Planet Fragment Discovered That Survived A Ringside Seat At The Death Of Its Star

Artist's impression of SDSS J122859.93+104032.9, the debris disk around it and the ring of gas trailing the rocky remnant of a former planet. University of Warwick/Mark Garlick

A small planet or asteroid has been found in what was previously considered an impossible orbit around a white dwarf star. It is thought the object represents a small and very dense remnant of an object that was pulled apart during its parent stars' death throes. Nevertheless, astronomers are astonished it survived at all.

Dr Christopher Manser of the University of Warwick studied a disk surrounding SDSS J122859.93+104032.9, a white dwarf 410 light-years away. Around a quarter of white dwarfs have these disks, made up of debris thought to be left over from planets destroyed as main sequence stars converted to white dwarfs.

In this case, the Gran Telescopio Canarias revealed a ring of gas within the disk, whose distribution suggests it is trailing behind an object we cannot see, like the tail of a comet. The object takes just over 2 hours to orbit.

In the journal Science, Manser reports the source could be anything from an asteroid just 1 kilometer across to a dwarf planet several hundred kilometers wide. It's also not clear if it is dissolving, shedding the gas in the process, or if what we see is a product of impacts between the object and dust particles in the disk.

White dwarfs are what is left behind when Sun-like main-sequence stars shed their outer layers, having run out of hydrogen to fuse. The remnant is much denser than its predecessor, in SDSS J122859.93+104032.9's case containing about a third of the original mass, but being so much smaller, that this planetesimal's orbit lies entirely within the volume the predecessor star once occupied.

Clearly whatever is causing this gas plume must once have orbited much further out. It is likely gravitational interactions with outer planets pushed it to its current orbit quite recently.

Its survival offers clues to the planetesimal's composition. "The white dwarf's gravity is so strong – about 100,000 times that of the Earth's – that a typical asteroid will be ripped apart by gravitational forces if it passes too close to the white dwarf,” Manser said in a statement. To survive any object would need to also be dense and probably have a strong internal structure, which Manser and co-authors think suggests a composition of iron and nickel.

This, in turn, suggests what survives is a remnant of a planet large enough to form a differentiated core, where heavier elements sink to its center.

One planetesimal has been found orbiting a white dwarf previously, but it was only detected because its orbit aligned with Earth. Since his method is more widely applicable, Manser hopes it will be used to see whether such objects are common, and may be the eventual fate of members of the Solar System, such as Mars, far enough out to survive the Sun's pre-death expansion.

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