AUSTIN, Texas – Fritz Benedict and Barbara McArthur, astronomers at
The University of Texas at Austin’s McDonald Observatory, have made
the first “positional” calculation of an extrasolar planet’s mass.
The work clearly determines the companion is a planet (not a low-mass
star), and is an incremental step in the process of discovering how
planets form around other stars.

The astronomers made the observations of the star Gliese 876 using
the Hubble Space Telescope’s (HST) Fine Guidance Sensors.

This research will be published in the Dec. 20 issue of The
Astrophysical Journal Letters.

The technique, called “astrometry,” involves multiple extremely
precise measurements of the star’s location as it orbits the center
of mass of the star-planet system. The mass measurement of the planet
announced today (Dec. 3) is about 100 times more accurate than
previously known. Until now, the planet was known to have a mass
between 1.9 and about 100 Jupiter masses. The HST-based calculations
pinpoint the mass as between 1.89 and 2.4 Jupiter masses.

“Until this work, the companion causing Gliese 876 to wobble back and
forth could have been anything from a planet to a garden variety
low-mass star,” Benedict said. “We have conclusively established the
planetary nature of the companion.

“Knowing the mass of extrasolar planets accurately is going to help
theorists answer lots of questions about how planets form,” Benedict
said. “When we get hundreds of these mass calculations for planets
around all types of stars, we’re going to see what types of stars
form certain types of planets. Do big stars form big planets and
small stars form small planets? From the case of Gliese 876, we now
know that a small star can form a big planet.”

An M-dwarf star, Gliese 876 is about one-third as massive as our Sun,
and about 500 times fainter.

“Our multiple measurements of Gliese 876’s location determined the
plane in which the planet orbits this star,” McArthur said. “Put
another way, the measurements determined the system’s orientation to
Earth – that is, face-on, edge-on, or a particular angle. We find
that the planet’s orbit is nearly edge-on to us.”

“Making these kinds of measurements of a star’s movement on the sky
is quite difficult,” Benedict said. “We’re measuring angles
equivalent to the size of a quarter seen from three thousand miles
away, or scientifically speaking, angles of one-half of a
milli-arcsecond.”

Benedict’s team combined the orientation information with the radial
velocity measurements (made in the planet’s discovery) to determine
the planet’s mass.

The planet under scrutiny is the more distant of two orbiting Gliese
876 and was discovered in 1998 by two groups led by Xavier Delfosse
(Geneva Observatory) and Geoffrey Marcy (U.C. Berkeley and San
Francisco State University). Marcy’s group discovered a smaller
planet closer to Gliese 876 a year later.

“There are a few more of stars where we can do this kind of research
with Hubble,” Benedict said. “Most candidate stars are too distant.
Astronomers can look forward to doing these kinds of studies on
literally hundreds of stars with SIM [the Space Interferometry
Mission],” he said.

SIM is a NASA space-borne telescope to planned for launch near the
end of this decade.

The planet around Gliese 876 is the second extra-solar planet overall
whose mass has been calculated to such accuracy. The first was able
to be calculated because the planet passed directly in front of the
star to Earth’s line of vision – an event known as a transit.

Fritz Benedict is a senior research scientist at The University of
Texas at Austin McDonald Observatory. He can be reached by phone at:
512-471-3448, or via email at: fritz@astro.as.utexas.edu.

Barbara McArthur is a research associate at The University of Texas
at Austin McDonald Observatory. She can be reached by phone at:
512-471-3411, or via email at: mca@astro.as.utexas.edu.

NOTE TO EDITORS: High resolution images to accompany this release are
available online at: http://oposite.stsci.edu/pubinfo/pr/2002/27
should be sent to the same address.

Electronic illustration files are available on the Internet at

The Space Telescope Science Institute (STScI) is operated by
the Association of Universities for Research in Astronomy,
Inc. (AURA), for NASA, under contract with the Goddard Space
Flight Center, Greenbelt, MD. The Hubble Space Telescope is a
project of international cooperation between NASA and the
European Space Agency (ESA).