In the past several days a number of news articles have touted the passage of a tidy astronomical milestone—the discovery of the 500th known planet outside the solar system. In the past 15 years, the count of those extrasolar worlds, or exoplanets, has climbed through single digits into the dozens and then into the hundreds. The pace of discovery is now so rapid that the catalogue of identified planets leaped from 400 to 500 entries in just over a year.

But the astronomer who tends to the exoplanet community's go-to catalogue tempered excitement surrounding the 500th-planet milestone in interviews and in an e-mail to fellow researchers, advising caution in assigning too much precision to the tally. Jean Schneider, an astronomer at the Paris Observatory, has since 1995 maintained The Extrasolar Planets Encyclopedia, a modest-looking Web site that charts a wealth of data on known exoplanets as well as those that are unconfirmed or controversial.

We spoke to Schneider about the difficulties in identifying any given planet as number 500, the future of exoplanet science and just how he came to be the unofficial record keeper of worlds beyond the solar system.

[An edited transcript of the interview follows.]

What motivated you to start the catalogue in 1995?
First, 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.

And so having not only a catalogue but also a Web site with a bibliography and links to conferences was something that I thought was a good thing to help build a community.

Also, at that time we did not know that we would find so many planets!

Is this something that you did on your own?
Yes. Now I have a computer assistant, and that's all.

Did you ever think that you would become the unofficial record keeper for the exoplanet community?
Not at all, not at all. It was just enthusiasm that made me do this.

Back in 1995, of course, 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 used to take me about half an hour every morning. But now 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 me information. And at this point I know everybody in the world, so I know what is going on.

Recently there has been a lot of talk about the "500th" extrasolar planet to be discovered. Why do you advise a little caution about celebrating that milestone?
There are several reasons for that. First, there is no consensus on what is a planet and what is a brown dwarf. We don't know exactly where the planets stop and the brown dwarfs start on the mass scale. In addition, the mass scale is not a good criterion. So there is some fuzziness there.

I have decided to take objects up to 20 Jupiter masses. But it's arbitrary, and it will always be arbitrary. There is no good solution for this problem.

Second, there are always errors in measuring the mass. If you have an object with 20.5 Jupiter masses, plus or minus two Jupiter masses, what should I do? This is another problem. To deal with this problem I decided to be flexible within one standard error. If the object is within one standard deviation of 20 Jupiter masses, I take it.

What is important is just to be clear about that. That is why there is a "readme" file which explains that.

The other comment I want to make is that in my opinion it is better to have a little bit more objects than those that are really well confirmed, because this catalogue is also a working tool to help astronomers around the world not to miss an interesting candidate so they can work on it.

Even so, I estimate that there have been only about five retracted planets, so that is 1 percent.

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.