Astronomers may have found an exomoon, and Hubble is going to check

The first evidence for an exomoon — a moon orbiting a planet orbiting a distant star — may have been spotted in data from the Kepler space telescope. But surprisingly, exomoons in general may be rare, at least around planets close to their stars.

Alex Teachey and David Kipping of Columbia University analyzed the dips in light from exoplanets passing, or transiting, in front of their stars. A second, smaller dip that appears ahead of or behind the planet could reveal a moon. Such exomoons, researchers have speculated, may be among the best places in the universe to look for extraterrestrial life. But because those signals are faint and inconsistent, they take a lot of computing power to find. Kipping has been searching for such signals for years in a project called the Hunt for Exomoons with Kepler.

In a paper posted online July 26 at arXiv.org, Teachey and Kipping present the first evidence for an exomoon candidate: Kepler 1625b i. The team analyzed 284 planets that seemed like good candidates for hosting detectable moons. “Out of those, this object popped out,” Kipping says.

The object, if it exists, orbits a planet slightly larger than Jupiter around a star about 4,000 light-years away. Because the potential moon is probably about the size of Neptune, the team nicknamed it “Neptmoon.” The team plans to check if the moon is really there by using the Hubble Space Telescope to watch for another transit on October 29.

“We threw all of our tests at it, and it passed them,” Kipping says. “But we were still pretty suspicious. We knew the best way to confirm it was to get more data. Hubble is the best telescope for the job.”

If confirmed, this moon would be almost in a class of its own. The team calculated that, statistically speaking, only 38 percent of Jupiter-like planets close to their stars are likely to host moons like Jupiter’s. That’s surprising, but given that there are thousands of exoplanets still to check, more moons may still be out there. The hunt continues.