In a surprise, first alien moon discovered is big and gaseous

Artist’s impression of the exoplanet Kepler 1625b transiting the star with the potential exomoon in tow

"And if validated, the planet-moon system - a Jupiter with a Neptune-sized moon - would be a remarkable system with unanticipated properties, in many ways echoing the expected discovery of hot Jupiters in the early days of planet hunting".

Two Columbia University researchers presented their tantalizing evidence Wednesday.

Instead, it looked like Kepler-1625b was orbited by a second body the size of Neptune - roughly four times the diameter of Earth.

And in one instance, they found "deviations and wobbles" in the light curve representing a planet known as Kepler-1625b.

In a paper published Wednesday in Science Advances magazine, researchers at Columbia University outline evidence supporting the existence of a moon orbiting the exoplanet Kepler-1625b, using data from the U.S. space agency NASA's Hubble Space Telescope. They were on the lookout for a second temporary dimming of starlight. There are no indications of tidal capture among our Solar System's moons.

The Hubble Space Telescope is a project of global cooperation between ESA and NASA.

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 the Hubble observations ended before the second dip was complete. That's why the astronomers need another look with Hubble, hopefully next spring.

Still, he and lead author Alex Teachey, also a Columbia astronomer, stressed that the observations don't constitute a definitive detection.

"It's our job as scientists to be as skeptical as possible about new ideas and hypotheses, even when those ideas are our own", Teachey said. The moon's Neptune size was on a scale that had "hardly been anticipated" and "defies easy explanation" based on current theories of moon formation.

But, he added: "We are excited about this result, certainly it is a tantalising result". Until now, astronomers have never found solid evidence for a moon.

Exomoons are hard to find because they are smaller than their companion planet and so their transit signal is weak; they also shift position with each transit because the moon is orbiting the planet.

And when E.T. phones home, there's no reason that home couldn't be a moon. The pair would have a similar mass and radius ratio to the Earth and its moon but scaled up by a factor of 11.

That's because most of the exoplanets we know about orbit close to their host stars.

Another planet could cause the same gravitational nudge, the researchers noted, although Kepler observations have come up empty in that regard.

That planet, called Kepler-1625b, is one of thousands that scientists have recently detected around distant stars.

This decrease in dimness is consistent with "a moon trailing the planet like a dog following its owner on a leash", Kipping said.

"We've tried our best to rule out other possibilities", Kipping told reporters. But the additional data both makes the case for the moon better and worse (don't worry, we will explain that).

A moon "is an excellent explanation" for these observations, he said.

Teachey and his co-author David Kipping found the moon - named Kepler-1625b-i - by using the intrepid Hubble Space Telescope. In the meantime, they're encouraging other scientists to join in.

Whether confirmed or not, the subject offers insight into how rare - or how common - our own solar system might be. "While most of these moons orbit Saturn and Jupiter, which are outside the Sun's habitable zone, that may not be the case in other solar systems", said Stephen Kane, an associate professor of planetary astrophysics and a member of the University of California Riverside's Alternative Earths Astrobiology Center. Of the eight planets in our solar system, only Mercury and Venus have none.

Likewise, Teachey is not discouraged by the fact that only one of almost 300 planets surveyed appeared to host a moon.

"This is not by itself a proof of an exomoon", Kipping said.

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