Over the past few decades, astronomers have slowly but surely added to the number of known exoplanets. Even the largest planets can be difficult to spot, and the moons are even smaller. Well, usually. A team working with data from the much-defunct Kepler space telescope has reported a possible moon orbiting an exoplanet about 5,000 light-years away. it falls somewhere between the size of the Earth and that of the gas giant Neptune. If confirmed, it could be the first known exomoon.
Currently, astronomers have confirmed the existence of almost 5,000 exoplanets, but we don’t have any confirmed exomoons in the books. There are about ten candidates, including one identified by the same team in 2018. This one has not yet been verified, and this is the case here. Kepler (see below) stopped working several years ago, but it has collected so much data that scientists are still dissecting it. There could be thousands of exoplanets and even more moons hidden in the archives.
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 just a little larger than Earth. At the upper end, it could be almost as large as Neptune. This means we can only guess at its composition, but a rocky moon could be habitable.
Like all of Kepler’s observations, the possible discovery of Kepler-1708 bi relied 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 star fields over long periods of time. By monitoring repeated dips, astronomers can find exoplanets invisible to traditional telescopes. The same goes for exomoons, we think. The signals from the moons around these planets are much more subtle, which is why we have yet to confirm the existence of any of them.
The team notes in the diary natural astronomy that there is a one percent chance that the detection of Kepler-1708 bi is in error. That might sound close enough for the rest of us, but it’s not good enough for science. While NASA’s Transiting Exoplanet Survey Satellite (TESS) can’t see that far, the recently launched James Webb Space Telescope might have enough power to spot objects like Kepler-1708 bi. It will be a few more months before we know Webb is working properly, but the launch and rollout went without a hitch.
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