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01/03/2001 08:00:00

New planets still being createdIn our stellar neighborhood, study shows

In a study that strengthens the likelihood that solar systems like our own are still being formed, an international team of scientists is reporting today that three young stars in the sun's neighborhood have the raw materials necessary for the formation of Jupiter-sized planets.

Data obtained from the European Space Agency's Infrared Space Observatory (ISO) indicate for the first time that molecular hydrogen is present in the debris disks around young nearby stars. The results are important because experts have long thought that primordial hydrogen—the central building block of gas giants such as Jupiter and Saturn—is no longer present in sufficient quantities in the sun's stellar vicinity to form new planets.

The paper appears in the January 4 issue of the journal Nature.

"We looked at only three stars, but the results could indicate that it's easier to make Jupiter-sized planets than previously thought," said Geoffrey Blake, professor of cosmochemistry at the California Institute of Technology and corresponding author of the study. "There are over 100 candidate debris disks within about 200 light-years of the sun, and our work suggests that many of these systems may still be capable of making planets."

The abundance of Jupiter-sized planets is good news, though indirectly, in the search for extraterrestrial life. A gas giant such as Jupiter, may not be particularly hospitable for the formation of life, but experts think the mere presence of such huge bodies in the outer reaches of a solar system protects smaller rocky planets like Earth from catastrophic comet and meteor impacts. A Jupiter-sized planet possesses a gravitational field sufficient to kick primordial debris into the farthest reaches of the solar system, as Jupiter has presumably done by sending perhaps billions of comets into the Oort Cloud beyond the orbit of Pluto and safely away from Earth.

If comets and meteors were not ejected by gas giants, Blake said, life on Earth and any other Earth-like planets in the universe could periodically be "sterilized" by impacts.

"A comet the size of Hale-Bopp, for example, would vaporize much of Earth's oceans if it hit there," Blake said. "The impact from a 500 km object (about ten times the size of Hale-Bopp) could create nearly 100 atmospheres of rock vapor, the heat from which can evaporate all of the Earth's oceans."

The researchers did not directly detect any planets in the study, but nonetheless found that molecular hydrogen was abundant in all three disks they targeted. In the disk surrounding Beta Pictoris, a Southern Hemisphere star that formed about 20 million years ago approximately 60 light-years from Earth, the team found evidence that hydrogen is present in a quantity at least one-fifth the mass of Jupiter, or about four Neptune's worth of material.

The debris disk of the star 49 Ceti, which is visible near the celestial equator in the constellation Cetus, was found to contain hydrogen in a quantity at least 40 percent of the mass of Jupiter. Saturn's mass is just under a third that of Jupiter. 49 Ceti, which is about 10 million years old, is about 200 light-years from Earth.

Best of all was a 10-million-year-old Southern Hemisphere star about 260 light-years away that goes by the rather unpoetic name HD135344. That star's surrounding debris disk was found to contain the equivalent of at least six Jupiter masses of molecular hydrogen.

"There may not be enough material to form Jupiters around Beta Pictoris or 49 Ceti, but our figures establish a lower limit that is well within the gas-giant planet range, which means we definitely detected a fair amount of gas. And there could be more," Blake said. "Around HD135344, there's at least enough material to make six Jupiters."

Not only does the study reveal that there is still sufficient molecular hydrogen to make gas giants but also that planetary formation is not limited to a narrow time frame in the early history of a star, as previously thought. Because molecular hydrogen is quite difficult to detect from ground-based observatories, experts have relied on measurements of the more easily detectable carbon monoxide (CO) to model the gas dynamics of developing solar systems.

But because results showed that CO tends to dissipate quite rapidly in the early history of debris disks, researchers assumed that molecular hydrogen was likewise absent. Further, the presumed lack of hydrogen limited the time that Jupiter-sized planets could form. However, the new study, coupled with recent theoretical models, shows that CO is not a particularly good tracer of the total gas mass surrounding a new star.

Blake said the study opens new doors to the understanding of planetary growth processes around sun-like stars. He and his colleagues anticipate further progress when the Space Infrared Telescope Facility (SIRTF) and the Stratospheric Observatory for Infrared Astronomy (SOFIA) are launched in 2002. SIRTF, which will have its science headquarters at Caltech, alone could detect literally hundreds of stars that still contain enough primordial hydrogen in their debris disks to form Jupiter-sized planets.

The other authors of the paper are professor of astronomy Ewine F. van Dishoeck and Wing-Fai Thi (the study's lead author), both of the Leiden University in the Netherlands; Jochen Horn and professor Eric Becklin, both of the UCLA Department of Physics and Astronomy; Anneila Sargent, professor of astronomy at Caltech; Mario van den Ancker of the Harvard-Smithsonian Center for Astrophysics; and Antonella Natta of the Osservatorio Astrofisico di Arcetri in Firenze, Italy.

Written by Robert Tindol