Dwarf Star With Oxygen Bolsters Theory That Earth Got Its Water From Outer Space

An asteroid rich in water had to have slammed into white dwarf SDSS J1242+5226 quite recently, or the heavy elements would have sunk out of its atmosphere.

Ruth Schuster
Ruth Schuster
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Artist's impression of a rocky, water-rich asteroid being torn apart by the strong gravity of a white dwarf.
Artist's impression of a rocky, water-rich asteroid being torn apart by the strong gravity of a white dwarf.Credit: Mark A. Garlick, space-art.co.uk, University of Warwick
Ruth Schuster
Ruth Schuster

The origin of water on Earth is a mystery. One pet theory of the astrophysics crowd is that earth began its existence dry as a bone: water arrived from outer space in the form of icy asteroids and comets smashing into the planet. Now new research in England bolsters that theory with evidence that a lot of space bodies, including asteroids and comets, contain large amounts of water.

The evidence is an unexpectedly large quantity of hydrogen and oxygen – the components of water – observed in the atmosphere of a white dwarf star, named SDSS J1242+5226. I think I'll name my next cat after it. For now let us call it SDSS for short.

A white dwarf is the incredibly dense corpse of a star that retains some faint luminescence, from residual thermal energy emission, not due to nuclear reactions – the hydrogen and helium "fuels" in its core are used up. It doesn't produce energy any more. Our Sun will shrink to a tiny fraction of its size and become a white dwarf in five to six billion years, cheerfully explains Professor Boris Gänsicke of University of Warwick, co-author on the paper.

Key to our story is that white dwarfs may be small in size, but their mass is vast and their gravity very strong indeed and like all stars – they have an atmosphere. "That's what we see when we look at stars," Gänsicke told Haaretz. "Stars are balls of gas. We can't look inside stars: we can only see their atmosphere."

Because white dwarfs are so dense and their gravity so powerful, heavy elements in their atmosphere don't stay there, but sink towards their core. (Think of scattering sand and gravel onto a bowl of water. Both will sink but the rocks will sink faster.)

So, one would expect to see hydrogen atoms in the atmosphere of the white dwarf, that being the lightest element; it will stay there; but not oxygen, which is heavier.

In the atmosphere of SDSS, the astronomers saw oxygen, as well as a bunch of metals such as calcium, iron and silicon.

"We deduced that the white dwarf had recently been hit by the equivalent of a large asteroid," says Gänsicke.

Recently, in astrophysical terms, means about a few hundred thousand years, he explained – that's about the time that it would take the heavier elements to sink and disappear from sight.

Anyway, there was enough water in SDSS' atmosphere to indicate that a water-rich asteroid had "recently" slammed into the star, which would have caused its water to mix into the atmosphere of the star.

Water-rich asteroids abound

One corollary of that theory is that other plants around the universe are also being bombarded by water-containing planetary bodies, which argues in favor of other suitable planets having life, suggest the scientists from the University of Warwick and published by the Royal Astronomical Society.

"Our research has found that, rather than being unique, water-rich asteroids similar to those found in our Solar System appear to be frequent," stated Dr Roberto Raddi of the University of Warwick's Astronomy and Astrophysics Group and the lead researcher. "It is believed that the Earth was initially dry, but our research strongly supports the view that the oceans we have today were created as a result of impacts by water-rich comets or asteroids."

Gänsicke elaborates. "That's what the people who work on the solar system think; that Earth formed very close to sun, and was too hot to have any water initially." (Not everybody agrees – some think the water came from inside.)

In any case, recent findings lean more towards the asteroid theory than the comet theory, Gänsicke says: a mission to a comet found the water had a different signature than the water on earth. The tentative conclusion is that the origin of water on earth is asteroids. "Comets are very rich in light elements – including carbon," Gänsicke explains. "They include carbon and carbon dioxide and methane, and so on. We do not detect carbon in the atmosphere of white dwarfs."

Yes, the carbon would sink but the point is, if we're seeing calcium and other metals and whatnot from a "recent" impact, we would see carbon too, if there was any. Ergo: the culprit was asteroid, not comet. Probably. Also, comets are lower in mass than asteroids, usually, Gänsicke adds.

One last point. That dead star, SDSS etc., may well have planets orbiting it, Gänsicke suggests. If the asteroid had been peacefully orbiting the star, as they do, it would have taken planetary perturbation of its path to cause it to crash into the star. Asteroid-dwarf collisions are not rare: this was however was exceptional because of the large amount of water in the asteroid.

Large? Huge. The amount of water found in the dwarf star's atmosphere was equivalent to a third of the total volume of oceans on Earth, Dr Raddi stated.

The research, "Likely detection of water-rich asteroid debris in a metal-polluted white dwarf," was published in the Monthly Notices of the Royal Astronomical Society by Oxford University Press.

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