Have astronomers found a moon in another solar system?

In a potential breakthrough for the field, astronomers David Kipping and Alex Teachey have discovered what they believe could be an exomoon – a moon orbiting a planet outside of our solar system.

Astronomers have been searching for exomoons for as long as they’ve been searching for exoplanets (and they’ve found thousands of those,) but, with the exception of a few false alarms, the search for moons has been pretty fruitless. Until now.

In July 2017, researchers started observing Kepler-1625b, an exoplanet orbiting the star Kepler-1625 in the constellation Cygnus. Kepler-1625b was discovered on May 10 2016, and is as far from its star as Earth is from the sun, placing it in the habitable zone. But before you get too excited over the idea of aliens, I should point out that Kepler-1625b isn’t exactly capable of supporting life – at least, as far as we know. It’s assumed to be a gas giant about the size of Jupiter, but ten times as dense. Any larger, and this planet would technically be a brown dwarf. Not really an ideal place to live.

Researchers have been studying Kepler-1625b as part of their hunt for exomoons and, out of a pool of 284 exoplanets, Kipping and Teachey found that Kepler-1625b was the best candidate. So, using the Hubble Telescope, they observed the planet as it passed in front of Kepler-1625.

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The event of a planet passing in front of a star is called a transit, and Kepler-1625b’s takes about 19 hours. Using the Hubble Space Telescope, Kipping and Teachey observed how the light of the star dimmed as the planet crossed in front of its star. They were also looking for a second, smaller, dimming, which would signify the existence of a moon trailing behind it.

They got exactly that. As detailed in the researchers’ publication, the Hubble telescope picked up a smaller dimming about 3.5 hours after the first one, suggesting a second body following the first one. However, the researchers’ alloted observation time ended before the planet could complete its transit.

The moon theory is also supported by the fact that Kepler-1625b completed its transit an hour earlier than anticipated. This is called a “planetary wobble” and is caused by gravitational pull of another celestial body. In this case, that celestial body is hypothesised to be a moon.

The exomoon candidate has been officially called Kepler-1625b-i, but its discoverers have nicknamed it the “Nept-moon,” since it’s predicted to be the size of Neptune. If real, Nept-moon would also be gaseous, and would appear twice as big as our moon in Kepler-1625b’s sky.

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But Kipping and Teachey aren’t popping open the champagne quite yet. The existence of this moon is only a hypothesis, based on the limited evidence gained from the transit study. “The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence,” said Teachey. They’re hoping to complete a second Hubble observation next year to either prove or disprove Kepler-1625b-i’s existence. I’ll be crossing my fingers, since I’m already pretty fond of Nept-moon. 

The team is doing their best to stay skeptical, in order to address any and all alternate possibilities. This was difficult for Kipping, who said in a press release that he could barely contain his excitement during testing. “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.” But he has admitted that the existence of an exomoon is the “simplest and most natural explanation.” Another explanation for the “wobble” could be the gravitational pull of another planet orbiting Kepler-1625. But so far, no other planets have been detected.

So, can we put money on Nept-moon’s existence? Personally, I’m not the betting type, but the evidence seems to be there. We won’t know for sure until researchers can do more tests, of course, but even so, what we do have is very promising. If confirmed, this discovery will change everything we know about moons, and I, for one, am beyond excited.

 

Photo Credit: NASA/ESA/L. Hustak

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