Watery asteroids like this one may be cosmic gravestones, revealing to astronomers past habitable worlds in dead …
One hundred and fifty light years away, there's a tiny white dwarf star named 'GD 61'. Once similar to our sun — although hotter and brighter — this star exhausting its nuclear fuel in about one-tenth the lifespan our sun will see, ballooned into a red giant, and then blew off its outer layers, leaving behind the stellar core we see today. Any planets that were orbiting close to the star would have been burned to a cinder by the red giant or torn to pieces by the powerful gravitational forces of the stellar core, and white dwarf stars (including GD 61) have been seen 'gobbling up' the remains of these planets from time to time.
As it turns out, a chunk of rocky debris that's pulled into the star can tell astronomers what kind of objects were orbiting the star when it was still 'alive', just like a gravestone tells a visitor to a cemetery who's buried there.
White dwarf stars are made of carbon and oxygen, but their atmospheres show only hydrogen and helium. Any elements heavier than that are pulled down into the core. However, if the star tears apart some asteroid or planetesimal, the elements from the object stick around in the star's atmosphere as 'pollution', and astronomers can see them show up in the spectrum of light they detect from the star.
Building on previous work, a team of astronomers led by Jay Farihi, from the University of Cambridge, UK, has been using the Hubble Space Telescope to examine close to 100 different white dwarf stars. For GD 61, they found that the 'pollution' created by an object that the star recently destroyed contains magnesium, silicon, and iron — the basic building blocks of rocky worlds — and they also found an abundance of oxygen. One thing they didn't find much of, though, was carbon. If they had, that would have pointed to the carbon monoxide or carbon dioxide ices from comets. However, since they didn't, the next most likely source of the oxygen is water.
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The amount of water in this rocky asteroid (over 25%) is very much like what we've seen from asteroids in our own solar system. Since collisions with this type of large, water-rich asteroid was one way Earth originally got its water, asteroids like this one may have supplied water to habitable worlds in GD 61's system too. There's no way to know for sure, but as Farihi said (according to Science): "the building blocks of Earth-like planets were there."
(Image courtesy: Mark A. Garlick/University of Warwick/University of Cambridge)
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