From Science News, Vol. 160, No. 21, Nov. 24, 2001, p. 326.

Week of Nov. 24, 2001; Vol. 160, No. 21

Is this young star ready to form planets?
                                                                          
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.

                                      
References:

Roberge, A., et al. Preprint. FUSE observations of possible infalling
planetesimals in the 51 Ophiuchi circumstellar disk. Available at
http://xxx.lanl.gov/abs/astro-ph/0111154.

Further Readings:

Cowen, R. 1998. Epsilon Eridani: An early solar system? Science News
154(Aug. 8):91.

van den Ancker, M.E., et al. 2001. The composition of circumstellar gas
and dust in 51 Oph. Astronomy and Astrophysics 369(April):L17-L21.
Abstract available at
http://www.edpsciences.fr/articles/aa/abs/2001/14/aada022/aada022.html.
Preprint available at http://xxx.lanl.gov/abs/astro-ph/0102281.

Sources:

Aki Roberge
Department of Physics and Astronomy
Johns Hopkins University
3400 North Charles Street
Baltimore, MD 21218

Alycia J. Weinberger
Department of Terrestrial Magnetism
Carnegie Institution of Washington
5241 Broad Branch Road, N.W.
Washington, DC 20015

From Science News, Vol. 160, No. 21, Nov. 24, 2001, p. 326.

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