A planet discovered far beyond our solar system has been for the first time accurately weighed, confirming the validity of the primary method -- the "wobble" method -- used to detect extrasolar worlds, astronomers announced today.
Researchers used the Hubble Space Telescope to watch the wiggle a star traced upon the sky as it was tugged back and forth by the gravity of the orbiting planet.
The star, called Gliese 876, is 15 light-years away. Its planet, named Gliese 876b, was previously discovered using an indirect method that measures a shift in light waves indicating the star is moving toward or away from Earth. This
But the Doppler method has stark limitations.
It cannot detect side-to-side motion of a star. It can't show how a planet's orbit is oriented, be it edge-on from our point of view or perhaps an effective circle drawn in the sky. Nor can the Doppler method determine the exact mass of the planet. Instead it sets a lower limit and leaves open the possibility that the companion object is something larger than a planet, such as a failed star called a brown dwarf.
The new observations, called astrometry, were planned and executed by George (Fritz) Benedict
of the University of Texas at Austin. They reveal the exact shape of the orbit and allow astronomers to precisely calculate the planet's mass. Gliese 876b is now known to weigh between 1.89 and 2.4 times as much as Jupiter. Previous estimates had put the planet's mass between 1.9 and 100 times that of Jupiter.
"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 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?"
How it works
Benedict explained the results, and how the method works, in an interview with SPACE.com in June, when he presented an early version of the findings to other astronomers at a conference on extrasolar planets. A scientific paper on the work has since been peer reviewed and will be published in the Dec. 20 issue of the journal Astrophysical Journal Letters.
"It's like a teeter totter," Benedict said. "You've got a big fat thing on this end of the teeter totter, with a very short lever arm. The other object, whatever mass it is, has to be a certain distance away from the fulcrum of the teeter-totter to balance."
Benedict and his colleagues knew how long it took the planet to orbit the star: 61 days. So they observed the star's position when the planet was in two positions on opposite sides of the star. They employed Hubble's Fine Guidance Sensors, which are used primarily to point and stabilize the observatory.
"The idea was to actually measure the wiggle [of the star] in the sky," Benedict said. "By measuring the amount wiggle in the sky and the orbit's shape we can determine the mass of the orbiting object."
The team found "a signature that can be interpreted as solid evidence for the existence of a planetary mass object around this star," he said. A second planet is thought to orbit the star, but its presence was not sought in the new study.
Why it matters
The new findings prove that the motion measured by the Doppler technique, in 1998, is actually due to a planet and not a low-mass companion star.
Few scientists had doubted the Doppler technique, but it had only been confirmed once before, by yet another method.
Last year, researchers confirmed a separate Doppler discovery around a different star by watching a dip in the star's light as a suspected planet crossed in front of it. This so-called
provides similar information to Benedict's astrometry technique, plus it can be used to probe a planet's atmosphere. But the transit method works only if the planet is aligned in such a way that it crosses in front of the star as seen from our point of view. Fewer than 10 percent of planets are set up this way. Astrometry will work with any planetary system, in principle.
So why has no one used astrometry before to examine other solar systems?
"Because it's damned difficult," Benedict said. "People have looked from the ground for years and years to see these wiggles. Any that have been announced have never been confirmed. The problem is the wiggle is very small. The wiggle we measured was just a half of a millisecond of arc. That's a dime 1,500 miles away."
The planet pulls the star around 100,000 actual miles in space, he said.
The observations are difficult and expensive. They took two years and were made on 27 separate Hubble orbits. The technique is therefore useful only for stars already suspected of having planets, Benedict said. Even then, it is only practical for relatively nearby stars with masses less than our Sun. Larger stars aren't swayed enough by the pull of a planet.
Benedict plans similar Hubble observations on two more stars, Upsilon Andromeda and Epsilon Eridini. Both have planets, according to Doppler studies. He expects further results over the next three to four years.
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