Extrasolar Planets

Since 1995, about 150 planets have been discovered orbiting stars other than the Sun. About 20 of them are in systems of two to four planets. Most of these planets revealed themselves when their gravitational pull caused their stars to wobble a bit.

Before any extrasolar planets were discovered, astronomers expected that most planetary systems would resemble our Solar System: all planets following fairly circular orbits, with small rocky planets similar to Earth closest to their star and gas giants like Jupiter further away. In reality, however, most of the extrasolar planets that have been found have elongated orbits, and gas giants have been found at varying distances. Many Jupiter-like planets outside the Solar System are closer to their stars than Mercury is to the Sun. Jupiter-like planets that are very close to their stars are called “hot Jupiters.”

In 1995, the first hot Jupiter to be discovered was found orbiting the star 51 Pegasi, with a period of only four days. In 1999, another hot Jupiter was found orbiting the star HD 209458, also in the constellation Pegasus; it was the first planet seen crossing the face of its star. Astronomers have analyzed the starlight diffusing through this planet’s atmosphere, and have determined that the atmosphere contains hydrogen, oxygen, sodium and carbon.

So far, the strangest planetary system that has been found was discovered in 1992 by Alex Wolszczan of Penn Sate University. It consists of at least four rocky planets, one of them smaller than Pluto, orbiting a pulsar 1500 light-years away.

The vast majority of extrasolar planets that have been discovered orbit ordinary stars within 200 light-years of the Solar System. This is only because it is easier to find nearby planets, and it implies that there must be an enormous amount of planets throughout the Milky Way.