ROYAL ASTRONOMICAL SOCIETY

30 March 1998

HUNT IS ON FOR MORE EXTRASOLAR PLANETS

The UK National Astronomy Meeting (NAM) at the University of St Andrews opens on Tuesday 31st March with astronomers expressing high hopes of finding more extrasolar planets in the near future, and of learning much more about planetary systems beyond our own, as major new research projects get under way at the Anglo-Australian Observatory (AAO) in New South Wales, and at the observatory on La Palma in the Canary Islands.

THE ANGLO-AUSTRALIAN PLANET SEARCH PROJECT

Commenting on the AAO programme, AAO astronomer Chris Tinney said, "This southern hemisphere program is exciting, because it is the first time the virgin territory of the southern sky has been searched. Every southern star we observe is being checked for the first time. And the experience of the northern hemisphere shows we can expect to find the first new planets within about a year. Altogether, we hope to double the number of nearby stars with known planets over the next five years."

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.

THE LA PALMA EXOPLANET PROGRAMME

Observations to start in May on La Palma will be described by Professor Keith Horne of the University of St Andrews, who is a member of a 20-strong team of astronomers from 15 different institutions, mainly in Europe. He explains, "We were awarded the 1998 'International Time Project' at La Palma and we have about 5% of the 1998 telescope time on all of the La Palma telescopes." Professor Horne is involved with searching for extrasolar planets by two different techniques - looking for any that act as gravitational lenses, and looking for evidence that a planet is passing in front of its parent star as it travels in orbit.

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.

MAKING SENSE OF EXTRASOLAR PLANETS

Dr William Cochran of the University of Texas, co-discoverer of the planet orbiting the star 16 Cygni B, opens the NAM on Tuesday 31st March with an invited talk in which he takes a long hard look at the facts on extrasolar planets as they stand today, about a year since the last discovery was announced. He will look at whether any of the 'planets' may really be brown dwarfs (failed stars) and whether it is possible to tell the difference.

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."

FURTHER INFORMATION ON THE INTERNET

Further information on these topics can be found at the following WWW sites:

Anglo-Australian Telescope Planet Search Programmme

G. Marcy and P. Butler's programme at Lick Observatory

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