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One of the week's biggest stories in science is a new paper in Nature from scientists who have used the Kepler satellite to measure the mass and size of a Mars-sized extrasolar planet. This is a tricky measurement, because Kepler uses the "transit method" to detect planets around other stars by the slight dimming when a planet passes between us and the star. (If you'd like to try this for yourself, check out the Planet Hunters citizen science project, which lets you look for planets in real Kepler data.) This tells you the orbital period very directly, and the size with a little bit of work, but teasing mass information out of the data relies on the presence of other planets in the system whose gravitational forces cause small changes in the period of the orbit. This is very tricky; thus, "paper in Nature"...
The details of the measurement are a bit outside my scientific wheelhouse, so I'll leave it to people with stronger astronomy credentials to explain exactly how they do that. The NASA press release does contain a bit that points toward stuff that's more up my alley, though, hidden in the third paragraph:
To determine a planet's mass, astronomers typically measure the minuscule movement of the star caused by the gravitational tug of an orbiting planet. For planets the mass of Earth detecting such a tiny tug is extraordinarily challenging with current technology.
This is a reference to the "radial velocity" method of planet detection, and as they say, it's very challenging with current technology. Coincientally, though, I have a couple of friends working on using lasers to improve that technology to a level that would allow the detection of Earth-like planets around distant stars. This has been in the works for a while, and they've recently advanced to the point of testing it by trying to detect an Earth-like planet in our own solar system: Venus.
The radial velocity technique works by detecting the small "wobble" cause by the gravitational tug of an orbiting planet. Like every other force, gravity works on both objects in an interacting pair, so when the Sun's gravitational pull bends the Earth into an orbit, the Earth pulls back on the Sun with exactly the same force, in the opposite direction. From the standpoint of someone looking at the Sun from a distant star, this produces a tiny back-and-forth motion of the Sun as the Earth goes around it. That motion is something that can be detected in the light, because the Doppler effect will shift the frequency of light up slightly when the Sun is moving toward the observer, and down slightly when the Sun is moving away.
This was the first method used to definitively detect an extrasolar planet, back in 1995. The earliest planets found were so-called "hot Jupiters"-- very massive planets very close to their stars. This is mostly due to the limitations of the technique: a big close-in planet produces a large variation in the frequency of the detected light, and a small orbit means you can observe a lot of orbits in a short period of time.
The fundamental problem here is that the variation in the velocity of the star is really tiny. this is relatively easy to back-of-the-envelope estimate for the Earth going around the Sun: the mass of the Earth is about 6x1024kg, and its orbital speed is close to 30,000m/s. If the Earth were the only planet orbiting the Sun, each would have the exact same momentum, in opposite directions. Since the mass of the Sun is about 2x1030kg, for the Sun to have the same momentum as the Earth, it would need to be moving at only 0.01m/s, or about a centimeter per second.
A centimeter-per-second velocity shift will leads to a Doppler shift that changes the frequency of visible light by tens of kilohertz, or less than a billionth of the frequency. This isn't outside the reach of precision spectroscopy, but it requires a spectrometer whose components are stable to within a billionth of the size of the instrument, and that's an extremely challenging problem for an astronomical instrument. It gets even worse when you realize that you need to maintain that stability over many orbital periods-- several years if you were looking for an Earth-like planet in an Earth-like orbit. That's why the press release above calls it "extraordinarily challenging," which is something of an understatement-- it's next to impossible to measure cm/s velocities and have confidence that you're not just seeing drift in the spectrometer.
One possible solution to this is to calibrate the spectrometer very well, comparing light from a distant star to a known source here on Earth to sort out which changes are due to the velocity of the star and which are due to Earthly effects. Traditional calibration sources aren't up to the task, though, which is where the ultrafast lasers come in. Half of the 2005 Nobel Prize in Physics went to John Hall and Theodor Hänsch for development of "optical frequency combs," a technique that turns out to be perfect for telescope calibration.
An optical frequency comb is just an ultrafast laser, with pulses only a few femtoseconds in duration (one fs=0.000000000000001s). If you take such a laser and shoot it into a spectrometer, you'll find that it's made up of a huge range of frequencies, spanning most of the visible band. The spectrum of an ultra-fast laser consists of lots of narrow lines spaced by a characteristic frequency determined by the size of the laser cavity; the typical representation is a plot of intensity versus frequency showing a bunch of upward-going spikes, like teeth in a comb (thus the name).
For a sufficiently fast laser pulse, the frequency of one of the low-frequency "teeth" can be compared to the frequency one at twice the frequency (this is an "octave-spanning" frequency comb). This provides a direct measurement of the small offset caused by effects within the laser cavity, and allows the frequency of any tooth to be determined absolutely. The whole comb can then be stabilized to something like an atomic clock, providing an ideal calibration source for planet-hunting: it gives a broad range of reference lines, covering a wide swath of the visible band, each line with a frequency known to incredible precision. Comparing light from a star to these reference lines allows radial velocity measurements at the centimeter-per-second level, and both Ron Walsworth's group at the Harvard-Smithsonian Center for Astrophysics and Ted Hänsch's group in Germany have developed "astro-combs" for use on telescopes.
Last week at DAMOP, Dave Phillips from the CfA presented on their astro-comb progress, which is now in the testing stage. While the technique is simple in concept, getting it to work in the real world is a challenge, and the sensitivity needs to be tested on a real spectrometer on a real telescope. Thus, before they start looking for Earth-sized planets that may or may not be orbiting other stars, they're looking for an Earth-sized planet that we know for sure exists: Venus.
The CfA astro-comb system is currently in the Canary Islands, attached to the HARPS spectrometer. During the day, when the spectrometer would otherwise be idle, they're using a small telescope to collect sunlight, and compare it to the astro-comb calibrator. They're looking for the tiny frequency shift caused by the gravitational tug of Venus (which is comparable to that of Earth, as Venus is a little smaller than Earth but also a little closer to the Sun), as a proof-of-principle test of the technique.
So far, according to Phillips, they've conclusively demonstrated that the Earth rotates-- they see a large and rapid variation in the frequency light from the Sun over the course of the day due to the relative velocity of their telescope as the Earth rotates. This is a well-known problem, something that extrasolar planet searches already correct for, and when they apply the usual corrections, it looks like their data are good to within several centimeters per second. Now it's just a matter of collecting data over several orbits of Venus (which takes about 225 days to go around the Sun), and checking to see if they can pick up the resulting motion of the Sun.
If everything works as expected, then it's on to look for small planets outside the solar system. And all of this is enabled by ultra-fast lasers here on Earth.
Chad Orzel is a physics professor, pop-science author, and blogger. His latest book is Eureka: Discovering Your Inner Scientist (Basic Books, 2014).