In the meantime, researchers have tracked down several thousand exoplanets, and their discovery is slowly becoming routine. They are also likely to be accompanied by thousands of exomoons, as dozens of satellites do on planets in our solar system. But the detection of these distant moons is much more difficult. David Kipping from Columbia University in New York and his team now present data in Nature Astronomy that at least strongly point to the presence of an exomoon in a solar system 6000 light-years away from us: Kepler-1708 b-i is the second promising candidate for an exomoon after Kepler-1625 b-i, a Neptune-sized companion orbiting a Jupiter-like exoplanet about 8000 light-years away from Earth.
The first evidence of the existence of Kepler-1708 b-i first appeared in 2018 during an investigation of archival data by David Kipping of Columbia University, one of the discoverers of Kepler-1625 b-i, and his research group. The team had studied transit data from NASA's Kepler space telescope of 70 cool gas giants. Their orbits are relatively far away from their star, which is why they need more than 400 Earth days for one orbit. These exoplanets can be detected by the fact that they shade the light of their star during a transit.
Large exomondes should then further weaken the light and send out corresponding signals that the astronomers searched for. The list of potential exomond candidates then worked off Kipping and Co and deleted each copy from the list, for which another explanation can be found. In the end, only Kepler-1708 B-I was left. "It was a stubborn signal," says Kipping. The light curvies speak very strongly for not only a gas giant, but also a very large companion passed the star.
Kepler-1708 b-i would therefore be 2.6 times larger than Earth and thus a third smaller than Kepler-1625 b-i. Both candidates are made of gas that they have accumulated through their strong attraction. They may even start as planets before being captivated by their gas giants. "It is probably a 'mini-Neptune'," says Kipping: objects that do not occur in our solar system, but can be found in large numbers near other stars.
However, the exomoon is far from being proven beyond doubt, as René Heller of the Max Planck Institute for Solar System Research in Göttingen tells "Scientific American". He was not sure that the transit signal came from a moon: "It doesn't convince me." Instead, the light dip could simply be the result of natural variations of the star, such as sunspots that pass over the surface of the star at the same time as the planet transit.
