Jupiter-sized object WD 1856 b is one special celestial body and a major puzzle by virtue of likely being the first known planet to have survived its star evolving into a white dwarf star. The potential discovery was possible thanks to NASA’s Transiting Exoplanet Survey Satellite (TESS) and the now-retired Spitzer Space Telescope.
As reported in Nature, WD 1856 b orbits only 3.05 million kilometers (1.9 million miles) from a white dwarf, the fate of sun-like stars. Once their fuel is spent and they have expanded into red giants, their core collapses in on itself. This is not enough to cause a supernova, but it compresses into a dense degenerate object known as a white dwarf. The one near WD 1856 b is half the mass of the Sun but in a body not much bigger than Earth.
“WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece,” lead author Professor Andrew Vanderburg, from the University of Wisconsin-Madison, said in a statement. “The white dwarf creation process destroys nearby planets, and anything that later gets too close is usually torn apart by the star’s immense gravity. We still have many questions about how WD 1856 b arrived at its current location without meeting one of those fates.”
The end days of a white dwarf allow for several scenarios in which a planet can migrate inward towards the star. It's possible the path of WD 1856 b was influenced by other Jupiter-sized objects in the system, pushing the planet much closer to the degenerate star, while the intense pull of the white dwarf helped stabilize its orbit over time. Other possibilities include the gravitational influence from two red dwarfs in the system or a star passing close at the right time and pushing the planet forward. So far, the model with extra planets is the most likely.
“We’ve known for a long time that after white dwarfs are born, distant small objects such as asteroids and comets can scatter inward towards these stars. They’re usually pulled apart by a white dwarf's strong gravity and turn into a debris disk,” added co-author Dr Siyi Xu, an assistant astronomer at the international Gemini Observatory. “That’s why I was so excited when Andrew told me about this system. We’ve seen hints that planets could scatter inward, too, but this appears to be the first time we’ve seen a planet that made the whole journey intact.”
WD 1856 b is only considered a "likely" planet because it is in the range between the heaviest planet and the lightest brown dwarf, another type of stellar object. Spitzer analysis strongly points toward "planet" but it is not 100 percent certain. More observations are necessary to confirm.
The system is located 80 light-years from Earth in the northern constellation Draco.