Name: Jean Schneider
Title:  Astronomer, Paris Observatory
Location: France

What motivated you in 1995 to start the Extrasolar Planets Encyclopedia, which is a Web site that charts data on known and unconfirmed planets outside our solar system?
I discovered the Web at the time, and I found it fantastic. I thought that the search for life in the universe is extremely important, and I wanted to make anything I could to encourage work on the search for life and other planets and possibly to unify the community.

Back in 1995 there wasn’t much to catalogue. Now there are hundreds of planets and more all the time. How much of your time does this occupy? It’s become one hour every morning. The thing is to be regular. You have to keep up with the literature and with people sending information. At this point I know everybody in the astronomy world, so I know what is going on.

Late last year astronomers reported finding the 500th extrasolar planet. Why do you advise caution about celebrating such milestones? There are several reasons for that. First, there is no consensus on what is a planet and what is a brown dwarf [an object that is more massive than a planet but less massive than a star]. We don’t know exactly where the planets stop and the brown dwarfs start. Second, there are always errors in measuring mass. But in my opinion, it is better to have a little bit too many objects than only those that are really well confirmed, because this catalogue is also a working tool to help astronomers around the world avoid missing an interesting candidate they can work on. Even so, I estimate that there have been only about five retracted planets, so that is 1 percent.

Still, the list of unconfirmed, controversial and retracted planets now numbers in the dozens. Do you ever get angry e-mails 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.

You’ve been keeping close watch on exoplanets for 15 years. Where do you see things headed in the coming years? I think the number of planets astronomers discover 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 cartography of the planet—to make a multipixel image, to really see the continents. But this is in 2050.

Once we find habitable planets, how do we find out if they are inhabited? For me, the first priority is 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. 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. [Editors’ note: The decadal survey is a report from the National Research Council that guides astronomy research.]