A RARE FIND: Hundreds of exoplanets have now been discovered and catalogued, but the planets orbiting the star HR 8799 are among the few that have been directly imaged. Image: GEMINI OBSERVATORY/NRC/AURA/C. MAROIS ET AL
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.