Astronomers say they have discovered what may be "planetesimals" —
precursors of Earth-like planets — forming at Earth-like distances and
temperatures around a Sun-like star 430 light years away.

Michael Meyer of the University of Arizona announced the group’s results at
the199th national meeting of the American Astronomical Society in
Washington, D.C.

The group includes Eric Mamajek, Philip Hinz, and William Hoffman of the
University of Arizona (Tucson, AZ), Dana Backman and Victor Herrera of
Franklin and Marshall College (Lancaster, PA), John Carpenter and Sebastian
Wolf of Caltech (Pasadena, CA), and Joseph Hora of the Harvard/Smithsonian
Center for Astrophysics (Cambridge, MA).

The star, HD 113766A, is located 430 light years away in the direction of
Centaurus. The star is number 113766 in Henry Draper’s late 19th-century
catalog of spectral classifications for bright stars. The letter ‘A’
indicates that it is one member of a binary star pair. Both stars are
similar to, but somewhat hotter, more massive, and more luminous, than our

Although bright by astronomers’ standards, the two stars’ combined light is
not visible to the naked eye. The pair are outlying members of the
Scorpio–Centarus association, a nearby and well-studied group of young
stars with ages of 5 – 20 million years visible from the southern

In 1998 astronomers Vincent Mannings and Michael Barlow included HD 113766
in their published catalog of "Vega-like" stars, otherwise normal stars
showing signs of excess heat emission from planetary debris material. This
class of objects was first identified by Fred Gillet and collaborators using
data from the Infrared Astronomy Satellite (IRAS), an international
collaboration led by NASA, in 1984.

Members of the present team Backman and Herrera independently identified the
HD 113766 system as being especially interesting because much of the
circumstellar material has comfortably Earth-like temperatures around 300
degrees Kelvin (80 Fahrenheit).

Meyer and colleagues made their observations with the 6.5-meter Magellan I
telescope at Las Campanas in Chile during August 2001 using an advanced
infrared camera system, MIRAC/BLINC, built by Hoffman, Hinz, Hora, and

They found, to their surprise, that heat emission from dust debris appears
around the slightly brighter of the two stars, component A, but not around B

The two stars are just 1.3 arcseconds apart, so close that they were
separately resolved in infrared images only by combined virtues of the large
aperture of the Magellan telescope, excellent observing conditions in Chile,
and the fine new camera. The astronomers also found significant amounts of
radiation from material hotter than Earth temperature that had been missed
in previous observations.

The HD 113766 system, estimated to be about 10 million years old, does not
show evidence of a massive hot gas+dust disk like the one around the
prototypical very young Sun-like star T Tauri. Rather, it appears to be in a
subsequent developmental stage in which the gas has mostly dispersed and
solid particles are supposed to be accumulating into asteroid- and
planet-sized objects. Planetary formation models indicate that at an age
comparable to this system, Jupiter and Saturn were mostly finished, but the
Earth and its neighbors in the inner solar were only partly constructed.

HD 113766 is similar to another famous inner debris disk system surrounding
one component in a multiple star system, HD 98800. Why one component in each
system appears to harbor a circumstellar disk system and the other does not
is still a mystery. The only other known star system with a similar
distributionof inner planetary debris material, apart from our solar system,
is zeta Lep, a more massive and somewhat older star, announced earlier this
year by Christine Chen and Michael Jura of UCLA, Meyer said.

Meyer presented a schematic (posted on the Internet at the address below)
based on models of the location and density of the planetary debris around
HD 113766A computed by members of the team using the new infrared
measurements from Magellan, millimeter-wave observations from the SEST
telescope, plus archived data from IRAS and the NASA-sponsored 2-Micron
All-Sky Survey.

HD 113766 is much too far away to map the actual structure of the dust belt
even using the best telescopes; instead, the structure must be inferred from
calculations constrained by the observations. The team finds that the solid
material around the star HD 113766A has temperatures ranging between 805 and
195 K (+990 and -110 F), which suggests it is distributed between distances
of 0.35 and 5.8 Astronomical Units (AU) from its parent star (the hottest
material is closest to the star). For comparison, Mercury, Earth, and
Jupiter respectively orbit 0.4, 1.0, and 5.2 AU from our Sun. The companion
star, component ‘B’, is located about 170 AU from ‘A’.

Models of the debris distribution indicate it has approximately constant
density (mass per area) between its inner and outer edges. This is exactly
the distribution that would be produced by an effect known as
"Poynting-Robertson" (P-R) radiation drag which can influence dust particles
only if there is no gas to impede motions.

Most importantly, P-R drag sets a timescale for destruction of small
particles. The observed grains should all spiral in toward the central star
in a few hundred thousand years at most. This means that they can’t be
primordial grains persisting since the birth of the system 10 million years
ago unless they are still embedded in a gas-rich disk, which seems unlikely.

"The fact that we observe copious amounts of dust means either that we are
seeing the system in a brief moment after the sudden creation of huge
amounts of it, or, more likely, that dust is produced and replaced
continuously, for example by destructive collisions of larger parent
bodies," Backman said.

"This line of reasoning applies as well to our solar system’s zodiacal dust
cloud, where particles produced by asteroid collisions continuously replace
previous generations of dust that drift to destruction in the Sun in times
much shorter than the age of the solar system," he added.

The astronomers estimate that the density of the HD 113766A debris belt is
250,000 times that of our solar system’s zodiacal dust cloud. Roughly 200
times the total mass of asteroids in our present asteroid belt is required
around HD 113766A to collide and replenish the observed small particles at
the required rate. This means the amount of loose raw material in the form
of asteroids and smaller objects located in the terrestrial-planet zone
around HD 113766A equals about one-tenth the Earth’s mass.

"The structure of the belt of material is consistent with current theories
about planet formation in our solar system as well as the dynamical
interactions of dust with large planetesimals," Meyer said. "Although there
is no direct evidence for planets surrounding HD 113766, the observations
suggest the emergence of a planetary system not unlike our own."

Future observations with NASA’s Space Infrared Telescope Facility will be
required to rule out the presence of remnant gas and refine models of the
dust distribution, he added.

**Web link to FIG 1, Walter Baade Telescope (Magellan 1)
CAPTION: The telescope enclosure for Magellan I, eclipsing the housing for
Magellan II at Las Campanas Observatory in the Chilean Andes. (PHOTO:
Courtesy of the Magellan Project, Observatories of the Carnegie Institution
of Washington).

**Web link to schematic based on an infrared image from Magellan and
artist’s illustration of the system . See this site for additional
CAPTION: Infrared image from the MIRAC/BLINC camera obtained at the Magellan
1 telescope and a schematic representation of the system (not to scale).