The first giant planet around a white dwarf was discovered

For the first time in history, a team of astronomers have discovered, thanks to the ESO’s Very Large Telescope, a Neptune-like planet orbiting the remnant of a star similar to the Sun.

The planet, which is much larger than the dead star it orbits, about four times larger than the white dwarf, was discovered through the disc of gas created by its evaporating atmosphere. The white dwarf is about the same size as the Earth. The planet orbits the star once every ten days; because of this short distance, it is leaving a trail of gas composed of hydrogen, oxygen and sulfur in its path, forming a disc of gas around the star. The situation is strange due to the fact that, so far, there has been no evidence of a planet that has survived the transition from a star to a white dwarf, researchers say.

This unique system gives us clues about how our own solar system could be in the distant future.

The data

Analysis of the spectral footprints of the white dwarf WD J0914 + 1914 revealed strange traces of a chemical belonging to a gas giant, revealing a star system very different from everything we’ve been exposed to so far.

Using data collected as a part of the Sloan Digital Sky Survey (SDSS), the international team of astronomers discovered the signs of a Neptune-like planet orbiting so close to the distant white dwarf that the planet is being dematerialized little by little.

White dwarfs represent the end of a star’s life, because once its fuel supply is depleted, these cosmic objects collapse under their own gravity, squeezing out all that mass, becoming hundreds of times larger and envelop the nearby planets.

In the case of our solar system, this future event will include Mercury, Venus and the Earth, which will be consumed by the Sun in about 5 billion years. Finally, Sun-like stars loose their outer layers, leaving only a burnt nucleus, a white dwarf. Such stellar debris can still host planets, and many of these star systems are believed to exist in our galaxy. However, until now, scientists had never found evidence of a surviving planet around a white dwarf.

The detection of an exoplanet in orbit around WDJ0914 + 1914, located about 1500 light-years away in the Cancer constellation, may be the first of many stars in orbit.

"It was one of those fortunate discoveries," says researcher Boris Gänsicke of the University of Warwick (UK), who led the study in Nature magazine. 

The researchers had already studied about 7,000 white dwarfs in the Sloan Digital Sky Survey and discovered something surprising: one was different from the others. By analyzing the subtle variations in the star’s light, they found traces of chemical elements in quantities that scientists had never before observed in a white dwarf. "We knew that there had to be something exceptional in this system, and we speculated that it might be related to some kind of planetary remnant".

By running the spectral data through a simulation program, the team was able to model the conditions we could expect in the closest environment of WD J0914 + 1914. The mixture of water and hydrogen sulfide suggested that a gaseous planet with a composition similar to that of our giants, such as Neptune and Uranus, was losing its atmosphere.

For the giant to vaporize like this, it would need to be close to the white dwarf. Researchers estimate it would be about ten million kilometers.

"The discovery also opens a new window on the final fate of planets," said lead researcher Gänsicke. 

Even with this evidence of a gas giant orbiting a white dwarf, it’s not clear if it’s an anomaly or if we just hadn’t looked hard enough to find other examples.

Given that our own Sun is destined to become a white dwarf, it would be interesting to find more examples that could give us an idea of the distant future of our own solar system.

"Until recently, very few astronomers stopped to think about the fate of planets orbiting dying stars. This discovery of a planet orbiting near a dying star core firmly demonstrates that the Universe is constantly challenging our minds, pushing us to go beyond our established ideas," concludes Gänsicke.

Reference: Accretion of a giant planet onto a white dwarf star, Nature (2019). DOI: 10.1038/s41586-019-1789-8 , https://nature.com/articles/s41586-019-1789-8