Hubble may have spotted the first known exomoon

The first suspected exomoon is coming into focus. Observations with the Hubble Space Telescope bolster the case for a Neptune-sized moon orbiting a gas exoplanet 8,000 light-years away, astronomers report October 3 in Science Advances. The moon’s existence, if confirmed, would challenge theories of how satellites are born.

Astronomers David Kipping and Alex Teachey of Columbia University trained Hubble on the star Kepler 1625 for 40 hours on October 28 and 29, 2017. The star was known to have a Jupiter-sized planet orbiting it every 287 days, thanks to observations by the Kepler space telescope, which detects dips in starlight that indicate a planet is transiting in front of the star.

Teachey and Kipping had seen signs in the Kepler data of a second dimming, either before or after the planet passed — exactly what they would expect if an exomoon were orbiting the planet (SN: 8/19/17, p. 15). The pair named the putative moon Kepler 1625b i, or “Neptmoon” for short. But the researchers needed more observations to be sure it really was a moon, not another planet or activity on the star.

Hubble, whose sensitivity is 3.8 times Kepler’s, spotted a secondary dip in light after the planet crossed the star. The planet also started its 19-hour transit 77.8 minutes earlier than expected, suggesting that something was tugging gravitationally on the planet.

Both signals are consistent with Neptmoon existing. Still, “we’re not cracking open champagne bottles just yet,” Teachey said in an Oct. 1 news conference. The team wants to check with Hubble again, hopefully during the next transit in May 2019, he said. “Things look exciting, tantalizing, maybe compelling.”

Astrophysicist René Heller of the Max Planck Institute for Solar System Research in Göttingen, Germany, says that while the data analysis is impressive, “I remain skeptical” about the exomoon’s existence. He also wants to see another transit and better observations of the star.

One reason for caution is the strangeness of the moon itself. In our solar system, moons form in one of three ways: by being knocked from a planet in an impact, by coalescing from gas and rock orbiting the planet or by being captured by the planet’s gravity. It’s unclear how these scenarios could create a moon as large as Kepler 1625b i.

“Kepler 1625b i, if real, would be about 10 times as massive as the mass of all moons and terrestrial planets in the solar system combined,” says Heller, who was not involved in the study. “This suggests that this moon would have formed in a completely different way than any moon in the solar system.”