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Forecasting Earth-Like Worlds

The French space agency CNES space telescope COROT detects planets larger than Earth on the basis of their transits. Image credit: ESA
by Astrobiology Magazine
Paris, France (SPX) May 29, 2007
Missions like COROT, KEPLER, and DARWIN mean we should know much more about the abundance of Earth-like exoplanets in the comming decades. Franck Selsis, an astrophysicist specializing in planetary atmospheres, discusses the characteristics of atmospheres that can reveal the presence of life, but is realistic about what data from these first missions can reveal. Interview with Franck Selsis

More than 200 extrasolar planets have been found so far, but none is like the Earth. Our past detection methods favoured gas giants like Jupiter. Such massive worlds exert a much greater and therefore more detectable effect on a star than a tiny planet like Earth does. Yet the dream of astrobiologists is to find many rocky Earth-like planets orbiting distant stars.

COROT, the first space telescope capable of detecting rocky planets around other stars, is already in orbit. The KEPLER and DARWIN missions are set to follow in the next two decades. In this interview with Astrobiology Magazine, Franck Selsis - planetary atmoshphere specialist - discusses the characteristics of atmospheres that can reveal the presence of life and what data from these missions is likely to reveal about the abundance of Earth-like planets. Astrobiology Magazine (AM): What attracted you to astrobiology?

Franck Selsis (FS): What attracted me was the fact that the detection of planets like the Earth should be feasible within about 10 or 20 years. I decided to do my PhD on that, knowing I wanted to be part of this kind of discovery.

AM: What will be the next space missions related to the discovery of Earth-like planets?

FS: The first mission that will unveil evidence of Earth-like planets is called COROT, which is a French and European mission. Then there's a mission called KEPLER by NASA. These missions won't be able to tell us much about the planets, but they will be able to detect them; to count them and to know the abundance of planets resembling the Earth in our galaxy. That will be a huge step in understanding the formation of planets like the Earth.

AM: How flexible is your definition of Earth-like planets? Does it have to be the same mass as the Earth? Does it need to have oceans, or liquid water?

FS: At this point we have only the solar system to guide us in defining Earth-like planets and to understand the formation of terrestrial planets. We expect that there must be quite a big diversity of solid planets. When I say solid planets I mean a planet that is not a gas giant planet like Jupiter or Saturn, or the hot Jupiters that we found in recent years, or the many icy planets which are different from Neptune or Uranus.

We expect that planets found at about the same star-planet distance as the Earth is from the Sun will resemble the Earth in some way. It will be made mainly of silicates, although some of them may be dominated by water. We hope that at least a few of these planets will be similar to our planets and might be also the cradle of life like it was on Earth. But we expect a huge diversity of planets. We expect to be surprised by what we find.

AM: We already know of over 200 planets orbiting other stars. How many of those planets would you characterize as being Earth-like?

FS: None of them.

AM:That is why we need missions like COROT and KEPLER?

FS: Yes. We are now able to detect planets of decreasing masses, but we haven't found any Earth-like planets. We can theoretically detect Earth-like planets by searching for gravitational microlensing events produced by planetary systems, but there will be always a big uncertainty in the characteristics of these planets since the only information we get is the ratio between the mass of the star and the mass of the planet. We don't even see the star itself. Microlensings will provide great statistics on the abundance of planets, though.

The radial velocity technique that is so successful in discovering giant exoplanets has been impressively improved and can now detect planets of less than 15 Earth masses. Finding Earth-like planets with this method seems unreachable at this point, but we said the same thing for 20 Earth masses planets a few years ago and we were wrong.

AM: One problem with the present ground-based search techniques is they have a strong selection effect for gas giants like Jupiter, since those planets are much easier to find than Earth-like planets. Hence the need for space missions.

FS: Exactly. So at the moment, we find planets around about 5 percent of the nearby stars that we search. But it doesn't mean that the remaining 95 percent of stars do not have planets. They could host solar systems very similar to our solar system, but we are not yet able to detect them. But this will be done in the next few years. To detect what we call habitable planets, such as a planet about the size of the Earth and at about the same distance from its star, we need space missions to detect them - first by transit and then later by direct detection. This next step will be taken by missions like NASA's Terrestrial Planet Finder and ESA's Darwin. These mission's objectives are to detect the photons coming from the planets to be able to do spectroscopy and to learn about the nature of these planets -- their atmospheres, their clouds, their surfaces, and maybe to find detectable signs of biological activity on these planets.

AM: What signs of biological activity would you, as a planetary atmosphere spectroscopist, want to look for?

FS: That is a difficult question. I would like to know as much information as possible about these planets, because it is difficult to extract or to search for a single biosignature. You'd have to know the size of the planet, you'd have to know a lot about the atmosphere and what kind of climate it has, and then if you are able to find atmospheric components like oxygen or ozone or methane, you'd have to consider the whole picture.

The first thing you'd have to ask yourself is, "Is there a way to have such a planetary atmosphere without involving life?" You will always find some exotic way to explain biosignatures like oxygen, ozone, or methane. But for a planet on which you detect ozone, or a planet on which you detect methane at the same time as oxygen or ozone, or a planet on which you detect nitrogen oxide, although we could imagine an exotic scenario in which these components accumulate without involving life, I would say that another explanation would be that there is a biosphere producing these components. Such a discovery would point to an extremely important target for future missions. But we have to be modest -- even if these missions are very successful, we are taking our first steps. We don't want to claim that we find life when we are not sure that we find life.

We want to distinguish, within all the planets we are going to find, any among them that could be future targets in the search for life. What are the planets that resemble the Earth? What are the planets that do not resemble the Earth, but which have characteristics that we cannot explain without involving the weird process that might be life?

AM: I understand the point of searching for oxygen, because oxygen is only sustained in the Earth's atmosphere from plants that are making it through photosynthesis. But if you got into a time machine and you traveled back to the first one billion years of life on Earth, you would not detect any oxygen because the organisms were not releasing it. How could you detect the presence of anaerobic life -- life that does not involve oxygen?

FS: There might be a lot of planets in the universe where there is life but it is very difficult to detect it remotely. If you think about Mars, some people still think there may be life there today, but it is very difficult to know that without going there and digging in the rocks to find it. It is the same for extraterrestrial planets. You may have life but you may not be able to recognize it. So we are searching for a big signal compared to what chemistry or photochemistry can do by itself.

We don't really know what the atmosphere of early Earth was. I am convinced that it did not contain significant amount of oxygen before about two billion years ago. If some extraterrestrial observer looked at the Earth three billion years ago, what did it see? I have studied the early Earth's atmosphere and it is a very frustrating topic because we have very few indicators of what the atmosphere was at that time and what the imprint of life on the whole planet was at that time.

I think it is really important to understand that our first aim is not detecting oxygen or ozone because we are not sure that there is oxygen or ozone on all planets with life. Our main goal is to understand the planets. We are pretty sure that we are going to find planets, and we first want to know about their orbit. We want to know about their size. We want to know if they have a dense atmosphere, the nature of their atmosphere, if they have a climate and seasons, if they have oceans -- very basic things about them.

If we are able to do all that, some of the atmospheric components would also be easy to detect, so we might be able to detect a biomarker. Life on our planet is tightly involved in all the geochemical cycles, and if it is the same elsewhere then we might be able to see it. But this might not always be the case. If the biomarkers are not obvious, we will miss them.

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Mapping Alien Worlds Beyond Sol
Boston MA (SPX) May 24, 2007
For the first time, astronomers have created a rough map of a planet orbiting a distant sun-like star, employing a technique that may one day enable mapping of Earth-like worlds. Since the planet just charted is a gas giant and lacks a solid surface, the map shows cloud-top features. Using the Spitzer infrared space telescope, astronomers detected a bright hot spot that is offset from "high noon," where heating is greatest.

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