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Planets that Orbit Another Star

For most of mankind's history, all the known planets in the universe numbered six--Mercury, Venus, Earth, Mars, Jupiter and Saturn--all of which orbited our sun. By the end of the 19th century, two more planets, Uranus and Neptune, would be added to the list; these too, belonged to our own solar system. In 1855, reports of evidence for an extrasolar planet would prove false. False reports persisted until 1994, with the confirmation of two planets orbiting a pulsar in the constellation Virgo discovered three years earlier. Today, many more planets are known outside our solar system than within our solar system.

First Exoplanets Discovered
- In 1992, the radio astronomers Alexander Wolszczan and Dale Frail, having used the giant radio telescope in Arecibo, Puerto Rico, announced the discovery of two rocky planets orbiting a distant pulsar with the unspectacular name of PSR 1257 in the constellation in Virgo. Though comparative in size to Earth and the moon, these planets are blasted with radiation from their dead star. Previously, in 1988, a team of astronomers announced the existence of a planet around gamma Cephei, but its existence would not be confirmed until 2002.
Exoplanet Discovery Methods
- Multiple methods are employed to detect exoplanets, including radial velocity, transit method and astrometric measurement. More than 300 planets have been discovered by radial velocity, which measures changes in a star's velocity as the star and planet orbit a common center. As the planet moves slightly closer and farther away from observers on Earth, the wavelengths of light from the star change. Large planets orbiting close to their star cause greater changes to the star's velocity and a corresponding shift in the wavelengths of light. The transit method measures the subtle dimming in light from a star as a planet moves across the front of the star. Astrometric measurement detects the slight back-and-forth motion of a star due to the orbit of a planet and around the star.
Earth-like Exoplanets
- The discovery of exoplanets has led to a search for Earth-like worlds capable of supporting life as we know it. Unfortunately, most of the almost 500 planets discovered are massive Jupiter-like planets, many of which are orbiting very near their star. However, in 2001, a planet orbiting the star HD 28185 was found orbiting its star at a distance comparable to the distance of the Earth from the Sun. Unlike Earth, this planet is over five times more massive than Jupiter. In 2010, two additional planets were discovered orbiting the star Gliese 581 near the constellation Libra, bringing the total known for this star to six. Of particular interest is one of these planets, Gliese 581g, which appears rocky and orbits in the "habitable region" of its star.

Astronomy