Have astronomers found 1st moon outside our solar system?

An illustration of the exoplanet Kepler 1625b with its large hypothesized moon

Three-and-a-half hours later, Hubble detected a much smaller dip in the star's brightness consistent with a large moon trailing the planet. Kepler-1625b is a Jupiter-sized planet that orbits far enough away from its star-about the same distance Earth is from ours-that a moon could be stable.

Researchers have found evidence of what may be a moon outside of our solar system.

"The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence", said David Kipping, an astronomer at Columbia University in NY who has spent more than a decade hunting for far-flung moons.

"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", said Dr Kipping, from Columbia University in NY.

Moons are abundant in our own solar system, with close to 200. The exoplanet is a gas giant, several times more massive than Jupiter [2]. That is roughly the same as the ratio of Earth's mass to the mass of the Moon. Kipping and Teachey discovered it among 300 exoplanets in Kepler's catalogue, all producing predictable dips in starlight that occur as an orbiting body passes in front of its sun - a phenomenon called a "transit".

Mr Kipping said: "We saw deviations and wobbles in the light curve that caught our attention".

To look for exomoons, the team analysed data from 284 Kepler-discovered planets that were in comparatively wide orbits, with periods greater than 30 days, around their host star.

Although the object itself can not be seen, there are hints it exists, according to the researchers: The planet moves around its star in a way that indicates something else is pulling on it gravitationally, probably a moon. However, Kepler 1625b and its moon are gaseous, not rocky, raising questions about how such a moon could have formed in the first place. An alternative explanation could be that a second planet is orbiting Kepler-1625 alongside Kepler-1625b.

So Kipping and Teachey asked for 40 hours of observing time with the Hubble Space Telescope, which is four times as precise as Kepler. But there is a chance we could detect the presence of a major feature of our Solar System elsewhere: exomoons. 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.

Thousands of planets have been detected around faraway stars in recent years.

"It was definitely a shocking moment to see that Hubble light curve, my heart started beating a little faster and I just kept looking at that signature".

They say they noticed a dip in the amount of light approximately 3 and half hours after a planet about the size of Jupiter transits past the star. In addition, the ideal candidate planets hosting moons are in large orbits, with long and infrequent transit times.

In principle this anomaly could also be caused by the gravitational pull of a hypothetical second planet in the system, but the Kepler Space Telescope found no evidence for additional planets around the star during its four year mission.

He added: "We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis".

The capture and impact scenarios are implausible for such a large moon, Kipping said. The moons of Jupiter and Saturn on the other hand coalesced from the debris left behind after the planets formed. The star was probably cooler in the past, so this heat could be a reason for the size of the moon, inflating the gas giant as the temperature rises.

Study co-author Alex Teachey, also of Columbia, admitted that the potential exomoon discovery is far from a slam dunk: "We are urging caution here". With a radius that's around a third that of its parent planet, this object is unlike any moon of a giant planet in the solar system. That would confirm that the find is an exomoon. NASA's upcoming James Webb Space Telescope could really "clean-up" in the satellite search, Kipping said. "If they see what we see, I expect some people will be convinced and other people will be skeptical".

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