Extrasolar planets were discovered during the 1990s as a result of improved telescope technology, such as CCD and computer-based image processing along with the Hubble Space Telescope. Such advances allowed for more accurate measurements of stellar motion, allowing astronomers to detect planets, not visually (the luminosity of a planet being too low for such detection), but by measuring gravitational influences upon stars (see astrometrics). In addition, extrasolar planets can be detected by measuring the variance in a star's apparent luminosity, as a planet passes in front of it (see eclipse).
Besides the detection of at least 80 planets (mostly gas giants), many observations point to the existence of millions of comets also in extrasolar systems.
The Polish astronomer Aleksander Wolszczan claimed to have found the first extrasolar planets in 1993, orbiting the pulsar PSR 1257+12. Subsequent investigation has determined that these objects are not "true" planets in that they are technically "sub-brown dwarf masses orbiting an object that is or once was a star"; it is believed that they are unusual remnants of the supernova that produced the pulsar, and did not form as conventional planets do.
The first "true" extrasolar planet was announced on October 6, 1995 by Michael Mayor and Didier Queloz; the primary star was 51 Pegasi. Since then dozens of planets have been detected, many by a team led by Geoffrey Marcy at the University of California's Lick and Keck Observatories. The first system to have more than one planet detected was Upsilon Andromedae. The majority of the detected planets have highly elliptical orbits.
There are two main methods of detecting extrasolar planets, which are too faint to be detected by present conventional optical means. The first involves measuring the displacement in the parent star's spectral lines due to the Doppler effect induced by the planet orbiting the star and moving it through mutual gravitation. The second involves catching the planet as it passes in front of the star's tiny disk which will cause the light of the star to "dip" in a distinctive way, and do so periodically as the planet completes multiple orbits. The second method is theoretically more sensitive, but is newer and has scored fewer successes. It also depends on the plane of the planet's orbit being aligned with the line of sight between the star and the Earth. As a result, any number of stars with planets that are not so aligned will be missed.
Most of the planets found are of relatively high mass (at least 40 times that of the Earth); however, a couple seem to be approximately the size of the Earth. This reflects the current telescope technology, which is not able to detect smaller planets. The mass distribution should not be taken as a reference for a general estimate, since it is likely that many more planets with smaller mass, even in nearby solar systems, are still undetected.
One question raised by the detection of extrasolar planets is why so many of the detected planets are gas giants which, in comparison to Earth's solar system, are unexpectedly close to the orbited star. For example, Tau Boötis has a planet 4.1 times Jupiter's mass, which is less than a quarter of an astronomical unit (AU) from the orbited star; HD 114762 has a planet 11 times Jupiter's mass, which is less than half an AU from the orbited star. One possible answer to these unexpected planetary orbits is that since astrometrics detects the extrasolar planets due to their gravitational influences and partially-ecliptic interference, perhaps current technology only permits the detection of systems where a large planet is close to the orbited star, rather than such systems being the norm.
On November 27, 2001, astronomers using the Hubble Space Telescope announced that they had detected the atmosphere of the planet orbiting HD 209458, from its absorption of light when passing in front of its star. Also during that year, a star was located which had the remnants of one or more planets within the stellar atmosphere - apparently the planet was mostly vaporized.
In 2002 a group of Polish astronomers (Professors Andrzej Udalski and Marcin Kubiak and Dr. Michal Szymanski from Warsaw, and Polish-American Professor Bogdan Paczynski from Princeton) during project OGLE (the Optical Gravitational Lensing Experiment) worked out a method of easily finding extrasolar planets, based on a photometrical method. During one month they claimed to find 46 objects, many of which could be planets.
On July 10, 2003, using information obtained from the Hubble Space Telescope, scientists discovered the oldest extrasolar planet yet. Dubbed Methuselah after the biblical figure, the planet is about 5,600 light years from Earth, has a mass twice that of Jupiter, and is estimated to be 13 billion years old. It is located in the globular star cluster M4, approximately 7200 light years from Earth in the constellation Scorpius.
The Kepler Space Mission will be launched in the next few years. It is a space-based telescope designed specifically to search large numbers of stars for earth-sized terrestrial planets.
The frequency of extrasolar planets is one of the parameters in the Drake equation, which attempts to estimate the probability of communications with extraterrestrial intelligence.
The following is a list of main sequence stars with confirmed extrasolar planets. Note that the masses of the planets are lower bounds only. If a planet is detected by the spectral line displacement method referred to above, no information is gained about the inclination of the planet's plane of orbit around its star, and a value for this is needed to calculate the mass. It has become customary to arbitrarily assume that the plane is exactly lined up with the line of sight from Earth (this produces the lowest possible mass consistent with the spectral line measurements). The planets are listed with indications of their approximate masses as multiples of Jupiter's mass (MJ), and some also have approximate distances in astronomical units (AU) from their parent stars.
Confirmed extrasolar planets
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