A Neptune-like exoplanet orbits an Earth-sized white dwarf star called WD J091405.30+191412.25 (WD J0914 for short) once every 10 days, leaving a comet-like tail of gas comprised of hydrogen, oxygen and sulfur, according to a new research led by University of Warwick astronomers.
This illustration shows the white dwarf WD J0914 and its Neptune-like exoplanet. Image credit: M. Kornmesser / ESO.
Stars like our Sun burn hydrogen in their cores for most of their lives. Once they run out of this fuel, they puff up into red giants, becoming hundreds of times larger and engulfing nearby planets.
In the case of the Solar System, this will include Mercury, Venus, and even Earth, which will all be consumed by the red-giant Sun in about 5 billion years.
Eventually, Sun-like stars lose their outer layers, leaving behind only a burnt-out core, a white dwarf. Such stellar remnants can still host planets, and many of these star systems are thought to exist in our Milky Way Galaxy.
However, until now, astronomers had never found evidence of a surviving giant planet around a white dwarf.
“Until recently, very few astronomers paused to ponder the fate of planets orbiting dying stars. The discovery of a planet orbiting closely around a burnt-out stellar core forcefully demonstrates that the Universe is time and again challenging our minds to step beyond our established ideas,” said lead author Dr. Boris Gänsicke, from the University of Warwick.
The white dwarf WD J0914, which is located about 1,500 light years away in the constellation of Cancer, was identified in a survey of 10,000 white dwarfs observed by the Sloan Digital Sky Survey.
Dr. Gänsicke and colleagues analyzed subtle variations in the light emitted from the star to identify the elements present around it.
They detected very minute spikes of hydrogen in the data, which was unusual in itself, but also of oxygen and sulfur, which they had never seen before.
“We knew that there had to be something exceptional going on in this system, and speculated that it may be related to some type of planetary remnant,” Dr. Gänsicke explained.
To get a better idea of the properties of WD J0914, the researchers analyzed it with the X-SHOOTER instrument on ESO’s Very Large Telescope.
The observations confirmed the presence of hydrogen, oxygen and sulfur associated with the white dwarf.
By studying the fine details in the spectra taken by the X-SHOOTER, the team discovered that these elements were in a disk of gas swirling into WD J0914, and not coming from the star itself.
“It took a few weeks of very hard thinking to figure out that the only way to make such a disk is the evaporation of a giant planet,” said co-author Dr. Matthias Schreiber, from the University of Valparaiso.
The detected amounts of hydrogen, oxygen and sulfur are similar to those found in the deep atmospheric layers of icy, giant planets like Neptune and Uranus.
The scientists showed through a set of calculations that the 28,000-degree-Celsius (five times the Sun’s temperature) white dwarf is slowly evaporating this hidden icy giant by bombarding it with high energy photons and pulling its lost mass into a gas disk around the star at a rate of over 3,000 tons per second.
“This is the first time we can measure the amounts of gases like oxygen and sulfur in the disk, which provides clues to the composition of exoplanet atmospheres,” said co-author Dr. Odette Toloza, from the University of Warwick.
The astronomers also found that the giant planet orbits WD J0914 at a distance of only 10 million km, or 15 times the solar radius, which would have been deep inside the red giant.
The unusual position of the planet implies that at some point after the host star became a white dwarf, the planet moved closer to it.
The study authors believe that this new orbit could be the result of gravitational interactions with other planets in the system, meaning that more than one planet may have survived its host star’s violent transition.
“The discovery opens up a new window into the final fate of planetary systems,” Dr. Gänsicke said.
The research is described in a paper in the journal Nature.
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B.T. Gänsicke et al. 2019. Accretion of a giant planet onto a white dwarf star. Nature 576: 61-64; doi: 10.1038/s41586-019-1789-8
