Kepler-1625b I: The First Exomoon…Maybe

Artist’s conception of a planet with an exomoon. Credit: NASA Wikimedia.

So, the big science news of the day is the potential discovery of the first known “exomoon”—that is, a moon orbiting an extrasolar planet. (Scientific paper here.) Scientists have been looking for such moons for years, only to turn up empty, but this is the first semi-convincing case that they’ve found one, orbiting the planet Kepler-1625b, and it’s a doozy. A moon the size of Neptune?! It could be. Here’s what you should know about this discovery.

It is not yet confirmed to exist

“Kepler-1625b I” was detected by David Kipping and his colleagues on the Hunt for Exomoons with Kepler (HEK) project based on what appears to be the planet and its moon passing in front of their star at the same time. However, there is not yet enough data to be certain. The planet definitely exists, but the moon is not so clear. Dr. Kipping will be observing the planet with the Hubble Space Telescope in October to determine for sure whether or not this moon exists. Until then, you should take the rest of this story with a grain of salt because it might not actually be there.

If it exists, it’s like nothing we’ve ever seen before (which we really should have seen coming).

Okay, seriously, we’ve been hunting for planets for twenty-five years, and pretty much the only thing we haven’t seen out there is what we actually expected. Kepler-1625b I is no exception. Its parent planet is ten times the mass of Jupiter (although only about the same size), but the moon itself could be called a planet, too. It’s about the size of Neptune at an estimated four times the diameter of Earth, and while we don’t know its mass yet, it’s probably in the ballpark of Neptune as well. This is totally not what we expected. Planets shouldn’t be orbiting planets! And yet, there it is (maybe).

The math does work out on this. This is a truly giant planet that turns out to be in an orbit similar to Earth’s with a moon which, although it’s the size of Neptune, has an orbit similar to Jupiter’s moon, Callisto, and if you work it out, it’s all gravitationally stable, so that’s not a worry. The problem is, how did it form? There’s not enough gas left over in the area where a gas giant forms to build a moon that is also a gas “giant”. This “moon” would have to have formed as a separate planet, which was then captured into an orbit around a bigger planet. I’ve talked to a number of other astrophysicists about this sort of thing, and the general conclusion has been that it is possible, but it should be very rare. Maybe Dr. Kipping just got lucky with this one case. Or maybe there’s something deeper we don’t understand about planets yet. It wouldn’t be the first time…or the tenth.

I will note here that this orbital capture scenario is believed to have occurred once in our own Solar System, with Neptune and its large, backwards-orbiting moon, Triton. However, Triton came from the Kuiper belt, where there’s a lot of junk orbiting around, and where there was a lot more junk in the early Solar System, making such orbital captures much more likely. I would contend that a full-sized planet winding up in orbit around another one should still be very rare.

Its name is a mish-mash of conventions.

Naming of planets and moons in astronomy is confusing (and it’s even more confusing because science fiction has pretty widespread conventions that are completely different). In this case, the name of the star is Kepler-1625, meaning the 1625th star in the Kepler survey that was confirmed to have a planet. In a solar system with multiple stars, the stars are marked with capital letters: Sirius A and Sirius B, for example. In keeping with this convention, planets are marked with lowercase letters in order of discovery. By convention, “a” is reserved for the star, even though it’s not uppercase, so Kepler-1625b is the first (and only) planet discovered orbiting Kepler-1625.

The convention for moons in our Solar System is that they are marked with Roman numerals in order of discovery. So Jupiter I through Jupiter IV are the Galilean moons: Io, Europa, Ganymede, and Callisto. (Unfortunately, science fiction tends to muddy the waters by using this naming system for planets instead.) In keeping with this convention, Dr. Kipping named his possible exomoon Kepler-1625b I.

Maybe we should just call it Bob.

They also estimated how many moons are out there.

This involves some complicated statistics, but based on the possible detection of Kepler-1625b I and the non-detection or uncertain detection of moons around other planets, Dr. Kipping and his team estimate that moons similar to Jupiter’s Galilean satellites are uncommon, occurring around about 16% of planets close to their stars, give or take. (Of course, the existence of six such moons orbiting three of the gas giants in our own Solar System demonstrates that they’re probably much more common farther from their stars…or our Solar System is just weird, which would surprise no one at this point.)

The paper also suggets that “super-Ios” may be an especially common type of exomoons: large, volcanically active moons that orbit close to their planets.

It was announced now because of the unintended consequences of social media.

The announcement of this exomoon was not ready for primetime. It’s a big discovery, and it’s considered bad form in science to announce big discoveries until you’re really sure about them, especially since there are upcoming observations by the Hubble Space Telescope already approved to confirm or refute this discovery in just a few months. But it was this very plan that got the discovery leaked by accident. You see, HST is a NASA mission, and all (or most) of NASA’s observations are public record. So when David Kipping, a man who has spent years putting a lot of time and effort into searching for exomoons, got approval for Hubble time, people put two and two together, and speculation started to circulate on Twitter.

As Dr. Kipping’s coauthor, Alex Teachy, explained in Scientific American, the best thing they could do at that point was to get in front of the story. Among other concerns: “It remains to be seen how much attention this will get, but we worried that getting the public excited about this object before we really know much of anything for sure is just bad for science.” Fortunately, the react to this move, both in the scientific community and in the mass media has been generally been measured and thoughtful, so it seems to have done the trick.

That this news got out before the science was ready was unfortunate, but probably unavoidable. Scientists in general and especially astronomers prefer to be open about our discoveries. Indeed, one thing I haven’t seen mentioned in the news stories is my personally impression that publishing a preprint of a paper before it is peer-reviewed is not only common in astronomy, but may even happen more often than not. So it’s easy for stories like this to get out fast. But we want to make sure what we’re saying is accurate first and foremost. This can be difficult when social media takes any rumor and runs with it, but all in all, I think the HEK team handled it well, and I’m excited to see whether or not this discovery pans out.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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