100th Extra-solar planet gives clues to origins of planets

British astronomers, together with Australian and American colleagues, have used the 3.9m Anglo-Australian Telescope [AAT] in New South Wales, Australia to discover a new planet outside our Solar System – the 100th to be detected. The discovery, which is part of a search for solar systems that resemble our own, will be announced today (Tuesday) at a conference on “The origin of life” in Graz, Austria. This takes the total number of planets found outside our solar system to 100, and scientists are now seeing a pattern in the orbits, giving clues to how they form.

The new planet, which has a mass about that of Jupiter, circles its star Tau1 Gruis about every four years. Tau1 Gruis can be found in the constellation Grus (the crane) and is about 100 light years away from Earth. The planet is three times as far from its star as the Earth is from the Sun.

`Now our searches have become precise enough to find many planets in orbits like those in our Solar System, we are seeing clues which may help us understand how planets are formed.` said UK team leader Hugh Jones of Liverpool John Moores University. `We are seeing a pattern for these planets to be of two types, those very close-in and another set with orbits further out. This Tau1 Gruis planet builds this second group. Why are there these two groups? We hope the theorists will be able to explain this.`

The long-term goal of this programme is the detection of true analogues to the Solar System. This discovery of a companion planet to the Tau1 Gruis star with a relatively long-period orbit and mass similar to that of Jupiter is a step toward this goal. The discovery of other such planets and planetary satellites within the next decade will help astronomers assess the Solar System`s place in the galaxy and whether planetary systems like our own are common or rare.

`The Anglo-Australian Telescope is providing the most accurate planet-search observations in the Southern Hemisphere`, said Dr Alan Penny, the other UK team member from the Rutherford Appleton Laboratory.

The researchers have found that as they probe for planets in larger orbits, the distribution of planets around stars is quite different from that of binary stars orbiting one another, where there is a smooth distribution of orbits. In contrast to the early discoveries of exoplanets, we now find that less than 1 in 5 exoplanets are to be found very close to their stars, a few orbiting with a period of 5 to 50 days but most giant planets are orbiting at large distances from their host stars. This supports the idea that they are formed at Jupiter-like distances from their host star. Dependent on the details of the early solar system, most giant planets probably spiral inwards towards their star until they reach a point where a lack of frictional forces stops their further migration.

To find evidence of planets, the astronomers use a high-precision technique developed by Paul Butler of the Carnegie Institute of Washington and Geoff Marcy of the University of California at Berkeley to measure how much a star “wobbles” in space as it is affected by a planet`s gravity. As an unseen planet orbits a distant star, the gravitational pull causes the star to move back and forth in space. That wobble can be detected by the `Doppler shifting` it causes in the star`s light. The AAT team measure the Doppler shift of stars to an accuracy of 3 metres per second – bicycling speed. This very high precision allows the team to find planets.