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News flash! The lowest mass planet yet found just so happens to orbit a very low mass star -- so low mass, in fact, that it might not even really be a star.

OK, first, the planet. Called MOA-2007-BLG-192Lb -- of course! -- it has a mass of about three times the Earth, making it the lowest mass planet found so far. It orbits its parent star at about the same distance Venus orbits the Sun. However, that doesn't make the planet terribly hot: the host star is itself may be a brown dwarf, in which case the planet may be as cold as Pluto! The star is right on the borderline of what can be called star. By consensus, a star can maintain fusion in its core over long periods of time. Fusion is what powers the star, heating up the core and making the star shine. The Sun fuses hydrogen into helium (700 million tons every second!) which is what powers our star. But that's maintained by the tremendous pressure at the Sun's core. An object less than about 8% of the Sun's mass won't be able to squeeze hydrogen together hard enough to fuse it. In that case, it's called a brown dwarf. It can stay warm for a few billion years just from the leftover heat of its formation, leaking out radiation slowly but never able to regenerate it. The thing is, the exact mass of the newly found star is not known, so it may be just below or just above that limit. This makes a difference to the planet, certainly -- its temperature depends directly on it! -- but also on our theories. There are competing ideas of how brown dwarfs form, and being able to have a planet form nearby will certainly have people scratching their heads and trying to figure out how to manufacture a system like this. There is a problem: the star (or whatever) and planet are 3000 light years away! They were not detected in the usual way by direct observation, but were found by gravitational lensing. The path a light ray takes bends when it passes a massive object. The gravity of the object can also amplify the light from stars behind it. In this case, both the planet and the dinky star were fund by this brightening of a background star. The amount of brightening can be used to determine the masses of the planet and the star, but not with perfect precision; hence the question on the stellar nature of the star. The planet's mass is pretty secure, at least that it's very low, only a few times that of the Earth. In the end, we have at least one very cool thing about this announcement, and that's the lowest mass planet yet found. At that mass, it's almost certainly a rocky or icy body, and not a gas giant like Jupiter. And, if it does indeed orbit a brown dwarf, well, that's pretty excellent too. But either way, say hello to our little friends.

browndwarf_planet.jpg

Artist's conception of the newly found system. Credit: NASA

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