Artist’s impression of a hot planet like Kepler-37b. Credit: ESO/L. Calçada.

I’ve said before that when looking for planets, we must expect the unexpected. NASA’s latest discovery, Kepler-37b, was not unexpected, but it raises interesting questions nonetheless.

Kepler-37b is a hot, rocky planet, but it’s not a super-Earth, because it’s smaller than Earth. If that was all it was, this wouldn’t be news–we’ve found exoplanets smaller than Earth before. The interesting thing is that it’s the smallest known planet…including the ones in our own Solar System. Check it out:

Size comparison of Mercury, Kepler-37b, and the Moon. Credit: NASA and Phil Plait.

That’s Mercury on the left, the Moon on the right, and an artist’s impression of Kepler-37b in the middle. Kepler-37b is quite a bit smaller than Mercury, the smallest planet in our own Solar System (since Pluto is only a dwarf planet), and even smaller compared with all the other exoplanets we’ve found.

Okay, technically, the pulsar planet PSR 1247+12 A is almost certainly smaller than Kepler-37b, but pulsar planets are weird objects that don’t form in the same way and therefore don’t tell us a whole lot about regular planets. Kepler-37b is the smallest known planet orbiting a live star.

But if Kepler-37b is so small, are we sure its a planet at all? Well, yes and no. Yes, because we know enough about its properties to determine that it is definitely round, and it almost certainly exerts gravitational control over the area around its orbit (what the IAU definition of a planet clumsily refers to as “clearing the neighborhood”). We can rest assured that Kepler-37b meets all the qualifications to be a full-fledged planet.

Well…except for one. The IAU definition also requires a planet to be “in orbit around the Sun”. Our Sun. They never actually addressed the classification of exoplanets. This means that at some point in the future, the IAU will either have to revise the definition of a planet to include exoplanets, or drop the definition entirely and let astronomers get back to calling things whatever they feel is appropriate.

Most astronomers I know–and I suspect a large part of the IAU itself–would prefer the second option. That won’t solve the Pluto problem. In fact, if most astronomers had their way, whether or not Pluto is a planet would depend not only on who you asked, but on how you phrased the question. But that’s a discussion for another day.