Astronomers identified a second possible exomoon

Some of the same researchers who found the first purported exomoon now say that they’ve found another.

Dubbed Kepler 1708 b i, the satellite has a radius about 2.6 times that of Earth, and circles a Jupiter-sized exoplanet that orbits its parent star about once every two Earth years, the team reports January 13 in Nature Astronomy. That sunlike star lies about 5,700 light-years from Earth.

To find this nugget, the team sorted through a database of more than 4,000 exoplanets detected by NASA’s now-retired Kepler space telescope. Because large planets orbiting far from their parent star are more likely to have moons large enough to be detected, the team focused on a subset of 70 exoplanets.

Each of these planets is between half and twice the size of Jupiter. They all either take more than 400 Earth days to orbit their star or have an estimated average surface temperature less than 300 kelvins (around 27° Celsius), slightly higher than that of Earth.

After further screening, including tossing out exoplanets that don’t have near-circular orbits (which are statistically less likely to host moons), the team identified a strong candidate for an exomoon. It, like its host planet, caused detectable dimming of the parent star’s light when moving across the face of the star.

Discovery of the first possible exomoon, dubbed Kepler 1625 b, has faced a lot of skepticism (SN: 4/30/19). Both proposed exomoons need to be confirmed by further observations by other instruments, such as the recently launched James Webb Space Telescope, the team notes (SN: 10/6/21).

But fresh observations will need to wait: The newfound exomoon candidate and its planet won’t pass in front of the parent star again until March 24, 2023, the researchers calculate.