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Has the first exomoon been found?

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The discoveries of distant exoplanets have become commonplace in the last few years, and now the first exomoon may have just been found.

Finding planets orbiting other stars was difficult enough at first because of the tiny size and dimness of the planets compared to their host stars. But the technology involved continues to improve, and we are now at the threshold of being able to find even smaller moons orbiting some of those planets.

As astronomer David Kipping of Harvard University noted, "This is the first serious candidate from any survey that I am aware of."

The planet and possible moon are about 1,800 light years from Earth, and this potential first discovery is also an unusual one - the planet is apparently a rogue one, floating free of any star. Other such planets have also been found in recent years and may actually be fairly common. Also, the purported moon is orbiting a long ways out from the planet at an estimated distance of 20 million kilometres (12.4 million miles) with an estimated mass of about half that of Earth. The planet itself is about four times the mass of Jupiter. A planet-sized moon orbiting a gas giant planet, reminiscent of the moon Pandora from the movie Avatar.

The odd nature of this discovery also means that a couple different scenarios are possible: this is either indeed a planet and moon or possibly a brown dwarf star orbited by a Neptune-mass planet, if it is actually farther away than estimated.

The objects were detected using the gravitational microlensing technique; when a planet passes in front of a star, as seen from Earth, the planet's gravity bends the star's light. The light is then focused like with a lens which makes the star appear temporarily brighter when it is observed from a particular angle. Unfortunately, this also makes the objects difficult to observe again, so we may never know which answer is correct.

But given the thousands of planets being discovered now, it is reasonable to think that there are a lot of moons out there as well. And if the many moons in just our own solar system are any indication, they probably come in countless varieties, with some being potentially habitable, such as Europa, Enceladus or Titan in our solar system. There might even be one like Pandora...

The published paper is available here.

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