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Exomoons could be bigger and weirder than nearby moons

Scientists may be on the verge of confirming that moons exist beyond our solar system. A likely candidate could be unlike any satellite we've ever seen.


The possible view of a distant system harboring what could turn out to be the first confirmed exomoon.


Most of the moons we know in our solar system tend to be either rocky or icy, with some weird features like Titan's methane lakes, the hidden oceans of Europa and Enceladus or the volcanic hell of Io. But exomoons around planets in other distant systems could be even weirder and large enough to rival gas giants like Saturn.

Astronomers have confirmed the existence of thousands of exoplanets beyond our solar system, but so far the existence of an exomoon circling one of them is not quite official. In July, researchers led by Columbia University's David Kipping and Alex Teachey said they might have spotted an exomoon orbiting the planet Kepler-1625 b some 4,000 light-years away. The team cautioned at the time that potential exomoon Kepler-1625 b-i is just a candidate and needs more observations before it can be confirmed.

Astrophysicist Rene Heller, from the Max Planck Institute for Solar System Research in Germany, has analyzed the existing observations of the possible exomoon. If it exists, Heller says, it may look nothing like any moon we've ever seen. 

"Possible extreme scenarios range from a highly inflated Earth-mass gas satellite to an atmosphere-free water-rock companion of about 180 Earth masses," Heller wrote in a paper submitted to the journal Astronomy and Astrophysics (PDF) last week.  

For reference, Saturn is about 95 Earth masses and Jupiter is over 300. In other words, the distant, unconfirmed moon could look something like Earth if it were nearly 100 times as massive as our home planet, or a gas world with the mass of Earth.

Heller reasons that the most likely scenario is somewhere in between these two extremes and that Kepler-1625 b-i would be very similar to Neptune.

If any of Heller's scenarios turn out to be true, the distant moon seems sure to be a unique addition to our currently limited catalog of existing confirmed moons.

We may not have to wait too long to find out the actual nature of Kepler-1625 b-i, if it's there at all: Kipping and Teachey's team is set to observe the system again using NASA's Hubble Space Telescope on Saturday evening.

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