The discovery of planets from outside the solar system is a new and fast-growing field of research. As of July 2001 there are 67 planets recorded from outside our own solar system. The first such planet was reported in the scientific journal Nature as recently as 1995 by Michel Mayer and Didier Queloz from the University of Geneva. This first discovery was around star 51 Pegasi. Shortly afterwards, in 1996, Geoffrey Marcy and Paul Butler from the USA reported a planet around the star Upsilon Andromedae — a discovery which subsequently revealed an entire planetary system associated with this star.
Compared to the intense brightness of a star, planets are invisible and cannot therefore be directly observed. It has long been known, however, that the presence of planets can be inferred by the gravitational pull they exert on their parent star. This should produce a tell-tale wobble in the motion of the star, which can be used to reveal the presence of an unseen planet.
Success in detecting wobble in the motion of a start has come from the analysis of the light emitted by the star. This allows us to measure the speed of the star. Spectroscopy — the analysis of starlight — splits up the light from a star into a spectrum and a measure of the shift of the absorption lines (the Doppler Shift) is used to calculate how fast the start is moving. [Visit http://exoplanets.org — go to 'Public Information', then 'The Doppler Detection Method'.] Stars which show a changing velocity are candidates for stars with planetary systems. If the change in velocity has a regular periodicity which is shown to be in accord with Kepler's Laws of planetary motion, then a planet may be inferred. By 1995 astronomers had improved the sensitivity of their spectroscopic methods such that the precision of velocity measurements had been increased from about 1 km/sec down to 20 m/sec. This paved the way for the first observations of a planet around star 51 Pegasi.
A second technique for detecting extrasolar planets is the observation of changes in star brightness. If a planet passes over the surface of the star, in certain conditions, the brightness of the star will be reduced. In the autumn of 1999 a 2% dimming of the star HD 209458 was observed at exactly the time predicted from calculations made of the orbit of an inferred planet. These observations provided important confirmatory evidence of results obtained using the Doppler method. [Visit http://exoplanets.org — go to 'Public Information', then 'Various Planetary Detection Methods'.]
Most of the recorded extra-solar planets to date have a mass close to that of Jupiter. Such planets are large gaseous planets, with a mass of about 1% of the sun. In March 2000 planets with a smaller mass were first detected and two planets with the mass of Saturn were found orbiting the stars HD 46375 and HD 16141.
Close analysis of the data from the star Upsilon Andromedae, 44 AU (astronomical units, i.e. earth-sun distances) away showed that there were three planets present. The innermost is ¾ the mass of Jupiter, 0.06 AU from the star and orbits the star in just 4.6 days; the middle planet is twice the mass of Jupiter, orbits at 0.3 AU from the star and takes 242 days to orbit. The outer planet has a mass four times that of Jupiter, is a distance 2.5 AU from the star and takes 3.5-4 years to orbit the star. These observations are of particular interest to astronomers and initially were a great surprise for these observations are very different from the pattern of planets in our own solar system. It was not expected that planets orbiting other stars would be large and very close to their parent star.
Details of planets associated with the star Upsilon Andromeda are at http://www.jtwinc.com/extrasolar/mainframes.html, select 'Planets of normal stars', select 'Upsilon Andromedae'.