The End of Planets

We had a colloquium yesterday from Ted von Hippel of Siena College, over on Route 9, about "White Dwarf Debris Disks and the Fate of Planetary Systems." The abstract was:

After a brief introduction to white dwarfs and debris disks, I will present observations from the Spitzer Space Telescope and ground-based telescopes of metal-polluted white dwarfs with circumstellar debris disks. We measure the constituents of the debris disks, the elemental abundances of the material being accreted, and the accretion timescale. Our measurements support the idea that disruptions of asteroids created these debris disks. Based on the properties of these stars, I interpret the majority of the metal-polluted white dwarfs as resulting from planetary system bodies being ground down during the late stages of stellar evolution.

It was a pretty cool talk, and covered a wide range of stuff. It also included a possible shout-out to Steinn.

Summarized and condensed for non-astronomers (a bold claim, as I'm not an astronomer, either), what he said was that there are a large number of white dwarf stars out there with spectra showing signs of "metals," a term of art in astronomy that means "anything heavier than helium." This is surprising, because the structure of these stars is such that any metals in the outer layers of the atmosphere should fall down into the core and become invisible in a matter of a couple of weeks.

Thus, if there are "metals" in the atmospheres of these stars, they must be constantly replenished by stuff falling into the star. Based on the spectra observed for these stars, which look more or less like the spectrum observed due to dust and gas in our solar system, he argued that the source of the "metals" was most likely debris from planets left behind from earlier in the star's life. The total mass required to produce the signals they observed is not all that much-- something like the mass of the asteroid belt, maybe as much as the mass of the Moon.

Since something like 25% of white dwarf stars have the spectral characteristics that he interprets as signs of debris disks, and since pretty much every kind of star eventually ends up as a white dwarf, he argued that this suggests that something like 25% of all stars have planets with Moon-sized masses. This is a much higher fraction than we can say contain planets due to direct observation-- around 5% have at least one Jupiter-sized planet-- that means there are a lot of planets out there that we have yet to see.

It was a pretty cool talk, and he did a good job of cutting the astro jargon down to a level where a dumb experimental AMO type could follow it. It was also a well-structured talk, going systematically through a bunch of different observations, and showing how each fit together with the others to lead to the eventual conclusion. I can't really say whether he was right-- there might be compelling evidence out there for some other model-- but it was a well-done presentation.

The possible shout-out to Steinn was a mention of a paper by somebody who may have been Steinn and somebody else whose name I forget, suggesting a mechanism by which stuff might fall into the star. The name was spelled slightly differently, but in a way that might've been a typo ("Sigardsson," I think it was). Or it might've been a different guy. Tough to say.