And the rest …
Uranus and Neptune are the notable exceptions. These gassy icy giants still lie pretty much unexplored with humankind only waving hello in 1986 and 1989 with the respective fly-bys of the Voyager 2 spacecraft.
The biggest difficulty in exploring these planets is that they are so far away that to reach them takes a spacecraft travelling at massive speeds – so fast that by the time they get there you need a massive amount of energy to kick them into orbit. With current technology missions to the outer fringes of the solar system, like New Horizons, are likely only to be fly-bys.
So, contrary to what you might think from recent media coverage, there really is so much more planetary exploration going on than that focused on Mars. To undertake these feats we’ve had to overcome technological hurdles and travelled massive distances at outrageous speeds.
I, for one, very much hope that we can continue to explore our solar system, Mars and beyond, at the same – or even faster – rate.
What’s a virus to do when it finds itself in an inhospitable environment such as hot water? Coating itself in glass seems to not only provide protection, but may also make it easier to jump to a more favorable location to spread.
Researchers led by a group from the Center for Life in Extreme Environments at Portland State University recently coated four different virus types in silica, a glassy substance found in certain types of hot springs. Three of the four viruses studied took on a silica coating and went into hibernation, reactivating when the coating was removed.
The NASA-funded finding has implications for seeking out viruses on other planets, including Mars — a big target for life studies over the decades. No microbes have been found on the Red Planet, but NASA is currently seeking evidence of habitable environments there, particularly with the Mars rover Curiosity. [Latest Amazing Photos from Curiosity]
Seeking the viral fossil record
Water and possible oceans once coated the surface of Mars, according to data from multiple missions. Pictures from orbit appear to show evidence of gullies and shorelines. Rovers and orbiters have also found extensive evidence of sulfates, which are rocks that form in water-enriched environments. Curiosity itself found evidence of an ancient streambed, including rounded pebbles, in the first year of its mission.
Whether microbes were in the supposed Martian water is still up for debate. One problem is scientists don’t even know how to look for viruses in the fossil record on Earth, said researcher Kenneth Stedman, an associate biology professor at Portland State who supervised the new virus research. One of his graduate students, James Laidler, led the work.
“I’m convinced there are viruses in the rock record, but we don’t have the technology to detect them,” Stedman said. “We really need to develop the technology [for virus detection] here before we can even think about going to look there [on Mars]. We’re trying to do just that.”
Stedman’s lab is in the early stages of looking at virus “biomarkers,” or naturally occurring characteristics that can give clues about virus origins. The challenge is, viruses use cells to replicate, so to find a virus biomarker one would need to figure out the difference between the two. If researchers could find the virus biomarkers in rocks, this could be a leftover of a past virus. Stedman is now working on a manuscript that examines this topic. [5 Bold Claims of Alien Life]
It’s possible that viruses can become part of the rock record when they are coated in silica. First, however, the researchers are looking at how the coating mechanism works. Previous work in Stedman’s lab showed that model viruses could be silica-coated in conditions similar to hot springs. Stedman’s most recent research, published online in the Journal of Virology,probed what happens to different virus types if and when they take on a silica sheen.
The team chose four viruses for study: the human smallpox vaccine virus (Vaccinia); bacteriophage PRD1 (which usually grows in Salmonella bacteria); bacteriophage T4 (a well-studied type that can infect E. coli bacteria); and archaeal virus SSV-K. The last virus is present in hot springs.
Team members placed each of these viruses in an environment similar to that of a hot spring, which is oftentimes acidic and features water at or close to boiling. All but the bacteriophage PRD1 developed the silica coating; “It just shrugged off the silica and said, who cares,” Stedman said. “We have no idea why PRD1 is resistant to silica coating, but probably there is something special about the structure.” [Extreme Life on Earth: 8 Bizarre Creatures]
Silica makes the viruses less effective at infecting because of the glass barrier surrounding them. Bacteriophage T4 was rapidly inactivated, but the SSV-K virus saw a moderate slowdown, while the smallpox virus was “incredibly” susceptible, Stedman said.
“That’s not terribly surprising,” he added. “The surface of the Vaccinia virus looks a lot like a bacterium in terms of having a membrane around the outside. Looking at bacterial mineralization that you find in hot springs, etc. [it's clear] why Vaccinia gets coated really well.”
Is interplanetary transport of viruses possible?
With the silica effects established, the researchers next turned to transport. If a virus is coated in glass and gets launched away from the hot spring somehow — perhaps by a geyser in the boiling water — could it remove that coating and become active again? The answer is yes, particularly for bacteriophage T4.
“At least 90 percent of the activity comes back, and so basically you can coat the viruses in silica, mistreat them, and then you can uncoat them,” Stedman said. “We call this the zombie experiment. We can inactivate them, and then they come back.”
What’s more, once the viruses are encased in silica, they are extremely resistant to desiccation, or drying. It’s possible that the viruses can survive deep freezes and other harsh environments, but drying is the only environment change tested at Portland State so far.
“The reason we tested desiccation is there are some rather strange data, particularly in hot spring viruses, that seem to indicate that these hot spring viruses are getting from one hot spring to another relatively quickly on a geologic time scale,” Stedman explained.
The dispersed viruses could land in a much drier environment if they were launched using a geyser, steam vent or volcanic explosion, he said. Then they could be protected by the silica coat that they acquired in the hot spring.
Previous research from volcanic springs worldwide revealed a remarkably similar genetic heritage of these viruses, he said. Dispersal by geysers, fumaroles or even volcanic explosions is only one possible explanation for having viruses that are extremely genetically similar, but there are other genetic or environmental explanations that could come into play, he added.
In tested conditions similar to those high in Earth’s atmosphere — which has a temperature and pressure similar to that of the frosty Martian surface — Stedman’s team found that the viruses can get their mojo back as long as they are out there for less than a month. Any longer and their activity ceases, for reasons Stedman’s team is still puzzling out.
“One of the issues with any astrobiology implications is, ‘Could something be transported from a meteorite, from one planet to another?’ In the conditions we looked at, No. There is a loss of that activity.”
While Stedman’s work has interplanetary implications, the NASA-funded research also could help protect flu vaccines on Earth. These sensitive and potentially life-saving vaccines often are ruined by something as simple as a power failure in the fridge used to store them. If there were a way to stabilize them with a silica coat until they were needed, this could make vaccines much easier to store and transport, particularly in developing countries.
The research is nowhere near those applications, Stedman cautioned, but it is something he is interested in furthering if he can. (He has filed a patent and is continuing research.)
“10 percent to 40 percent of vaccines spoil and have to be thrown away,” Stedman said, adding that the Bill & Melinda Gates Foundation has poured money into transportable refrigerators to get these vital shipments into the developing world. But Stedman hears of examples in the United States, as well: “One of my colleagues, his wife is a pharmacist. Their fridge died and they had to throw out the flu vaccine.”
Major funding for this research came from the NASA Astrobiology Institute Director’s Discretionary Fund as well as the National Science Foundation. Stedman’s results will be printed in the Journal of Virology on Dec. 15.
An alien signal could take many forms. From radio to light, and even genetic manipulation only visible in DNA, extraterrestrial communication could be extremely diverse.
Whatever method aliens use to communicate, scientists at the Toronto Science Festival on Sept. 29 said they want to be ready for it. Staying ready means keeping on top of new technologies as they are introduced.
One example comes from Shelley Wright, an astronomer at the University of Toronto. Wright is building a near-infrared optical detector for use in the search for extraterrestrial intelligence (SETI). The device is designed to look for very short laser light pulses in space. These brief flashes could be thousands of times brighter than a typical star, making them excellent beacons, she said.
“We’ve had 40 years of radio searching and a decade of optical SETI,” Wright said at the panel. “We’re asking how to take this further, in different directions. What are we not thinking of?”
Only a glassful of observations
The first SETI search took place between April and July of 1960, when radio astronomer Frank Drake’s team pointed a National Radio Astronomy Observatory telescope at the sky for six hours each day. The team was searching for pulses in the 21-centimeter emission range. While Drake found nothing coming from outside of the planet, his technique sparked decades of observations of radio signals.
That search continues today. In the past decade, optical searchers have also contributed to research seeking out intelligent life beyond Earth. Those efforts, although frequent, have only sampled a tiny bit of the sky. If the sky’s volume were equivalent to that of Earth’s oceans, researchers would have only sampled one 8-ounce glass of water using all wavelengths so far, said retired SETI astronomer Jill Tarter.
“We’ve hardly begun to search, and what’s left to be done — you just can’t underestimate the significance of that search,” she said.
“But the good thing is that the technology is not increasing lineally. It’s increasing exponentially, and with an exponential [curve], the good stuff comes at the end. That last doubling is doing as much as everything else as you ever done. It’s not discouraging. It’s exciting.”
Communicating across large distances
Astronomy’s biggest discoveries are now coming in the form of planets, Wright said. While astronomers are still on the hunt for a second Earth, she said, the range of planets coming to light indicates that microbial life is more and more likely in other areas. Intelligent extraterrestrial life, however, is another search entirely.
Credit: Elizabeth Howell
“This development in astronomy, where we may be able to look at a planet far away and say that looks alive, with it will come a challenge,” said York University anthropologist Kathryn Denning, who participated in the panel by phone.
“We won’t be able to send direct instrumentation to go there and look at and see that particular life forms are there. We’ll just be able to look at the planet and say it looks alive.”
Some astronomers are also asking what kind of message Earth itself can send into space. The Voyager 1 and 2 “golden records” were one example, but Tarter said they only portrayed the positive side of Earth: no warfare, no hunger and no disease.
To try again, Tarter cited a new campaign to allocate computer space on the Pluto-bound New Horizons spacecraft. The initiative, if it is successful would see a message from humanity crowdsourced from the masses and then placed into storage on New Horizons’ hard drive as a message in a bottle for any aliens that come across it. Tarter added that the message would be placed on the computer after the spacecraft’s primary mission is complete..
“It’s another opportunity to talk among ourselves, across the planet, about who we are and what our future will be,” Tarter said.
The exoplanet Kepler-78b, whose supertight orbit baffles astronomers, is just 20 percent wider and about 80 percent more massive than Earth, with a density nearly identical to that of our planet, two research teams report in separate papers published online today (Oct. 30) in the journal Nature.
“This is the planet that, in many respects, is the most like Earth that’s been discovered outside our solar system,” said Andrew Howard, of the University of Hawaii at Manoa’s Institute for Astronomy and lead author of one of the studies. “It has approximately the same size. It has the same density, which means it’s made out of the same stuff as Earth, in all likelihood.”
Kepler-78b, whose discovery was announced last month, orbits a sunlike star in the constellation Cygnus, about 400 light-years from Earth.
The alien world circles 900,000 miles (1.5 million kilometers) or so from its parent star — just 1 percent of the distance between Earth and the sun— and completes one lap every 8.5 hours. Surface temperatures on Kepler-78b likely top 3,680 degrees Fahrenheit (2,000 degrees Celsius), Howard said.
The planet was found by NASA’s prolific Kepler space telescope, which has spotted nearly 3,600 potential exoplanets since its March 2009 launch. (Kepler was hobbled in May of this year when the second of its orientation-maintaining reaction wheels failed, but scientists are still sifting through the instrument’s huge databases.)
Kepler flagged alien worlds by noting the telltale brightness dips they caused when passing in front of, or transiting, their parent stars from the spacecraft’s perspective. Kepler’s measurements allow researchers to estimate an exoplanet’s size but not its mass, meaning that other strategies are required to get a handle on a world’s density and composition. [Gallery: A World of Kepler Planets]
One such method is the radial velocity technique, which measures the wobble in a host star’s light induced by the gravitational pull of an orbiting planet. Both new studies employed this method to investigate the Kepler-78 system, with Howard’s group using the HIRES spectrograph at Hawaii’s Keck Observatory and another team, led by Francesco Pepe of the University of Geneva, relying on the new HARPS-N instrument on the Telescopio Nazionale Galileo in the Canary Islands.
The two teams came to very similar conclusions. Howard’s group determined Kepler-78b’s mass to be 1.69 times greater than that of Earth, while Pepe’s team calculated it to be 1.86 times higher than Earth’s. The results of the Pepe-led study suggest a density of 5.57 grams per cubic centimeter for Kepler-78b, while those of Howard’s team imply a density of 5.3 grams per cubic cm.
These numbers agree to within the error range independently estimated by both teams, suggesting that they are quite accurate, Howard said.
“The fact that we agree to within our errors — in science, that’s basically as good as you can do,” Howard told SPACE.com.
Earth’s density is about 5.5 grams per cubic cm, so Kepler-78b probably has an Earth-like composition, complete with a rocky interior and an iron core, both studies suggest.
A mysterious origin
The extremely tight orbit of Kepler-78b puzzles astronomers. According to prevailing theory, the alien world shouldn’t exist where it does, because its host star was significantly larger when the planet was taking shape.
“It couldn’t have formed in place because you can’t form a planet inside a star,” Dimitar Sasselov, of the Harvard-Smithsonian Center for Astrophysics and a member of the Pepe-led team, said in a statement. “It couldn’t have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma.”
What is clear, however, is that Kepler-78b’s days are numbered. The planet will continue circling lower and lower until the immense gravity of its host star tears it apart, likely within 3 billion years or so.
“Kepler-78b is going to end up in the star very soon, astronomically speaking,” Sasselov said.
The search for another Earth
The hellishly hot Kepler-78b is not a good place to hunt for alien life. But the determination of its density marks a milestone in the ongoing search for a true “Earth twin” — a planet very much like Earth in size, composition and surface temperature.
“The existence of Kepler-78b shows that, at the very least, extrasolar planets of Earth-like composition are not rare,” astronomer Drake Deming, of the University of Maryland, writes in an accompanying commentary article today in the same issue of Nature.
Deming points to NASA’s upcoming Transiting Exoplanet Survey Satellite mission, or TESS, which is slated to launch in 2017 to hunt for transiting planets around nearby stars (as contrasted with Kepler, whose gaze was more distant).
“By focusing particularly on small stars cooler than the sun, TESS should find exo-Earths whose mass can be measured by trading the close-in orbit of Kepler-78b for more distant orbits around low-mass stars, approaching orbital zones where life is possible,” Deming writes. “That trade-off probably cannot be pushed to the point of measuring an Earth twin orbiting once per year around a sun twin, but it will allow future scientific teams to probe habitable planets orbiting small stars.”
If scientists ever find a true diamond planet in an alien solar system, they shouldn’t expect to see life teeming on its surface.
The same carbon-rich conditions that might give rise to a so-called diamond planets could also create waterless environments too hostile to support life, new theoretical research shows.
“It’s ironic that if carbon, the main element of life, becomes too abundant, it will steal away the oxygen that would have made water, the solvent essential to life as we know it,” study collaborator Jonathan Lunine, of Cornell University, explained in a NASA statement. [The Strangest Alien Planets Ever (Gallery)]
Though carbon is the main building block of life on Earth, it is relatively scarce in our solar system. Our sun is actually a quite carbon-poor star, and our home planet is comprised mostly of silicates, scientists said.
But to imagine what a more carbon-based exoplanet system might look like, a group of researchers funded by NASA turned to theoretical models. The team found that carbon-rich alien solar systems would likely lack the icy comets and asteroids thought to have given rise to Earth’s oceans.
These icy space rocks are thought to have originated far our in our solar system, past a boundary called the “snow line” before slamming into Earth and depositing water on its surface. But in the models of carbon-rich star systems, this water was nonexistent.
“There’s no snow beyond the snow line,” Torrence Johnson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., said in a statement.
The results could influence how scientists view potentially habitable rocky planets like Earth that lurk outside of our solar system. Exoplanets in the “habitable zone” lie in an orbit around their star where temperatures would be warm enough for water to pool on the surface. But the chemical composition of the planet could influence whether or not liquid water is even a possibility for these words, the research suggests.
“All rocky planets aren’t created equal,” Lunine said in a statement. “So-called diamond planets the size of Earth, if they exist, will look totally alien to us: lifeless, ocean-less desert worlds.”
The computer model results were published in the Astrophysical Journal last year. The research also was presented this month at the American Astronomical Society Division of Planetary Sciences meeting in Denver.
An alien planet thought to be made largely of diamond may be less than glittering inside, new research shows.
The notion of a diamond planet hinges on an abundance of carbon. A few years ago, scientists reported that the star at the center of a solar system 40 light-years from our own had a carbon-to-oxygen ratio greater than one.
“This observation helped motivate a paper last year about the innermost planet of the system, the ‘super-Earth’ 55 Cancri e,” University of Arizona astronomy graduate student Johanna Teske, explained in a statement.
Planet 55 Cancri e is what’s known as a super-Earth because it is likely a rocky world orbiting a sun-like star, but it has a radius twice as large as that of our own planet, and a mass eight times greater. The hot planet also races around its star at such a close distance that one year lasts just 18 hours.
Drawing on indirect observations of the planet, researchers of the 2012 study suggested its interior contained more carbon than oxygen. But in a new analysis, Teske and colleagues found that the host star doesn’t appear as carbon-rich as previously thought, which might dash hopes for a diamond-packed 55 Cancri e.
Instead of just using one measure for the distant star’s chemical signatures, Teske’s group averaged different indicators of chemical abundance that were not considered previously.
“We find that because this particular host star is cooler than our Sun and more metal-rich, the single oxygen line analyzed in the previous study to determine the star’s oxygen abundance is more prone to error,” Teske explained. “Averaging all of these measurements together gives us a more complete picture of the oxygen abundance in the star.”
The researchers found that the host star may actually contain nearly 25 percent more oxygen than carbon. That’s about halfway between the carbon ratio of our sun and the ratio suggested by the previous study.
“In theory, 55 Cancri e could still have a high carbon to oxygen ratio and be a diamond planet, but the host star does not have such a high ratio,” Teske said in a statement. “So in terms of the two building blocks of information used for the initial ‘diamond-planet’ proposal — the measurements of the exoplanet and the measurements of the star — the measurements of the star no longer verify that.”
Teske also explained that the compositions of planets and stars don’t always match; the makeup of a planet also seems to depend on planet-forming processes that are not fully understood, she said.
“Depending on where 55 Cancri e formed in the protoplanetary disk, its carbon-to-oxygen ratio could differ from that of the host star,” Teske said in a statement. “It could be higher or lower. But based on what we know at this point, 55 Cancri e is more of a ‘diamond in the rough.’”
Chunks of diamonds may be floating in hydrogen and helium fluid deep in the atmospheres of Saturn and Jupiter. What’s more, at even lower depths, the extreme pressure and temperature can melt the precious gem, literally making it rain liquid diamond, researchers said.
“The new data available has confirmed that at depth, diamonds may be floating around inside of Saturn, some growing so large that they could perhaps be called ‘diamondbergs,’” officials from California Specialty Engineering in Pasadena, Calif., wrote in a statement announcing the discovery today (Oct. 9). Planetary scientists Mona Delitsky of CSE and Kevin Baines of the University of Wisconsin-Madison conducted the research.
Diamonds can form when elemental carbon, like graphite or soot created by huge lightning storms on Saturn, falls into the deep atmosphere of the planet where it is crushed into the gem, Baines and Delitsky said. Those solid diamonds then move farther into the depths of the planet, where they turn into a liquid near the core.
Scientists have known that stable diamonds may exist in the relatively chilly cores of Neptune and Uranus, but until now, Jupiter and Saturn were thought too hot to allow for solid, stable diamond formation.
“Diamonds are forever on Uranus and Neptune and not on Jupiter and Saturn,” Delitsky and Baines wrote.
In a book called Alien Seas (Springer 2013), Baines and Delitsky detailed the story of how robotic mining ships may eventually be able to roam the interior of Saturn sometime in the future to collect diamonds and return them to Earth.
Although it is still a mysterious process, scientists think that, on Earth, diamonds form naturally when carbon is buried about 100 miles (160 kilometers) below the surface. The future diamond then needs to be heated to approximately 2,000 degrees Fahrenheit (1093 degrees Celsius) and squeezed under pressure of around 725,000 pounds per square inch. It also needs to quickly move to the Earth’s surface — usually catching a ride with some fast-moving magma — to cool down.
The new diamond research was presented today (Oct. 9) at the American Astronomical Society’s Division for Planetary Sciences 45th annual meeting in Denver.
Alien planets could also play host to diamonds. An exoplanet 40 light-years from the solar system is made largely out of diamond, astronomers have said. Scientists think the planet (named 55 Cancri e) is more carbon-rich than Earth, an ideal environment for diamond formation.
A new study from the University of Arizona casts doubt on that conclusion, however. The new research suggests that 55 Cancri e might not be quite so diamond laden because its host star is not as carbon-rich as expected.
By analyzing photos taken by the Hubble Space Telescope, scientists at the SETI Institute in Mountain View, Calif., have caught sight of Naiad, the innermost of Neptune’s moons. The 62-mile-wide (100 kilometers) moon has remained unseen since the cameras on NASA’s Voyager 2 spacecraft discovered it in 1989.
Scientists recently tracked Naiad across a series of eight archival images taken by Hubble in December 2004 after using a different technique to help cancel out Neptune’s glare. Neptune is 2 million times brighter than Naiad, so Naiad is difficult to see from Earth, SETI officials said. [See photos of Neptune, the mysterious blue planet]
“Naiad has been an elusive target ever since Voyager left the Neptune system,” SETI scientist Mark Showalter said in a statement. Showalter announced the new findings today (Oct. during a session at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences, held in Denver.
Now that scientists have spotted the small moon again, there are other mysteries to be solved. Naiad seems to have drifted off course: The new observations show that the moon is now ahead of its predicted path in orbit around Neptune, SETI officials said.
Scientists expect that the new trajectory could have something to do with Naiad’s interaction with one of Neptune’s other moons that caused the innermost moon to speed up in its orbit. The exact cause of the moon’s new orbit won’t be known until researchers collect more data.
The images taken in 2004 also reveal something about the ring arcs surrounding Neptune. Voyager observed four arcs during its flyby of the system, but the newly processed images show that the two leading arcs are absent, while the two trailing arcs haven’t changed, SETI officials said. Scientists aren’t sure what is causing this change, but the arcs have been shifting since their discovery.
“It is always exciting to find new results in old data,” Showalter said. “We keep discovering new ways to push the limit of what information can be gleaned from Hubble’s vast collection of planetary images.”
The same images taken by Hubble also helped Showalter and his colleagues find another small moon orbiting Neptune — a discovery they announced in July. The newfound moon, called S/2004 N 1, is much smaller than Naiad, at 12 miles (20 km) across, but it was easier to spot in the images because its orbit takes it farther from Neptune than Naiad’s orbit takes it from the planet, SETI officials said.
S/2004 N 1 evaded Voyager 2′s cameras in 1989 because of its tiny size. During its flyby, Voyager revealed six previously unknown moons circling Neptune. Scientists have now discovered 14 moons in orbit around the blue planet.
Astronauts sent to Mars on future space missions may have to contend with a toxic chemical known as perchlorate that’s thought to be widespread in Red Planet dirt. But perchlorates are already proving problematic for researchers using robots to hunt for possible traces of Martian life, a new study has found.
As part of its science mission, NASA’s Mars rover Curiosity heats up scoops of Red Planet dirt to test for organic carbon compounds — the building blocks of life on Earth.
But that heat can cause perchlorates in soil samples to set off a chemical reaction that destroys organics, researchers discovered. [The Hunt for Martian Life: A Photo Timeline]
“The presence of perchlorates isn’t good news for some of the techniques we’re currently using with Curiosity,” study lead author Daniel Glavin, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Md., said in a statement. “This may change the way we search for organics in the future on Mars.”
Perchlorates, which are salts comprised of chlorine and oxygen, were first detected in Martian polar soil by NASA’s Phoenix lander in May 2008. More recently, Curiosity found perchlorates while trekking around the Rocknest sand dune in November 2012.
Curiosity’s Sample Analysis at Mars (SAM) system uses a pyrolysis gas chromatograph mass spectrometer, which is an instrument that breaks soil down into its chemical components and measures the concentration of each type of molecule.
But when perchlorates in these soil samples are heated above 392 degrees Fahrenheit (200 degrees Celsius), they release pure oxygen, the researchers say. This oxygen then causes organic molecules in the sample to combust into carbon dioxide.
However, Glavin said not all of the organic carbon would be destroyed in this reaction; some might be preserved inside more heat-resistant materials, or the molecules could possibly be detected before the breakdown of perchlorates. Scientists might be able to account for the organic carbon that has combusted if they assume a certain baseline of perchlorate in Martian dirt, he added.
The recent findings at Rocknest could help scientists establish this baseline.
“It will be absolutely critical as we move on to other samples to compare them to the Rocknest dune to infer the presence or absence of Martian organic material,” Glavin said in a statement.
The sandy site dubbed Rocknest is inside Gale Crater, where Curiosity landed in August 2012. Based on the rover’s analyses of Rocknest dirt, scientists also reported this week that surface soil on the Red Planet contains about 2 percent water by weight. That means astronauts on Mars could one day extract roughly 2 pints (1 liter) of water out of every cubic foot (0.03 cubic meters) of dirt they dig up.
The soil-water study was one of five published in the journal Science on Thursday (Sept. 26) describing what scientists have learned about Martian surface materials from Curiosity’s first 100 days on the Red Planet.
The study on Martian perchlorates was detailed in the Journal of Geophysical Research: Planets.
Scientists have long debated the possibility of that the microbial seeds of life did not originate on Earth, but were perhaps delivered here from an alien source, encased in comets or meteorites from Mars.
But to get here, simple life forms would have had to endure a litany of harsh cosmic conditions, including ejection into space, freezing temperatures, fiery re-entry and impact.
Now, a team of researchers found new evidence that a terrestrial algae just might be able to survive the physical strains of space travel, a discovery that may support the possibility that panspermia, the concept that microbial life is everywhere in the universe and can spread between planets, could potentially occur. [5 Bold Claims of Alien Life]
The scientists, who presented their findings at the European Planetary Science Congress in London on Sept. 12, focused on a type of single-celled ocean-dwelling algae called Nannochloropsis oculata.
Using a two-stage light gas gun, the researchers shot frozen pellets of the algae into water at extremely high speeds and then analyzed their sample to see if any of the organisms came out alive.
“As you might expect, increasing the speed of impact does increase the proportion of algae that die, but even at 6.93 kilometers per second (4.31 miles per second), a small proportion survived,” study researcher Dina Pasini, of the University of Kent explained in a statement. “This sort of impact velocity would be what you would expect if a meteorite hit a planet similar to the Earth.”
The researchers say their findings suggest that alien life and panspermia may not be impossible, though the theory still remains unproven. Pasini and colleagues noted that space travel might not be so bad for a tiny life-form. Enclosed in a natural spaceship of rock and ice, alien organisms might be protected from radiation and extreme heat.
“Our research raises several questions,” Pasini said in the statement. “If we find life on another planet, will it be truly alien or will it be related to us? And if so, did it spawn us or did we spawn it? We cannot answer these questions just now, but the questions are not as farfetched as one might assume.”
Pasini’s research is not affiliated with another study announced Thursday (Sept. 19) that claims to have found evidence of alien life in Earth’s upper atmosphere.
That separate study is based on findings by British scientists who deployed a research balloon over England. The balloon returned a sample containing the cell wall of a diatom, a type of microscopic algae.
The British researchers, led by astrobiology researcher Milton Wainwright, of the University of Sheffield in the United Kingdom, took the discovery as proof that life is continually arriving to Earth from space and perhaps didn’t originate on our planet, but the claims have been met with wide skepticism.
Landing astronauts on Mars is a tall order, but bringing them back to Earth promises to be even trickier — especially if Red Planet explorers get the sniffles on the long flight home.
Sick astronauts could conceivably have been infected on Mars, some parts of which may be capable of supporting life as we know it. So the world may be reluctant to welcome such travelers home, leery of possibly unleashing an extraterrestrial superplague on Earth’s 7 billion people.
NASA is already thinking about how to deal with this concern as it works toward getting people to the vicinity of Mars by the mid-2030s. The key is to monitor the health of astronauts meticulously during all phases of Red Planet missions and any other deep-space efforts, said Cassie Conley, NASA’s planetary protection officer. [The Boldest Mars Missions in History]
“The ability to have documentation to justify to the rest of the Earth why this really isn’t some nasty disease from Mars, it’s actually something totally normal and we expected it — we saw it when we went to the moon, we saw it when we went to asteroids, we know this is a result of nonliving exposure, it has nothing to do with some potentially Martian disease — that, I think, is going to be the most important aspect of doing planetary protection on human missions [to Mars],” Conley said last month during a presentation with NASA’s Future In-Space Operations working group.
Of course, NASA will also be doing its best to minimize the chances that astronauts could pick up a potentially pathogenic Martian organism while roaming the surface. For example, human explorers will steer clear of “special regions” — defined as areas where Earth microbes could likely survive and reproduce — and they won’t set foot in a Martian locale that hasn’t been visited and vetted by a robot first.
These and other guidelines are laid out in a rough planetary protection protocol drawn up in 2008 by the Committee on Space Research (COSPAR), which is part of the International Council for Science. NASA and the European Space Agency have committed to follow this protocol, whose top priority is to protect Earth from any possible “back contamination” from Mars. (The policy seeks to safeguard the Red Planet against “forward contamination” from Earth as well.)
But astronauts on the surface will inevitably come into contact with some Martian material no matter what precautions mission planners devise, Conley and other experts say, potentially lending a sinister edge to the slightest sneeze or cough.
And Red Planet explorers are highly likely to get sick.
“They’re going to have runny noses, they’re going to have some skin rash,” Conley said. “People who are in a small, contained environment for hundreds of days — that happens to them.”
Astronauts should assiduously track the nature and severity of these various illnesses as part of their concerted health-monitoring efforts, she added, with one crewmember assuming primary responsibility for implementing planetary protection protocols throughout the entire mission.
NASA’s working plan also calls for quarantine capabilities and appropriate medical testing to be provided to crews returning from Mars — and for everyone associated with any manned Red Planet mission to keep things in the proper perspective.
“Six people going on a mission to Mars — if something happens to them, that’s a really bad tragedy and we want to prevent that as much as possible,” Conley said. “But six people bringing some horribly infectious, horribly damaging organisms back from Mars to Earth is a global tragedy, and there is a difference in scale there that has to be recognized.”
NASA’s Voyager 1 probe won’t rest on its laurels after becoming the first manmade object ever to reach interstellar space.
Voyager 1 arrived in interstellar space in August 2012 after 35 years of spaceflight, researchers announced Thursday (Sept. 12). While this milestone is momentous enough in its own right, it also opens up a new science campaign whose potential already has scientists salivating.
“For the first time, we’re actually going to be able to put our hands in the interstellar medium and ask what it does and what characteristics it possesses,” Gary Zank, director of the Center for Space Plasma and Aeronomic Research at the University of Alabama in Huntsville, told reporters Thursday. “It’s a tremendous opportunity.”
Into the unknown
Voyager 1 and its twin, Voyager 2, launched a few weeks apart in 1977 to study Jupiter, Saturn, Uranus and Neptune, as well as the moons of these outer planets.
The probes completed this historic “grand tour” in 1989, then embarked on a quest to study the outer reaches of the solar system and beyond.
Voyager 1 finally popped free of the heliosphere — the huge bubble of charged particles and magnetic fields that the sun puffs out around itself — on or around Aug. 25, 2012, becoming humanity’s first envoy to the vast realms between the stars.
“This is truly a remarkable achievement,” Zank said. “We’ve exited the material that’s created by the sun, and we’re in a truly alien environment. The material in which Voyager finds itself is not created by the sun; it’s created, in fact, by our neighboring stars, supernova remnants and so forth.”
Many discoveries to come
This new vantage point should yield big scientific dividends, Zank added. For example, Voyager 1 should now help researchers get a much better look at galactic cosmic rays, charged particles accelerated to incredible speeds by far-off supernova explosions.
Observations of galactic cosmic rays made from within the heliosphere are not ideal, since the solar wind tends to affect these high-energy particles substantially.
“Being outside the heliosphere allows us an opportunity to, in a sense, look at the undiluted galactic cosmic ray spectrum,” Zank said. “That will tell us a great deal more about the interstellar medium at very distant locations. It’ll tell us about how the galactic cosmic rays propagate through this very complicated interstellar medium.”
Voyager 1 should also be able to shed light on the nature of the instellar medium, and how material from other stars flows around the heliosphere, researchers said.
“Now we will be able to understand and measure and observe that interaction, which is a very important part of how the sun interacts with what’s around it,” Voyager chief scientist Ed Stone, a physicist at the California Institute of Technology in Pasadena, told SPACE.com.
In short, reaching interstellar space does not mark the end of the road for Voyager 1, which should be able to continue gathering data for a dozen more years as long as nothing too important breaks down. (The probe’s dwindling power supply will force the mission team to turn off the first instrument in 2020, and all of Voyager 1′s science gear will be shut down by 2025.)
“This mission is not over,” said Voyager project manager Suzanne Dodd, of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Many, many more discoveries are out there, yet to come.”
The headlines have been coming thick and fast.
A trio of super-Earths found in the habitable zone of the star Gliese 667C, two probably rocky planets in the Goldilocks zone around Kepler-62 and possible super-Earths orbiting Tau Ceti and HD 40307 at just the right distance for liquid water to exist on their surfaces, albeit under certain conditions.
These are all just from the past twelve months. Should those exoplanet hunters who are seeking out Earth 2, a planet where life as we know it could possibly exist, start to feel excited? [The Strangest Alien Planets (Photos)]
Not yet. Our knowledge of these planets is woefully incomplete. However, the times may be changing. While we cannot yet determine whether a planet is hospitable to life, David Kipping of the Harvard–Smithsonian Center for Astrophysics has led a team of astronomers to develop a new theoretical model that can tell us with one swift glance whether a super-Earth — a world with two to 10 times the mass of our planet and up to twice the diameter — has an atmosphere that might not be suitable for life.
Consequently, we could rule such worlds out of our search for analogs to Earth. It’s all about whether a planet has an atmosphere and how that atmosphere is connected to the relationship between a planet’s mass and diameter.
The two main exoplanet detecting techniques are beautifully complementary. When a planet transits its star — that is, passes in front of its star, blocking a fraction of the starlight — we can determine the diameter of the planet from the size of the transit. Meanwhile, that orbiting planet also exerts a gravitational tug on its parent star. If we can detect that tug we can calculate the planet’s mass based on the extent by which the planet is pulling on the star.
The only problem is that not all planets orbit their star at an appropriate angle for us to see a transit, while some exoplanets and their stars are too distant and faint for us to accurately measure their “radial velocity” tug (many of the Kepler spacecraft’s candidate planets fall into this category).
However, for those worlds where we are fortunate to know both properties, we can work out a planet’s volume and then divide the mass by the calculated volume to determine the planet’s density, which tells us whether it is likely rocky, gaseous or icy.
The computer model that Kipping has developed, along with Harvard’s Dimitar Sasselov and Princeton’s David Spiegel, allows an astronomer to plug in these numbers for mass and radius and, with the knowledge of the density, figure out if a planet — in particular a super-Earth — has a light but extended atmosphere or a relatively thin, heavy atmosphere.
That’s important because Earth’s atmosphere is the latter kind — a 100 kilometer (62 mile) layer filled with the likes of nitrogen, oxygen, carbon dioxide, argon, water vapor and neon that contributes just 1.5 percent of Earth’s radius. We don’t know if an extended atmosphere of mostly hydrogen and helium — similar to Uranus’ or Neptune’s atmospheres but warmer — could support life, and so searches for Earth’s twin may want to avoid such worlds.
Solid, liquid or gas
The way Kipping, Sasselov and Spiegel’s model makes use of a graph that plots a planet’s mass against its radius, and where a world falls on that graph, tells us whether it is solid rock, partly watery or has a significant fraction of gas.
“There’s a full range of models that we think a super-Earth can be built out of,” Kipping said. “You can make them out of iron, or out of silicate, or out of water, or some mixture of those things.”
However, when a planet transits a star, not only does the solid body of the planet block some of the starlight, but so too does its atmosphere. By simply detecting the planet’s silhouette we cannot automatically figure out which part is solid and which part is gaseous atmosphere. The mass-radius diagram, however, offers a way around this problem. [9 Exoplanets That Could Host Alien Life (Countdown)]
Kipping and his cohorts have calculated theoretical limits — boundary conditions — for each type of planet. The lower boundary condition denotes a super-Earth made of solid rock with an iron core and lacking an atmosphere. The top boundary signifies a planet made entirely of water that, Kipping said, is probably impossible — there needs to be a solid core in there somewhere — and thus you cannot get a super-Earth less dense than a water-world (purely gaseous planets, it is thought, cannot exist as small as super-Earths and even Neptune-type worlds have a large rocky core lurking inside them).
Therefore, if you discover a planet and plot its mass against its radius only to discover that it resides on the graph above the impossible pure-water line, then the only way to explain its apparent density given its radius is that it must have a large atmosphere.
Such mass-radius models have been around for a while, but what makes Kipping’s different is that they are based on a new understanding of the physics of materials placed under the enormous amounts of pressure that the interior of a super-Earth would impose on them. Dimitar Sasselov, along with his student Li Zeng, was able to create superior models of the interior of super-Earths using new laboratory technology that is able to simulate those pressures.
They published their work in the March 2013 issue of the Publications of the Astronomical Society of the Pacific and Kipping’s mass-radius diagram, itself to be published in the Monthly Notices of the Royal Astronomical Society, is modeled around those interior structures derived by Sasselov and Zeng.
What does the model tell us about super-Earths we have already discovered? Kipping, Spiegel and Sasselov concentrated on GJ 1214b, a world with six and a half times the mass and two and a half times the diameter of our planet that is orbiting a red dwarf star 47 light years away.
Prior to now the planet had been a puzzle — no matter what wavelength it was observed in, the size of the planet was always the same, which shouldn’t happen because an atmosphere should be more opaque to some wavelengths than others. Was its atmosphere extended and topped with thick, opaque clouds, or was its atmosphere thin enough not to be noticed? Employing the mass–radius diagram settles the matter.
“Our method says that 20 percent of this planet’s radius is pure atmosphere, which strongly favors the idea of a very light, extended hydrogen-helium atmosphere with clouds on top,” Kipping said. “So we are able to come into this discussion with these two possibilities and say which is more likely, just based on the simple measurement of the mass and the radius of the planet.”
Another intriguing world is Kepler-22b, which was the first habitable zone planet to be discovered by NASA’s Kepler spacecraft. Around 620 light-years from Earth, it orbits a sun-like star at a distance of 0.85 astronomical units (one astronomical unit is the average distance between Earth and the sun, 149.6 million km) and has a diameter two and a half times that of our planet. [Gallery: A World of Kepler Planets]
“We tried to apply our technique to this planet but unfortunately the mass measurement is very poor because it is a very distant star,” Kipping said. “What we found was that the data was unable to say one way or another what kind of planet it is; it sits right on the blue [water-world] line, so we can’t tell whether it is a rocky planet with an extended atmosphere or a water-world with very little atmosphere.”
Unfortunately that’s also the story for the rest of the potentially habitable planets discovered so far, a list of which is maintained by Professor Abel Mendez of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, in the form of the Habitable Exoplanets Catalog.
One dozen planets currently reside on the list, meeting the criteria of being (probably) rocky and existing within their star’s habitable zone. However, as we found with Kepler-22b, in most cases either the mass or the radius is little more than an estimate, and as such the majority tend to sit on that boundary condition.
“Astronomers estimate mass or radius from the assumption that smaller planets are more rocky in composition and those larger planets close to two Earth radii are water-worlds,” Mendez said. “This seems to be a good estimate for most cases but there is a lot of uncertainty; for example, Kepler-11f has just over two Earth masses but it is a gas planet, while Kepler-20b with about nine Earth masses is rocky.”
Kipping’s mass–radius diagram is only half the job. Without good data the new mass-radius relationship is limited in what it can tell us. For Kepler planets, better mass measurements from radial velocities are required, but this is tricky given that most of the stars around which Kepler discovers planets are faint and distant.
For those worlds discovered by radial velocity, we need more luck in observing transits to give us their diameter. The approval of the Transiting Exoplanet Survey Satellite (TESS), which is scheduled to launch in 2017 and will systematically survey all the brightest stars in the sky for transiting planets, will be a massive boon to the field.
“The TESS mission promises to dramatically change this picture,” said Heather Knutson, a planetary astronomer at the California Institute of Technology whose research is focused in the area of exoplanet atmospheres. “At the moment there are currently only three transiting super-Earths that are suitable for detailed characterization and all three have been observed with either the Spitzer or Hubble space telescopes, or both. In the era of TESS we will have far more super-Earths than we can reasonably study and Kipping’s criterion will provide a useful means to select targets that are likely to have detectable atmospheric signatures.”
The launch of the James Webb Space Telescope (JWST) a year after TESS will also dramatically boost the nascent science of exoplanetary atmospheric investigations. JWST, with its 6.5-meter mirror, will extend its observations well into the near-infrared, perfect for picking up the tenuous signatures of water, methane, oxygen, carbon monoxide and carbon dioxide in atmospheres, which could be interpreted as biosignatures depending upon their concentrations. TESS will identify the planets, the mass-radius model will decide which ones we want to observe, and JWST will tell us about them. It is going to be an exciting time and the wait will be excruciating for scientists. [See a video about the JWST]
“At this point almost anything is possible!” Knutson said.
Finding a potentially habitable planet
There are many factors that go into making a planet habitable, from the presence of a magnetic field to protect its atmosphere to the question of whether it has plate tectonics to recycle carbon. A stable rotational axis, a moderate impact rate and sufficient gravity are also plausible necessities.
Yet, the possession of an atmosphere, especially one that contains some form of greenhouse gas, is one of the most crucial factors, essential for maintaining cozily warm temperatures that permit all-important liquid water to exist on its surface. That said, the range of suitable atmospheres may not be as narrow as we may think.
“I don’t think that thick hydrogen–helium atmospheres will rule out the potential for life on these planets as long as the pressure at the surface/water transition allows for liquid water,” Mendez said.
So a super-Earth, with a thick envelope of hydrogen swathing a rocky core deep down could still have watery conditions at depths where the pressure, according to Mendez, drops below 10,000 atmospheres, although of course temperature will also have a say where and if this transition point occurs.
There is one more intriguing possibility. On Earth, convection currents and air flows are strongly influenced by what is on the surface, be it oceans, continents or mountains. Could a careful study of the atmosphere of a super-Earth tell us things about the terrain below that are otherwise beyond the capabilities of our telescopes?
“Yes, potentially, but the atmosphere would need to be thin enough for our observations to detect the atmosphere flows from the region close to the surface,” said Knutson, who also points out that a thin atmosphere will be transparent enough for us to spectroscopically measure the surface of the planet and determine whether there are oceans, desert or even plant life.
“When we get these new super telescopes in the future [such as the Thirty Meter Telescope, the Giant Magellan Telescope and the European Extremely Large Telescope] we’ll be able to go down to sort of Earth-like atmospheres,” Kipping said. “In special cases we could probably go down to these very small atmospheres that are potentially life-harboring.”
But we’re getting ahead of ourselves; the new mass–radius model only provides us with a way of saying which planets don’t have a thin atmosphere. If we come to the conclusion that a super-Earth does not have an extended atmosphere, then it might be worth pointing JWST at it to measure the spectrum of any atmosphere present and see whether it is analogous to Earth’s atmosphere. [7 Ways to Discover Alien Planets (Countdown)]
“If you are really hunting Earth-like planets and our method tells you it has a big extended atmosphere, then you are probably wasting your time,” Kipping said. “So it’s a way of making our searches for Earth-analogues more efficient.”
With TESS and JWST and the next generation of extremely large telescopes on the horizon, Kipping’s new model is timely indeed. The way things are going, the next decade might be the decade of the super-Earth. All the hints are it is going to be an exciting time.
Planet hunters keep finding distant worlds that bear a resemblance to Earth. Some of the thousands of exoplanet candidates discovered to date have similar sizes or temperatures. Others possess rocky surfaces and support atmospheres. But no world has yet provided an unambiguous sign of the characteristic that still sets our pale blue dot apart: the presence of life.
That may be about to change, says exoplanet expert Sara Seager of the Massachusetts Institute of Technology in Cambridge. Upcoming NASA missions such as the Transiting Exoplanet Satellite Survey (TESS) and the James Webb Space Telescope, both due to launch around 2018, should be able to find and characterize Earth-like planets orbiting small stars.
Spotting signs of life on those planets will be possible because of progress in detecting not only planets, but their atmospheres as well. When a planet passes in front of its host star, atmospheric gases reveal their presence by absorbing some of the starlight. Oxygen, water vapor or other gases that do not belong on dead worlds could very well provide the first evidence of life elsewhere. [5 Bold Claims of Alien Life]
In 1961, astronomer Frank Drake developed an equation that summarizes the main factors to contemplate in the question of radio-communicative alien life. These factors include the number of stars in our galaxy that have planets and the length of time advanced alien civilizations would be releasing radio signals into space.
Instead of aliens with radio technology, Seager has revised the Drake equation to focus on simply the presence of any alien life. Her equation can be used to estimate how many planets with detectable signs of life might be discovered in the coming years. Presented at a meeting earlier this year, the Seager equation looks like this:
N = N*FQFHZFOFLFS
N = the number of planets with detectable signs of life
N* = the number of stars observed
FQ = the fraction of stars that are quiet
FHZ = the fraction of stars with rocky planets in the habitable zone
FO = the fraction of those planets that can be observed
FL = the fraction that have life
FS = the fraction on which life produces a detectable signature gas
Focusing on M stars, the most common stars in our neighborhood that are smaller and less luminous than the sun, Seager plugged in values for each term. Her calculation suggested that two inhabited planets could reasonably turn up during the next decade.
What was the inspiration behind this equation?
Sara Seager (SS): People have been thinking about trying to find signs of life for a hundred years. This equation is a purposeful take-off on the Drake equation, which was about the search for intelligent extraterrestrial life. Frank Drake wrote that equation because he was using radio telescopes to look for life. It was relevant then and still is. SETI [the search for extraterrestrial intelligence] has been going on now for 50 years.
I wanted to explain that we have a new search in progress. We’ll use TESS to find rocky planets transiting small stars. Then we’ll use the James Webb Space Telescope to observe the atmospheres of those planets, during transits or secondary eclipses. The punchline here is that if we’re really lucky and everything works in our favor, we will be able to infer signs of life on those planets. We have a shot — I’d call it a remote shot — of finding life within the next decade.
Is your approach specific to intelligent life as well?
SS: No. The equation focuses on the search for planets with biosignature gases, gases produced by life that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes. If we find gases that we might attribute to life we will not know if the gases are produced by intelligent life or simple bacteria. [6 Most Likely Places for Alien Life in the Solar System]
Could someone like Drake have sketched out your equation 50 years ago, before the first discovery of an exoplanet?
SS: Somebody probably could have written the equation back in Drake’s day. But back then people didn’t like the idea of habitable planets around M stars. All life requires liquid water, which can only exist on a planet that’s not too hot and not cold. A planet in the “Goldilocks” zone around an M star ends up being tidally locked, like Earth’s moon. It shows the same face to the star all the time and is always hot on one side and cold on the other.
In the old days, people thought that wouldn’t be amenable to life. Modern studies with computers have shown that it’s okay to be tidally locked. If a planet heats up on one side and not the other, the atmosphere can still circulate, because heat wants to move around. Back then people also didn’t know the frequency of habitable-zone planets around M stars and that we would have the capability for detection by 2020.
How confident are you about the values you plugged into this equation?
SS: For some of the terms, you can get a number that’s an estimate with an error bar. We start with the number of stars bright enough to be seen by James Webb. What we need are enough photons to see the starlight shining through the atmosphere of a planet. We know that number: it’s 30,000.
Then we select stars that are quiet. Some stars are like our solar maximum all the time, with flares and other activity. We don’t like those noisy stars. It’s hard to spot a planet transiting in front of the noise, hard to spot the dimming that occurs. Also the ultraviolet light from many active stars would destroy biosignature gases through a complicated series of chemical reactions.
The fraction of planets that can be observed, that are transiting, is just simple geometry. It’s easy to calculate.
What about the fraction of rocky planets in the habitable zone? Calculating that was the Kepler space telescope’s mission, but Kepler broke down earlier this year.
SS: Astronomers have largely completed Kepler data analysis for small-star statistics. Small stars are what we’re interested in. So we have this number, where the number is for quiet stars. It’s 0.15. [Gallery: A World of Kepler Planets]
And the other terms?
SS: Not all of the terms in the equation can be calculated. The last two are just guesses. For the fraction of planets that have life, I put in one. I wanted to be optimistic. It really matters what you speculate for this term. You can put your own number in.
Detectable signatures of gas could mean a lot of things. As human beings, we exhale carbon dioxide. That’s our biosignature gas. But that’s not useful because carbon dioxide in the atmosphere is naturally occurring. There are other possible gases we could look for. Oxygen is produced by plants and photosynthetic bacteria. We have also considered ammonia as a biosignature gas.
I carefully crafted the last term of this equation so one could actually add more information in. Does life produce a detectable signature? Are there systematic effects that rule out some biosignature gases being detected in some planets? Can we not find the signature for technical reasons? We just don’t know how many planets have life that is producing biosignature gases that are detectable by us.