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'Death star' reaches out invisible hand, rips planet apart

Kepler K2 mission spots distant planetary death throes

By Richard Chirgwin, 22 Oct 2015

Pity the latest planet spotted by NASA's K2 (the re-birthed Kepler) mission: boiled by starlight from its host, WD 1145+017, and racked by gravity, it's only got a million years left.

The planet, about the size of Ceres and circling a star shrunk down to roughly Earth-size, is already followed by so much debris that Harvard-Smithsonian Centre for Astrophysics graduate student spotted an odd asymmetry in the characteristic “dimming” that identifies exoplanets.

Having already survived WD 1145+017's red-giant phase, the planet whips around the white dwarf in a breakneck 4.5 hours. The star is so small the passing planet dims it by around 40 per cent.

That provided the characteristic transit dimming that Kepler's designed to spot – but as lead author of this paper at Nature, graduate student Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics explains, the shape of the light curve didn't match a planet.

“The shape and changing depth of the transit were undeniable signatures”, NASA's release quotes Vanderburg here.

Rather than the neat U-shaped dip of an undamaged planet, the wreckage circling WD 1145+017 produces an asymmetric slope, suggestive of a trail of debris (rather like a comet's tail, only much bigger).

Transit light plot of 'death star' transit

The light signal from WD 1145+017 suggests debris follows the planet. Image: CfA/A, Vanderburg

As well as Kepler, Vanderburg and his co-authors made extra measurements on the 1.2-meter and MINERVA telescopes at Whipple Observatory, the MMT, MEarth-South, and Keck.

Their observations also found heavy elements polluting the atmosphere of the star – stuff like calcium, silicon, magnesium and iron – instead of a chemically pure surface of hydrogen and helium.

Astro-boffins have long suspected that such pollution is leftover from planets that white dwarfs have consumed, and K2 and NASA Ames research scientist Fergal Mullally says the WD 1145+017 could provide a “smoking gun”. ®

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