NASA Hubble Space Telescope's crisp view has allowed an international team of astronomers to apply a previously unproven technique (called astrometry) for making a precise measurement of the mass of a planet outside our solar system. The Hubble results place the planet at 1.89 to 2.4 times the mass of Jupiter, our solar system's largest world. Previous estimates, about which there are some uncertainties, place the planet's mass between 1.9 and 100 times that of Jupiter's.

This is an artist's concept of a gas giant planet orbiting the cool, red dwarf star Gliese 876, located 15 light-years away in the autumn constellation Aquarius. Illustration Credit: NASA and G. Bacon (STScI)

A Hubble set of instruments called Fine Guidance Sensors (FGSs), which are also used to point and stabilize the free-flying observatory, measured a small "side-to-side" wobble of the red dwarf star Gliese 876. This is due to the tug of an unseen companion object, designated Gliese 876b (Gl 876b) and first discovered in 1998 with ground-based telescopes.

Gl 876b is only the second extrasolar planet (after HD 209458) for which a precise mass has been determined, and it is the first whose mass has been confirmed by using the astrometry technique.

Now that this technique has been proven viable for space-based observatory planet confirmations, it will be used in the future to nail down uncertainties in the masses of dozens of extrasolar planets discovered so far.

The observations were made by George F. Benedict and Barbara McArthur (University of Texas at Austin), members of the international observing team led by Thierry Forveille (Canada-France-Hawaii Telescope Corporation, Hawaii and Grenoble Observatory, France). The results are being published in the December 20 issue of Astrophysical Journal Letters.

Benedict had to observe the star's yo-yo motion for over two years, using a total of 27 orbits worth of Hubble Space Telescope observations. "Making these kinds of measurements of a star's movement on the sky is quite difficult," Benedict emphasizes. "We're measuring angles (.5 milliarcsecond) equivalent to the size of a quarter seen from 3,000 miles away.

The target planet, Gl 876b, is the more distant of two planets orbiting Gliese 876. It was originally discovered by two groups, led by Xavier Delfosse (Geneva/Grenoble 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, in 1999. These initial discoveries were made by measuring the star's subtle "to-and-fro" speed. This is called the radial velocity technique.

Benedict and McArthur combined the astrometric information with the radial velocity measurements (made in the planet's discovery) to determine the planet's mass by deducing its orbital inclination. If astronomers don't know how the planet's orbit is tilted with respect to Earth, they can only estimate a minimum mass for the planet. But without knowing more, the mass could be significantly larger if the orbit was tilted to a nearly face-on orientation to Earth. The star would still move towards and away from us slightly, even though it had a massive companion. "You can't hide massive companions from the Hubble Space Telescope," says McArthur. "The planet's orbit turns out to be tilted nearly edge on to Earth. This verifies it is a low-mass object."

"There are a few more stars where we can do this kind of research with Hubble," Benedict says. "Most candidate stars are too distant. Astronomers can look forward to doing these kinds of studies on literally hundreds of stars with the planned NASA Space Interferometry Mission, called SIM, which will be far more precise than Hubble.

"Knowing the mass of extrasolar planets accurately is going to help theorists answer lots of questions about how planets form," Benedict adds. "When we get hundreds of these mass determinations 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?"

Measuring stellar wobbles on the sky has been used to search for planets for decades. But extremely high precision and telescope optical stability are required. The Hubble FGSs are the first astrometric tool to accomplish this ultra-precise kind of measurement for an extrasolar planet.

The gas giant plant orbiting the sunlike star HD 209458 is the very first planet to have its mass verified by using transit and radial velocity data. This was only possible because the planet was discovered to be passing in front of the star every four days, slightly dimming the star's light. This is proof the orbit is edge on, yielding a mass that agrees with the lower limit estimate of .7 Jupiter masses.