"It was one of those fortuitous discoveries," says the researcher who led the study, Boris Gansicke, from the University of Warwick (United Kingdom). According to ESO, the team had studied around 7,000 white dwarfs and discovered that one was different from the others. By analyzing subtle variations in the star's light, they found traces of chemical elements in quantities that scientists had never observed before in a white dwarf.
"We knew there had to be something exceptional in this system, and we speculated that it could be related to some kind of planetary remnant," Gansicke explains in the ESO statement.
The scientists discovered that very close to the unusual star, called WDJ0914 + 1914, there is a disk of hydrogen, oxygen and sulfur gas that turns towards it but does not come from it.
"It took several weeks of work to reach the conclusion that the only way to make such a disk is the evaporation of a giant planet," says Matthias Schreiber, of the University of Valparaiso, in Chile, who computed the evolution Past and future of this system.
The detected amounts of hydrogen, oxygen and sulfur are similar to those found in the deep atmospheric layers of giant frozen planets such as Neptune and Uranus.
The ESO report also explains that if such a planet were orbiting near a hot white dwarf, the star's extreme ultraviolet radiation would strip its outer layers. Part of this torn gas would swirl in a disk, curling on the white dwarf. This is what scientists think they are seeing around WDJ0914 + 1914: the first evaporating planet that orbits a white dwarf.
Characteristics of the planetary system found
- The white dwarf is small but five times hotter than the Sun: it has a temperature of 28,000 degrees Celsius.
- On the contrary, the planet found is icy and large, at least twice as large as the star.
- It is located about 1,500 light years away from Earth, in the constellation Cancer.
- The planet is orbiting the star at close range (10 million kilometers, or 15 times the solar radius), and completes its orbit in just 10 days. According to ESO, the short distance indicates that in the past the white dwarf must have been submerged in the depths of the red giant star. It also indicates that sometime after the host star became a white dwarf, the planet approached her.
For astronomers, this new orbit could be the result of gravitational interactions with other planets in the system. In other words, more than one planet may have survived the violent transition of its host star.
"Until recently, very few astronomers stopped to think about the fate of planets orbiting dying stars. This discovery of a planet that orbits near a finished star nucleus firmly demonstrates that the Universe is challenging our minds again and again , driving us to go beyond our established ideas, "concludes Gansicke.
Most of the planet's gas escapes, but a part is trapped in a disk that swirls around the star at a speed of 3,000 tons per second, the Observatory details. This disk is the one that makes the planet of the Neptune type visible that would otherwise remain hidden.
"It is the first time we can measure the amounts of gases such as oxygen and sulfur in the disk, which provides clues about the composition of exoplanet atmospheres," says ESO Odette Toloza of the University of Warwick, who developed a model for the gas disk that surrounds the white dwarf.
"The discovery also opens a new window to learn more about the final destination of planetary systems," Gansicke adds.
What are white dwarf stars?
Stars like our sun burn hydrogen in their nuclei for most of their lives. According to ESO, once they run out of this fuel, they swell, turning into giant red stars, hundreds of times larger and enveloping nearby planets. In the case of the Solar System, this will include Mercury, Venus and even the Earth, which will be consumed by the red giant Sun in about 5,000 million years.
In the end, sun-like stars lose their outer layers, leaving behind only a dying core, a white dwarf. These stellar remains can still house planets and it is believed that there are many of these star systems in our galaxy.