How many planets are outside our Solar System?

Sun and other planets
Artist's impression of extrasolar planets orbiting another sun
9 July 2003
Since 1995, when Michel Mayor and Didier Queloz discovered the first planet orbiting another star like the Sun, over hundred more extrasolar planets have been found.
Detected by watching the star wobble, the discovery by Mayor and Queloz of Geneva Observatory, Switzerland, stunned the world because the planet was initially considered the wrong type for the orbit in which it was found.

Since then, the number of planets detected by this method has grown rapidly. In June 2003, the Geneva-based team announced the discovery of seven new planets orbiting other stars, bringing the total number of known extrasolar planets to 115.
Michel Mayor and Didier Queloz
Michel Mayor and Didier Queloz
Hot Jupiters
Astronomers had initially expected that other solar systems would follow the pattern of our own: small rocky worlds close to the star and gas giants further out. Mayor and Queloz's planet, called 51 Pegasi b, was at least half the mass of Jupiter, the largest planet in our Solar System. However, it was in an orbit eight times smaller than Mercury's, the closest planet to the Sun.
More detections followed, by both the European team and an American one led by Geoff Marcy, then of San Francisco State University. It became obvious that 51 Pegasi b was not an isolated freak. There was a class of large planets in tight, circular orbits. Astronomers dubbed these new worlds 'hot Jupiters'. They are in such small orbits that hot Jupiters circle around their stars in a matter of days.
Eccentric planets
Other data indicated another distinct type of planet known as 'eccentric planets'. These are also gas giant planets but move in larger, elliptical orbits. In our Solar System, the gas giants all follow more-or-less circular paths.

An eccentric planet may be created when two large planets pass close to one another. During the encounter, one is thrown into the eccentric orbit and remains in the Solar System while the other is ejected into interstellar space where it wanders forever. Some astronomers call these planetary rejects, or 'planetars'.
A planetary system that looks similar to our own
Artist's impression of another planetary system like our own
Jupiter analogues
In our Solar System, Jupiter takes 12 years to orbit the Sun, so an observer would have to watch the Sun for a dozen years to see a full cycle and be certain of Jupiter's presence. Astronomers are watching other stars closely for such long-term orbits. As time goes by, astronomers have found planets in larger and larger orbits.

The discovery of the first Jupiter analogue was announced by astronomers from the Geneva Observatory in June 2002. The planet, that has a mass equivalent to Jupiter, takes about seven years to revolve around its sun which is located 553 million kilometres (3.7 AU) from it.

Another bonus from long observations is that, as the data accumulate for stars that are already known to have planets, small discrepancies betray other planets in the system. For example, Upsilon Andromedae is now known to possess a system of three large planets.
Better observations from space
On average, astronomers discover about 15 planets a year. So-called hot Jupiters (or eccentric planets) are found around about 5% of the stars studied. It is still too early to say how many of the other 95% have Jupiter analogues.

There are currently more than 30 planet-search programmes worldwide using ground-based telescopes. The current method of detection will never find Earth-sized worlds because they simply do not pull their parent star enough to be detectable. To find other Earths, astronomers need new techniques to detect planets. Such techniques work best from space.

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