EMBARGOED for Wednesday 5/9 2 p A distant Sun-like star has been accused of gobbling a giant planet, possibly the result of a deadly game of cosmic billiards in which the planets of a
fledgling solar system are lured to disaster, flung into odd orbits or even ejected from the game altogether. | A Star Swallows A Gas Giant Planet |
| How a near encounter with another planet could send a gas giant spinning into its parent star. [Watch the video] |
The scenario, from a study to be published in the May 9 issue of the journal Nature, may help researchers understand why several dozen other
solar systems, all found in the past six years, are so unlike our own. It could also shed light on how our own nine planets settled into their inexplicable positions.The new evidence takes the form of lithium 6, an isotope of the element lithium that is rare in stars but common in planets. Lithium 6 is destroyed over time by a star's internal heat. And so while it is sometimes thought to exist in
young stars, it is virtually absent in the Sun, which is about 4.5 billion years old.But the study found a considerable amount of lithium 6 in a star called HD 82943, which is at least as old as the Sun. The stuff must have been added sometime after the star was 30 million years old, though no firm estimate can be made, the scientists write in the May 9 issue of the journal Nature.
The logical source is the ingestion of a giant planet, the scientists say.
"There is enough lithium 6 isotope even in
the Earth to make an observable difference in a star which has no lithium 6 at all," said Garik Israelian of the Astrophysics Institute of the Canary Islands and lead author of the report.Israelian and his colleagues figure that a gas planet possibly twice the size of Jupiter, or a number of smaller planets adding up to the same mass, was eaten by HD 82943. If it had an Earth-like composition, the researchers say it must have had a metal-rich, rocky core several times the size of Earth.
Far away and hard to see
The star HD 82943 is nearly 89 light-years away, so studying it is a challenge. While scientists are confident that lithium 6 would be present in extrasolar planets, this has never been directly observed.
"There is no evidence for this because so far we cannot observe the light from the known extrasolar planets," Israelian said. (Researchers spot the planets by observing wobbles in the host star, or by noting a dip in starlight as the planet crosses in front of the star.)
But Israelian told SPACE.com that lithium 6 is assumed to exist in
extrasolar planets because it is known to exist on Earth and the other planets of our solar system, and because it has been measured in consistent quantities, with good confidence, throughout the interstellar medium from which stars and planets eventually form.While the results do not necessarily mean that most or all stars engage in such activity, they do mesh with other research into how stars and their planets interact.
Cosmic billiards
Since the first extrasolar planet was discovered in 1995, researchers have found 58 with masses 13 times that of
Jupiter or less. The number grows monthly and and is not agreed upon, as some of the planets may actually be failed stars called brown dwarfs. Many are lone giants orbiting a star or pair of stars. At least six of the systems involve two or more planets.Collectively, they paint a picture of solar system formation very unlike our own. Though huge, most known extrasolar planets orbit very close to their host stars -- often as near as Mercury is to our Sun. And unlike planets in our solar system, exoplanets typically carve highly non-circular, or elliptical orbits.
Researchers have come up with various ways to account for these odd orbits.
One possibility is that the gravitational dance between planets and their host star creates a sort of cosmic billiard game, explains Maria Rosa Zapatero Osorio, an exoplanet hunter at the California Institute of Technology who previously worked with the Canary Island group and is familiar with the new study.
"Some of these planets may be ejected from the system, some may remain in very elongated orbits and others may migrate inwards, being finally swallowed by the star," Osorio said. "The observations presented by Garik Israelian and colleagues seem to support this scenario."
Osorio said numerous comets might also have added the lithium 6, though this is not the preferred explanation.
Closer to home
While the formation of solar systems outside our own is a very young science, researchers don't even agree on how our own nine planets came to be where they are.
When researchers run one popular computer model designed to show how our solar system formed, the planets Uranus and Neptune turn up missing. In 1999, a group of researchers suggested these two planets perhaps formed closer to the Sun but
were bullied outward by Jupiter and Saturn."By investigating other planetary systems we will learn about our own solar system and will be able to provide scientific explanations of our origin and future," Osorio said.
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