Image Credit: NASA / Spitzer / JPL-Caltech
White dwarfs, the slowly cooling remains of stars that have completed their life cycles, often seem to be the zombies of the night sky, devouring anything that happens to stray within their grasp. In an article that will be appearing in an upcoming issue of the Astrophysical Journal, astronomers Patrick Dufour, Mukremin Kilic and collaborators discuss a recently-discovered white dwarf that seems to have devoured a dwarf planet. Its name: "SDSS J073842.56+183509.6" (its nickname: 5877352363341268816. Seriously.)
White dwarfs have so much matter squeezed into such a little size that their pull of gravity is very strong. If you were to try and stand on the surface of the white dwarf remains of the sun, you would weigh one hundred thousand times more than you do now. So, if you weigh 150 pounds, on a white dwarf you would weigh 15 million pounds. Ouch.
That gravity is strong enough that the lightest element in a white dwarf's atmosphere will float to the top. The lightest element in the universe is hydrogen, and most white dwarfs have atmospheres of pure hydrogen. If you were to dump a heavy element, like iron, into a hydrogen-atmosphere white dwarf, it would disappear in a matter of weeks. That's fast in a science like astronomy when a million years is the blink of an eye! Take the sun, for instance. The sun has been around for four billion years, and yet its surface still has almost as much iron as it did when it formed.
Some white dwarfs have helium atmospheres (the second lightest element in the universe). In these white dwarfs, iron might not sink out of sight for a thousand years. Again, that's still very short in astronomy. The take-away point here is that if you see any element heavier than hydrogen or helium in the atmosphere of a white dwarf, it just got there within the last thousand years.
Okay, back to the white dwarf studied by Patrick, Mukremin and their collaborators. This white dwarf has a helium atmosphere, but all kinds of other heavier elements are also seen, including oxygen, magnesium, silicon, calcium, and iron. Remember, this means that these elements just recently landed on the surface of the white dwarf. These elements are similar to the composition of asteroids and comets, so when we find them in the atmosphere of a white dwarf, we assume that an asteroid or comet recently came too close to the white dwarf, was shredded and devoured like in the picture above.
By carefully analyzing the spectrum of the white dwarf, Dufour's team was able to determine that the white dwarf is about 90% the mass of the sun, meaning the original star was about 4 or 5 times the mass of the sun. With some more careful modeling, they were even able to add up the estimated total amount of rock swallowed by the white dwarf. The result was big: the asteroid eaten by the white dwarf was the size of Ceres, the largest asteroid in the Solar System. Ceres is so much bigger than the typical asteroid that it is now officially a "dwarf planet", the same designation that former planet Pluto was awarded as a consolation prize. (Pluto has about 10 times the mass of Ceres.) This is also at least 10 times more asteroidal material than any other known white dwarf has in its atmosphere. And remember, this amount of material has been swallowed in recent past!
Even better, it's possible to guess at the past history of the recently-deceased dwarf planet. The white dwarf shows evidence for a little hydrogen, but not much. This means that the doomed planet had little water or ice, just like asteroids in our own Solar System (and unlike comets). Of course, the red giant star that became a white dwarf would likely have evaporated any water ice, so it is hard to know whether the planet may have had water before its parent star died.
There is also some evidence in infrared light that the white dwarf has not swallowed all of the asteroid yet. It's hard to tell exactly what fraction of the asteroid material remains around the white dwarf. It could be that the white dwarf has already swallowed all but one percent of the asteroid, or it could be that half the shredded asteroid remains to be swallowed.
So, why is this surprising? After all, we see comets falling into the sun every few days.
When a star dies, it swells up into a red giant star. The Sun will expand in size about 80 times, reaching out to the orbit of the Earth and swallowing any planet, asteroid or comet inside of that distance. A star four times the mass of the sun, like the progenitor of the white dwarf in this study, will swell up twice as big as that -- beyond the orbit of Mars and to the inner region of the asteroid belt.
Red giant stars then shed their outer layers, with the core of the star shrinking to an orb about the size of the Earth. In order for an asteroid to get shredded and swallowed by the white dwarf, it has to come within about a million miles of the white dwarf star. But the red giant star cleared out everything within about 200 million miles of the star. Somehow the asteroid has to live further from its parent star than 200 million miles for almost a billion years, and then somehow get flung in very close to the white dwarf. In other words, the white dwarf needs and accomplice.
In order for the doomed asteroid to get flung in close to the white dwarf, the hidden accomplice has to be bigger than the asteroid. So we know that there must something at least the size of another dwarf planet around this white dwarf. But the bigger the accomplice, the easier it is to throw the asteroid in toward the white dwarf.
In our Solar System, Jupiter is a bully. Its gravity has been constantly tossing asteroids and comets around, sometimes flinging them in toward the sun, often flinging them out of the Solar System and into the icy depths of deep space. And Jupiter is far enough away from the Sun to survive the Sun's death throes. Therefore, I would not be surprised if there is a Jupiter-sized planet lurking around the white dwarf, and this big planet is responsible for sending the dwarf planet to its doom.
This sounds like the beginning of a bad science fiction movie. A zombie star (dead, but yet alive) being fed the occasional dwarf planet by an unseen accomplice. An international team of astronomers on Earth have uncovered this sinister plot. Can they save other innocent dwarf planets from this horrible fate? Stay tuned...
P. Dufour, M. Kilic, G. Fontaine, P. Bergeron, F. -R. Lachapelle, S. J. Kleinman, & S. K. Leggett (2010). "The Discovery of the Most Metal-Rich White Dwarf: Composition of a Tidally Disrupted Extrasolar Dwarf Planet" Accepted for publication in The Astrophysical Journal arXiv: 1006.3710v1