Sifting Through the Ashes For Shredded Planets

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You’d think the last place you’d go looking for evidence of Earth-like planets would be around a burned-out star that used to be sun-like — a white dwarf. So you may be surprised to hear that astronomers are employing interstellar forensics to find evidence for disintegrated Earths around these stellar husks.

White dwarfs are too small and too dim as targets for NASA’s Kepler planet-hunting observatory. And besides, nothing we’d call “Earth like” would be surviving in the post-Apocalypse age after a star has burned out and collapsed to a fiery hot cinder.

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In fact, Earth-like planets would be destroyed in a star’s final days. When the fuel-starved sun puffs up into a red giant around 4-5 billion years from now, the planets’ orbits will become unstable in a life-and-death game of interplanetary billiards. The sun will shed mass and this will weaken its gravitational grip on the planets, triggering dynamical chaos. Even worse, the ballooning sun will engulf most, or all, of the inner planets, turning Earth to ash.

After the sun gravitationally collapses into an ultra-compact white dwarf, the mayhem continues. Surviving asteroids or planets bodies may be thrown into elliptical orbits due to the gravitational tug of the remaining outer giant planets. Call this the era of Planets Gone Wild!

A new type of stellar forensics is unearthing evidence for planets that were cannibalized by white dwarfs. All the evidence that’s left is the chemical composition of the disintegrated planets that were ripped apart and devoured by the white dwarf.

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It’s like archeological digs that sift though debris in ancient fire pits to examine the mobility and food-storage strategies of the site’s prehistoric occupants.

This is a “backdoor” approach to estimating how many planets, and what kinds of planets, whirled around a progenitor star before it burned out.

White dwarfs should have atmospheres of pure hydrogen and helium, but spectroscopic observations show that they are polluted with heavier elements. Where does this pollution come from?

Previously, the idea was that white dwarfs had excess dust simply sucked in from interstellar space. But the chemical forensics point to devoured planetary systems much like ours.

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Using the Keck I telescope on Mauna Kea in Hawaii, Beth Klein of UCLA and colleagues did spectroscopic observations of two white dwarfs. One candidate, called PG1225-079, has an atmosphere containing magnesium, iron and nickel in ratios that resemble Earth’s chemical recipe.

The other target, HS2253+8023, contains more than 85 percent oxygen, magnesium, silicon and iron. These would have only condensed around a body that was born at high temperatures, very close to the star, such as Earth was 4.5 billion years ago.

The researchers report that they now know of four heavily polluted white dwarfs that swallowed objects which formed within a close warm zone around their star, analogous to the inner planetary region of our solar system.

“It is clear that extrasolar planetary systems produce rocky bodies that are compositionally similar to terrestrial planets in our own solar system. Earth-like planets apparently do form elsewhere in the galaxy,” Klein’s group write in their Astrophysical Journal paper.

Images: An asteroid getting shredded by a white dwarf’s tidal forces (top); planetary nebula NGC 2440 created after a star died as a red giant, completing its life cycle as a white dwarf (bottom). Credit: NASA