For the first time, astronomers have measured the mass of a planet circling a distant star by tracking the star's tiny side-to-side displacements on the sky.
When a planet orbits a star, its gravitational pull causes a slight wobble in the star's position. The stellar wobble, or "reflex motion," is a tiny mirror image of the planet's orbit. Astronomers have already tracked the orbits of about 100 giant planets of nearby stars by measuring the part of the reflex motion that's oriented toward or away from Earth (radial velocity). But decades of attempts to observe the sideways components of the motions have yielded nothing until now.
Using the Hubble Space Telescope's Fine Guidance Sensors, a dozen astronomers in the United States, France, and Switzerland collaborated to detect the orbit-induced motion of Gliese 876, a 10th-magnitude red dwarf star located 15 light-years away in Aquarius. The star was already known to have two planets based on radial-velocity studies. The astronomers were barely able to detect the side-to-side motion induced by the outer planet, which is about four times as massive as the inner one and orbits the star every 60 days.
This success is important because radial-velocity studies have one big weakness. They can't tell how much the planet's orbit is tilted with respect to the plane of the sky, and therefore they can give only the minimum possible mass of the planet, not its true mass. Statistically speaking, the planet's true mass will be only about 15 percent greater than the measured mass in a typical case. But from time to time the measured mass will be a gross underestimate.
The astronomers found that the reflex motion of Gliese 876 amounts to only 1/2,000 of an arcsecond on the sky, the equivalent of an ellipse 3 feet (1 meter) long on the surface of the Moon. The displacement was just at the detection limit of the Hubble's exquisitely precise Fine Guidance Sensors, whose normal function is to keep the telescope motionless during exposures.
After making 133 measurements over a span of nearly two years, the astronomers were able to determine that we see the planet's orbit nearly edge-on (its inclination is between 78° and 90 °). This means the planet's minimum mass measured by the radial-velocity technique, 1.9 Jupiter masses, is close to the truth. Considering all sources of uncertainty, the astronomers say the planet should be no heavier than about 2.4 Jupiters.
The only other extrasolar planet with such a well-constrained mass is the one orbiting HD 209458 in Pegasus. Its 0.7-Jupiter-mass planet transits (crosses) the star's face from our viewpoint, dimming it slightly once every 3.5 days and thus proving that the orbit is essentially edge-on to our line of sight.
Gliese 876 was an excellent target for the study because of its nearness and its low mass as a type M4 red dwarf (estimated at about 0.32 of the Sun's mass). Both factors make its reflex motion appear relatively large. Only a few other stars with known planets offer this kind of opportunity for Hubble. Far more, however, will be in range of NASA's Space Interferometry Mission (SIM), scheduled for launch in 2009.
The Gliese 876 system is especially interesting because its two planets seem to be locked in a 2:1 orbital resonance; the outer one orbits in 60 days, the inner one in 30 days. Accurate masses for both planets will be crucial for determining what this system has to say about the formation, evolution, and stability of planetary systems in general.
In addition, the astronomers measured the parallax (distance) of Gliese 876 with unprecedented precision. The star is 15.19 light-years from the Sun with an uncertainty of only one part in a thousand.
The astronomers' paper will appear in the December 20th Astrophysical Journal Letters.