Hidden giant planet revealed around tiny white dwarf star

Pablo Tucker
December 5, 2019

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We think of stars as huge objects that dominate everything in their considerable neighborhood, but for the first time astronomers have seen a giant planet that surrounds a dying dwarf star of only a quarter of its size.

Its size is similar to that of Neptune and is more than four times as large as the Earth-sized white dwarf it orbits.

Due to the fact that the massive planet is located at such close distance to the white dwarf, the astronomers say it should have been eaten during the star's explosion phase; unless it moved inward after the star shifted into a white dwarf.

The white dwarf WD J0914, which is located about 1,500 light years away in the constellation of Cancer, was identified in a survey of 10,000 white dwarfs observed by the Sloan Digital Sky Survey. But the wavelengths of the light coming from the system revealed materials that wouldn't normally be found in a pair of white dwarfs alone or even around the subset of "polluted" white dwarfs that show evidence of material falling onto them. The Sun will burn all its hydrogen reserves in about 4.5 billion years.

There is no guarantee that the outer gas giants won't be affected either. Still, the white dwarf won't be gaining much weight from this meal. The blast of high-energy photons this potential world receives from the white dwarf evaporates its atmosphere, which is mainly composed of hydrogen, oxygen and sulphur.

The team then analysed the system using a spectrograph called the X-Shooter on the Very Large Telescope in Chile.

"It was one of those chance discoveries", says researcher Boris Gänsicke, from the University of Warwick in the United Kingdom, who led the study, published today in Nature.

"At first, we thought that this was a binary star with an accretion disk formed from mass flowing between the two stars", Gänsicke explained. "However, our observations show that it is a single white dwarf with a disk around it roughly ten times the size of our Sun, made exclusively of hydrogen, oxygen, and sulfur. Such a system has never been seen before, and it was immediately clear to me that this was a unique star", Gaensicke said in a statement.

When the astronomers averaged all the spectra they obtained over two nights in Chile, it was clear that WDJ0914+1914 was accreting sulphur and oxygen from the disc. Meanwhile, oxygen and sulfur-in addition to smaller amounts of hydrogen-from the planet fall towards the white dwarf, creating the disk of gas that was detected.

The giant planet is orbiting the white dwarf just outside the gas disc.

"This is the first time we can measure the amounts of gases like oxygen and sulphur in the disc, which provides clues to the composition of exoplanet atmospheres", says Odette Toloza from the University of Warwick, who developed a model for the disc of gas surrounding the white dwarf. "However, some of those planets might [be] detectable using the transit method once the Large Synoptic Survey Telescope goes on sky".

"Over the past two decades we had growing evidence that planetary systems survive into the white dwarf stage".

Perhaps this exoplanet-eating white dwarf system is how our own solar system will look in the future: The progenitor red dwarf expands, swallowing up the inner planets and leaving just the ice giant outer planets behind.

The white dwarf observed in the study in its heyday used to be a star similar to the sun but eventually ran out of fuel.

In a companion paper by Schreiber and Gaensicke in Astrophysical Journal Letters, they detail how this will radiate enough high energy photons to evaporate Jupiter, Saturn, Uranus, and Neptune. The light signature of one star - called WDJ0914+1914 - was found to contain trace elements of chemicals that had never before been associated with a white dwarf.

Scientists have discovered evidence of a star system that might foreshadow the fate of our own.

Dr Gaensicke said: "This star has a planet that we can't see directly, but because the star is so hot it is evaporating the planet, and we detect the atmosphere it is losing".

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