Sometimes, if you look deep into the soft glow of a dying sun, you might see the rocky remains of small planets slowly being digested. For the first time, astronomers have watched a collapsed star pull apart the remains of something a little more hearty.
An analysis of the spectral fingerprints of the white dwarf WD J0914+1914 has revealed rare traces of a chemistry belonging to a gas giant, revealing a star system quite unlike any others we’ve seen to date.
Using data collected as part of the Sloan Digital Sky Survey (SDSS), researchers from the UK, Chile, and Germany have found signs of a Uranus-like planet orbiting so close to a distant white dwarf, the planet’s being stripped apart.
White dwarfs are the end of the line for stars much like our own Sun. Once their fuel supply has been used up, these cosmic objects collapse under their own gravity, squeezing all of that mass into an Earth-sized sphere.
Even though they are technically retired, the bodies of these stars remain hot enough to radiate for hundreds of billions of years. That warm glow – while still rather dull – can provide astronomers with just enough information about their chemical makeup.
It’s not uncommon to find hints of metals flashing in that gentle light, telling a story of nearby rocky planets evaporating in the white dwarf’s heat.
For that to happen, the solar system has to be in a bit of a mess, though. Planets need to be close enough for the white dwarf to bake, and that usually points to a shake-up caused by a nearby heavyweight.
Until now though, no sign of such a gas giant orbiting a white dwarf has been glimpsed. It’s not hard to believe some should be out there – but even the most reasonable assumptions in science demand hard evidence before they can be taken seriously.
Now, we finally have our first real signs of one.
At a distance of around 2,000 light-years, WD J0914+1914 is hard to see even by white dwarf standards. Initially, researchers studying its spectrum thought it was a binary star system, thanks to the signature of its hydrogen component. A closer look soon found clear signs of oxygen, with tentative hints of sulphur.
The fluctuating ratios of hydrogen and oxygen were weird enough to prompt researchers to return for a better look, this time using a tool at the Very Large Telescope of the European Southern Observatory called the X-Shooter spectrograph.
By running the spectral data through a simulation program, the team was able to model the conditions we might expect in WD J0914+1914’s immediate surroundings, revealing important details about the kinds of objects we would expect to see.
The mix of water and hydrogen sulphide suggested a gas planet similar in composition to our own behemoths, such as Neptune and Uranus, was shedding its atmosphere.
“It took a few weeks of very hard thinking to figure out that the only way to make such a disc is the evaporation of a giant planet,” says astronomer Matthias Schreiber from the University of Valparaiso in Chile.
For the giant to be vaporising in this fashion, it would need to be right up close to the white dwarf. The researchers estimate it would be around 15 solar radii – or about 10 million kilometres – with an orbit of just ten of our days. (By comparison, Mercury has an orbit that brings it no closer than 46 million kilometres to our Sun.)
“The discovery also opens up a new window into the final fate of planetary systems,” says lead researcher Boris Gänsicke from the University of Warwick in the UK.
Even with this evidence of a gas giant in orbit around a white dwarf, it’s not clear if it’s an anomaly or if we’re just not looking hard enough to find other examples. A close inspection of some 7,000 or so other white dwarfs in the SDSS’s databanks failed to find anything similar to WD J0914+1914.
Still, around 260,000 examples of these collapsed suns have been identified using the European Space Agency’s Gaia mission, so we have a long way to go before we can draw any solid conclusions.
Given our own Sun is destined to become a white dwarf, it’d be interesting to find more examples that just might give us a glimpse of our own Solar System’s distant future.
This research was published in Nature.