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The hunt for moons outside our solar system has just turned up another possible lunar world, a moon bigger than Earth that's orbiting a Jupiter-like planet. The planet and its moon — if it really is a moon — orbit a Sun-like star that's over 5,000 light years away, according to a report in the journal Nature Astronomy. "The moon is pretty alien compared to any moon in the solar system," says David Kipping, an astronomer at Columbia University. "We're not sure if it's rocky, we're not sure if it's gaseous. It's kind of in between the size of Neptune,...

Last week, a team of astronomers reported the first potential discovery of an exomoon–a satellite orbiting a planet around another star. Part of what is so striking about the report is the scale of this possible planet-moon system. In this case, the “moon” appears to be about the size of Neptune; the planet it orbits is some 10 times the mass of Jupiter, or about 3,000 times the mass of Earth! The system pushes at the limits of how we normally categorize objects in space and invites questions about where we stand in the scale of things. What is the...

First Exomoon Candidate is discovered by the astronomers The scientists used an innovative technique called gravitational microlensing. The technique takes advantage of chance alignments between stars: when a foreground star passes between us and a more distant star, the closer star can act like a magnifying glass to focus and brighten the light of the more distant one. These brightening events usually last about a month. If the foreground star has a planet circling around it, the planet will act as a second lens to brighten or dim the light even more. By carefully scrutinizing these brightening events, scientists can...