Stars eat their young
Two new studies have revealed that the exoplanets are doomed to premature deaths even before they could get close to be ripped apart by the host stars' gravity, a finding that may help explain why few exoplanets are found next to host stars.
According to the studies, a star's gravity can put a nearby planet on a fast track to spiralling into the star and may also cause the planet to lose much of its atmosphere, the New Scientist reported. More than 300 exoplanets have been catalogued to date. Many are situated close to the host stars. But the closest-in ones are commonly found some 0.05 astronomical units (AU) from their host stars.
But, no one is sure why the planets seem to pile up there. Very close to a star, at a boundary called the Roche limit, planets are dismembered by the star's gravity. But the migration of planets seems to stop well outside this limit. So why do planets seem to stop there? Some models suggest gas and dust in the disc around a star could drag the planets inward. If the star managed to clear away the debris close to it, that could stop planet migration.
However, Brian Jackson of the University of Arizona in Tucson and colleagues offer an alternative explanation. There may be planets that orbit closer in, but they will not do so for very long before they get dragged inwards by their host star's gravity. The tugging is caused by tidal forces between the planet and its star u2014 differences in the pull of gravity on the objects' near and far sides. Counter-intuitively, the same force is causing the moon to slowly widen its orbit around Earth. But in that case, the moon orbits the Earth more slowly than the Earth spins, and that causes the moon's distance to increase. "Once a planet gets that close, the tide raised on the star by the planet causes the planet to migrate in so quickly they're hard to catch," Jackson was quoted as saying.
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