NASA exoplanet researchers have recently announced the TRAPPIST-1 system, a solar system like our own with 7 planets, 49 light-years away. Three of these planets are in the habitable zone, meaning that there is a possibility that life will form on them, just like Earth. While many have been speculating about whether life exists on these planets or whether we will ever be able to visit the system, there are still many issues that astronomers are dealing with. I spoke with Professor Kipping of the Cool Worlds Lab, about the future of exoplanet research.
Professor Kipping is a professor of astronomy at Columbia University. There he runs a group called the Cool Worlds Lab, where he studies extra-solar planets. The name, Cool Worlds Lab, comes from an interest in planets within the habitable zone and beyond it. He is particularly interested in planets like the TRAPPIST-1 system, and planets with moons and rings. Before he was a researcher at Columbia, he received his BA in Natural Sciences at Cambridge University, UK, and his PhD from the University College London, UK.
I spoke with Professor Kipping early in the morning, and he was excited as ever to share what he knew of the subject.
I – Can you tell me the process of finding an exoplanet?
K – There’s a few different methods. The method used to find the 7 new planets, is called the transit method. You can really think of it as looking for eclipses. The moon sometimes eclipses the sun, and that’s very obvious and clear to see. But if the moon was way way smaller it wouldn’t block out all of the light from the sun, it would just block out a small amount. So that’s what we’re doing when we’re looking for these transit exoplanets. We’re looking at distant stars, and although they’re so far away we can’t resolve the planets themselves, we can tell they’re there because sometimes they pass in front of the star and make the star look a bit dimer.
I – You mentioned the habitable zone, can you tell me why that’s so important?
K – Ultimately, one of the major drivers in the quest for doing this kind of science is to hopefully discover life one day; and answer this very ancient question as to whether we’re alone in the universe. So if we want to get to that point, researchers have to think about what types of planets could or could not harbor life on them. That’s a very difficult question to ask, but what we know from looking at all life on Earth is that all life requires liquid water in order to survive. There’s very good arguments from chemistry as to why that happens…
So when we look for planets that could have life on them, what we’re really doing is looking for planets that could have liquid water on their surfaces. A couple of things go into that, the planet needs to have the right temperature, it needs to have an atmosphere, it needs to have a surface, and many of those things we can’t measure directly from these eclipses… Once we know how far from the star we are, and how hot the star is, you can kind of calculate how hot the planet should be. If the planet is in the right zone for liquid water, meaning if the surface is in the temperature range from 0-100 degrees Celsius, then we would say that it’s in the habitable zone.
View of the Habitable Zone
Credit: Chester Harma, Penn State
I – Do you think one of these worlds could contain life, and what are the chances that that life could be intelligent?
K – I think we’re very hopeful, but at this point anything more than that is kind of speculative. So the really interesting thing about this system, is that it doesn’t have just one planet in the habitable zone, it has at least three. And depending on how different researchers define the habitable zone, it might even have four or five. So that’s incredibly exciting! And, there’s not just one opportunity for the system to have life, there’s several. Maybe some of those planets in that zone just doesn’t have the chemistry for life. For example, Venus, is in the habitable zone of our own solar system, and so is Mars.
So our solar system has three planets in the habitable zone, but only one of them, Earth, is truly inhabited, and truly has the right conditions for life…. In Venus’ case the atmosphere is too thick, so it has this greenhouse gas effect, and in Mar’s case the planet is not quite big enough so it loses its atmosphere over time. So Earth is just right in that sense, and in this case all three of these planets have the right size, they’re all bigger than Mars. But we still don’t know if any of these planets have this very thick atmosphere, like Venus, causing this green-house effect. So having three planets in the habitable zone is great, because maybe one of them is bad, but it would be strange if all three had no chance at life…
The other thing I should say is that this star is very different than our sun, and that is one of the big question marks about this system right now. The star is about 8% the mass of our sun… It is the smallest type of star that nature can build. It is extremely faint and small, and the planets have to stay very close to the camp fire in order to stay warm, so we see a very compact system. The danger with that is that being close to the star might have some problems… It’s unclear what that means for life on the planet, it may have sterilized the chance for life on the planet. So we have to be conscious that this star is extremely different, it is a very alien star, compared to the Sun.
I – So how do exoplanet researchers find more information about these planets, in terms of the atmosphere, whether or not tidal locking is happening?
This diagram compares the orbits of TRAPPIST-1 planets with the
Galilean moons of Jupiter and the inner Solar System.
Image credit: ESO / O. Furtak.
K – We have to do remote observing with the telescopes that we have, and there are a couple of techniques that we use. One is the same eclipse method, but to do this in different colours, and this is what we call doing spectroscopy. If the planets have an atmosphere, and the atmosphere contains molecules that like to absorb light, such as say oxygen, ozone in particular likes to absorb light in very specific wavelengths. So what will happen is that at that particular wavelength, more light will be absorbed. We can use this technique over many other colours, and can learn more about the atmosphere’s make up.
This is a technique that we have known about for quite a long time, we’ve had successes with it, but we’ve never successfully done this on an Earth sized planet. The eclipses are so small; we may not be able to detect even smaller changes due to absorption. It’s not infeasible, the new telescope that is being built, the James Webb Telescope, the successor to Hubble, will probably make this one of its science priorities. To smell the atmosphere, if you like, Proxima Centauri b and the TRAPPIST-1 system will be high priority targets.
I – Since TRAPPIST-1 is 39 light years away, do you think that there is any possibility that humans can visit?
K – Well, even if we travel at the speed of light it’s going to take us 40 years! I don’t think exo-planet astronomers are excited about TRAPPIST-1 because they think they can visit it. However, the planet Proxima Centauri b, that’s only 4-lightyears away, instead of 40-light years away. That’s a factor of 10 closer. That is a much more interesting target, if you’re thinking seriously about visiting someday. Even then, 4-light years is hard…The strategy that’s most feasible, is probably not to send a person, not even a robot, but a micro-chip… And the idea, is we get a tiny micro-chip, that you accelerate with a laser in orbit… The photons hitting the microchip act as a force from the laser, so it’s almost like spraying it with a water jet or something. So we can accelerate those extremely fast, and build them very cheaply. We can build thousands of these chips, and if half of them go off course it won’t really matter because hopefully one will get to its destination. So that’s the most realistic strategy.
In terms of sending people, the only way to do it right now would to be very patient, and to build a huge generational ship, or to send DNA in cryogen and have some machine try to do some in vitro birth or something crazy like that. But we can’t send people within our lifetime, that’s just complete science fiction.
I – Does exoplanet research tell us anything about our own planet?
K – I guess the weird thing about our planet, is why do we live around a sun-like star? Sun-like stars are actually rare, and only make up 10% of stars in the universe, while these endwarfs, like TRAPPIST-1, make up something like 75% of the stars in the universe. And that’s kind of interesting because if these endwarfs, have multiple planets in the habitable zone, which seems to be the case, then it is very peculiar that we do not live around one. Because not only are they more numerous, but they seem to have rocky planets in the right zone for life, and they also live way way longer than the Sun. The Sun only lives a few billion years, maybe 10 billion years in total from birth to death. The endwarfs live for trillions of years, they can live much longer than the age of the universe so far. So there is plenty of temporal real-estate if you like. So there’s something kind of odd, or maybe there’s something we don’t understand.
Artist Representation of TRAPPIST-1
Credit: NASA/JPL- CalTech
I – So why does it seem so rare to find an Earth-like planet in the habitable zone?
K – I don’t think they are rare, maybe you’re alluding to why haven’t you heard more press releases in the past? And that’s because they’re very hard to detect, the Earth passing the sun is only blocking about 80 parts per million in terms of the amount of light. Whereas these stars block out a percent of the total light of the stars, and when you block out a percent it’s much much easier to notice in a telescope. For endwarfs it’s easier to see the light being blocked, but the problem with that is that endwarfs are really really faint because they’re tiny stars. If you look into the sky you will not see a single endwarf in the sky even though they make up ¾ of the stars in the universe. So this is the problem, yes there are a heap-load of them, and yes they give you nice big signals, but the downside is that they’re so faint they’re difficult to see with a telescope. It’s always going to be difficult to do this kind of science. So we’re very lucky to have seen this system.