PLANET-HUNTING TELESCOPES have recently taken a huge leap in their ability to find “exoplanets,” or planets orbiting other stars. In just the past six months, astronomers have announced the discovery of more than 700 such worlds, bringing the total to more than 1700. These discoveries include the first Earth-size planet found in what’s called the habitable zone of a star, where liquid water could exist; the oldest known planet that could support life; and the first rocky “mega-Earth,” a planet that’s much like Earth except that it’s 17 times more massive.
On July 9, 2014, three exoplanet hunters came together to discuss the recent uptick in known planets beyond our solar system, consider the next steps in the hunt for habitable worlds, and ponder the odds of finding evidence of life on another planet. Below is a modified transcript of the discussion. Edits and changes have been made by the participants to clarify spoken comments recorded during the live webcast.
- ZACHORY BERTA-THOMPSON – is the Torres Fellow for Exoplanetary Research at the MIT Kavli Institute for Astrophysics and Space Research.
- BRUCE MACINTOSH– is a Professor of Physics at Stanford University and a member of the Kavli Institute for Particle Astrophysics and Cosmology.
- MARIE-EVE NAUD – is the University of Montreal PhD student who led analysis that recently uncovered a previously unknown giant planet, GU Pisces b, using infrared light.
THE KAVLI FOUNDATION: Most surveys that look for planets in other solar systems look for them orbiting big, bright stars that are easy to see. But, Zachory, you’re part of a project hunting for small planets orbiting nearby, very small stars. What’s the name of that project and why are you focused on the small?
ZACHORY BERTA-THOMPSON: I focus on the small because one of the things that’s exciting in exoplanets right now is this push toward finding planets that are more and more like Earth. So in exoplanet speak that’s a small exoplanet.
To find a small planet, it’s easier to look around a small star and so with our survey, we’re looking around the very nearby, very small stars. And those small stars in astronomy are called M dwarfs. This is just the name that astronomers have given to very small stars. Marie-Eve will probably tell us a little more about M dwarfs later. So we’re looking for Earth-like stars around M dwarf stars and so we call the project the MEarth project (we pronounce it “mirth” because it makes us happy!). The survey has been going on for a number of years now, using a bunch of telescopes out in Arizona to monitor a bunch of these new small stars to look for new transiting exoplanets.
TKF: Bruce, I’d like to turn to you now. I’m going to show everyone the picture you took – the best-ever direct photo of a planet outside our solar system. What was your reaction when you saw this, and more importantly, what do you see in this picture that makes it so exciting? What do you see that someone with a less trained eye, like myself, can’t see?
BRUCE MACINTOSH: As you said, this is an image where you’re actually seeing an extrasolar planet. For most of the extrasolar planets that have been discovered so far, we don't see the planet. We guess that it's there, inferring that it’s there based on its effect on the parent star as it makes the parent star wobbles or as it blocks out the light of the parent star.
What we've been working on is actually blocking enough of the light from the star that you can see the planet directly. This has been done before – depending on how you count, there’s on the order of half a dozen to a dozen directly imaged extrasolar planets that have been seen by various groups including the one that Marie-Eve is going to talk about. What was exciting about this one is that this is a planet that was previously seen by other groups, but with the previous generation of technology it took hours to successfully detect these planets. The new instrument we built – the Gemini Planet Imager on the Gemini South Telescope in Chile – managed to do it in a minute. So that’s just incredibly exciting.
The version you’re showing now is actually about 30 minutes worth of data stacked and combined. The little dot in it is a planet about ten times the mass of Jupiter. It’s very young and hot, so we're seeing it shining with leftover energy from when it formed. It’s got a temperature of maybe 1500 or 1800 degrees. What was really exciting, to me at least, is not that we could see this planet, which we knew was there, but that the new instrument is so sensitive we could have seen this planet if it were 10 times fainter or three or four times closer to its parent star. So we can start to see systems like our own, planets the size of Jupiter as close to their star as Jupiter was. Being able actually make images of planets like that, and then get a spectrum – we can tell from this information what that planet is composed of – is really enormously exciting. It should make the direct imaging of extrasolar planets catch up to the transit and Doppler techniques that people have been using for longer.
TKF: Marie-Eve, you discovered a huge and really unusual planet at an incredible distance from its star. And you’re a Ph.D. student. What’s your secret? And what makes this discovery so interesting?
MARIE-EVE NAUD: My secret is entirely the team I'm working with. It’s a very dynamic team that’s been working on this topic for a certain amount of time now, so I'm relying on them.
Like Zach told you, the small stars are the ones around which it’s easier to find small planets. But it’s also true that it's easier to find a big planet around small stars. And I can also build on what Bruce has said. He was talking about the very new generation of instruments, which are awesome and allow us to find planets that are super close, but we had a surprise with this planet. We realized that it was actually possible - in rare systems - to find planets that are super, super far from their host star. And for these planets we don’t need instruments like the one that Bruce is in charge of. We can use easier methods and more simple instruments. So we were really happy we found this planet, that allows us to better understand exoplanet systems in general and also giant exoplanets.
TKF: This really seems an amazing time for planet hunters. We’re starting to discover Earth-size planets in what’s called the habitable zone of a star, where liquid water could exist. We recently also discovered what’s the oldest known planet that could support life; and just last week astronomers announced the discovery of an Earth-like planet in a two-sun system. I’m curious to know what each of you think is the most amazing planetary discovery we’ve made so far.
ZACHORY BERTA-THOMPSON: It's a tough question. I really can’t pick. One of the things I’ve been most excited about in the past few years is the diversity that we’ve seen in exoplanets. So singling out any particular one is really challenging. So I’ll pick maybe two to represent this.
One of these is this planet called Kepler-10c. This is one that we learned about very recently. It’s about the mass of Neptune, which is this giant ice planet in our own solar system. It’s as much stuff as Neptune but it’s crammed into a very small space. It’s very, very dense. It’s almost entirely made up of rock. It’s weird to have a planet that big, but it’s just a giant ball of rock.
Then there’s another really weird system that’s less massive than Kepler-10c but that’s much, much, much bigger; it’s very, very, very puffy. This is Kepler-51b. It’s only a few Earth masses but it’s almost the size of Jupiter. The fact that there's this huge range of planet densities that we’ve discovered now means there's this huge range of compositions out there. And that means that these planets have formed in vastly different ways. And so there's a lot left to explore.
BRUCE MACINTOSH: That’s probably similar to the answer I would give, which is not to pick out individuals but just statistics. We’re in this era now where there are 1700 planets known, and for many of those planets there’s really no analogy to these in our solar system. With the exception of a few specialized individuals, we have no idea what the heck they’re made of. And so this enormous puzzle about how the universe has made systems so different from our own is extraordinarily exciting.
MARIE-EVE NAUD: I’ve always been super interested – of course! – in Earth-like planets orbiting Sun-like stars. And even though I'm not doing my research on that, I'm always following what's going on in that area of research because it’s exciting for everyone to wonder if there's something similar to our system out there. I was particularly interested by the discovery of Kepler-186f, which is a very similar planet to the Earth but that orbits a super small star, in the habitable zone of that star.
What we mean by “habitable zone” is just the range of distance from a star where water on the surface of the planet can remain liquid. So of course, the Earth is in the habitable zone of our Sun. And this planet, Kepler-186f, is in the habitable zone of this much smaller star. It’s motivating for me to learn the existence of planets that are more and more Earth-like.
BRUCE MACINTOSH: I may cheat and bring up one more, just so giant planets don't get neglected. And that’s the HR 8799 system, which is the first really good direct-imaged system. It has four planets that are maybe three to seven times the size of Jupiter and you can see them all on a single picture. We’ve been watching them long enough that we can actually see them orbiting around their parent star, moving the way Kepler – the astronomer - predicted 500 years ago that planets ought to go: the ones close to the star go fast, and the ones far away from the star go slow. That’s not a surprise since everyone believes Kepler's laws, but actually seeing another solar system slowly revolving around like clockwork is pretty amazing.
MARIE-EVE NAUD: I agree with you Bruce. That was a major revolution. I think the first discovery of an exoplanet was in 1992 or 1995, depending on how you define an exoplanet, but these were always indirectly detected. So in 2008 when HR 8799 was discovered, it was a breakthrough for exoplanet science.
TKF: Let’s transition a bit to talk about techniques. Bruce talked about the direct imaging of exoplanets, which is new and very exciting. How else do we go about seeing planets in other solar systems?
ZACHORY BERTA-THOMPSON: This is what I’ve worked on most – these other methods. The technique I use most often is called the transit technique. If you have a planet that is so close to its star, even if you have a really, really powerful camera like the Gemini Planet Imager, you won’t be able to pick out the light from that planet because of the big glare of the star that’s so close to that star. So you have to use some other method. And what we do is to watch a star very carefully and look for little dips in the brightness of that star. We measure these little dips when the planet is passing that star, blocking the star’s light. This is great because it allows us to see these planets that would otherwise be invisible.
But it gives us a little bit more than that, too. It allows us to make really fundamental measurements about a planet’s properties. We know how big the planet is by measuring how much of the star’s light the planet blocks. And then we can couple this with other techniques, like the Doppler technique that allows you to measure how much a star is wobbling toward you and away from you. That can tell you whether or not there's a little planet that’s tugging on the star, causing it to wobble, and so the mass of the planet. With those two numbers – a size and a mass – you can start to calculate the density and learn about the properties of a planet in much more detail than you could otherwise. This method gives you nice really clean measurements.
TKF: A follow-up question just came in from a viewer: When you're searching for planets by these light-dimming or wobbling methods, how do you determine if it’s one large body or it’s two smaller bodies that are having this effect on the star?
ZACHORY BERTA-THOMPSON: I guess it’s a very specific shape that we're looking for. The manner in which the star dims and comes back, that’s very easy to predict. So you can calculate the very simple geometry: if there were two planets right next to each other passing in front of the star at the same time then you would see two dips and a much more complicated structure.
MARIE-EVE NAUD: And indeed, you can see it sometimes. With stars that have many planets around them, you see the starlight dimming more if it’s a bigger planet, less if it's a smaller planet. Kepler has a couple of these multiple planet systems.
ZACHORY BERTA-THOMPSON: Exactly. As Marie-Eve says, if you watch a star over a very long time then you'll see one dip from one planet and you’ll see another dip from another different planet, at different times and maybe different sizes. But if you just watch it over a day, you’ll see just the one dip from one planet at a time. So it’s pretty easy to separate all of the planets that could be passing in front of the star.
BRUCE MACINTOSH: With the Doppler technique, it’s harder. That’s one of the problems with disentangling very complicated systems with Doppler measurements. Especially if you have planets whose periods are close multiples of each other – maybe one goes around in 60 days and another goes around in 30 days – it's hard to disentangle the signals and sometimes it’s possible to be fooled by complicated arrangements.
TKF: We actually have another follow-up question from a viewer, who says that if you're using this transit method or any of the other ones you just described, it seems like it would only work for systems that are aligned to our view, where the planet actually passes between us and the star. Do you have a rough percentage of what systems are expected to align like and many there may be that we’re not seeing?
ZACHORY BERTA-THOMPSON: That’s a great question. It's true that we're only seeing a tiny fraction of the planets that are out there using this method. The percentage depends on what kind of planet you’re looking at. If you’re looking at a planet like a “hot Jupiter,” a really massive planet which orbits its star maybe once every three days or so, then it’s very close to it and the chance that it's lined up so it blocks the star’s light is about 1 in 10. So for every one we see, there are 10 out there that we miss. But if you look at a planet more like the Earth, then the numbers turn into more like one out of every 200. So for every Earth-like planet around a Sun-like star you find, there are 200 others out there that you’ve missed.
So yes, you're missing a lot of planets, but the ones that you do find are really useful. Sometimes we refer to these as “useful laboratories” because you have everything set up so you can take the measurements that you want to take. But of course we also want to know about planets that don’t transit. That's why you need to use these other methods and to build cameras to detect these other planets that we can’t see.
TKF: So you’re able to determine a planet’s size, its mass and so its density. What other characteristics can we find out about planets?
BRUCE MACINTOSH: The other thing that one can do is to try to study more about a planet’s composition, the composition of its atmosphere. We can see that in detail using spectroscopy, and that's done both for transiting and for direct imaging planets. In the case of direct imaging, you could use a scientific instrument called a spectrograph that takes the light in different wavelengths and then look for the chemical signatures of different compounds that could be present in the planet’s atmosphere. In this way we can see evidence of water – although for these giant very hot planets, water means extremely superheated steam. We can also see evidence of carbon monoxide, methane, and all these other elements that may be present. And then the ratios of those can tell you things like the history of the planet – how it formed. We think we understand enough about the process that formed planets in our solar system to see that it left a chemical signature in the atmosphere of, say, Jupiter, and we can try to look for that same chemical signature in the atmosphere of other planets.
MARIE-EVE NAUD: I guess what is really fascinating at this stage of exoplanet science is that we have many methods and all the methods can help define planets. All of them bring different information. So when we are able to combine different methods, we are able to see more. That’s what’s really fun right now.
TKF: One of our viewers would like to know whether it's possible to detect magnetic fields around exoplanets.
BRUCE MACINTOSH: It’s hard at the current state of the art, I think. There are experiments you could imagine where you might look for the spectral signatures induced by magnetic fields, but those are very tiny. You might look for variability in the planet. The planets, especially the ones we are seeing with direct imaging, are almost like tiny stars since they're so hot. Magnetic fields and stars can produce complicated activity and flares in the radiation they give off, and so there have been a couple of proposals to try and look for that signature in some of the hotter, younger planets, but nobody has done it successfully to my knowledge.
TKF: And one more follow-up question: This viewer would like to know how close you think researchers are to finding a planet with molecular oxygen in its atmosphere.
BRUCE MACINTOSH: Proving that a planet has oxygen is probably five or 10 or 20 years away still. The techniques we use just don't quite have the sensitivity. It’s possible that the James Webb Space Telescope – for a couple of the transiting planets that people like Zach will discover – might be able to see signatures of oxygen-like materials, but failing that it’s really going to take either an enormously giant telescope on the ground or a space telescope beyond even James Webb that’s designed especially to do this extrasolar planet detection mission.
ZACHORY BERTA-THOMPSON: I would say that in the next decade, we are just barely getting to the edge of being able to detect molecular oxygen in a planet’s atmosphere. So with the wind at our backs, we may just be able to do that if we find the right planet—meaning it’s the right size and the right temperature, and around one of the closest very small stars, so it’s very easy to observe. Then you just might be able to do it. But it’s going to be a challenge. So I think one of the things we really need to think about now is what we need to do in the next ten years to make that a possibility.
MARIE-EVE NAUD: I agree with my colleagues. We are really close to doing that in maybe a decade. But if the underlying question is if we will figure out if life is creating a given amount of oxygen in the atmosphere, that is much more tricky. I’m not sure that we’ll be able to prove that, if we detect oxygen in a given planet, it’s there because there’s a life form similar to our own on that planet.
BRUCE MACINTOSH: Right. We still don't know if chemistry on Mars indicates that there’s life there, and we can practically drive there [with the various Mars rovers]. It’s hard and super exciting, but not impossible.
TKF: I've heard it said by some pretty reputable people that they think if there is life on other planets we’re likely to find it within the next 10 years. Do you agree with that statement, or do you think it’s a little further out?
BRUCE MACINTOSH: I would take the no side of the bet against those reputable people. It really requires everything to be just right, for that perfect start to be close enough with a transiting planet – which, as we said earlier, requires the geometry to line up just right. And that system has to be close enough to get measurements by James Webb or the future ground-based telescopes. So I would be pessimistic that we will see it on the 10-year time scale.
BRUCE MACINTOSH: Right. If chemistry on Mars indicates that there’s life there, then you could just drive there [with the Mars Rover]. It’s hard and super exciting, but not impossible.
MARIE-EVE NAUD: I think I could be on the optimistic side, but I would agree with Bruce that we're maybe not talking a strict 10 years. It’s maybe just a way of saying we’re getting closer. But it’s still super optimistic to say that we will find life soon. I also have to add that it does not only depend on us and our capacity to find life. It also depends on what’s out there. Which we don’t know.
ZACHORY BERTA-THOMPSON: I’ll also land on the slightly optimistic side of this – let’s say cautiously optimistic. I think it will be really tough. But there’s one big thing that we do have going in our favor here, which is that we kind of know what’s out here. If you asked this question five years ago, we would have said, well, hmmm, maybe habitable planets around M dwarf stars are like one in a million, we don't really know. But now we know, thanks to the Kepler mission, whose job was to figure out statistics of close planets around stars in the Galaxy, we now know that the rate of occurrence of potentially habitable planets around M dwarf stars – those small stars. The numbers keep popping around a little bit, but it's somewhere from 0.1 planet per star up to maybe about 0.5 planets per star that are in the habitable zone.
And so this does really increase our chances for finding a habitable planet that transits its star – so that we can study it in lots of detail – and it’s still close enough that we get enough photons – enough light from that star – so we can use our telescopes to study its atmosphere in enough detail to start saying something about molecular oxygen. But Marie-Eve’s point about whether molecular oxygen in the atmosphere of a planet actually corresponds to life on the planet, well that's a slightly more complicated question.
BRUCE MACINTOSH: What we don't know is whether all these small planets in the M dwarf habitable zones are in fact rocky things that might have thin atmospheres or whether a lot of them are micro-Neptunes.
So I bet you dinner, restaurant of the other person’s choosing, if 10 years from now, oxygen is discovered with 5-sigma certainty.
ZACHORY BERTA-THOMPSON: So that’s 5-sigma detection of oxygen in any exoplanet? I’ll take that bet.
MARIE-EVE NAUD: I’ll take that bet too, because if we don’t find oxygen, well then at least we’ll have dinner!
I guess what I meant was that from the biological point of view we don’t know. We have clues of how life formed on Earth, but we only have one ecosystem to study – our own. Extrapolating from one system is all pretty speculative, from my astronomical point of view.
TKF: Changing gears a bit, we have a question from a viewer who's curious about technology. They want to know if there's a measurement that you're just dying to make that you aren’t able to make right now with the current technologies – whether that means finding alien life or maybe it's something else.
MARIE-EVE NAUD: Personally, I want to take spectra of an Earth-like exoplanet. That would need to be made from space, and we don't have these tools in line yet, I mean we might have them in the future but they’re not as close as the James Webb Space Telescope. Maybe with the James Webb Space Telescope we will find one specific type of planet, but I would really like to have a dedicated instruments do that.
ZACHORY BERTA-THOMPSON: The measurement that I would want to make is studying the atmosphere of a planet that’s something like the Earth. I want to be able to make that measurement for two reasons. I’m really interested in that measurement by itself, but I’m also interested because it’s a good target to aim for. Even if we miss that target we’re doing tons of interesting stuff along the way. So that will teach us a lot about how planets work.
We still are in this position that a lot of what we know about planets comes from our own solar system, so the more we can study individual planets outside our solar system, the more we’ll learn about the evolutionary histories of planets.
BRUCE MACINTOSH: In the nearer-term, in direct imaging, I’d like not just to see these young, hot planets radiating in the infrared, but I’d like to reach the point where we could see a planet like Jupiter sitting there reflecting light. No technique we use right now would be expected to detect a planet in other solar systems like Jupiter at a high level of confidence. So we’ve reached a point where our direct imaging technology lets us block out enough starlight that we could see these Jupiters – which are a billion times fainter than their sun – directly. And that would open up a huge piece of planet space that we can’t access right now.
ZACHORY BERTA-THOMPSON: Bruce, what would that take in terms of a telescope?
BRUCE MACINTOSH: Either the 30-meter class telescopes on the ground or there's a proposal to put a coronagraph on a mission called WFIRST. It was going to be a small telescope to mostly do dark energy science (which is exciting, but not as exciting as exoplanets!), but NASA was given larger (Hubble-size) telescopes by another government agency that it could use for this WFIRST mission, which would give it the ability to start to do direct imaging of Jupiter-like or even Neptune-like planets around nearby stars.
TKF: One of our viewers says that a few years ago, there was a proposal to create a spacecraft pair that would act as a giant pinhole camera, to better images of the closer exoplanets. Do you know if this is still planned?
BRUCE MACINTOSH: There’s a variation on that, I think, which is not a pinhole camera but what's called an occulter. If you want to see something, like a bird flying close to the Sun, you hold up your hand and put your thumb in front of the Sun to see the bird. There’s a proposal for the space version of that, except the thumb is about 50 meters across and the you is a telescope 50,000 kilometers away from it. And that technology could produce, with pretty high confidence, images of planets orbiting around the nearby stars.
But it really requires a dedicated space mission, and it's not obvious that that's going to happen on a 10-year timescale – but might on a 20-year time scale or maybe it could be merged with this WFIRST mission. I think that's the most promising technology out there right now for doing direct imaging but it’s a decade or two away.
TKF: We’re running out of time and I just have one last question. It’s a very general one. What’s the burning question that we have haven’t answered yet? What makes you get out of bed in the morning, what makes you get to the office early and try to answer it?
MARIE-EVE NAUD: I have a very cheesy answer to that question. It’s whether or not there is life out there. For me it’s the primordial question and I think it will remain so until the end of my life. I have professors who, like the people you were talking about earlier, are very confident we will find life in the next 10 years. I am not that confident, but I still dream I will have a hint of the answer by the time I leave this planet. That’s my motivation.
ZACHORY BERTA-THOMPSON: I would agree with Marie-Eve. In the big picture, that’s the thing that I want to figure out. It’s a good time to just be entering the field of exoplanets. But in the more near-term, I want to bite off small pieces of that question. One of those pieces that I’m interested in right now is: What is the boundary between a rocky planet like Earth, which has rocks, oceans and a thin atmosphere around it, and planets like Neptune and Jupiter that have these very thick gaseous envelopes around them. And so we’re just starting to get clues of where that might be and trying to figure out what we can measure to figure out whether a planet is rocky or gaseous. That’s one of the questions I’m really interested in right now, and trying to look at that from as many different directions as possible is, this year, what I’m really interested in.
BRUCE MACINTOSH: I guess since the two big ones are already taken, I guess my question would be: How do planetary systems form? We have this enormous diversity of planetary systems discovered through a wide variety of techniques. If we're being completely honest, we have very little consensus on how they form. The models that I was taught in graduate school – which was a long time ago – for how planets form really only work for our solar system. They don't do a good job of reproducing the populations that we're seeing.
And so all of these different handles on understanding the composition of planets and their orbits and their distribution, ought to encourage theorists – since the three of us are all observers – to get their act together and come up with some model that can actually produce these. And, in turn, model should help us answer questions like how likely is that planets will have gone through a history that would allow them to have life.