Free Astronomy Magazine January-February 2020

18 JANUARY-FEBRUARY 2020 SPACE CHRONICLES blowing away the planet’s atmosphere. Most of the gas es- capes, but some is pulled into a disc swirling into the star at a 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 atmos- pheres,” says Odette Toloza from the University of Warwick, who de- veloped a model for the disc of gas surrounding the white dwarf. “The discovery also opens up a new window into the final fate of plane- tary systems,” adds Gänsicke. 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 en- gulfing nearby planets. In the case of the Solar Sys- tem, this will in- clude 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 stel- lar remnants can still host planets, and many of these star systems are thought to T his animation shows the white dwarf WDJ0914+1914 and its Neptune-like exoplanet. Since the icy giant or- bits the hot white dwarf at close range, the extreme ultra- violet radiation from the star strips away the planet’s atmosphere. While most of this stripped gas escapes, giv- ing the planet a comet-like tail, some of it swirls into a disc, itself accreting onto the white dwarf. [ESO/M. Kornmesser] A rtist’s animation of the Sun becoming a red giant. [ESA/Hubble (M. Kornmesser & L. L. Christensen)] puted the past and future evolution of this system. The detected amounts of hydrogen, oxygen and sulphur are similar to those found in the deep atmospheric layers of icy, giant planets like Neptune and Uranus. If such a planet were orbiting close to a hot white dwarf, the extreme ul- traviolet radiation from the star would strip away its outer layers and some of this stripped gas would swirl into a disc, itself accreting onto the white dwarf. This is what scien- tists think they are seeing around WDJ0914+1914: the first evaporat- ing planet orbiting a white dwarf. Combining observational data with theoretical models, the team of as- tronomers from the UK, Chile and Germany were able to paint a clearer image of this unique system. The white dwarf is small and, at a blistering 28,000 degrees Celsius (five times the Sun’s temperature), extremely hot. By contrast, the planet is icy and large — at least twice as large as the star. Since it or- bits the hot white dwarf at close range, making its way around it in just 10 days, the high-energy pho- tons from the star are gradually ! exist in our galaxy. However, until now, scientists had never found evi- dence of a surviving giant planet around a white dwarf. The detec- tion of an exoplanet in orbit around WDJ0914+1914, located about 1500 light years away in the constellation of Cancer, may be the first of many orbiting such stars. According to the researchers, the ex- oplanet now found with the help of ESO’s X-shooter orbits the white dwarf at a distance of only 10 mil- lion kilometres, 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 astronomers believe that this new orbit could be the result of gravitational interactions with other planets in the system, mean- ing that more than one planet may have survived its host star’s violent transition. “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 be- yond our established ideas,” con- cludes Gänsicke.