For the first time, astronomers have discovered a planet the size of Jupiter orbiting a white dwarf star. The narrow orbit of the companion is particularly surprising for the scientists. Because white dwarfs are the final stages of stars similar to our sun. When their nuclear fuel is used up, they initially expand into red giants and then collapse to form dwarf stars, which are roughly the size of the earth. By expanding into a red giant, all celestial bodies in narrow orbits should be destroyed. According to the researchers in the journal “Nature”, the planet that has now been tracked down must have reached its unusual orbit later.
“So far, no intact planets have been discovered in close orbits around white dwarfs,” said Andrew Vanderburg from the University of Wisconsin-Madison in the USA. Observations so far have only indicated dust and rocks in the vicinity of these stars – possibly the remains of earlier planets that did not survive the expansion into a red giant. Nonetheless, Vanderburg and his colleagues scoured the data from the NASA space telescope TESS – short for Transiting Exoplanet Survey Satellite – for celestial bodies orbiting white dwarfs. Around the dwarf star WD 1856 + 534, which is 81 light years away, the team encountered a strange flashing light: every 34 hours, its brightness decreases by 56 percent for eight minutes. Observations with further telescopes from the ground confirmed the researchers’ suspicion that a planet with a maximum of 14 times Jupiter’s mass in an extremely narrow orbit is causing the flickering.
How the proven celestial body got into its current orbit remains a mystery. Vanderburg and his colleagues consider it impossible that a gas planet survives the red giant phase so close to its star. So they design a different scenario: a red giant sheds large parts of its outer layers into space before it collapses into a white dwarf. This causes it to lose mass, which affects the orbits of existing planets. In principle, the companions migrate outwards due to the loss of mass of the central star. But the conditions in this phase are chaotic, according to the Vanderburg team. As a result, the planets can influence each other through their gravitational pull and thus transport them to orbits further inward. These processes can take a long time – but time is not a problem: WD 1856 + 534 became a white dwarf about six billion years ago.