Astronomers have hunted for "exoplanets" - planets outside our solar sytem - for decades, but have never spotted an "exomoon", until now.
"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 NY, told reporters earlier this week.
Jupiter itself has over 70 known moons, and four of these are comparable in size to our own, so multiple moon signals are not unexpected from exoplanet systems. Astronomers think numerous gas giants' moons are captured asteroids. That is roughly the same as the ratio of Earth's mass to the mass of the Moon. The planets are revealed by a momentary dimming of starlight as they pass in front of their host star, in what astronomers call a transit. Current theories cannot therefore explain how a Neptune-sized moon could have formed in orbit around a Jupiter-sized planet.
In the meantime, a new release of Kepler data smoothed away many of those bumps, weakening the original case for a moon search.
Editor's note: This story has been corrected from an earlier version that included one reference to the primary planet of the potential exomoon as WASP-1625b.
Hubble was also able to measure that the planet began its transit earlier than expected, consistent with the "wobble" that occurs when a planet and moon orbit the same center of gravity. The data suggested that a Jupiter-sized planet, Kepler-1625b, may be orbited by an exomoon.
WASHINGTON, Oct 4 ― Astronomers have pinpointed what appears to be the first moon detected outside our solar system, a large gaseous world the size of Neptune that is unlike any other known moon and orbits a gas planet much more massive than Jupiter. But because it's so big, the object would be about twice as big in Kepler-1625b's skies as Earth's moon is in ours, Teachey and Kipping said.
This artist’s impression depicts the exomoon candidate
The possible moon was documented by Kepler, the powerhouse planet-hunting space telescope, when it cast a shadow by crossing in front of a star. In this study, they analysed 284 light curves from the Kepler satellite of planet-hosting stars that were considered to be plausible candidates for systems containing exomoons. But Kepler didn't find any other planets around this star. "It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature". In addition, the ideal candidate planets hosting moons are in large orbits, with long and infrequent transit times.
Although the researchers can not say with certainty that an exomoon caused these effects, Kipping argues that the moon hypothesis offers the best single explanation for both anomalies.
"But going forward, I think we're opening the doors to finding worlds like that", Teachey said. "We think that they're at the right temperature to have liquid water on the surface, but of course there is no real surface because they are gas giant planets".
How did this moon form in the first place?
Faced with that skepticism, Teachey and Kipping are undoubtedly applying for more Hubble time during its next transit. However, given that the existence of such a large, oddball moon had not even been predicted until now, astronomers will struggle to explain how it got there. There are no indications of tidal capture among our Solar System's moons. A giant moon like Neptune points to other mechanisms.
Both the study team and critics alike say that further confirmation is still needed, however, adding an element of caution to what would be a landmark discovery. That would confirm that the find is an exomoon. "We'd be very grateful if we had the means to use James Webb, because then we could really clean up", Kipping said.
"If this does pan out and turn into a true discovery, it would be really revolutionary, but I don't think we're quite there yet", says Megan Bedell, an astronomer at the Flatiron Institute in NY.