Exomoon?

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lpetrich
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Exomoon?

Post by lpetrich » Mon Jan 15, 2018 12:54 pm

NASA telescope hunts first moon beyond our solar system - CNET: "NASA's space telescope is taking a closer look at a star system where astronomers think they may have spotted the first moon beyond our solar system."

Why astronomers reluctantly announced a possible exomoon discovery : Nature News & Comment [1707.08563] HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I [1710.06209] The nature of the giant exomoon candidate Kepler-1625 b-i (Wikipedia)Kepler-1625 (Wikipedia)Kepler-1625b
Astronomers on Saturday pointed the Hubble Space Telescope in the direction of a star system 4,000 light-years away, where they hope to confirm the presence of an exomoon orbiting a distant exoplanet for the first time. Confirming the existence of such satellites beyond our own solar system opens a new door in the search for life elsewhere in the universe.
They are being cautious because previous proposed exomoons have turned out to be false alarms. Now word on this one yet, however.

Kepler-1625 is a rather Sunlike star, and Kepler-1625b is about as big as Jupiter. Its mass is unknown, being anything from a Saturn-ish 0.4 Jupiter masses to being a brown-dwarf 75 Jupiter masses -- or even a very low-mass star (112 Jupiter masses ~ 0.11 solar masses). This big range of masses is because this planet is close to the maximum size for a cold object.

Kepler-1625b-i has a size roughly 4 times the Earth's, about as big as Uranus and Neptune. It could be a gassy body the mass of the Earth, a rock-and-water one about 180 Earth masses, or somewhere in between, like Uranus and Neptune themselves.

It orbits at roughly 20 times its planet's radius, or something like the orbit radii of Ganymede, Callisto, and Titan.


Whatever its mass is, its formation is a theoretical challenge, since its mass is relatively high compared to its planet's mass. Did some other planet collide with one of its moons? Something like a scenario proposed for Neptune's moon Triton.

But if it is real, it would indicate that Earthlike exomoons might possibly exist.

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lpetrich
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Post by lpetrich » Mon Jan 15, 2018 1:12 pm

Relative masses of moons:
  • Pluto: 0.12 (Charon, total)
  • Earth: 0.012 (Moon, total)
  • Saturn: 0.00024 (Titan), 0.00025 (total)
  • Neptune: 0.00021 (Triton, total)
  • Jupiter: 0.000047, 0.000025, 0.000078, 0.000057 (Io, Europa, Ganymede, Callisto), 0.00021 (total)
  • Uranus: 0.000015, 0.000014, 0.000039, 0.000033 (Ariel, Umbriel, Titania, Oberon), 0.00010 (total)
So if Kepler-1625b is anything like Jupiter or Saturn, then its combined relative mass of moons ought to be 0.0002. Multiplying by 10 Jupiter masses gives 0.67 Earth masses. So Kepler-1625b-i likely has 20 to 200 times that mass.

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