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Giant ice planet orbits hot star 1/4 its size

The giant planet is orbiting the white dwarf just outside the gas disc. The blast of high-energy photons that it receives from the white dwarfs evaporates its atmosphere, which is mainly composed of hydrogen, oxygen and sulphur. Whereas much of the hydrogen is forced away from the white dwarf by its ultraviolet photons in a comet-like tail, oxygen and sulphur fall towards the white dwarf, forming the disc researchers have detected. The light from the disc comes only from a few strong emission lines: oxygen lines, which we would see as red light, light up the inner part of the disc, and sulphur lines, which would appear blue, are stronger in the outer regions of the disc. (Credit: University of Warwick/Mark Garlick)

Researchers have found the first evidence of a giant planet orbiting a dead white dwarf star in the form of a disc of gas formed from its evaporating atmosphere.

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 work is the first evidence of a giant planet orbiting a white dwarf star and suggests that there could be many more planets around these stars waiting to be discovered. Until now, there has never been evidence of a planet that surviving a star’s transition to a white dwarf.

Researchers identified the star WDJ0914+1914 in a survey of ten thousand white dwarfs the Sloan Digital Sky Survey observed. Scientists analyzed subtle variations in the light the system emitted to identify the elements present around the star.

The sun sits at the center of planetary orbits, with planets around it looking as though they're fading or evaporating
The sun will evolve into a white dwarf in about 6 billion years from now. Mars and the outer gas giants of our solar system will survive this metamorphosis. For the first few million years after its formation the white dwarf will be extremely hot and its strong EUV emission will evaporate gas from the outer atmospheres of the gas giants. The white dwarf will accrete a fraction of this gas and produce atmospheric lines detectable for future generations of alien astronomers. (Credit: Mark Garlick)

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.

“At first, we thought that this was a binary star with an accretion disc formed from mass flowing between the two stars,” says lead author Boris Gaensicke, a professor of the physics department at University of Warwick. “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.”

The research appears in the journal Nature.

Giant planet, tiny 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. Analyzing 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.

Matthias Schreiber from the University of Valparaíso showed through a set of calculations that the 28,000 degrees Celsius (50,432 degrees Fahrenheit) 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.

“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,” Gaensicke says. “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.”

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

When the sun runs out of fuel

The white dwarf we see today was once a star similar to the sun but eventually ran out of fuel and swelled up into a red giant, a few 100 times the size of the sun. During that phase of its life the star 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 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. Just as on WDJ0914+1914, some of that atmospheric gas will end up on the white dwarf the sun leaves behind y, 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.

“We were stunned when we realized that when observing hot white dwarfs, we are potentially seeing signatures from extrasolar planet atmospheres,” Schreiber says. “While this hypothesis needs further confirmation, it might indeed open the doors towards understanding extrasolar planet atmospheres.”

The Science and Technology Facilities Council and the Leverhulme Trust supported the UK authors.

Source: University of Warwick