Planet four times the size of its star discovered - BBC Focus Magazine

Evidence of a giant planet orbiting a dead white dwarf star has been found for the first time.

Astronomers say the evidence for the distant planet comes in the form of a disc of gas created from its evaporating atmosphere. The Neptune-like planet is thought to be more than four times the size of the Earth-sized white dwarf star it orbits.

The giant planet orbits the star about once every 10 days, leaving a trail of gas comprised of hydrogen, oxygen and sulphur in its wake.

Until now, there has been no evidence of a planet that has survived a star’s transition to a white dwarf, researchers say.

The discovery by astronomers from the University of Warwick’s Department of Physics and the Millennium Nucleus for Planet Formation (NPF) at the University of Valparaiso is published in the journal Nature.

The star, WDJ0914+1914, was identified in a survey of 10,000 white dwarfs observed by the Sloan Digital Sky Survey. Researchers say the star is around 2,000 light years from Earth.

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Astronomers at Warwick analysed subtle variations in the light emitted from the system to identify the elements present around the star. They detected very minute spikes of hydrogen in the data, but also of oxygen and sulphur, which they had never seen before.

Using the Very Large Telescope of the European Southern Observatory in Chile they found the shape of the gases are typical indicators of a ring of gas.

Lead author Dr Boris Gaensicke, from the University of Warwick, said: “At first, we thought that this was a binary star with an accretion disc formed from mass flowing between the two stars.

“However, our observations show that it is a single white dwarf with a disc around it roughly 10 times the size of our Sun, made solely of hydrogen, oxygen and sulphur. Such a system has never been seen before, and it was immediately clear to me that this was a unique star.”

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Analysis of the data suggests the composition of the disc matches what scientists expect for the deeper layers of our own Solar System’s ice giants, Uranus and Neptune.

Dr Matthias Schreiber from the University of Valparaiso calculated that the 28,000°C hot white dwarf is slowly evaporating this hidden icy giant by bombarding it with high energy photons. It is pulling its lost mass into a gas disc around the star at a rate of more than 3,000 tonnes per second.

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.

“There could be many cooler white dwarfs that have planets but lacking the high-energy photons necessary to drive evaporation, so we wouldn’t be able to find them with the same method.

“This discovery is major progress because over the past two decades we had growing evidence that planetary systems survive into the white dwarf stage. We’ve seen a lot of asteroids, comets and other small planetary objects hitting white dwarfs, and explaining these events requires larger, planet-mass bodies further out.”

He added that having evidence for an actual planet was an “important step”.

Dr Schreiber added: “In a sense, WDJ0914+1914 is providing us with a glimpse into the very distant future of our own Solar System.”

Artist’s impression of future of the solar system © Mark Garlick

The white dwarf was once a star similar to the Sun but eventually ran out of fuel, and swelled up into a red giant, a few hundred times the size of the Sun. During that phase of its life, the star will have lost about half of its mass and what was left has shrunk, ending up size of the Earth. It is essentially the burnt-out core of the former star.

Once the Earth’s Sun runs out of fuel in about 4.5 billion years it will shed its outer layers, destroying Mercury, Venus, and probably the Earth, eventually exposing the burnt-out core – the white dwarf.

In a companion paper led by Dr Schreiber and Dr Gaensicke, published in Astrophysical Journal Letters, they detail how this will radiate enough high energy photons to evaporate Jupiter, Saturn, Uranus and Neptune.

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Planet four times the size of its star discovered - BBC Focus Magazine

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