Extrasolar planets became a subject of scientific investigation in the mid-19th century. Astronomers generally supposed that some existed, but it was not known how common they were and how similar they were to the planets of the Solar System. The first confirmed detections were made in the 1990s; since 2000, more than 15 have been discovered every year. The frequency of detection is increasing with 61 planets detected in 2007. It is estimated that at least 10% of sun-like stars have planets, and the true proportion may be much higher. The discovery of extrasolar planets sharpens the question of whether some might support extraterrestrial life.
Currently, Gliese 581 d, the third planet of the red dwarf star Gliese 581 (approximately 20 light years from Earth), appears to be the best example yet discovered of a possible terrestrial exoplanet which orbits close to the habitable zone of space surrounding its star. Going by strict terms, it appears to reside outside the "Goldilocks Zone", but the greenhouse effect may raise the planet's surface temperature to that which would support liquid water.
Claims about detection of exoplanets have been made from the 19th century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855, Capt. W. S. Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton soon published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star. Astronomers now generally regard all the early reports of detection as erroneous.
In 1991, Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.
The following year, additional observations were published that supported the reality of the planet orbiting Gamma Cephei, though subsequent work in 1992 raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR 1257+12. This discovery was quickly confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then spiraled into their current orbits.
On October 6 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the detection of many new exoplanets at a rapid rate. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Several extrasolar planets were eventually also detected by observing the variation in a star's apparent luminosity as a planet passed in front of it.
To date, 312 exoplanets have been found, including a few that were confirmations of controversial claims from the late 1980s. The first system to have more than one planet detected was υ And. Twenty such multiple-planet systems are now known. Among the known exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of circumstellar dust disks also suggest the existence of millions of comets in several extrasolar systems.
Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. In addition to the intrinsic difficulty of detecting such a faint light source, the parent star causes a glare that washes it out.
For those reasons, current telescopes can only directly image exoplanets under exceptional circumstances. Specifically, it may be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.
The vast majority of known extrasolar planets have been discovered through indirect methods:
Not counting a few exceptions, all known extrasolar planet candidates have been found using ground-based telescopes. However, many of the methods can yield better results if the observing telescope is located above the restless atmosphere. COROT (launched in December 2006) is the only active space mission dedicated to extrasolar planet search. Hubble Space Telescope has also found or confirmed a few planets. There are many planned or proposed space missions such as Kepler, New Worlds Mission, Darwin, Space Interferometry Mission, Terrestrial Planet Finder, and PEGASE.
The most common way of naming extrasolar planets is almost similar to the naming of binary stars, except a lowercase letter is used for the planet (while an uppercase letter is for stars). A lowercase letter is placed after the star name, starting with "b" for the first planet found in the system (51 Pegasi b). The next planet found in the system could be labeled the next letter in the alphabet. For instance, any more planets found around 51 Pegasi would be cataloged as "51 Pegasi c" and then "51 Pegasi d", and so on. If two planets are discovered around the same time, the closest one to the star gets the next letter, while the last planet would get the last letter. For example, in the Gliese 876 system, the most recently discovered planet is referred to as Gliese 876 d, despite the fact that it is closer to the star than Gliese 876 b and Gliese 876 c. The suffix "a" was intended to refer specifically to the primary, as opposed to the system as a whole, but this did not catch on. The planet 55 Cancri f is currently the first and only planet to have "f" in its name (being the fifth planet found in the 55 Cancri system), with no letters currently beyond "f" (the highest letter currently in use).
Only two planetary systems have planets that are named "unusual". Before the discovery of 51 Pegasi b in 1995, two pulsar planets (PSR B1257+12 B and PSR B1257+12 C) were discovered from pulsar timing of their dead star. Being that there was no official way of naming planets at the time, they were called "B" and "C" (similar to how planets are named today). However, uppercase letters were used, most likely because of the way binary stars were named. When a third planet was discovered, it was designated PSR B1257+12 A (simply because the planet was closer than the other two). Some nomenclatures (generally in science fiction) use Roman numerals in the order of planets' positions from the star, but for the above reason, this is not practical.
If the planet orbits in a non-circumbinary system, the letter of the star is added to the name. If the planet orbits the primary star of the system, and the secondary stars were either discovered after the planet or are relatively far form the primary star and planet, the name is usually omitted. For example, Tau Boötis b orbits in a binary system, but because the secondary star was both discovered after the planet and very far from the primary star and planet, the term "Tau Boötis Ab" is rarely to never used. However (in the cases of 16 Cygni Bb and 83 Leonis Bb), if the planet orbits a secondary star of the system, the star's name is always used. Some planets have received unofficial (informal) names that can be compared to the planets of the Solar system. The most noted planets that have been given names include: Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b). The International Astronomical Union (IAU) currently has no plans to officially name extrasolar planets, considering it impractical, but the idea may work if only a few planets get officially named (similar to how only a few stars have traditional names and always use it).
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
- Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
There have also been reports of free-floating planetary-mass objects (ones not orbiting any star), sometimes called "rogue planets" or "interstellar planets". Such objects are not discussed in this article since they are outside the working definition of "planet". For more information, see rogue planet.
Stars are composed mainly of the light elements hydrogen and helium. They also contain a small fraction of heavier elements such as iron, and this fraction is referred to as a star's metallicity. Stars of higher metallicity are much more likely to have planets, and the planets they have tend to be more massive than those of lower-metallicity stars.
In careful spectroscopic observations it is found that rotational velocity drops off abruptly after spectral class F2 stars. It should be noted that the Sun is a G2 Class star (which is, after F2.) Ninety eight percent of the angular momentum of the solar system derives from the orbital motions of the planets. In an isolated system, angular momentum must be conserved, so, of course, the remaining 2 percent lies with the sun. Therefore it seems that the angular momentum of the Sun has been transferred to the planets, that would otherwise cause the Sun to rotate 50 times faster than it currently does (approximately 2 km/sec.) If this hypothesis is correct, slowly rotating stars are so because a large portion of their angular momentum has been transferred elsewhere, perhaps to orbiting planets. Since ninety three percent of all main sequence stars are later than F2, it would seem that the bulk of stars in the galaxy may have planets, unless alternative methods of angular momentum transfer are proven likely.
Spectroscopic measurements during the transit can be used to study a transiting planet's atmospheric composition. Secondary transit (occurs when the planet is behind the star) can be used for direct detection of infrared radiation from the planet. In addition, infrared observations can be used to study heat patterns on the surface of a closely-orbiting planet.
The vast majority of exoplanets found so far have high masses. As of August 2008, all but twelve of them have more than ten times the mass of Earth. Many are considerably more massive than Jupiter, the most massive planet in the Solar System. However, these high masses are in large part due to an observational selection effect: all detection methods are much more likely to discover massive planets. This bias makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. In addition, the fact that astronomers have found several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common. According to 2008 data from the Harps (High Accuracy Radial velocity Planet Searcher) spectrograph instrument in Chile, about one star in 14 may have gas giant planets, while one in three probably has rocky planets of below 30 Earth masses.
Many exoplanets orbit much closer around their parent star than any planet in our own Solar System orbits around the Sun. Again, that is mainly an observational selection effect. The radial-velocity method is most sensitive to planets with such small orbits. Astronomers were initially very surprised by these "hot Jupiters," but it is now clear that most exoplanets (or at least, most high-mass exoplanets) have much larger orbits, some located in habitable zones where suitable for liquid water and life. It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in our own Solar System.
The eccentricity of an orbit is a measure of how elliptical (elongated) it is. Most known exoplanets have quite eccentric orbits. This is not an observational selection effect, since a planet can be detected about a star equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits. One possible theory is that small companions such as T dwarfs (methane-bearing brown dwarfs) can hide in such solar systems and can cause the orbits of planets to be extreme. This is also an indication that our own Solar System may be unusual, since all of its planets except for Mercury do follow basically circular orbits.
Many unanswered questions remain about the properties of exoplanets, such as the details of their composition and the likelihood of possessing moons. The recent discovery that several surveyed exoplanets lacked water showed that there is still much more to be learned about the properties of exoplanets. Another question is whether they might support life. Several planets do have orbits in their parent star's habitable zone, where it should be possible for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if these planets have large moons, the moons might be a more plausible abode of life. Detection of life (other than an advanced civilization) at interstellar distances, however, is a tremendously challenging technical task that will not be feasible for many years, even if such life is commonplace.
The first verified discovery of an exoplanet (51 Pegasi b) orbiting a main sequence star (51 Pegasi) was announced by Michel Mayor and Didier Queloz in Nature on October 6, 1995. Astronomers were initially surprised by this "hot Jupiter" but soon set out to find other similar planets with great success.
2003, PSR B1620-26 b: On July 10, using information obtained from the Hubble Space Telescope, a team of scientists led by Steinn Sigurdsson confirmed the oldest extrasolar planet yet. The planet is located in the globular star cluster M4, about 5,600 light years from Earth in the constellation Scorpius. This is the only planet known to orbit around a stellar binary; one of the stars in the binary is a pulsar and the other is a white dwarf. The planet has a mass twice that of Jupiter, and is estimated to be 13 billion years old.2004, Mu Arae c: In August, a planet orbiting Mu Arae with a mass of approximately 14 times that of the Earth was discovered with the European Southern Observatory's HARPS spectrograph. Depending on its composition, it is the first published "hot Neptune" or "super-Earth".
2004, 2M1207 b: The first planet around a brown dwarf. The planet is also the first to be directly imaged (in infrared). It has 5 Jupiter mass while other estimates give a slightly lower mass. It orbits at 55 AU from the brown dwarf. The brown dwarf mass is only 25 Jupiters. The temperature of gas giant planet is very hot (1250 K), mostly due to gravitational contraction. In late 2005, the parameters changed to 41 AU and has mass of 3.3 Jupiters as a result that the star is closer to Earth than it was originally expected. In 2006, the dust disk was found around 2M1207, providing evidence for a planet formation about the same as typical stars.2005, Gliese 876 d: In June, a third planet orbiting the red dwarf star Gliese 876 was announced. With a mass estimated at 7.5 times that of Earth, it is currently the second-lightest known exoplanet that orbits an ordinary main-sequence star. It may be rocky in composition. The planet orbits at 0.021 AU with a period of 1.94 days.2005, HD 149026 b: In July, a planet with the largest core known was announced. The planet, HD 149026 b, orbits the star HD 149026, and has a core that was then estimated to be 70 Earth masses (as of 2008, 80-110), accounting for at least two-thirds of the planet's mass.
2006, OGLE-2005-BLG-390Lb: On January 25, the discovery of OGLE-2005-BLG-390Lb was announced. This is the most distant and probably the coldest exoplanet found to date. It is believed that it orbits a red dwarf star around 21,500 light years from Earth, towards the center of the Milky Way galaxy. It was discovered using gravitational microlensing, and is estimated to have a mass of 5.5 times that of Earth, making it the least massive known exoplanet to orbit an ordinary main-sequence star. Prior to this discovery, the few known exoplanets with comparably low masses had only been discovered on orbits very close to their parent stars, but this planet is estimated to have a relatively wide separation of 2.6 AU from its parent star.2006, HD 69830: A planetary system with three Neptune-mass planets. It is the first triple planetary system around a Sun-like star without any Jupiter-like planets. All three planets were announced on May 18 by Lovis. All three orbit within 1 AU. The planets b, c, d have masses of 10, 12, and 18 Earths respectively. The outermost planet d appears to be in the habitable zone, sheparding the asteroid belt.
|First planet discovered||PSR B1257+12B, C||PSR B1257+12||1992|| first extrasolar planets discovered |
|First discovery by a method|
|First planet discovered using the pulsar timing method||PSR B1257+12B, C||PSR B1257+12||1992|
|First planet discovered by radial velocity method||51 Pegasi b||51 Pegasi||1995|
|First planet discovered by transit method||OGLE-TR-56b||OGLE-TR-56||2002|| |
|First planet found by gravitational lensing method||OGLE-2003-BLG-235Lb||OGLE-2003-BLG-235L/MOA-2003-BLG-53L||2004|
|First discovery by system type|
|First planet around a solitary star||PSR B1257+12 B, C||PSR B1257+12||1992|| first extrasolar planets discovered |
|First free-floating planet discovered||S Ori 70||n/a||2004|| has mass of 3 MJupiter, needs confirmation |
|First planet in a multiple star system discovered||55 Cancri b||55 Cancri||1996|| 55 Cnc has distant red dwarf companion |
|First planet orbiting multiple stars discovered||PSR B1620-26 b||PSR B1620-26||1993||orbits pulsar - white dwarf pair|
|First multiple planet system discovered||PSR 1257+12A, B, C||PSR 1257+12||1992||a pulsar planetary system|
|First planet in star cluster||PSR B1620-26 b||PSR B1620-26||1993||located in Globular Cluster M4|
|First discovery by star type|
|First pulsar planet discovered||PSR B1257+12 B, C||PSR B1257+12||1992|
|First known planet orbiting a Sun-like star||51 Pegasi b||51 Pegasi||1995|
|First known planet orbiting a red dwarf||Gliese 876b||Gliese 876||1998|
|First known planet orbiting a giant star||Iota Draconis b||Iota Draconis||2002|
|First known planet orbiting a white dwarf (confirmed 2003)||PSR B1620-26 b||PSR B1620-26||1993||in December 2007, GD 66b was discovered orbiting a solitary white dwarf star GD 66, but has not been confirmed|
|First known planet orbiting a brown dwarf||2M1207b||2M1207||2004||first directly imaged planet|
|First free-floating planet discovered||S Ori 70||n/a||2004|| has mass of 3 MJupiter, needs confirmation |
|Firsts by planet type|
|first cool, possibly rocky/icy planet around main-sequence star||OGLE-2005-BLG-390Lb||OGLE-2005-BLG-390L||2006|
|First transiting planet||HD 209458b||HD 209458||1999|| |
|First directly imaged planet||2M1207b||2M1207||2004||first planet found around brown dwarf|
|First imaged planet orbiting a 'normal' star||??||1RXS J160929.1-210524||Sep 2008||first planet orbiting a Sun-like star|