Planet vapourised by star light, ripped apart by dead star

Scientists using NASA's repurposed Kepler space telescope have uncovered strong evidence of a tiny, rocky 'planet' being torn apart as it spirals around a white dwarf star.

This discovery validates a long-held theory that white dwarfs are capable of cannibalising possible remnant planets that have survived within its solar system, researchers said.

"We are for the first time witnessing a miniature "planet" ripped apart by intense gravity, being vaporised by starlight and raining rocky material onto its star," said Andrew Vanderburg, graduate student at the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts.

As stars like our Sun age, they puff up into red giants and then gradually lose about half their mass, shrinking down to 1/100th of their original size to roughly the size of Earth. This dead, dense star remnant is called a white dwarf.

The devastated planetesimal, or cosmic object formed from dust, rock, and other materials, is estimated to be the size of a large asteroid, and is the first planetary object to be confirmed transiting a white dwarf.

It orbits its white dwarf, WD 1145+017, once every 4.5 hours. This orbital period places it extremely close to the white dwarf and its searing heat and shearing gravitational force.

During its first observing campaign from May 30 to August 21, last year, K2 trained its gaze on a patch of sky in the constellation Virgo, measuring the minuscule change in brightness of the distant white dwarf.

When an object transits, or passes in front of a star from the vantage point of the space telescope, a dip in starlight is recorded. The periodic dimming of starlight indicates the presence of an object in orbit about the star.

A research team led by Vanderburg found an unusual, but vaguely familiar pattern in the data. While there was a prominent dip in brightness occurring every 4.5 hours, blocking up to 40 per cent of the white dwarf's light, the transit signal of the tiny planet did not exhibit the typical symmetric U-shaped pattern.

It showed an asymmetric elongated slope pattern that would indicate the presence of a comet-like tail. Together these features indicated a ring of dusty debris circling the white dwarf, and what could be the signature of a small planet being vaporised.

"The eureka moment of discovery came on the last night of observation with a sudden realisation of what was going around the white dwarf. The shape and changing depth of the transit were undeniable signatures," said Vanderburg. The research was published in the journal Nature.