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The Star That Ate a Mars

Astronomers study white dwarf pollution for clues to extrasolar planet ingredients

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2:24pm, July 3, 2009
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For several years, UCLA astronomers have studied GD 362, a peculiarly dirty white dwarf star 165 light-years away in the constellation Hercules. Now they are pretty sure why the atmosphere of this dense, hot but slowly cooling ghost of a once much larger star is so polluted. It ate a planet.

“We probably have a destroyed world here,” says Michael Jura, coleader of the UCLA team. Apparently a planet with the mass of Mars — a billion trillion metric tons or so of rock, iron, dissociated water and other ingredients — was dismembered and atomized, its remains now bobbing in the thin but dense, 10,000 kelvins atmosphere that GD 362’s powerful gravity holds close around itself.

Like a specimen on the ultimate autopsy table, the supposed planet has its insides spread wide for inspection. It would thus appear to provide science its first look at the composition of an alien, rocky and roughly Earthlike planet in an exosolar planetary system. In fact, the material marring GD 362 appears to closely match what you would get by grinding up Earth, Mars or Venus, the UCLA team and collaborators report in a paper to appear in The Astrophysical Journal.

The report provides some of the first hard data for a nascent field of science — let’s call it terrestrial exoplanetology —inching ever closer to the day when Earthlike planets might finally be visible directly to ever more powerful telescopes planned for construction on Earth and for orbit. Viewing white dwarfs with such instruments may tell astronomers whether rocky planets orbiting other stars all resemble Earth or come in diverse varieties.

“We have a unique tool for studying extrasolar planetary composition,” Jura says. “If we are clever enough, we might even know something about their structure, not just their bulk ingredients.”

The UCLA team has an additional dozen or so polluted white dwarfs under study. So far, all look as though they contain similar stuff. 

Like all white dwarfs, GD 362 is the steadily cooling core of a once normal-sized star, now shriveled into a sphere only about the size of the Earth with a mass nearly equal to the sun’s. All white dwarfs are crushed so tightly that their atoms (mostly carbon and oxygen) lose individual identities in a soup of jostling nuclei and electrons, a condition that earns from physicists the sobriquet “degenerate matter.”
But unlike most white dwarfs, GD 362 does not have a pristine, highly compressed skin of pure helium.

Jura, coleader Benjamin Zuckerman and the rest of the team have aimed some of astronomy’s most powerful telescopes at the tiny star, including NASA’s infrared Spitzer Space Telescope, the European Space Agency’s XMM-Newton orbiting X-ray observatory, and the Keck Observatory in Hawaii. The astronomers identify not only hydrogen mixed in with the dominant helium, but also at least 17 heavier elements, including oxygen, silicon, magnesium, calcium, titanium and iron. None of those are expected in the normally immaculate outer layers of a white dwarf — its strong gravity should pull them to greater depth in a few tens of thousands of years.

For a while the astronomers thought dust was responsible, or perhaps an asteroid got too close and was shredded by GD 362’s intense gravitational field. Then, with the help of German colleague Detlev Koester of Kiel University, the team reanalyzed how much water it would take to provide the extra hydrogen seen in the star, and by implication, how large a body it would take to deliver it.

The recent upgrade of the Hubble Space Telescope included a new ultraviolet spectrometer. With it, the number of elements observed in the white dwarf could grow to two dozen, Zuckerman believes. As the study expands to more white dwarfs carrying material absorbed from their former exosolar systems, the chemistry set for building planets should become generally known. And given the range of possibilities, he says, some of those planets “will be unlike that of anything known in our own solar system.”

Planetary lunch

Just how a fully formed planet would be ground up to dust and ultimately consumed by a tiny white dwarf star — the previously normal star that it once circled — is a remarkable scenario.

While the details are uncertain, it goes roughly like this: Hundreds of millions to a billion years ago or so, GD 362 was born as a normal star with a mass perhaps three times the sun’s. It presumably had several planets big and small. For a long time the star lived a normal life. That changed as it exhausted its core load of hydrogen fuel, burning helium instead, and generating vast amounts of oxygen and carbon as a sort of nuclear ash. It expanded enormously into a red giant, perhaps consuming some of its closer planets. The seething, bloated outer atmosphere blew away into space, taking most of the star’s mass with it. Surviving planets, responding to the lower gravity, migrated farther away. Then the star’s fires, not hot enough to burn carbon and oxygen, went out.

The remains of the star’s core — a fused mass of almost pure carbon and oxygen — collapsed into the white dwarf seen today. As surviving planets retreated to farther distances, their gravitational interactions produced ever-more unstable orbits. And thus, the tale goes, one of those planets, loaded with internal water but about the size of Mars, looped perilously close to the glowing ember of its former sun. It passed within perhaps half a million miles. Powerful tidal forces shredded it into a disk of debris resembling the ring system around Saturn. The disk’s contents have, ever since, been filtering onto the white dwarf —perhaps as rapidly as 100 million tons per second.

A few colleagues have reservations about the details. “It is not clear that distant planets could be efficiently re-directed so close to the star, or that it all would be accreted,” says Philip Chang, an astrophysicist at the University of California, Berkeley. “It also could be a new planet that formed close to the white dwarf after the red giant phase,” he says. “But the evidence that something like a planet has been absorbed by the white dwarf is pretty convincing.”

However it happened exactly, an infrared glow near the star indicates that the process may not be done yet. If the debris has quit raining onto the star, it did so only recently on astronomical time scales.

Weird worlds

The inside-out look at an atomized planet comes as astronomers who specialize in planet formation are champing at the bit for their first looks at actual, fully intact planets in the same size and habitability category as Earth. In the meantime, a sort of academic parlor game is underway to guess how diverse and Earthlike or utterly alien rocky planets around other stars will turn out to be.

In April, for instance, when prominent scientists and thinkers met at the Origins Symposium at Arizona State University to discuss the roots of many things — the universe, life, language and consciousness — one panel confronted “How Common Are Earthlike Planets?” One thing that already should be expected, declared UCLA geochemist and cosmochemist Edward Young, is that for most alien planets “rocks are rocks.” Other worlds may be too dry, too wet, too wobbly in their orbits, or too something else to be suitable for life’s easy evolution, he said. But with the data on white dwarfs as evidence, he argued that in addition to likely iron cores, such planets will probably still have familiar silicate minerals such as olivine, quartz, feldspar, mica and other common ingredients of Earth’s granites, schists, lavas and sandstones. 

But Jade Bond, of the University of Arizona’s Steward Observatory, suspecting that far more exotic worlds are awaiting discovery, argued, “Rocks are rocks, yes, but with some exceptions.” She is working out the implications for planets if some budding exosolar systems grow in an environment not so different from that of the Earth’s birth, but with one subtle change: What if carbon atoms outnumbered oxygen atoms?

Carbon and oxygen bond tightly to form carbon monoxide, a gas in the pre-planetary nebula not substantially incorporated in solid planets. The sun and most stars are also made of relatively more oxygen, but the margin is not large. The leftover oxygens bond to silicon atoms to form the solid on which silicate minerals are based. Were the comparative abundances reversed in the disk where planets form near a sun, she said, carbon but very little oxygen would be left over for incorporation into planets. Such worlds might congeal with startlingly different mineralogy and perhaps a far different chance for evolving life — or perhaps for a distinctly different sort of life.

On such planets may be plains of graphite, cliffs of carborundum and sheets of carbide. Below them — covering an iron core — could be thick strata of pure diamond. There might even be literally dark continents washed by oceans of tar; who knows? Bond showed a schematic cross section of such a planet. She conceded that such a world would have a “giggle factor,” but added, “If we ever find a planet with a global diamond layer as is pointed out here, I call dibs on it right now.”

It is not a totally far-fetched idea. In the past 10 years several astronomers have imagined such things, and three years ago astronomers reported ultraviolet evidence that a carbon-rich disk orbits the star Beta Pictoris, 60 light-years away, and could spawn carbon planets. “It is easy to imagine it happening,” says Eric Gaidos of the University of Hawaii. “But it is not easy to imagine it happening a lot.” 

Looking for little

Small planets, dead or alive, carbon or oxygen-rich, are big news. An aching hunger to find them has intensified among many astronomers since 1995. That is when a Swiss team, quickly followed by others, began detecting the gravitational influence of huge planets — many far more massive than Jupiter — circling other stars. These exoplanets are mostly bizarre places, typically seared to scorching temperatures because of their close-in orbits. The list of “hot Jupiters” and other large and uninhabitable-looking worlds now exceeds 350.

Among the smallest is COROT-7b, named for the European satellite COROT that measured its size by detecting its transit across the face of its star from a slight dimming in the star’s brilliance (SN: 2/28/09, p. 9). COROT-7b has a radius perhaps 75 percent larger than Earth’s and a mass six times greater or more. It orbits its star at such a roasting close distance that it goes around every 20 hours. Some suspect it is a former Neptune whose outer atmosphere evaporated away. That’s hardly Earthlike.

NASA’s newest space telescope — the Kepler satellite launched in March and just starting operation — may provide proof in coming years of how many sunlike stars have planets similar to Earth in size and orbiting at distances suitable for life of the sort biologists can easily imagine. Kepler will be able to detect them only from subtle changes in the brightness of their stars, but won’t be able to analyze them. Geoff Marcy, a University of California, Berkeley astronomer, is a former student of Jura’s at UCLA and is now a leading pioneer in discovery of exoplanets; weird giant planets are getting old for people like him. “There is a science fiction appeal to discovery of other Earths,” Marcy says. “Kepler is fantastic, we need it. But the sad news is that all the stars it is looking at are about a thousand light-years away. It will give our first accounting of small planets, but unfortunately none will be near enough to tell us much else.”

Waiting in the wings at NASA is a multibillion-dollar design for a huge orbiting instrument called the Terrestrial Planet Finder, originally to have been launched this decade but now on ice due to heavy space science budget cuts. The European Space Agency’s somewhat similar project, called Darwin, is also stalled by money woes.
That might change if Kepler hits paydirt.

“If we do learn that lots of stars, say 50 percent of them, have Earthlike planets, that will give a huge kick [to Congress and other nations] to pay for follow-up missions,” says Victoria Meadows, a University of Washington astronomer and leader of the Virtual Planet Lab project to simulate how such planets might look.

She is involved with one spaceborne project called Epoxi that has already scored. The craft measured reflected light as a distant terrestrial planet rotated while illuminated in a crescent view by its star. The variations permitted the team to derive a crude map. “We could see the Atlantic and Pacific ocean basins!” enthused Meadows. The planet was Earth itself, as seen from several tens of millions of miles away. The exercise used sensors aboard the Deep Impact spacecraft — while it was between missions to comets — to watch Earth while manipulating the data to mimic what might be gathered by larger instruments but across many light-years of space.

There is one thing that everybody in the planet-hunting business expects: surprise. They already know that a delicate balance of factors went into making the sun’s family of planets, including Earth. Those details included not just the ratio of carbon and oxygen to tip the balance toward silicate rocks, but an infusion of radioactive elements from a nearby supernova or perhaps from a giant cluster of large, young stars as the sun’s progenitor cloud began its contraction. The infusion of radioactivity, its signature clear in ancient meteorites, would have allowed greater heat to build up in the interiors of rocky planets as well as the minor planets and asteroids that orbit the sun beyond Neptune and Pluto.

With just slightly different starting conditions, the inner solar system could be drier than it is today, or far wetter. As it is, Venus and Mars are “Earthlike,” with similar masses and orbits, yet profoundly different.

“We sometimes call Earth the water planet, but the real question is, why does Earth have so little water?” UCLA’s Young said at the Origins symposium. Earth’s water layer is only about two hundredths of one percent of the planet’s mass — not even enough liquid to cover the mountains. “It wouldn’t take much more to have a true ocean planet,” Young said.

Thus Earthlike planets — really Earth-like — may be exceedingly rare, but only because with so many variations possible, to get two in separate systems that are the same down to fine detail would be unusual. David Stevenson, a Caltech planetary scientist, said at the Origins panel that “there will be a tremendous richness in outcomes. There will be things not like Earth — maybe carbon- rich planets, maybe super Ganymedes [a moon of Jupiter], half rock and half ice. What I think will come out from Kepler and later missions is a diversity of things different from anything that we have thought about so far. And that will be great.”

Charles Petit is a freelance science writer in Berkeley, Calif.

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