‘Evaporating Planet’ spotted in orbit around White Dwarf Star

The atmosphere of a surviving giant planet orbiting a white dwarf star is being stripped away — a hint as to what our solar system may look like in the future.

This illustration shows the white dwarf WDJ0914+1914 and its Neptune-like exoplanet. Since the icy giant orbits the hot white dwarf at close range, the extreme ultraviolet radiation from the star strips away the planet’s atmosphere. While most of this stripped gas escapes, some of it swirls into a disc, itself accreting onto the white dwarf. (ESO/M. Kornmesser)

Researchers have used the ESO’s Very Large Telescope (VLT) to spot a giant planet orbiting a white dwarf star. The first time astronomers have spotted a surviving planet orbiting such a star. If that is not remarkable enough, the planet’s close orbit is causing this remnant of a Sun-like star to strip away its atmosphere. This gives researchers a hint as to what the solar system may look like in the distant future when the Sun is reaching the end of its life.

The discovery was made by researchers inspecting 7000 white dwarfs catalogued by the Sloan Digital Sky Survey (SDSS). They observed that one white dwarf was unlike the others. Analysing subtle variations in the light from the star uncovered traces of chemical elements in amounts never before observed around a white dwarf.

“It was one of those chance discoveries,” says researcher Boris Gänsicke, from the University of Warwick in the UK, who led the study, published in the journal Nature. “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.”

This animation shows the white dwarf WDJ0914+1914 and its Neptune-like exoplanet. Since the icy giant orbits the hot white dwarf at close range, the extreme ultraviolet radiation from the star strips away the planet’s atmosphere. While most of this stripped gas escapes, giving the planet a comet-like tail, some of it swirls into a disc, itself accreting onto the white dwarf. (ESO/M. Kornmesser)

To investigate the properties of this unusual star — named WDJ0914+1914 — -the team analysed it with the X-shooter instrument at the ESO’s Very Large Telescope in the Chilean Atacama Desert. These follow-up observations confirmed the presence of hydrogen, oxygen and sulphur associated with the white dwarf. The team discovered that these elements did not originate from the star itself, but was located in a disc of gas swirling into the white dwarf.

“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,” adds Matthias Schreiber from the University of Valparaiso in Chile, who computed the past and future evolution of this system.

The amounts of hydrogen, oxygen and sulphur the team detected resemble those found in the deep atmospheric layers of icy, giant planets like Neptune and Uranus.

Were such a planet were to orbit a hot white dwarf at close proximity the extreme ultraviolet radiation emanating from the star would strip away its outer layers. This stripped gas would swirl into a disc, which would fall onto the surface of the white dwarf in a process called accretion. Exactly what the team think have spotted around WDJ0914+1914 — the first evaporating planet orbiting a white dwarf.

By combining observational data and theoretical models, the team of international astronomers were able to paint a clearer image of this unique system.

The white dwarf is small and extremely hot — five times the temperature of the Sun, a blistering 28 000 degrees Celsius. The giant exoplanet is the complete contrast of its parent star — icy and at least twice as large as the white dwarf.

The planet has an extremely tight orbit, making its way around the parent star in just 10 days. As a result, high-energy photons from the star are bombarding the planet, gradually blowing away its atmosphere.

Whilst most of the gas is able to escape, some material is pulled into an accretion disc swirling into the star at an incredible rate of 3000 tonnes per second. It is this disc that makes the otherwise hidden Neptune-like planet visible.

“This is the first time we can measure the amounts of gases like oxygen and sulphur in the disc, which provides clues to the composition of exoplanet atmospheres,” says Odette Toloza from the University of Warwick, who developed a model for the disc of gas surrounding the white dwarf.

Looking into the future of the solar system

“The discovery also opens up a new window into the final fate of planetary systems,” points out Gänsicke. But how does this unusual and extraordinary system relate to our solar system, in which the gas giants are much further from their parent star?

The answer has grim connotations for our home planet and its closest neighbours.

Stars such as 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. As shown in this animation, 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 stars exist in our galaxy. However, until 2019, scientists had never found evidence of a surviving giant planet around a white dwarf. The detection of a Neptune-like exoplanet at WDJ0914+1914 may be the first of many orbiting such stars. (ESA/Hubble (M. Kornmesser & L. L. Christensen))

Stars such as our Sun, burn hydrogen in their cores for most of their lives — a phase that is known as the main sequence. During this period they exist in a state of ‘hydrostatic equilibrium’ — meaning that the outward pressure caused by radiation created by hydrogen burning balances the inward pressure exerted by gravity.

Once the hydrogen in the star’s core is exhausted the core collapses rapidly and hydrogen burning can begin in the outer layers. This results in the star's outer layers ‘puffing up’. This creates a red giant, a star with a radius hundreds of times larger than its initial radius. The result for nearby planets is a firey end, engulfed by the expanding red star.

In the case of the Solar System, this is the fate that awaits 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.

These stellar remnants can still host planets, and many of these star systems are thought to exist in our galaxy. Despite this, the research marks the first time that astronomers have found evidence of a surviving giant planet around a white dwarf — the first of many orbiting such stars.

This chart shows the location of WDJ0914+1914 in the constellation of Cancer (The Crab). This map shows most of the stars visible to the unaided eye under good conditions, and WDJ0914+1914 itself is highlighted with a red circle on the image. This white dwarf is orbited by a Neptune-like exoplanet that is evaporating, the first-ever giant planet found around a white dwarf. ( ESO, IAU and Sky & Telescope)

According to the researchers, the exoplanet now orbits the white dwarf found in the constellation of Cancer at a distance of only 10 million kilometres — about 15 times the solar radius — which would have been deep inside the red giant.

Thus the planet’s unusual position and close orbit imply that at some point after the host star became a white dwarf, the planet must have moved closer to it. According to the astronomers, this new orbit is likely the result of gravitational interactions with other planets in the system. Thus meaning that more than one planet may have survived its host star’s violent transition to a red dwarf.

“Until recently, very few astronomers paused to ponder the fate of planets orbiting dying stars. This 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,” concludes Gänsicke.