Just over five years ago, astronomers had no evidence for planets circling Sun-like stars outside our solar system. This situation changed in 1995 when Michel Mayor and Didier Queloz at the Geneva Observatory discovered a massive planet orbiting just 7 million kilometres from its star. Now, planets are routinely detected around other Sun-like stars. Indeed, 30 extrasolar planets have been discovered so far, compared with just nine planets in our solar system.
Until now, however, the evidence for these extrasolar planets has remained indirect. Astronomers have relied on detecting the motion of the star around the centre of gravity of the system - the so-called radial-velocity technique. As the planet orbits the star, it causes the star to wobble back and forth. Although the motion is slow, the variations in the velocity of the star cause a Doppler shift in the wavelength of the absorption lines in the star's spectrum. However, radial-velocity measurements tell us little about the planets themselves. We can only derive the orbital distance, the eccentricity (which tells us how elongated the elliptical orbit is) and the mass of the planet multiplied by the sine of the inclination of the orbit.
Now Andrew Collier Cameron of St Andrews University in the UK and co-workers may have detected the light reflected by one of these extrasolar planets directly (Nature 1999 402 751). The research team looked at a remarkable system in the Boötis constellation using the 4.2 m William Herschel telescope at La Palma on the Canary Islands. However, until further observations are made confirming their discovery, there is a 1 in 20 chance that the signal is just spurious background noise.
Meanwhile, a Harvard team has recently found the first unambiguous proof for the presence of an extrasolar planet - in a system where a planet crosses the star HD209458 every 3.5 days (Astrophys. J. 2000 529 L45-L48). They carefully monitored the light from the star and saw that it dimmed by about 2% when the planet moved in front, which allowed them to determine that the radius of HD209458b was about 1.3 times larger than Jupiter.
In the March issue of Physics World magazine, Tristan Guillot from the Observatoire de la Côte d'Azur, Nice, France describes the latest observations of extra-solar planets.