NOIRLab: A White Dwarf’s Surprise Planetary Companion

In this illustration, WD 1856b, a giant planet, orbits its dim white dwarf star every day and a half. Credit: NASA’s Goddard Space Flight Center

Astronomers detect first-of-its-kind exoplanet around dead star

For the first time, an intact, giant exoplanet has been discovered orbiting close to a white dwarf star. This discovery shows that it is possible for Jupiter-sized planets to survive their star’s demise and settle into close orbits around the remaining stellar ember, near the habitable zone. This foretells one possible future for our own Solar System when the Sun ages into a white dwarf.

Astronomers have used the international Gemini Observatory, a Program of NSF’s NOIRLab, and other telescopes around the globe and in space to find and characterize a giant planet, less than 13.8 times as massive as Jupiter [1], orbiting a white dwarf star [2][3]. The research is published in the journal Nature.

This is the first example of an intact giant planet orbiting close to a white dwarf star — in this case a particularly cool and dim stellar ember known as WD 1856+534. “The discovery came as something of a surprise,” according to lead author Andrew Vanderburg, assistant professor at the University of Wisconsin-Madison. “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.” [4]

After detecting the planet with the TESS satellite, which observed it transiting its white dwarf star, the team took advantage of the tremendous light-collecting power of Gemini North’s 8.1-meter mirror and used the sensitive Gemini Near-Infrared Spectrograph (GNIRS) to make detailed measurements of the white dwarf star in infrared light from Maunakea, Hawai‘i. The spectroscopic observations captured the unique fingerprint of the star, but not that of the planet or any debris surrounding this system [5][6]. “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 Siyi Xu, an assistant astronomer at Gemini Observatory and one of the researchers behind the discovery.

Notes

[1] The upper limit of the object’s mass is 13.8 Jupiter masses. This mass is close to the dividing line astronomers use to distinguish between a planet and a brown dwarf.

[2] White dwarfs are common stellar remnants left behind by the deaths of low-mass stars like the Sun. Though they have a mass comparable to the Sun’s, they are roughly the size of Earth, making them incredibly dense. White dwarfs generate no energy of their own and glow faintly with leftover thermal energy, slowly fading over billions of years.

[3] The discovery of WD 1856b relied on observations from facilities including Gemini North, NASA’s Transiting Exoplanet Survey Satellite (TESS), NASA’s Spitzer Space Telescope, various professional telescopes around the world, and a handful of privately operated telescopes.

[4] Result reported by NASA.

[5] The light from a star is spread over many wavelengths, and not all these wavelengths radiate equally. The distribution of emission at different wavelengths makes up the emission spectrum of a star, and features of this spectrum act as very recognizable “fingerprints.” When an orbiting planet gravitationally tugs at a star, it causes a star to wobble and these spectral fingerprints shift slightly. This technique is often used to gather information about exoplanets, but in the case of WD 1856, the stellar spectrum obtained by Gemini North showed no identifying features — no “fingerprints” — showing that the orbiting planet is intact.

[6] The first “polluted white dwarf” — a white dwarf with planet debris in its outer layer — was discovered in 1917 by Adriaan van Maanen using Mount Wilson observatory’s 60-inch telescope. The star is known as van Maanen’s Star and has an interesting backstory.