Hubble Makes Precise Measure of Extrasolar Planet’s True Mass

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.

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.

-end-

Electronic illustration files are available on the Internet at

• http://oposite.stsci.edu/pubinfo/pr/2002/27
• http://oposite.stsci.edu/pubinfo/latest.html
• http://oposite.stsci.edu/pubinfo/pictures.html
• http://hubblesite.org/go/news

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).