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SYFY WIRE Bad Astronomy

A white dwarf is cooking a giant planet and slowly eating it

By Phil Plait
Artwork depicting a white dwarf evaporating a close-orbiting giant planet, with some of that material forming a disk around the dead star. Credit: ESO/M. Kornmesser

Over 1,500 light years from Earth, a dead star is eating one of its giant planets.

The stellar corpse is a bit of a gourmet, too: It's cooking the Neptune-like planet first, and being picky about what it eats, throwing away most of its planetary meal and even then pickily eating only certain ingredients.

The star is called WD J091405.30+191412.25 — it's a white dwarf, and the alphanumeric salad after that is its coordinates on the sky — so let's call it J0914 for short. A white dwarf is the core of a star like the Sun after it dies, a complicated process where it expands into a swollen red giant, blows off its outer layers, and finally reveals its extremely dense and hot core to space. The Sun will go through this process in about 7 billion years, but judging from the light it gives off, J0914 just finished up ejecting its outer layers about 13 million years ago.

It started off life as a star pretty much like the Sun, but that was many billions of years ago. Now, as a white dwarf, it's extremely hot (27,500°C / 50,000° F) and phenomenally dense, with a surface gravity 70,000 times Earth's! This gravity is so extreme that any elements heavier than hydrogen and helium should have long ago been dragged down beneath its surface.

And that's where the weirdness of this star begins. Sometimes stars like this had planetary systems before they died, and sometimes asteroids or even rocky planets can be disrupted by the dead star's fierce gravity. They break apart, fall onto the star, and pollute its surface with elements like calcium and iron. Astronomers looked through the spectra of 7,000 white dwarfs to look for the fingerprints of these heavy elements, hoping to survey stars eating their rocky planets.

That's when they found J0914. But instead of calcium and iron, they found oxygen and sulfur. That's pretty weird; no other white dwarf was seen in their data with such strong signals of these elements. These are more like what you'd find deep in the atmosphere of a giant planet, one like Neptune. This implies that the star isn't eating an asteroid or small planet. It's eating a giant!

Furthermore, there's no iron or calcium seen at all, meaning this was certainly not a planet like the Earth. All evidence points to this planet being a giant.

The planet is close enough to the star (about 10 million km away) that the intense radiation coming off the white dwarf is vaporizing its atmosphere. Gases like water vapor (H2O) and hydrogen sulfide (H2S) are stripped off, torn apart by the intense light, and mostly blown away. But some of those materials (the oxygen and sulfur, mostly) fall toward the star, forming a disk around it some 7 million km in radius. As the material whips around the star the light it gives off is strongly Doppler shifted, and this is seen in the star's spectrum, confirming the star is accreting matter from the planet.

The star is gobbling down the planet's atmosphere at a rate of over 3,000 tons per second, which makes it something of a gourmand, too. While that sounds like a lot, it's actually very small compared to the mass of the planet. The white dwarf is slowly cooling, and in about 350 million years will no longer be able to vaporize the planet. But by that time it will have accumulated only about 4% of the planet. Mind you, only a fraction of that vaporized material is actually falling onto the star; most of it is being blown away. Still, the white dwarf won't be gaining much weight from this meal. It's more of an amuse bouche.

It's interesting that out of 7,000 stars examined, only this one was found to be eating a giant planet. That implies this is a rare circumstance, despite us knowing that gas giants are common around Sun-like stars. The astronomers posit that the gas giant may have encountered another massive planet in that system, dropping it down toward the star. At the planet's distance this means it would have been inside the star when it expanded into a red giant — stars like that can be well over one hundred million kilometers in radius! Unless, that is, the planet was dropped down after the star turned into a white dwarf. That seems unlikely to me, since the white dwarf stage only started a dozen or so million years ago.

We do know of a lot more white dwarfs in the galaxy — the Gaia mission has observed over a quarter million such stars! So it's possible we'll find a lot more like J0914 sometime soon.

Artwork depicting a white dwarf evaporating a close-orbiting giant planet, with some of that material forming a disk around the dead star. Credit: ESO/M. Kornmesser

It's also interesting to wonder how much our own solar system will resemble this system in seven or so billion years. It's possible, kinda sorta, if somehow the outer planets interact in a way to drop Neptune or Uranus down to inside Mercury's orbit. Mercury will be gone once the Sun expands into a red giant, as will Venus. Earth may escape being consumed, since the Sun will lose mass at that stage in its life, weakening its gravity and therefore its hold on our planet; Earth may recede from the Sun during that epoch enough that it won't be physically inside the star (not that that matters much; even if it's outside the swollen Sun will fill the sky and cook the Earth very thoroughly).

Still, the more we learn about how planets affect their stars, even after the stars' deaths, the more we're informed about our own fate all those eons from now. A bit morbid, perhaps, but it also shows us the fantastic variety and diversity the Universe exhibits — a giant planet being cooked and eaten by its star! How amazing the cosmos is, and I'm just glad we get to be around to see it.