White dwarf’s surprise companion

A giant intact exoplanet has been found orbiting close to a white dwarf star for the first time.

The discovery suggests that planets can end up in or near a white dwarf’s habitable zone, and potentially be hospitable to life even after their star has died. And this, astronomers say, foretells one possible future for our Solar System when the Sun ages into a white dwarf.

An international team used a suite of telescopes around the globe and in space to find and characterise the planet (designated WD 1586 b) transiting the white dwarf WD 1856+534 every 1.4 days. They estimate that WD 1586 b is no more than 14 times the mass of Jupiter.

The research was led by Andrew Vanderburg, from the University of Wisconsin-Madison, US, and is reported in a paper in the journal Nature.

“The discovery came as something of a surprise,” Vanderburg says. “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.”

A white dwarf is the collapsed core of a Sun-like star left over after it has shed its atmosphere. They are typically the size of Earth, with masses about half that of the Sun.

Most exoplanets 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, making it unlikely that any close planets could remain to orbit the star in its white dwarf phase.

Although some white dwarfs show evidence that planets might be found in their orbit, no intact planets have previously been found.

Vanderburg and colleagues detected WD 1586 b with NASA’s TESS satellite, then used the sensitive Gemini Near-Infrared Spectrograph (GNIRS) to make detailed measurements of the white dwarf star in infrared light from Hawai’i.

“We were using the TESS satellite to search for transiting debris around white dwarfs, and to try to understand how the process of planetary destruction happens,” says Vanderburg. “We were not necessarily expecting to find a planet that appeared to be intact.”

The authors assume that to avoid destruction when the progenitor star of the white dwarf evolved into a red giant, WD 1586 b must have been more than one astronomical unit (the distance between Earth and the Sun) away from the host star.

However, it is unclear how it arrived in the close orbit observed, estimated to be four solar radii from the white dwarf (about one-twentieth the distance that Mercury is from the Sun).

On the basis of their simulations, the authors suggest that when the star evolved into a white dwarf, WD 1586 b was thrown into a close orbit due to interactions with other planets in the remnant planetary system.

“We’ve had indirect evidence that planets exist around white dwarfs and it’s amazing to finally find a planet like this,” says Gemini’s Siyi Xu.