Scientists have spotted a few thousand exoplanets, or planets orbiting stars other than our sun. Exomoons are another story. There have been some potential exomoons, but nothing definitive. Now, a team from Columbia University thinks it might have the first solid evidence of a moon outside our solar system, and Hubble is going to give it a look.
While we haven’t been able to identify any exomoons in the past, we have every reason to think they exist. After all, most planets in our solar system have at least one moon. In the case of gas giants, there can be dozens of moons. Many of the exoplanets that can be reliably detected by current methods are gas giants, so there ought to be moons. Alex Teachey and David Kipping at Colombia may have found evidence of one in data from the planet-hunting Kepler spacecraft.
The exoplanet in question is known as Kepler 1625b and orbits a star about 4,000 light-years away. The potential exomoon has been dubbed “Kepler 1625b i.” The Kepler mission uses the transit method of detecting exoplanets. As a planet passes in front of its star, Kepler registers a small drop in luminance. By analyzing the repeating pattern of these drops, scientists can determine a planet’s size, orbital characteristics, and temperature. In the case of Kepler 1625b, there’s a second, smaller dip in brightness that occurs ahead of or behind the main one, which could indicate a moon in orbit.
The team analyzed 284 planets looking for exactly this kind of signal, and Kepler 1625b “popped out.” The transit method is not sensitive enough to detect what we would usually think of as a “moon,” but Kepler 1625b i isn’t like any moon in the solar system. Kepler 1625b is about the size of Jupiter, and possibly as much as 10 times as massive. The exomoon, if it exists, could be the size of the planet Neptune. Thus, it’s been nicknamed “Neptmoon.” Something that large is within the detection limits of Kepler, but many smaller moons would still go unnoticed.
Kepler 1625b will transit in front of its star again on October 29th, and the team intends to be watching with the Hubble telescope when that happens. This could offer proof that Neptmoon is indeed a giant moon in orbit of Kepler 1625b.
Astronomers are keen to prove that exomoons exist because these objects are considered a potential home for alien life in orbit of uninhabitable gas giants. Future instruments like the Webb Telescope should also help identify smaller exomoons that are more hospitable to life as we know it.
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