The list of unconfirmed, controversial and retracted planets now has dozens and dozens of objects. Do you ever get angry e-mails or phone calls from astronomers about their planets being demoted?
Almost never. In 15 years I have received perhaps five to 10 angry messages and hundreds of encouraging messages.
Why do you think it is that people have been so civil?
Because it is reasonable [laughs]. I have reasonable argumentation; it's not just a ... I don't know the word for it, but in French it's caprice.
You've been keeping close watch on the field of exoplanets for the past 15 years. Where do you see things headed in the coming years?
I think that radial velocity measurements will provide several hundred to a few thousand planets and no more. Astrometric measurements, and in particular the GAIA mission, are expected to provide a few thousand planets by astrometry, because they are surveying one billion stars. As for microlensing, if a mission like WFIRST is finally launched in 2020, they could have, say, a few hundred planets. Direct imaging will provide certainly more than one hundred but not more than a few hundred, because with direct imaging you cannot go very far away in the galaxy. And the Kepler mission will provide many, at least several tens, of Earth-size planets in the habitable zone of their parent star.
The number of planets will increase until about 2030 and then begin to stop. Another step will start, which will be to characterize more and more closely these planets. Detect more and more molecules, investigate the climate of these planets, et cetera. Another thing we could eventually do is to make the cartography of the planet—to make a multipixel image, to really see the continents. But this is 2050.
Do you think we will ever reach a point where we will stop finding planets?
Except with the so-called microlensing method, we cannot detect planets too far away. Yes, it is true that by the radial velocity method, and with very large telescopes, people can also detect planets up to, say, one kiloparsec [about 3,000 light-years] away. But these planets will not be very interesting; they might be interesting on statistical grounds, but it will be very hard to investigate them in detail because they are too far away and therefore too faint and too difficult to separate from their parent star.
And we will be sufficiently busy with planets that are closer than, say, 20 or 30 parsecs [65 to 100 light-years].
Once we've found nearby habitable planets, how do we go about finding out if they are inhabited? Are we on the right track to do that?
For me, the first priority is to be able to make a spectroscopic investigation of the planets. That means to make an image of the planetary system and to measure the colors, if you want, of the planets in orbit to see what molecules are in the planets, what is the climate evolution around the orbit—to see seasons. By direct imaging we can even measure the duration of a day on the planet.
And for that we will need a direct imaging of the planetary system. This is the top priority. And it is too bad that the decadal survey did not go this way. [Editor's note: The decadal survey is an influential report produced by the National Research Council intended to guide astronomy and astrophysics research in the coming years.]
So that would be something like the long-discussed Terrestrial Planet Finder [TPF] mission?
Yes. But we can start modestly, with not a full TPF but a small TPF that can monitor giant planets and very close-by Earth-like or super-Earth planets. Because a super-Earth two times larger in radius than the Earth can also be habitable, but it is four times easier to detect.
Here at the Paris Observatory, along with some American colleagues, we are submitting a proposal to the European Space Agency to do that.