Spaceflight Now: Breaking News

Planets orbiting other stars could be more plentiful

Posted: January 5, 2000

  Beta Pictoris
Beta Pictoris. Photo: G. Blake, Caltech
The number of stars with extrasolar planets may be much larger than previously thought, scientists studying several nearby stars concluded this week.

In research published in the journal Nature, astronomers using a European astronomy satellite found that clouds of molecular hydrogen gas, the raw material for gas giant planets like Jupiter, may last millions of years longer than once believed, making it much easier for such planets to form.

Planetary scientists had believed that hydrogen in protoplanetary disks -- clouds of gas and dust that surround newborn stars -- would have been locked up into planets within a few million years of the star's formation as instabilities formed in the disk. Moreover, during this phase dust made the disk opaque to optical telescopes, preventing astronomers from observing planet formation or even the star itself.

To test this explanation, a team of Dutch and American astronomers studied several young stars with the Infrared Space Observatory (ISO), a European Space Agency astronomy satellite that operated from late 1995 through mid 1998. The three stars -- Beta Pictoris, 49 Ceti and HD135344 -- have all lost most of their original protoplanetary disks, with only thin rings left behind. According to theory, these disks should have little, if any, hydrogen left, yet the ISO data turned up significant quantities of it, enough in some cases to form ten planets the mass of Jupiter.

These findings "suggest that Jovian planet formation can occur on timescales up to 20 million years," the authors noted in their paper. That period of time, several times longer than previously though, opens the door to an alternative, more gradual model for planet formation.

"The second model says that a small 'Earth-like' core is formed first, and then the lighter material in the disk, the gas, is attracted by gravity," explained Ewine van Dishoeck of Leiden University. "In this second model planet formation takes longer than just a few million years. Our results imply that it cannot be ruled out. You don't need to make planets that quickly."

Left column: Scattered light or thermal emission images of the debris disks surrounding beta Pictoris, 49 Ceti and HD 135344. The relative sizes of the images depict the angular sizes of these disks, located some 63, 200 and 260 light years distant, respectively. Right column: Infrared Space Observatory Short Wavelength Spectrometer scans of the molecular hydrogen from each. Prepared by G. Blake, Caltech.
If hydrogen lasts longer around new stars, it should also make it easier for planets to form regardless of how they form. "The discovery of larger amounts of gas in the disks of older systems suggests that Jovian planets can form on timescales of up to 10-20 million years," said Jack Lissauer, a planetary dynamicist at NASA's Ames Research Center, in an essay accompanying the Nature paper. "This means that the formation of giant planets is likely to be fairly common, at least around isolated Sun-like stars."

Lissauer believes that studies of protoplanetary disks and models for planetary formation may explain why many of the extrasolar planets discovered in the last five years have unusual orbits, with planets the size of Jupiter orbiting only a few million kilometers from their parent stars. "The manner in which gas is removed from a protoplanetary disk could have as much influence on the ultimate configuration of the planetary system as does the lifetime of the disk," he said. "A planet gravitationally tugs surrounding disk material, and this interaction can alter planetary orbits substantially."

Because molecular hydrogen cannot be detected from the ground at the long infrared wavelengths used by the now-defunct ISO, astronomers will have to wait until the launch of NASA's Space Infrared Telescope Facility (SIRTF) in July 2002 to conduct followup studies of protoplanetary disks. That telescope should provide much higher resolution data than ISO, enabling more detailed studies of planet formation.