A giant, intact planet has been discovered orbiting close to a white dwarf star for the first time, a new study suggests.
The finding indicates it is possible for Jupiter-sized planets to survive their star’s demise and settle into close orbits around what remains, researchers say.
A white dwarf is the collapsed core of a Sun-like star, left over after shedding its atmosphere. They are typically the size of Earth, with masses about half that of the Sun. Most exoplanets – planets outside the Solar System – discovered so far orbit stars that will eventually form white dwarfs.
As these stars use up their reserves of hydrogen, they first evolve into red giants, expanding and then engulfing any planets in a close orbit. This makes it unlikely that any close planets could remain to orbit the star in its white dwarf phase, according to the study in Nature.
The authors estimate that the exoplanet, designated WD 1856 b, is around the same size as Jupiter, and up to 14 times its mass.
Lead author Vanderburg, assistant professor at the University of Wisconsin-Madison, said: “The discovery came as something of a surprise. A previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid.”
The scientists assume that to avoid destruction when its parent star evolved into a red giant, WD 1856 b must have been more than one astronomical unit – the average distance between Earth and the Sun – away from the host star.
On the basis of their simulations, the authors suggest that when the star evolved into a white dwarf, the exoplanet was thrown into a close orbit due to interactions with other planets in the remnant planetary system.
“Because no debris from the planet was detected floating on the star’s surface or surrounding it in a disk we could infer that the planet is intact,” said Dr Siyi Xu, an assistant astronomer at Gemini Observatory and one of the researchers behind the discovery.
Dr Xu added: “Additionally, because we didn’t detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we’ve ever found.”
The precise upper limit of the planet’s temperature was measured by NASA’s Spitzer Space Telescope to be 17°C, which is similar to the average temperature of Earth.
“We’ve had indirect evidence that planets exist around white dwarfs and it’s amazing to finally find a planet like this,” said Dr Xu.
What does it mean if an exoplanet is ‘habitable’?
All forms of life that we know of depend on one critical component: liquid water. So, in the search for life, astronomers focus on planets where liquid water could exist, which they call ‘habitable’.
Every star has a ‘habitable zone’, also called the ‘Goldilocks zone’, where it is not too hot and not too cold. A planet in the habitable zone gets the right amount of energy from the star to support liquid water. Any closer in to the star and water would boil, and any further out and it would freeze.
However, this doesn’t guarantee that liquid water would exist on a planet in the habitable zone. The planet’s atmosphere could be too thick, raising the temperature even higher. And even if liquid water does exist on the planet, habitable doesn’t mean inhabited.