Doomed planet may have been drenched in water

BY KEITH COOPER

ASTRONOMY NOW

Posted: 19 May, 2009

A small exoplanet full of water may have been swallowed up by a dead white dwarf star, according to anomalous readings of hydrogen in the star’s helium-rich atmosphere. This is the possible conclusion put forward by a team of astronomers based in the United States and the UK, and would be a sure sign that water is present in some form deep within sizeable planetary bodies around other stars.

An artist’s illustration of debris rings formed by fragmented asteroids around the white dwarf GD 362. Did a water-rich planet get too close?

Image: Gemini Observatory/Jon Lomberg.

The white dwarf in question, called GD 362 and located 150 light years away in the constellation Hercules, has been in the news ever since odd amounts of heavy elements were discovered in its atmosphere in 2004, followed a year later by the detection of a ring of broken-up asteroids encircling it. It is easier to understand this ring when you consider what a white dwarf is: the final evolutionary step of a Sun-like star, after its hydrogen fuel in its core has run out and it has swollen into a red giant, before ejecting its outer layers to, leaving behind its helium-rich core in the shape of a white dwarf. During the red giant phase, the star engulfs and destroys its innermost planets, and disrupts the orbits of outlying bodies. So what we see today in the GD 362 system is a dead solar system.

The heavy elements found in GD 362’s atmosphere are believed to have come from a large rocky body that fragmented and was pulled in by the white dwarf’s gravity. The new observations using the European Space Agency’s XMM-Newton X-ray observing spacecraft and NASA’s infrared Spitzer Space Telescope, described in a paper available on the Astro-ph website, have now found an over-abundance of the lightest element in GD 362’s atmosphere: hydrogen.
The hydrogen in question amounts a hundredth of the total mass of Earth. This could be explained if an object, at least as massive as Jupiter’s moon Callisto and possibly as massive as Mars, and containing more water (the source of the hydrogen) deep inside it than even Earth does, was ripped apart by the white dwarf’s gravity and the remnants pulled in by the dead star. However, this was unlikely to be the destruction of an Earth-like planet with oceans, says one of the researchers, Jay Farihi of the University of Leicester.

“If it had surface water, it was all evaporated during the giant phase of the host star,” Farihi told Astronomy Now. “This makes it unlikely, but not impossible, to have retained internal water. Also, to be in the ‘Goldilocks Zone’ close to a star where liquid water temperatures are maintained makes a planet a candidate for being engulfed and destroyed during the giant phase, prior to [the star] becoming a white dwarf.”

Instead, Farihi suspects it may have been an object similar in structure to Europa or Callisto, which are moons suspected to contain oceans deep underground. Or perhaps a Mars-like world that dried up on the surface but retained a large amount of water underground would be a plausible explanation.
The alternative is that around a 100 water-rich asteroids the size of Ceres – our Solar System’s largest asteroid – crashed into the white dwarf, but would a solar system contain so many asteroids of that size? One hundred Ceres-sized objects would contain more mass than our Solar System’s entire asteroid belt put together, but there is evidence of sufficiently large enough belts around other stars, such as zeta Leporis, which may have an asteroid belt weighing in at 4 x 10^23 kilograms, compared to 2 x 10^21 kilograms for our Solar System. To prove it either way, it would be necessary to show whether the hydrogen and heavy elements came from the same object or many different ones.

“If we were able to show the hydrogen we see was accreted from the same parent body as the metals, it would be an unambiguous detection of extra-solar watery rocks,” says Farihi. It would be ironic should a doomed planet prove a boon to astronomers searching for habitable worlds containing water.
The other researchers include Michael Jura and Ben Zuckerman of the University of California, Los Angeles, and Michael P Muno from Caltech. To see a video of Jay Farihi talking about heavy elements detected in the atmospheres of white dwarfs during April’s European Week of Astronomy and Space Science at the University of Hertfordshire, click here.