Back in 1992, Aleksander Wolszczan and Dale Frail announced the discovery of a multi-planetary system around pulsar PSR 1257+12. In this case, by “planet” they meant lumps of rock roughly four earth masses in size orbiting at distances slightly larger than Mercury’s orbit around our Sun. At least we assumed they were lumps of rock since these objects would have had to survive a blast from the supernova explosion that formed the pulsar. 

These were the first exoplanets discovered, and they’d be followed in short order by the discovery of hot Jupiters in 1995. While most of our attention on exoplanets has focused on planets around regular stars, astronomers have continued to find worlds around dead stars, including the 2000 discovery of a planet orbiting a neutron star/white dwarf binary, and many subsequent discoveries of rocky (assumed to be) wreckage around white dwarfs and heavier neutron stars. 

During the roughly soon to be 30 years of exoplanet exploration, many have wondered if white dwarfs, which represent the future evolution of our Sun, are capable of having regular gas giants orbiting them. Fundamentally, we want to know, when our Sun dies, what worlds will have a chance of sticking around. We know the Sun will expand out into a red giant and eat a few worlds in the process; we know it will undergo significant mass loss, especially as it forms a planetary nebula and sheds its outer atmosphere, and that could alter planets in myriad ways. What exactly will happen to our solar system… we just don’t know.

Well, we now know, at least, that gas giants can be found around white dwarf stars. 

In a new paper published in Nature, a team lead that includes Andrew Vanderburg as first author finds that a gas giant orbits a white dwarf. Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), Spitzer Space Telescope, Gemini North, and the Hobby-Eberly Telescope (HET), they found that the planet is no larger than 13.8 Jupiter masses, and it orbits every 34 hours. This is an odd-looking pair, with the star being about four times the radius of the Earth, and the planet being an order of ten Earth radii. What struck me about this discovery is the gas giant appears to have a temperature not too different from Earth’s temperature – about 17 C. This makes me imagine an Endor-like moon evolving in this new configuration.

And let me make it clear – this is a new configuration. The planet had to migrate to where it is today. It’s not entirely clear how planets like this migrate into a new and stable orbit, but we know they can. In all likelihood, this planet started farther out and somehow got knocked into an elliptical orbit through interactions with other planets, and tidal effects that included stretching this world out were able to circularize the orbit. However it happened, this means we get to imagine cool new futures for stellar remnants. Let the science fiction writing begin!

More Information

University of Wisconsin-Madison press release 

NSF’s NOIRLab press release 

McDonald Observatory press release 

NASA press release 

“A Giant Planet Candidate Transiting a White Dwarf,” Andrew Vanderburg et al., 2020 Sep. 16, Nature