Hidden giant planet revealed around tiny white dwarf star

The Neptune-like planet orbits a star a quarter of its size about once every ten days, leaving a comet-like tail of gas comprised of hydrogen, oxygen and sulphur in its wake.

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 Valparaíso is published today (4 December) in the journal Nature. It is the first evidence of a giant planet orbiting a white dwarf star and suggests that there could be many more planets around such stars waiting to be discovered.

Until now, there has never been evidence of a planet that has survived a star's transition to a white dwarf.

The star WDJ0914+1914 was identified in a survey of ten thousand white dwarfs observed by the Sloan Digital Sky Survey. Scientists 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, which was unusual in itself, but also of oxygen and sulphur, which they had never seen before. Using the Very Large Telescope of the European Southern Observatory in Chile to obtain more observations of this star, they found that the shape of the hydrogen, oxygen and sulphur features 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 ten 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."

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. Analysing the data, they were able to measure the composition of the disc, and concluded that it 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 Valparaíso showed through a set of calculations that the 28,000 degrees Celsius hot white dwarf is slowly evaporating this hidden icy giant by bombarding it with high energy photons and pulling its lost mass into a gas disc around the star at a rate of over 3,000 tons 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. However, some of those planets might detectable using the transit method once the Large Synoptic Survey Telescope goes on sky.

"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. Having evidence for an actual planet that itself was scattered in is an important step."

Dr Schreiber adds: "In a sense, WDJ0914+1914 is providing us with a glimpse into the very distant future of our own solar system."

The white dwarf we see today was once a star similar to the sun but eventually ran out of fuel, swelled up into a red giant, a few 100 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 dramatically ending up size of the Earth -- the white dwarf is essentially the burnt-out core of the former star.

Extraordinarily, today's orbit of the planet around the white dwarf would have been deep inside the red giant, so scattering with some other planets in the system, a kind of cosmic pool game, moved it close to the white dwarf after the red giant's outer layers were lost.

Once our own sun runs out of fuel in about 4.5 billion years it will shed its outer layers, destroying Mercury, Venus, and probably the Earth, and eventually expose the burnt-out core -- the white dwarf. In a companion paper led by Dr Schreiber and Dr Gaensicke and published in Astrophysical Journal Letters, they detail how this will radiate enough high energy photons to evaporate Jupiter, Saturn, Uranus and Neptune. Just as on WDJ0914+1914, some of that atmospheric gas will end up on the white dwarf left behind by the sun, and will be observable for future generations of alien astronomers.

The astronomers argue that this planetary evaporation and subsequent accretion by young white dwarfs is probably a relatively common process and that it might open a new window towards studying the chemical composition of the atmospheres of extrasolar gas giant planets.

Dr Schreiber comments: "We were stunned when we realised that when observing hot white dwarfs, we are potentially seeing signatures from extrasolar planet atmospheres. While this hypothesis needs further confirmation, it might indeed open the doors towards understanding extrasolar planet atmospheres."