Researchers have now tracked down several thousand exoplanets, and their discovery is slowly becoming routine. They are also likely to be accompanied by thousands of exomonds, as are dozens of satellites of planets in our solar system. But detecting these distant moons is much more difficult. David Kipping from Columbia University in New York and his team are now presenting data in “Nature Astronomy” that at least very strongly suggest the presence of an exomond in a solar system 6000 light-years away from us: Kepler-1708 bi is the second promising one Candidate for a post-Kepler-1625 bi exomond, a Neptune-sized companion orbiting a Jupiter-like exoplanet about 8000 light-years from Earth.
The first evidence of the existence of Kepler-1708 bi first appeared in 2018 when David Kipping of Columbia University, one of the discoverers of Kepler-1625 bi, and his group examined archival data. The team had examined transit data from NASA’s Kepler space telescope of 70 cool gas giants. Their orbits are relatively far from their star, which is why they need more than 400 Earth days to orbit. These exoplanets can be proven by the fact that they shade the light of their star during a transit.
Large exomonds should then further weaken the light and emit corresponding signals, which the astronomers were looking for. Kipping and Co then worked through the list of potential exomond candidates and crossed off every specimen for which another explanation could be found. In the end only Kepler-1708 bi remained. “It was a persistent signal,” says Kipping. The light curve data strongly suggests that not only a gas giant but also a very large companion is passing the star.
Kepler-1708 bi would therefore be 2.6 times larger than Earth and thus a third smaller than Kepler-1625 bi. Both candidates are made of gas accumulated by their strong gravitational pull. They may even have started out as planets before falling under the spell of their gas giants. “It’s probably a ‘mini Neptune’,” says Kipping: Objects that don’t occur in our solar system but are found in large numbers near other stars.