Although extrasolar planets were long posited, no planets orbiting main sequence stars were discovered until the 1990s. The discovery of extrasolar planets raises the question of whether they support extraterrestrial life.
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History of detection
Discoveries regarding extrasolar planets were first published in 1989  , when variations in the radial velocities of HD 114762 and Alrai (γ Cephei) were explained as being caused by sub-brown dwarf masses, possibly giant planets (11 MJ & 2–3 MJ respectively). Alrai had been the subject of a paper  the year before, but the question of a planetary companion as the cause was left open. Subsequent work in 1992 however concluded that no planet was likely present . The case for HD 114762 has yet to be disproven.
The Polish astronomer Aleksander Wolszczan also claimed to have found the first extrasolar planets in 1993, later confirmed, orbiting the pulsar PSR 1257+12. They are believed to be formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or the rocky cores that remain of gas giants that survived the supernova, and spiralled in to their current orbits.
Extrasolar planets around solar-type stars began to be discovered in large numbers during the late 1990s as a result of improved telescope technology, such as CCD and computer-based image processing. Such advances allowed for more accurate measurements of stellar motion, allowing astronomers to detect planets, not visually (the luminosity of a planet is generally too low for such detection), but by measuring gravitational influences upon stars (see astrometrics and radial velocity). Extrasolar planets can also be detected by measuring the variation in a star's apparent luminosity as a planet passes in front of it (see eclipse).
The first definitive extrasolar planet around a main sequence star (51 Pegasi) was announced on October 6, 1995 by Michel Mayor and Didier Queloz. Since then dozens of planets have been detected, and original claims from the late 1980s confirmed, 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 υ Andromedae. The majority of the detected planets have highly elliptical orbits. All of the planets so far discovered are high-mass and most are larger than Jupiter.
As of mid 2004, there were 108 known planetary systems around main sequence stars, containing at least 152 known planets. In July, 2004, it was announced that Hubble had been used to detect an additional 100 planets, but the presence of these planets could not yet be confirmed. Besides this, many observations point to the existence of millions of comets also in extrasolar systems.
Methods of detection
There are currently six methods of detecting extrasolar planets which are too faint to be directly detected by present conventional optical means.
The first method used to discover extra-solar planets was to observe anomalies in the regularity of pulses from a pulsar. This led to the 'discovery' of the first planet with the orbital period of one year. That was later retracted, as it was actually the failure to account for the motion of the Earth through its orbit. However, this method did lead to the discovery of the first planets, and first stellar system outside of our own, by Aleksander Wolszczan. It also led to the discovery of the oldest known planet, by Steinn Sigurdsson's team, around PSR B1620–26's binary stellar core. This planet is the only known planet to orbit two stars. The pulsar timing method involves precise measurements of the signal of a pulsar in order to determine if there are any timing anomalies in the period of the pulses. Subsequent calculations are used to determine what could cause the anomalies. This method is commonly used to detect pulsar companions but is not used to specifically find planets.
Astrometry is the oldest method used in the search for extrasolar planets, used as early as 1943. A number of candidates have been found since but none of them are confirmed and most astronomers have given up on this method for more successful ones. The method involves measuring the proper motion of a star in the search for an influence caused by its planets, but, unfortunately, changes in proper motion are so small that the best current equipment cannot produce reliable enough measurements. This method requires that the planets' orbits should be nearly perpendicular to our line of sight, and so planets detected by it could not be confirmed by other methods.
The radial velocity method measures variations in the speed with which the star moves away from us or towards us, i.e., the component along the line of sight, of the relative velocity of the star with respect to us. The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. Its variations are induced by the planet orbiting the star, because both orbit their mutual barycenter (see two-body problem). The velocity of the star around the barycenter is much smaller than that of the planet (the radii of the orbits and hence also the velocities are inversely proportional to the masses). Velocities down to 1 m/s can be detected.
This is the first and by far most successful technique used by planet hunters. It is also known as the "Doppler method" or "Wobble method". But it works only for relatively nearby stars out to about 160 light-years. It easily finds planets that are close to stars, but struggles to detect those orbiting at great distances. The Doppler method can be used to confirm findings made by using the transit method.
The gravitational microlensing effect occurs when the gravitational field of a planet and its parent star act to magnify the light of a distant background star. For the effect to work the planet and star must pass almost directly between the observer and the distant star. Since such events are rare, a very large number of distant stars must be continuously monitored in order to detect planets at a reasonable rate. This method is most fruitful for planets between earth and the center of the galaxy, as the galactic center provides a large number of background stars.
Gravitational microlensing has a checkered past. In 1986, Bohdan Paczyński of Princeton University first proposed using it to look for mysterious dark matter, the unseen material that is thought to dominate the universe. In 1991 he suggested it might be used to find planets. Successes with the gravity lensing method date back to 2002, when a group of Polish astronomers (Andrzej Udalski, Marcin Kubiak and Michał Szymański from Warsaw, and Bohdan Paczyński) during project OGLE (the Optical Gravitational Lensing Experiment) perfected a workable method. During one month they claimed to find objects, many of which could be planets.
Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than 1,000 stars have been detected in microlensing relationships over the past ten years.
The key advantage of gravitational microlensing is that it allows low mass (i.e. earth-mass) planets to be detected using available technology. A notable disadvantage is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations would not be possible. However, if enough background stars can be observed with enough accuracy then the method can be used to determine how common earth-like planets are in the galaxy.
In addition to the NASA/National Science Foundation-funded OGLE, the Microlensing Observations in Astrophysics (MOA) group is working to perfect this technique. Astronomers expect that it may be possible to detect an earth-sized world within five years.
A recently developed method detects a planet's shadow when it transits in front of its host star. This "transit method" works only for the small percentage of planets whose orbits happen to be perfectly aligned from our vantage point, but can be used on very distant stars. It is expected to lead to the first detection of an Earth-size planet when employed by NASA's forthcoming space-based Kepler observatory.
Most of these extrasolar 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 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 planetary systems, are still undetected.
An even newer approach is the study of dust clouds. Many solar systems contain a significant amount of space dust that is present due to frequent dust generation activity such as comets, asteroid and planetary collisions. This dust forms as a disc around a star and absorbs regular star light and re-emits it as infrared radiation. These dust clouds can provide invaluable information through studies of their density and distortion, caused either by an orbiting planet "catching" the dust, or distortion due to gravitational influences of orbiting planets.
Unfortunately this method can only be employed by space-based observations because our atmosphere absorbs most infrared radiation, making ground based observation impossible. Our own solar system contains enough dust to make up about 1/10th the mass of our moon. Although its mass is negligible, its surface area is so great that at a distance, its infrared emissions would outshine all our planets by a factor of 100.
The Hubble Space Telescope is capable of these observations using its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument, but was unable to do so due to a cooling unit malfunction that left NICMOS inoperative between 1999 and 2002. Even better images were then taken by its sister instrument, the Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility), in 2003. The Spitzer Telescope was designed specifically for use in the infrared range and probes far deeper into the spectrum than the Hubble Space Telescope can.
In March 2005 it was announced that scientists using the Spitzer Space Telescope were able to detect infrared radiation emitted from two extrasolar planets. The two teams, from the Harvard-Smithsonian Center for Astrophysics, led by David Charbonneau and the Goddard Space Flight Center, led by L. D. Deming studied the planets HD 209458b and TrES-1. They were able to measure the temperatures of the planets: 1,060 kelvins (1,450°F) for TrES-1 and about 1,130 kelvins (1,570°F) for HD 209458b.
In early 2005, two groups, both using the European Southern Observatory's Very Large Telescope array in Chile announced direct infrared images of extrasolar planets: GQ Lupi b and 2M1207b. Both planets are believed to be several times the mass of Jupiter (planet) and orbit at distances greater than 50 AU from their primary star. As of May 2005, their status as planetary objects (as opposed to being small brown dwarf stars) has not been firmly established.
Solar system formation processes
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, that is closer to the star than Mercury orbits the sun. 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.
Notable extrasolar planets
- In 1992, Wolszczan and Frail published results indicating that pulsar planets existed around PSR B1257+12 in Nature, volume 355, 145–147. Wolszczan had discovered the millisecond pulsar in question in 1990 at the Arecibo radio observatory. These were the first exoplanets ever verified, all the much more rare, that they orbit a pulsar.
- The first verified discovery of an exoplanet (51 Pegasi B) orbiting a main sequence star (51 Pegasi) was announced on October 6, 1995 by Michel Mayor and Didier Queloz in Nature, volume 378, page 355.
- A microlensing event in 1996 of the gravitationally lensed quasar Q0957+561, observed by R. E. Schild in the A lobe of the double imaged quasar, has led to a controversial, and unconfirmable speculation that a 3 Earth mass planet is possibly in the unknown lensing galaxy, between Earth and the quasar. This would be the most distant planet, if it could be confirmed, and is assumed to reside at redshift 0.39; 2.4 Gpc away (7.8 billion light years or 74 Ym), where the lensing galaxy is. (The double-image quasar itself, (called The Twin Quasar, or Old Faithful) Q0957+561 A/B, resides at redshift z=1.41)
- In 1999, HD 209458b was the first exoplanet seen transiting its parent star, conclusively proving that the radial velocity measurements that were planets actually were planets.
- On November 27, 2001, astronomers using the Hubble Space Telescope announced that they had detected the atmosphere of the planet orbiting HD 209458 (known as HD 209458b and provisionally dubbed "Osiris"). 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. It has been suggested that there may be planets that orbit so closely to their suns that most of their mass has been stripped away by the heat, provisionally referred to as Chthonian 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, in the constellation Scorpius.
- On April 15, 2004, separate teams announced the discoveries of three planets outside our solar system.
- One of which is 17,000 light years away, more than three times farther away than the previous record holder. The background star that was used in the gravitational lensing is 24,000 light-years away. The newly-discovered exoplanet is estimated to be about 1.5 times the mass of Jupiter and presumed to be similarly gaseous. It orbits the star about 3 astronomical units (AU). Jupiter is 5.2 AU from the Sun.
- The same day, a European team of planet hunters based at the Geneva Observatory found two giant planets using the transit method. Both planets are called "hot Jupiters," close to one Jupiter-mass but orbiting its star so closely that it completes an orbit in less than two earth days.
- In August 2004, a planet orbiting mu Arae with a mass of approximately 14 times that of the Earth  was discovered with the ESO HARPS spectrograph. It is the lightest extrasolar planet orbiting a main sequence star to be discovered to date, and could be the first terrestrial planet around a main sequence star found outside the solar system.
- In August 2004, a planet was discovered using the transit method with the smallest aperture telescope to date, 4 inches. The planet was discovered by the TrES survey, and provisionally named TrES-1, orbits the star GSC 02652–01324. The finding was confirmed by the Keck Observatory, where planetary specifics were uncovered.
See the list of stars with confirmed extrasolar planets for a list of confirmed observations.
- Aleksander Wolszczan — discovered first extrasolar planets, pulsar planets, 'solar system', pulsar planetary system.
- Steinn Sigurdsson — discovered oldest planet, and first circumbinary planet.
- Michel Mayor — with Queloz, discovered first planet around a main-sequence star.
- Didier Queloz — with Mayor, discovered first planet around a main-sequence star.
- Geoffrey Marcy — discovered more extrasolar planets than anyone else
- R. Paul Butler — co-discoverer with Marcy
Planets and their Stars:
- PSR B1620–26c, the oldest planet, also known as Methuselah, around neutron star PSR B1620–26, and white dwarf binary companion.
- 51 Pegasi B, a hot Jupiter, and the first planet found around a main-sequence star (51 Pegasi).
- PSR 1257+12's first extra-solar planets, planetary system, pulsar planets ever found.
- Astrometric binary
- Hypothetical planet
- Interstellar planet
- List of stars with confirmed extrasolar planets
- Pulsar planets
- Planetary system
- University of California Planet Search Project
- The Geneva Extrasolar Planet Search Programmes
- German Center for Exo-Planet Research Jena/Tautenburg
- Astrophysical Institute & University Observatory Jena (AIU)
- The Extrasolar Planets Encyclopaedia
- Table of known planetary systems
- Extrasolar Planet XML Database
- Andrew Collier Cameron, Extrasolar planets, Physics World (January 2001). (See the online version.)
- Diagram of planet detection methods – PDF