J. Brit. Astron. Assoc., 106, 2, 1996
At the January 1996 meeting of the American Astronomical Society in Texas, Geoffrey Marcy and Paul Butler of San Francisco State University announced that two naked-eye solar-type stars are harbouring planets in orbit around them. This brought to three the total of known 'ordinary' planetary systems apart from our own, taking into account the 51 Pegasi discovery announced in October 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory, but discounting the planets around pulsars.
Marcy and Butler have refined a technique for detecting the tiny wobble in the motion of a star induced by the presence of one or more planets with masses from one to several times that of Jupiter. The January announcement represented the culmination of a programme that has been in progress for eight years. Their survey has covered 120 stars similar to the Sun within 100 light years. Data from about half of the sample were subject to detailed analysis, producing the two positive results.
The discovery opens up a whole new area of research in astronomy – the study of comparative planetary systems. Bigger telescopes and more resources will be devoted to the search and theorists are already being spurred into action. There is of course a huge gap to bridge between demonstrating the existence of planets and being able to make any statement about the likely abundance of life in the Galaxy – but finding the planets is an important first step.
Much interest has focused on how the newly-discovered planets look in relation to our own solar system. A 0.6-Jupiter-mass orbiting 51 Peg in just over 4 days sounded intuitively implausible. Nevertheless, theorists have quickly shown that a Jupiter-like planet could survive in such an orbit, though it could not have formed there in the first place. It must have been at least three astronomical units from the star originally. Later interaction with material remaining in the proto-planetary disk could have caused it to spiral inwards.
The two planets found by Marcy and Butler seem more like those we are familiar with. That around 70 Virginis is in an eccentric 116-day orbit and has a mass about nine times that of Jupiter. Calculations suggest its surface temperature should be around 85°C, in the narrow range compatible with liquid water. The other new planet, belonging to 47 Ursae Majoris, is about three Jupiter masses and takes 1100 days to orbit its star at a distance of just over two AU. In a related development, new Hubble Space Telescope observations of the disc surrounding Beta Pictoris show that the disc is warped. Such a warp could not survive long-term unless something is continually disturbing the disc. That 'something', according to Chris Burrows of the Space Telescope Science Institute, could be a Jupiter-size planet orbiting in the clear zone between the star and its disc.
Jacqueline Mitton
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