White dwarfs are the cores of stars that were once similar to the Sun. At some point, these stars have exhausted the lighter elements that fueled their earlier existence, flared up into a bloated red giant, and burned down into carbon and oxygen rich cores not much larger than the Earth (but far more massive). With fusion shut down, they gradually radiate away the remaining temperature, fading out of our ability to detect them.

We now know that a large number of stars have planets around them. So what happens to a planet orbiting a star that puffs up on its way to becoming a white dwarf? Some hints of that have come from a number of these stars that have material similar to that of a rocky planet embedded in their surface. But a new example has been found with gas that has been drawn off from a Neptune-like planet.

One weird dwarf

The white dwarf in question, WD J091405.30+191412.25 (the authors refer to it affectionately as WD J0914+1914), was initially identified as having hydrogen on its surface. This, on its own, is not unusual. While it would have burned much of its hydrogen during its past life, many white dwarfs pull material off nearby stars. But in this case there was no sign of a nearby star. And, even more oddly, there was sulfur on the surface of the white dwarf as well. Sulfur is generally a very rare element on stars, which suggested that the material did not have a stellar origin.

So how did it get there? The key to sorting that out was a careful look at the lines in the spectrum produced by sulfur, hydrogen, and oxygen. All of these were split into two peaks. This is the sign of a rotating disk of material. On one side of the white dwarf, it rotates away from us, adding a slight red shift to the light; on the other, it rotates toward us, adding a slight blue shift. The presence of a disk like this indicates that the material is actively being drawn in from a nearby source.

If the material isn't coming from a star and isn't coming from a rocky planet, the most likely candidate is a gas giant of some sort. If the planet is orbiting close enough to the white dwarf, the residual heat will cause some of the atmosphere to evaporate off. And planets like Neptune have a composition consistent with the materials found in the disk. While Neptune is far more hydrogen-rich, the researchers conclude that the radiation from the white dwarf would be enough to drive most of the hydrogen off.

The calculations done by the researchers indicate that the white dwarf is drawing in approximately three million kilograms of material a second and forming a disk that extends out to about 10 times the radius of our Sun. That's about 7 million kilometers, or over 18 times the distance between the Earth and the Moon. To be warm enough to evaporate off this much material and produce a disk this size, the planet has to be orbiting at a bit over 10 million kilometers, or about one-sixth the distance of Mercury from our Sun.

(It's also worth noting that a Neptune-sized planet has a radius of about 25,000km, while a typical white dwarf's radius is under 10,000km. So, the planet literally dwarfs the white dwarf.)

How did that happen?

How does a large planet end up this close to a tiny star? One possibility is that it was close enough to be swallowed up by the star when it expanded near the end of its fusion-fueled days. If this occurred, the planet would have orbited inside the star, and friction would have slowed it, bringing the planet ever closer to the star's core. But the researchers indicate that this requires very fine tuning of the initial conditions and only works with planets that are initially larger than Jupiter.

Instead, the researchers favor the idea of gravitational interactions among multiple planets, which could rearrange the orbits after the star had entered its white dwarf phase. This suggests there are additional planets nearby, which would be the first planets ever found to orbit a white dwarf.

Despite the addition of new material, the white dwarf itself will continue to cool and will eventually be unable to evaporate any more of the planet's atmosphere. The authors estimate that it will take about 350 million years for this to happen. (Don't worry, the planet will be fine—they estimate it will only lose around 0.002 percent of Jupiter's mass.)

That does, however, suggest we'd only see something like this within a fairly narrow time window. So the researchers searched a database of 7,000 white dwarfs for similar materials. None turned up, indicating that this is the only system of its sort we've ever observed. But there are hopes for others, as the Gaia mission is expected to identify more than a quarter million white dwarfs. Even if something like this is rare, those sorts of numbers make it likely that we'll find another.

Nature, 2019. DOI: 10.1038/s41586-019-1789-8 (About DOIs).