The mass of a planet orbiting a distant star has been measured using a new and precise method. Astronomers say it will shed fresh light on how planets are formed.
George Benedict and colleagues at the University of Texas in the US used the Hubble space telescope to monitor the motion of a star called Gliese 876 with unprecedented precision.
The measurements enabled the researchers to calculate the centre of gravity about which the star and the planet orbit. This allowed them to determine the planet鈥檚 orbit and, from that, the planet鈥檚 mass.
Advertisement
Previous estimates suggested the planet was between 1.9 and 100 times the mass of Jupiter. The new measurements narrow this range sharply, indicating the planet has 1.9 to 2.4 times Jupiter鈥檚 mass. Gliese 876 has a third of our Sun鈥檚 mass and is 15 light years from the Earth.
鈥淚t鈥檚 neat that a star with a third of the mass of the Sun can crank out a planet twice as massive as Jupiter,鈥� Benedict told 快猫短视频. 鈥淭hat will box in the planetary formation theorists.鈥�
Star wobble
Detecting the 鈥渨obble鈥� caused by a planet as it circles its parent star has been used many times to detect extrasolar planets. But previous observations have not studied this wobble in fine detail.
Benedict鈥檚 team measured the circular motion of Gliese 876鈥檚 wobble using Hubble鈥檚 three Fine Guidance Sensors over a period of two years. These instruments are normally used to track the position of celestial objects to keep the telescope correctly positioned.
Although this is not the first time an extrasolar planet鈥檚 mass has been calculated so precisely, it is a much improved method. In November 2001 a team from the University of Arizona determined the mass of a planet orbiting the star HD 209458. But this depended on the target planet passing directly in front of its parent star. Only around one in every 1000 planets is thought to pass directly in front of its host star.
Benedict says Hubble could be used to perform the same measurement for around five more of the 100 or so known extrasolar planets. But even more precise instruments will be needed to determine the mass of other extrasolar planets.
鈥淚 think the golden age for this technique will come when the Space Interferometery Mission is launched at the end of this decade,鈥� he says. This new NASA space-based telescope should be around 1000 times more precise than Hubble.
The research will appear in the 20 December 2002 edition of the Astrophysical Journal.