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twentieth century, we have finally begun to find other planets around nearby stars, opening the door to a new era whose impact upon human history may ultmately be more significant than the discovery of the New World. The recent detection of planets around other stars is based on finding the small cyclic motions due to the gravitational pull of a companion planet. The biggest effects are produced by the most massive planets: for example, in our solar system, Jupiter (by far our largest planet, at 318 times the mass of the Earth) causes the Sun to wobble in a 12-year cycle at a speed of 13 metres per second. To detect such tiny orbital motions of other stars that might have their own Jovian-type gas-giant planets, astronomers must be able to measure stellar velocities to an accuracy of just 3 metres per second, the speed of a fast walker! Moreover, these velocity changes must be monitored over many years for their cyclic nature to reveal itself.

The spectroscopic techniques and instruments capable of such high precision measurements were pioneered by a Canadian team over a decade ago. Building on these techniques, several other international groups have now surveyed over 300 nearby solar-type stars; the first discoveries of Jovian-size planets around a few such stars were announced in 1995. Twenty such planets have now been found, most of them with measured masses between 0.5 and 5 times the mass of Jupiter. A major surprise about these new planets, however, is that many of them are much closer to their parent suns than the Earth is to our Sun. Most astonishing of all is the discovery this year of the first planetary system: three Jovian-mass planets have been detected to be orbiting the star Upsilon Andromedae. We now have the first concrete evidence that entire families of planets revolve around stars like our Sun!

Jupiter, the giant of our Solar System, is seen here through the eyes of the Hubble Space Telescope. In the foreground is one of its satellites, Io, casting its shadow (the dark circle at center) on Jupiter's cloudtops. Io is the same size as Earth's Moon.

Image by John Spencer
(Lowell Observatory) and NASA

The presence of a Jupiter-sized planet around nearby star 51 Pegasi reveals itself by the small, periodic "wobble" of the star as the planet orbits around it. Highly precise measurements of the motion of the star show that it changes by more than 100 metres per second with a period of 4.2 days.

Graph by G. Marcy and P. Butler, San Francisco State University