Astronomers keep turning up new exoplanets, and as the count rises, they keep edging closer to finding worlds like our own pale blue dot. Astronomer Jay Farihi thinks Earth-like worlds might be even more common in the universe than previously expected, based on evidence from rocky planets few astronomers are studying: The ones that don’t exist anymore.
Farihi’s research subjects are white dwarfs. In our galaxy, about 90 percent of stars will end their lives in this incredibly dense state once the star sheds its outer material and only the core remains. This is the fate of our sun. White dwarfs usually have atmospheres composed of the light elements helium and hydrogen, as the heavy elements have settled to the core. But about 20 percent of white dwarfs are different, showing heavy elements—what astronomers call “metals”—in their atmospheres. For decades, astronomers attributed this metallic pollution to the interstellar medium, the thin gas that permeates the space between stars. The idea was that white dwarfs were old stars that had been on several orbits around the Milky Way and had picked up bits of the interstellar medium as they went around [Space.com]. But Farihi thinks those elements are evidence of something else.
His hypothesis: the heavy elements came not from the interstellar medium, but rather from the remains of rocky planets that once orbited the stars back in their younger days. For a study he presented at the Royal Astronomical Society meeting in Scotland this week, he looked at 146 white dwarfs that showed calcium pollution in their atmosphere, and which haven’t hung around near the interstellar medium anytime in the recent past. If the heavy elements in these stars had come from the medium, he argues, it would’ve sunk to the core long ago—it wouldn’t be dancing around with the light elements in the upper atmosphere. Ruling out the interstellar medium, Farihi says there are two possibilities: the debris could come from an asteroid belt similar to our own, which essential represents a planet that didn’t form, or the pieces of a shattered planet [Space.com].
If he’s right it could be further evidence that rocky planets are rather common around stars like our sun. The proportion could be even greater than 20 per cent, as some planetary systems might be entirely destroyed and leave no trace rather than leaving behind a debris ring to pollute their parent star [New Scientist].
Farihi also argues that white dwarfs could hold secrets about long-gone watery planets, too. He saw a lot of stars with atmospheres of nearly pure helium. But atmospheres that showed hydrogen traces also tended to contain the heavier elements. If the two are connected, he surmises, then the hydrogen would have come on board the same rocks as the metals, and that means the rocks could have carried water as well.
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Image: NASA / Casey Reed