AUSTIN, Texas — McDonald Observatory astronomers Fritz Benedict and Barbara McArthur 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. They made the observations of the star Gliese 876 using the Hubble Space Telescope’s Fine Guidance Sensors.

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 is about 50 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. We have conclusively established the planetary nature of the companion," Benedict said.

"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.

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

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.

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NOTE TO EDITORS: High resolution images to accompany this release are available online at the Space Telescope Science Institute website at: http://oposite.stsci.edu/pubinfo/pr/2002/27