By Ron Cowen
Planets coalesce from the placental cloud of gas and dust that surrounds a newborn star. Over time, gravity gathers the gas and dust into ever larger clumps. Dust grains form pebble-size bodies, which stick together to make objects as big as boulders. These bodies, called planetesimals, may grow large enough to form planets as massive as Jupiter.
New observations by Aki Roberge of Johns Hopkins University in Baltimore and her colleagues suggest that a mere stripling of a star, which might be as young as 300,000 years, already has planetesimals. If confirmed, the star, known as 51 Ophiuchi, may rank as the youngest star known to have initiated the planet-forming process. Roberge cautions, however, that the star’s age is not well determined.
Last year, another team reported that 51 Ophiuchi may already have formed a planet that has since broken up. The standard theory of planet formation holds that it takes a million or so years to make the core of a Jupiterlike body. If the mass of the putative planet turns out to be as heavy as Jupiter, the theory may need revision.
In the new study, Roberge’s group used the Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft to study the absorption of ultraviolet light by gas surrounding 51 Ophiuchi. They posted their report on the Internet (http://xxx.lanl.gov/abs/astro-ph/0111154). The team confirms the observations of an earlier spacecraft, the International Ultraviolet Explorer, which revealed that gas is falling toward the star.
Moreover, data from FUSE indicate that the gas is not pristine interstellar material but has a higher abundance of iron. That suggests the gas came from planetisimals such as asteroids and fragments of an Earthlike planet.
“The FUSE observations dramatically narrow the explanations for the observed infalling gas because it’s the first time we’ve analyzed [its] composition,” Roberge notes. From the data, her team infers that 51 Ophiuchi has a debris disk of planetesimals, some of which are being vaporized as they fall toward the hot star. A few other stars, most notably Beta Pictoris (SN: 8/8/98, p. 91), have such disks.
The findings “are telling us that there’s a lot more variety and complications to [debris-] disk evolution than was supposed when Beta Pictoris was the only example,” says Alycia J. Weinberger of the Carnegie Institution of Washington (D.C.).
No one has yet detected a debris disk around 51 Ophiuchi, perhaps because the star lies 487 light-years from Earth, much farther than Beta Pictoris, Roberge says.
The star is four times as heavy as the sun, and its gravity and the intensity of its ultraviolet light would tend to rapidly destroy the cloud of material from which planetesimals and planets form. The ability of 51 Ophiuchi to make planetesimals despite these obstacles suggests it must have done so in a hurry.
Determining how rapidly planetesimals can form is critical for calculating the abundance of planetary systems in our galaxy, Roberge adds. “If the timescale is very short compared with the lifetime of the typical circumstellar [cloud], then maybe every system can form planetesimals,” she says. That timing may be particularly crucial for building Earthlike planets, which astronomers consider to be agglomerations of kilometer-size planetesimals, including asteroids and comets.
