I was interviewed by the Living on Earth radio show about my book, Under Alien Skies. It was a lot of fun! Also, the show producer, El Wilson, asked me if it’s safe to drink Saturn’s rings, which inspired me to write a Scientific American article on the question!

Also here’s a link to my book, because why not.

I’m fascinated by the evidence of white dwarfs eating planets. White dwarfs are the super-dense leftovers of stars like the Sun once they die; the core is compressed so hugely that it’s only about the size of Earth. Mind you, the Sun has a mass 330,000 times that of our planet, so squeezing that down into such a small volume makes the white dwarf dense—roughly a ton per cubic centimeter. The gravity on the surface is evilly strong as well, tens of thousands of times Earth’s gravity. 

We know that stars commonly have planets, so we expect there to be planetary systems around white dwarfs, too. However the death of the star can play havoc on the system, with the planets moving farther out or getting ejected, and causing some planets to interact gravitationally as well. This can drop a planet too close to the white dwarf, where it will get shredded by the immense gravity. The material then forms a wide disk around the star, with the material at the inner edge falling down onto the white dwarf.

Typically, the gravity on a white dwarf is so strong that heavier material will sink beneath the surface rapidly leaving lighter materials like hydrogen and helium behind in their atmospheres and on their surfaces. And sure enough, most white dwarfs have spectra (like a stellar fingerprint in the light emitted) that indicate the surface is almost entirely those two light elements. But some show heavy elements that shouldn’t be there, which we now know indicates a planet that got torn apart and is falling onto the white dwarf. The first known evidence of the existence of exoplanets is from a white dwarf spectrum “polluted” with calcium, found in 1917!

Mostly, the elements found are pretty durable, like calcium, magnesium, and the like. More volatile substances like water and nitrogen are almost never seen. Part of this is due to selection effect: for example, the kinds of white dwarfs typically examined are too cool to have detectable volatiles.

A team of astronomers used the Cosmic Origins Spectrograph, an instrument on Hubble, to look at hundreds of white dwarfs and see if they are polluted with interesting elements. Two of their targets did, with evidence of silicon and carbon, so they followed up with deeper observations to get better data [link to journal paper].

For the white dwarf WD1647+375 they hit paydirt: they found strong evidence of carbon, nitrogen and sulfur, which are extremely rare or nonexistent in other white dwarfs. Moreover, they found a ton of oxygen in the data, a huge excess that indicates a big fraction — about 2/3rds — of the material falling onto the white dwarf is water.

That means that either the compact object snared a comet (those have lots of water ice), or a larger body like a dwarf planet similar to Pluto. A comet fits the data, though the ratio of oxygen to nitrogen fits better with what we know about dwarf planets.

The amount falling onto the white dwarf is about 200 metric tons per second. The thing is, they observed the star over the course of 13 years and saw a steady amount falling, which means the white dwarf has accreted about one hundred billion tons of material at least. That could still be a comet, but theoretical work has shown that material from a disintegrated object can fall onto the dwarf for 100,000 years, which would indicate this is a planetary mass object, not something small like a comet. Also, what are they odds they just happened to look at the dwarf in the short period it was eating a comet? Statistically speaking it’s more likely this was a bigger object that been chewed up and falling down for a long, long time.

How could this happen? There are many ways such a body could drop down close enough to the white dwarf to get et. A bigger planet farther out could have tugged on the smaller object gravitationally, disrupting its orbit and letting it fall closer down to the star. A collision between two objects could do the same thing. It’s also possible this was an interstellar object like ‘Oumuamua, Comet Borisov, or 3I/ATLAS; comets that passed through our own solar system from deep space. The scientists show statistically this happens relatively often enough to account for us catching one getting disrupted and eaten by the white dwarf.

So I hope you can see why I love this topic so much. I’ve always been interested in stellar deaths (I studied them for both my Master’s and PhD), but this is even weirder and cooler. Whole planets torn apart with pieces slamming down at huge velocities onto the surface of a white dwarf! How cool is that?

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