|
||||||||
  |
||||||||
|
||||||||
|
||||||||
|
||||||||
|
||||||||
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
Hubble 'weighs' faraway planet
By Richard Stenger
(CNN) -- With observations from the Hubble Space Telescope, astronomers announced that they had used a new measurement technique to make a precise estimate of the mass of a planet in another star system. The international team of scientists placed the mass of the so-called extrasolar planet to between 1.89 and 2.4 times the mass of Jupiter, the largest planet in our solar system. Previous calculations indicated that the mass of the planet, which is about 15 light-years away, was between 1.9 and 100 times that of Jupiter's. "There are a few more stars where we can do this kind of research with Hubble," said George Benedict, co-author of a report in the December 20 edition of Astrophysical Journal Letters. Knowing the mass of extrasolar planets could help astronomers answer many questions about how planets form. "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," said Benedict, a researcher at the University of Texas at Austin. Wobble vs. wiggleThe planet, known as Gl 876b, is only the second extrasolar planet for which a precise mass has been calculated and the first whose mass has been verified with a technique known as astrometry, the scientists said. Dozens of planets have been discovered indirectly using the Doppler or radial velocity method, where astronomers measure the shift in light waves from a star as it moves toward or away from us, a spectral "wobble" caused by an orbiting planet's gravitational tug. But the Doppler technique does not reveal the angle of a planet's orbit, whether it is edge-on or almost circular from our perspective. The drawback means that scientists can estimate the minimum mass of suspected planets but have more difficulty with the maximum. It leaves open the question of whether a star's companion is much larger than a planet, perhaps a brown dwarf. In contrast, the astrometry technique measures the miniscule "wiggle" in the motion of a star caused by a companion object. A sophisticated instrument on Hubble can show the shape of the orbit, permitting researchers to estimate the mass of an extrasolar planet within a much narrower range. "You can't hide massive companions from the Hubble Space Telescope," said co-author Barbara McArthur, also of the University of Texas. A planet orbiting the sun-like star HD 209458 is the first planet to have its mass verified. Scientists used the radial velocity technique and transit observations, which was possible only because the planet passed in front of the star every four days, slightly dimming the star's light.
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||
