[Artwork depicting a gas giant exoplanet with a gas giant exomoon. Credit: Shutterstock / Jurik Peter]
In a paper just published on the astronomy preprint server called arXiv, a group of astronomers has made an astonishing claim: They have found evidence of the possible existence of an exomoon, a moon orbiting a planet orbiting another star. If true, this will be the first moon orbiting an exoplanet ever discovered!
However, I want to be really, really clear here: The evidence is very interesting, and I might even use the word compelling. But it’s not conclusive, and certainly not confirmed. Importantly, the paper has not yet been peer-reviewed, either (it was released early because news of upcoming observations leaked, and the authors wanted to get ahead of any news stories that might be sensationalized; normally, I wouldn’t write about a non-peer-reviewed paper, but I agree with them, and hopefully this will curtail inaccurate coverage). Assuming it passes peer review, what they have here is evidence, which means it needs to be examined carefully, and follow-up observations must be made to further investigate it. If it pans out, it’s a major discovery, and a very exciting one. But please keep in mind the preliminary nature of this while reading.
The exomoon, if it exists, is orbiting an exoplanet named Kepler-1625b, which itself is orbiting a star called Kepler-1625 (and so, if the moon is confirmed it will be called Kepler-1625b-I). The star is similar to the Sun, with about the same mass, though 1.8 times the Sun’s size. This indicates it’s old, and may be starting to swell into a red giant. It also means the star has a low density, which turns out to be important (more on that below). The star is about 4,000 light years from Earth, which makes it apparently pretty dim.
The planet is a gas giant, about the size of Jupiter but with ten times its mass, and takes just under 290 days to orbit the star once. It was found in 2016 using the…