Astronomers have discovered a large Neptune-sized planet orbiting a white dwarf star. The planet is four times bigger than the star, and the white dwarf appears to be slowly destroying the planet: the heat from the white dwarf is evaporating material from the planet’s atmosphere, forming a comet-like tail.
A white dwarf is the end state for stars like our Sun. As a Sun-like star runs out of fuel, it expands into a red giant. During that phase, much of the star’s mass is shed into space. After that, what’s left is a compact, relatively cool, white dwarf.
A white dwarf is mostly a spent force in stellar terms. It’s in its end state, not putting out near the same energy it used to. But it’s still radiative enough to strip the atmosphere from the planet. And that’s what’s happening with the star in this study, named WDJ0914+1914.
The white dwarf in this study was among 10,000 surveyed by the Sloan Digital Sky Survey (SDSS.) Working with the Sloan data, astronomers at Warwick analyzed the light coming from the white dwarf. Subtle but detectable variations in light came from the system that allowed the astronomers to identify the elements present.
They detected tiny spikes in hydrogen, oxygen, and sulfur. The hydrogen was unusual, since white dwarfs are mostly comprised of oxygen and carbon, and the oxygen and sulphur had never been seen before in a situation like this. They looked closer with the Very Large Telescope (VLT) and found that the shape of the three elements indicated the presence of a ring of gas around WDJ0914+1914.
At first, the researchers thought they were seeing a binary star.
Dr. Boris Gaensicke from the University of Warwick is the lead author of one of the studies. In a press release he 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.”
After further analysis, the astronomers found that the white dwarf was accreting oxygen and sulphur from the disc. They were able to analyze the composition of the disc itself, and discovered that it matches the deeper layers of planets in our own Solar System like Neptune and Uranus, both ice giants.
Even though fusion ceased a long time ago, the white dwarf is still about 28,000 Celsius. It’s putting out enough energy to bombard the unseen planet, and to evaporate away its material. The team’s calculations show that about 3000 tons of material is stripped away into the disc every 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.”
Not all white dwarfs are as hot as this one. That means that detecting any planets orbiting them with this same method likely isn’t possible. But there must be more of them. In fact, there may be as many as 10 billion white dwarfs in the Milky Way. What percentage of them are hot enough to have planets interacting with them like this one is an open question.
“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,” said Dr. Gaensicke. “However, some of those planets might be detectable using the transit method once the Large Synoptic Survey Telescope goes on sky.” (That telescope might be re-named the Vera Rubin Survey Telescope.)
There’s a growing body of evidence showing that planets may survive into their star’s white dwarf phase. This discovery adds weight to that evidence.
“This discovery is major progress because over the past two decades we had growing evidence that planetary systems survive into the white dwarf stage,” said Dr. Gaensicke. “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.”
According to Dr. Matthias R. Schreiber, lead author of the second paper, the WDJ0914+1914 system is a glimpse into the future. “In a sense, WDJ0914+1914 is providing us with a glimpse into the very distant future of our own solar system.”
Eventually our Sun will share the same fate as this white dwarf. It will become a red giant, expanding and enveloping Mercury, Venus, and Earth. (Maybe even mars.) After that, about 6 billion years from now, it will be a white dwarf. If the gas giants and ice giants in our Solar System migrate close enough to the star, their material might be stripped away into a disc just like WDJ0914+1914 is doing to its ice giant.
In a companion paper, the astronomers point out what will happen. Our future Sun will emit enough high energy photons to strip away material from Jupiter, Saturn, Uranus, and Neptune. Some of that gas will make its way into the disc, and then into the Sun itself. Future astronomers, if there are any, will be able to see it the same way we have.
The team may also have inadvertently found a way to study the atmospheres of exoplanets.
In the press release, Dr. Schreiber commented: “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.”
This is the second time astronomers have found a planet in an unexpected place. In November, astronomers using TESS data found a planet orbiting a red giant. In both of these cases, any planets in these positions should have been destroyed when their stars expanded into red giants.
But now that two have been discovered, it’s likely there will be more, many more. In the second paper, the authors infer a rate of 1 in 10,000 white dwarfs stripping the atmospheres from planets. If there are 10 billion white dwarfs in the Milky Way, there would be 1 million of them with planets that cold be detected spectroscopically. And that’s bound to re-shape our understanding of how solar systems evolve.
Astronomers at the University of Warwick in the UK and the University of Valparaiso in the US made the discovery. Their research was published in two papers. One paper is titled “Accretion of a giant planet onto a white dwarf star” and was published in Nature. The second paper is titled “Cold Giant Planets Evaporated by Hot White Dwarfs” and was published in Astrophysical Journal Letters.
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