Almost 3,800 exoplanets have been discovered so far, and if our solar system is anything to go by, many of those should also harbor moons. Although it's basically a sure thing that these exomoons are out there, astronomers haven't been able to spot any – until now. Scientists using NASA's Hubble and Kepler space telescopes have found possible evidence of a large moon orbiting a gas giant planet some 8,000 light-years away.
Orbiting an exoplanet known as Kepler 1625b, the new moon appears to be about the size of Neptune. That's much bigger than any currently-known moon, but in relation to its host planet that ratio lines up – it's about 1.5 percent the mass of Kepler 1625b, which itself is several times bigger than Jupiter.
Finding distant worlds isn't just a matter of seeing them through a telescope – at those distances planets and moons are just too dim to be directly visible. Instead, the majority of known exoplanets have been discovered through the transit method, where the light from the host star dims temporarily as the planet passes between it and us. Unfortunately, exomoons are harder to see this way, mostly due to the fact that they're smaller and may be hiding behind or in front of their home planet during a transition.
That said, astronomers from Columbia University believe they've found some intriguing fingerprints that may indicate an exomoon. The team examined data from 284 exoplanets that took at least 30 days to orbit their host stars, and found one – Kepler-1625b – that had a few anomalies to it.
With their attention drawn to that oddball, the researchers then used Hubble to peer closer and gather more data. Over 40 hours of observations, the team watched the planet transit the star, and noticed that about three and a half hours after the event, a second, much smaller dimming began. That suggested a moon was following the planet, although the team's scheduled time with the telescope ended before it could be confirmed.
That wasn't the only clue to the existence of a moon. The team noted that the planet's transition began more than an hour earlier than was expected, which is consistent with the gravitational wobble caused by swinging a moon around.
"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," says David Kipping, co-author of the study. "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. 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."
Strangely enough, both moon and planet appear to be gas giants. If that turns out to be the case, this would be the first known gaseous moon, which raises new questions about how these natural satellites form.
As intriguing as the data is, the exomoon's existence remains unconfirmed. Follow-up observations will be required by Hubble and other instruments, such as the upcoming James Webb Space Telescope. Other techniques may help identify other distant moons too, including searching for radio signals that result from a moon's gravity interfering with the host planet's ionosphere.
The research was published in the journal Science Advances.
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