The Great Exoplanet Debate, part 7: Questioning Exoplanet Missions

The transit method of detection. The amount of starlight reaching Earth dips while the exoplanet passes in front of it star. Image credit: Hans Deeg

At the 2012 Astrobiology Science Conference, Astrobiology Magazine hosted a plenary session titled: “Expanding the Habitable Zone: The Hunt for Exoplanets Now and Into the Future.” Originally formulated as part of our “Great Debate” series, this panel of exoplanet hunters and thinkers held a lively discussion about some of the most important issues facing the search for and understanding of alien worlds orbiting far-distant stars.

watch the video here:

David Grinspoon: I would like to know if anybody in the audience has comments or questions related to this topic of missions, and what we can do in the future to move ourselves forward?

Audience: In this community and the Astrobiology community, people seem to talk a lot about space missions and about transit and RV techniques specifically. I am wondering how you guys think ground-based, direct imaging, adaptive optics techniques fit into this picture. Specifically, we haven’t pushed it past the low mass planets that we’re finding with transit and RV techniques, but maybe it offers a different regional or parameter space. Soon we’ll be discovering, I think we’ll be imaging these disks, during the epoch of planet formation and getting compositional information. I was just wondering how you guys think that will fit into this picture in terms of priorities?

Eric Ford: I’m glad I get to be the optimist for once. I am actually really excited about that because so far, a lot of the astronomy is focused on these short period planets. And we say may be we can reach out to one year orbital periods. And maybe with RVs for larger planets, several AUs. But we haven’t focused close in. Yet there is this whole other regime of things from ten to hundred AU that we’re just barely beginning to learn about. And I think direct imaging will provide a very complimentary set of information about the larger planets, at cooler separations.

The radial velocity method of exoplanet detection relies on the Doppler effect: when the starlight is red-shifted, it means the star is moving away from us, and when it is blue-shifted it means the star is moving towards us. This effect occurs because a planet doesn’t orbit around a motionless star, but instead the planet and star orbit a common center of mass. Image Credit: ESO

But in terms of our picture of planet formation, and trying to understand what happened in all these planetary systems, how common is it to form Earth-like planets? Do they form in situ at these close locations? Did they migrate throughout their disks? What were the possibilities for them to acquire volatiles? I think that those will actually fit into that bigger picture context very nicely. Now, if you actually what to go study their atmospheres to look for life signatures, then it is extremely difficult from the ground. And certainly not the current next generation of instruments domain. But for the overall science prospective I am really excited about it.

Sara Seager: I agree, in terms of where planets come from and when they’re formed in the relation to the debris disks that you see, its going to be phenomenal. So we hope that you, if that’s what you’re doing, will actually find some really great things for us.

And I just wanted to be pessimistic again. Not for you, but because there is a misconception of why can’t we just do better things from the ground. And actually the part we haven’t mentioned yet is the atmosphere is really bad, we don’t like it at all. Even though we want to find one like that.

Vikki Meadows: We want to breathe it.

Sara Seager: Yeah, we want to breathe it, that’s a problem. The best analogy is like when you are swimming in a pool, you look up and you can never see anything really clearly. And adaptive optics is a great way to try and get rid of that. To decode what the atmosphere is doing. But I just wanted you to know that the reason we can’t reach Earths is because we don’t have enough information to decode the atmosphere. And that really is in the form of a bright star. You need an extremely bright star in order to get enough information going through the atmosphere to know what the atmosphere is doing. Now you could imagine a laser, because there are laser guide stars, generating a really bright star on your own, but the cost of generating a star bright enough would probably be like the cost to put a space mission up to begin with. So we don’t think it can reach Earths, but in the terms of the waves of new exoplanets science, I believe that direct imaging is the next wave. And we’re actually going to wait to see what it can help us understand about planet formation.

Eric Ford: And also, even if you could find some technology to overcome the variables of doing great adaptive optics, you still have the issue the atmosphere has a lot of water, carbon dioxide. And so it would be very difficult to get the sort of life signatures that you’d be looking for and disambiguate with the Earth’s atmosphere.

The Juno spacecraft will make the five-year, 400-million-mile voyage to Jupiter and orbit the planet, collecting data for more than one Earth year. Artist concept. Credit: NASA/JPL-Caltech

Audience: This might be kind of going along with the topic of piggy-backing on other missions and in other fields. But how important do you think Juno is going to be to Exoplanet science? What kind of Exoplanet stuff can you do based on what Juno is going to be doing on Jupiter?


David Grinspoon: Wow, you stumped us.

Sara Seager: Well, you didn’t stump us. So Juno is going to Jupiter and measure the gravitational moments. Hopefully resolve whether or not Jupiter has a core and what the internal structure of Jupiter is. So that may be helpful for us, but again from exoplanets, we’re not going to be getting gravitational moments. It’s not really clear what the connection would be to understanding a population of exoplanets and what their internal structure is, versus just wanting to learn about planets for the sake of understanding fundamentals.

Audience: But if Juno is going to understand the formation and evolution of Jupiter, if you’re doing a lot of complex modeling of exoplanets, is that going to improve the models and help when you are doing more theoretical modeling?

David Grinspoon: In an indirect way of course. Any understanding of planets, and you know Juno going to help us hopefully understand magnetic fields and how dynamos generate magnetic fields more. And that relates to atmospheric evolution. And so it’s all related. This is Astrobiology; we’re all one. But as far as you know, I think, the reason why there was that silence is that it’s in that indirect sense. I don’t think that exoplanet people are thinking of that as really a mission that very directly helps them.

Sara Seager: Right, it won’t help us directly, but indirectly in terms understanding models of our biggest planet, our biggest sign post. It will be useful that way. But it doesn’t have a direct connection, unfortunately.

Eric Ford: The diversity of extrasolar planetary systems is so big that even if the planetary community did a great job of understanding every planet in our solar system, we would still have a humongous task ahead of us.

David Grinspoon: Okay, a question from Shawn.

The transit of an exoplanet allows us to observe the planet’s atmosphere. As the planet passes between its star and us, some of the starlight shines through the planet’s upper atmosphere before reaching Earth. Different parts of the atmosphere absorb different amounts of the starlight at certain wavelengths, so imprinted on the observed starlight are the tale-tale signatures of various gasses.

Audience: Shawn Domagal-Goldman, I am currently a Postdoc at NASA Headquarters, but soon moving to Goddard. I want to address something that David actually brought up earlier. Which is his question on how do we divide and conquer these Decadal Surveys? And I think the answer is, however they end up being divided, wherever exoplanets sit, the onus is on this community to get involved. So if you’re sitting in the audience and saying, ‘ Gosh, the astrophysicists really screwed that one up, I wished they’d gone and done more exoplanet research, recommended more, so I could get my hands on some more exoplanet data.’ Well, the next time the Decadal comes around, wherever exoplanets are sitting, whether its planetary… I don’t think it’ll be heliophysics. If it’s in planetary astrophysics, wherever it is, get involved. As a community, we’ve been very good about getting involved with Mars, with Venus, with other planets in our solar system. And other objects in our solar system for the planetary Decadal. And I think we need to start doing it for the astrophysics. It doesn’t have to just exoplanets either. It can be all the other things that affect habitability that we think about and study. Make your voice heard on both of those Decadal surveys. That’s just the point I wanted to make.

Dirk Schulze-Makuch: That is absolutely right.

David Grinspoon: Yeah, it may seem like an uphill battle, but of course part of the Decadal process is these townhall meetings. And if we show up and flood the townhall meetings with people that are really passionate exoplanet missions, then I think that would have an effect. Another question.

Audience: Lucy Ziurys, University of Arizona. So I hear this this talk about [how] we’ll be able go see molecules in these atmospheres. So I want to know, really over the next ten years, what kind of spectral resolution you can really obtain, realistically. And what spectral features do you really think you can identify conclusively in a planet…

Sara Seager: Are you talking about a habitable planets, or in any atmospheres?"

Lucy Ziurys: Let’s just say any atmospheres just to start with. Then you can go to habitable.

Sara Seager: Yeah, good question.

Vikki Meadows: Well, at the moment we can, using transit transmission, detect molecules like CO2, methane, water, in these atmospheres. For JWST we were hoping to get a habitable planet and we would go again after things like CO2 and water vapor and potentially oxygen that’s in the wavelength band. Might be quite challenging. But again, many of these observations to get a single molecule could be several hundred hours. So, it’s challenging.

Artist concept of the planet GJ 1214b. Image Credit: Harvard-Smithsonian Center for Astrophysics

Sara Seager: Are you asking anything about the current state of exoplanet atmosphere molecules identification? Or was it just a general interest question?

Lucy Ziurys: Just for the next decade. How well can you do?

Vikki Meadows: Well JWST will be the only thing that can do a habitable Earth-like planet or Super-Earth-like planet in the habitable zone.

Sara Seager: Well, I don’t know. I mean, the best example now, let’s leave the hot Jupiters aside. There’s this planet GJ 1214b, which is kind of a mini-Neptune. Some people call it a Super-Earth, whatever. But in that case, things have really been happening from the ground. Although it’s only one object for now. As we speak, pretty much anyone with any big ground-based telescope is trying to look at this thing. And in terms of spectral resolution, they just clued in to the fact that one or two transits won’t cut it. And they need like ten or fifteen; they could really make progress in identifying molecules. Now which molecules? Water is a good one to think about because water is very spectroscopically active and it’s ubiquitous. The other ones that have been mentioned are maybe methane. The spectral resolution may be, I don’t know, twenty.

Vikki Meadows: Filter bands.

Sara Seager: Yeah. People are trying to do spectrophotometry. So its sort of a more in-depth discussion, but I think it will be really great from what we can do from the ground on bright objects. We just need more Super-Earths to be discovered that we can actually observe.