In recent decades, astronomers have slowly but surely increased the number of known exoplanets. However, even the largest planets can be difficult to detect, and the moons are even smaller. Well, usually. A team working with data from the defunct Kepler space telescope has reported a possible moon orbiting an exoplanet some 5,000 light-years away. falls somewhere between the size of Earth and that of the gas giant Neptune. If confirmed, it could be the first known exomoon.
Currently, astronomers have confirmed the existence of nearly 5,000 exoplanets, but we don’t have any confirmed exomoons in the books. There are a dozen candidates, including one identified by the same team in 2018. That hasn’t been verified yet, and it’s the same case here. Kepler (see below) stopped working several years ago, but it collected so much data that scientists are still analyzing it. There could be thousands of exoplanets and even more moons hidden in the file.
The potential moon is in a star system known as Kepler-1708. The planet (Kepler-1708 b) is thought to be about the size of Jupiter, so it’s not unthinkable that it could have a very large moon orbiting it. The candidate moon has been dubbed Kepler-1708 bi. At the lower end of the size range, Kepler-1708 bi could be slightly larger than Earth. At the high end, it could be almost as big as Neptune. That means we can only guess at its composition, but a rocky moon could be habitable.
Like all Kepler observations, the possible discovery of Kepler-1708 bi was based on solar transits. When Kepler-1708 b passes in front of its star, the luminance drops slightly. Kepler worked by recording the brightness of vast fields of stars over long periods of time. By looking at the repeated dives, astronomers can find exoplanets that are invisible to traditional telescopes. The same goes for exomoons, we think. The signals from the moons around those planets are much more subtle, so we have yet to confirm that any of them exist.
The team’s notes in the diary. nature astronomy that there is a one percent chance that the detection of Kepler-1708 bi is a mistake. That may sound close enough for the rest of us, but that’s not good enough for science. While NASA’s Transiting Exoplanet Survey Satellite (TESS) can’t see that far, the newly launched James Webb Space Telescope could have enough power to detect objects like Kepler-1708 bi. It will be a few more months before we know that Webb is working properly, but the launch and deployment went smoothly.