Astronomers find first moon beyond our solar system

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At more than three times the radii of the Earth, these gaseous planets are less common than terrestrial planets, but each is expected to host several large moons.

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. The researchers' investigations showed that the HST-recorded transit of Kepler-1625b occurred almost 80 minutes earlier than expected, a pattern suggesting the presence of transit timing variations, or TTVs, which are among the first proposed methods to confirm the presence of exomoons.

Astronomers may have found the first moon outside our solar system, a gas behemoth the size of Neptune. It's comparable to so-called hot Jupiters, gas giant exoplanets that are closer to their stars than Jupiter is to its own, and warmer. The only snag is that to impart such a large TTV, the moon must have a mass similar to Neptune.

Still, it is possible that there is a second planet in the system external to Kepler-1625b whose transit we can't see, Teachey said. The two spent hundreds of hours analyzing data from 284 planets discovered by Kepler, which had orbits longer than 30 days around their host star.

In the meantime, a new release of Kepler data smoothed away many of those bumps, weakening the original case for a moon search. So, did Kepler-1625b capture its satellite? This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location. What's more, the telescope detected an additional decrease in the star's brightness after the planet's crossing was complete, suggesting that a Neptune-size moon was in front of the star. The Kepler outcomes were sufficient enough for the group to achieve 40 hours of time with Hubble to vehemently study the planet acquiring the data four times more accurate than that of Kepler.

Given the abundance of moons in our own Solar System, you'd think there are heaps orbiting exoplanets, too. There's no analog for such a large moon in our own system.

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Kipping said there's already another potential planet-moon combination worth looking at.

According to the authors of the study, it's possible that the second dip in light could be caused by another planet orbiting the star, but the Kepler Space Telescope didn't find any evidence of a second planet in the system during its observations of the star. Like Kepler, Hubble tracked the faint dimming of light from Kepler-1625, a sunlike star about 8,000 light-years from Earth, as the two worlds passed over the star's disk.

"If we want to do moon hunting in the future, we will have to look at planets further [than one astronomical unit, or the distance between the sun and Earth]", Teachey said. These are generated because, as the moon orbits its planet, it exerts a gravitational tug on it. Kipping and Teachey weren't even sure how such a moon might have formed.

Based on the moon's distance from its star, its surface temperature was estimated to be 80C, which is considered just on the upper end of conditions that could support life. It makes sense, Teachey said, that the first moon scientists spot would also be a giant. 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. Of the eight planets in our solar system, only Mercury and Venus have none.

Teachey and Kipping are submitting proposals for more time on Hubble to observe this planet and its moon during another transit. To further confirm the exomoon, the team will need to continue to observe the transit events in this system. That would confirm that the find is an exomoon.

"The mass ratio that we derive [between the moon and the planet] is only about 1.5%, so I'd call it a moon", he says. "We'd be very grateful if we had the means to use James Webb, because then we could really clean up", Kipping said.

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