[7 Greatest Alien Planet Discoveries by NASA's Kepler Spacecraft (So Far)]

A very big moon

The Kepler and Hubble observations, along with modeling work, suggest that the moon is about the size of Neptune and 1.5 percent as massive as Kepler-1625b. The remarkable find, described in a study just published in the journal Science Advances, points to a Neptune-sized exomoon around an exoplanet known as Kepler-1625b, which orbits a star codenamed Kepler-1625 located in the constellation of Cygnus (“the Swan”). “If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of.”

Since moons outside our solar system – known as exomoons – cannot be imaged directly, their presence is inferred when they pass in front of a star, momentarily dimming its light. "We've tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system or stellar activity, but we're unable to find any other single hypothesis which can explain all of the data we have," co-author David Kipping, an astronomer at Columbia University in New York, told reporters earlier this week. The astronomers first got a hint of Kepler-1625b-i when they studied Kepler data on 284 exoplanets in search of an exomoon - what they describe as "little deviations and wobbles" beyond the normal transit dips in the light curve coming from the star as the planet moves across it. “We’re not cracking open champagne bottles just yet.”

Teachey and colleague David Kipping (Columbia University) got their first hints of an exomoon after looking through data on 284 planetary systems from the Kepler space telescope. The announcement of the discovery at HubbleSite explained:

Based upon their findings, the team spent 40 hours making the observations with Hubble to study the planet intensively — also using the transit method — obtaining more precise data on the dips of light. The team logged in about 40 hours of Hubble time to take a better look at Kepler-1625 and managed to observe not one, but two dips in starlight brightness during one of the exoplanet’s 19-hour transits. In a statement, Kipping sait it was consistent with “a moon trailing the planet like a dog following its owner on a leash.” Unfortunately, however, the scheduled Hubble observations ended before the complete transit of the supposed exomoon could be measured. “Basically, when a planet has a moon, the planet is of course pulling on the moon gravitationally, but the moon is also pulling on the planet, and they’re both actually orbiting their common center of mass—a point in space along the line between the centers of the two objects,” explained Teaching. But we knew our job was to keep a level head and essentially assume it was bogus, testing every conceivable way in which the data could be tricking us.”

In a paper published in the journal Science Advances, the scientists report the candidate moon is unusually large – potentially comparable to Neptune. For some perspective, according to René Heller (Max Planck Institute for Solar System Research, Germany), that one moon is about 10 times as massive as all the moons and rocky planets in our solar system combined. “But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation.”

Similar to the mass-ratio between our Earth and its satellite, this moon is estimated to be 1.5 percent the mass of its companion gas-giant planet, which is likely several times the mass of Jupiter. Despite the evidence, Teachey stressed, “we are urging caution here.”

“The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence,” Teachey said.
Kepler-1625b, an exoplanet proposed in 2017 to be orbited by an exomoon, the first possible example of a moon orbiting a planet in another stellar system, is confirmed with observations from the Hubble Space Telescope.