5,000 planets, more or less

We still don’t have a clear definition of what constitutes a planet, even if it’s been about thirty years since the discovery of the first exoplanets, or planets orbiting stars other than our sun. Far from a very general standard, it was adopted in 2003 by the IAU: any object with a mass greater than 13 times the mass of Jupiter is classified as a star.

At the other end of the spectrum, the smaller bodies of our planets are excluded from the outer planets, but since these smaller bodies are only just beginning to be discovered, this criterion is still very arbitrary: there is no guarantee that our solar system is in the “average” of the solar systems.

Like other criteria likely to elude astronomers, in 2018, the IAU Exoplanet Working Group he heard On two additional sides:

• To qualify as a planet, the mass of an object must be less than 1/25 of the mass of the object it orbits;
• And he must have “cleaned his neighborhood”, which means, obviously, that it is not just a disk of matter in the center of which a larger body appears.

But not everyone is happy with these new standards, Details this week The new world. Among other things because with current telescopes it is impossible to know whether the object has been cleaned, because the “disk” consists of asteroids and other objects too small to be detected. Even the oldest criteria are ambiguous: with a mass over 13 times that of Jupiter, an object with the same chemical composition as Jupiter begins a process of nuclear fusion that could qualify it as a star. But there really is a gray area between the two.

These doubts explain why NASA announcementon March 21, as we had Threshold Of the 5,000 exoplanets (5,005 more accurate) are still up for debate, depending on the exact definition of what a planet is. But it at least allows us to gauge the progress that was made not so long ago when astronomers did not have evidence that planets orbit other stars.