30 March 1998
The AAO observations form part of an international project to examine nearby stars similar to the Sun for the presence of planets. The project stems from the highly successful search by Geoffrey Marcy and Paul Butler at Lick Observatory in California. Marcy and Butler are the world's leading discoverers of extrasolar planets with 6 independent finds to their credit. Because many stars in the far southern sky are not visible from Lick, Paul Butler will be working on the 3.9-metre Anglo-Australian Telescope at the AAO in collaboration with Chris Tinney, Hugh Jones (Liverpool John Moores University) and Alan Penny (Rutherford Appleton Laboratory), who will talk about the project at the NAM.
Other members of the La Palma consortium will observe stars with known planets, where there is a very large planet orbiting close to the star. They will be looking for signs of gas boiling off a giant planet similar to Jupiter. A third study is concerned with disks of dust around stars where planetary systems may still be forming. The researchers will investigate the presence of comet- like objects which have already been discovered in the disk around the star Beta Pictoris.
Dr Cochran says, "The question of whether these objects are planets or brown dwarfs is much more than an issue of semantics. I define a brown dwarf as a sub-stellar object formed in the same manner as a star. On the other hand, a planet is an object formed in the way we believe the planets in our solar system formed, by accretion in a circumstellar disk. Very different physical processes were involved. So there is no reason to believe that there should be a nice clean boundary between the masses of planets and brown dwarfs. The mass ranges of the two kinds of object may overlap, or there may be a 'mass gap' between planets and brown dwarfs. Those of us who have discovered the low mass objects have labelled them as planets, but mostly through wishful thinking. For example, the companion to 70 Virginis is commonly called a "planet", while the virtually identical object in orbit around the star HD114762 was called a "brown dwarf" by its discoverer. I believe that two different physical processes are indeed at work in the formation of these objects, and that the dividing line is somewhere around 10 Jupiter masses. The companions to 70 Virginis and HD114762 could well be either planets or brown dwarfs, but the lower mass companions are most likely true planets."
Anglo-Australian Telescope Planet Search Programmme
G. Marcy and P. Butler's programme at Lick Observatory