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Extrasolar planet

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   An extrasolar planet, or exoplanet, is a planet beyond the Solar
   System. As of 11 November 2006, 209 extrasolar planets have been
   discovered (see list of stars with known extrasolar planets).

   Known exoplanets are members of planetary systems that orbit a star.
   There have also been unconfirmed reports of free-floating
   planetary-mass objects (that is, ones that do not orbit any star).
   Since such objects do not satisfy the working definition of "planet"
   adopted by the International Astronomical Union, and since their
   existence remains unconfirmed, they will not be discussed in this
   article. For more information, see interstellar planet.

   For centuries, extrasolar planets were a subject of speculation.
   Astronomers generally supposed that some existed, but it was a mystery
   how common they were and how similar they were to the planets of the
   Solar System. The first confirmed detections were finally made in the
   1990s. Since 2002, more than twenty have been discovered every year. It
   is now estimated that at least 10% of sunlike stars have planets, and
   the true proportion may be much higher. The discovery of extrasolar
   planets raises the question of whether some might support
   extraterrestrial life.

History of detection

   Claims have been made for the detection of exoplanets going back many
   decades. Some of the earliest involve the binary star 70 Ophiuchi. In
   1855 Capt. W. S. Jacob, working at the Madras Observatory of the East
   India Company 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 claimed 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. But shortly afterward Forest Ray Moulton 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 these early "detections" as erroneous.
   Our solar system compared with the system of 55 Cancri
   Enlarge
   Our solar system compared with the system of 55 Cancri

   The first published discovery to have received subsequent confirmation
   was made in 1988 by the Canadian astronomers Bruce Campbell, G.A.H
   Walker, and S. Yang. Their radial-velocity observations suggested that
   a planet orbited the star Gamma Cephei (also known as Alrai). They
   remained cautious about claiming a true planetary detection, and
   widespread skepticism persisted in the astronomical community for
   several years about this and other similar observations. Mainly that
   was because the observations were at the very limits of instrumental
   capabilities at the time. Another source of confusion was that some of
   the possible planets might instead have been brown dwarfs, objects
   intermediate in mass between planets and stars.

   The following year, additional observations were published that
   supported the reality of the planet orbiting Gamma Cephei. But
   subsequent work in 1992 raised serious doubts. Finally, in 2003,
   improved techniques allowed the planet's existence to be confirmed.

   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.
   Our inner solar system superimposed behind the orbits of the planets HD
   179949 b, HD 164427 b, Epsilon Reticuli ab, and Mu Arae b (all parent
   stars are in the center)
   Enlarge
   Our inner solar system superimposed behind the orbits of the planets HD
   179949 b, HD 164427 b, Epsilon Reticuli ab, and Mu Arae b (all parent
   stars are in the centre)

   In early 1992, the Polish astronomer Aleksander Wolszczan (with 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 spiralled in to 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
   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.

   As of October 9, 2006, 210 exoplanets have been found, including a few
   that were confirmations of controversial claims from the late 1980s.
   Many of these discoveries were made 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. 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.

Detection methods

   Planets are extremely faint light sources compared to their parent
   stars. In 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 young (so that it
   is hot and emits intense infrared radiation).

   The vast majority of known extrasolar planets have been discovered
   through indirect methods. At the present time, six different indirect
   methods have yielded success:
   This diagram shows how a smaller object orbiting a larger produces
   changes in the position and velocity of the latter.
   Enlarge
   This diagram shows how a smaller object orbiting a larger produces
   changes in the position and velocity of the latter.
     * Astrometry Consists of precisely measuring a star's position in the
       sky and observing how that position changes over time. If the star
       has a planet, then the gravitational influence of the planet will
       cause the star itself to move in a tiny circular or elliptical
       orbit.

     * Radial velocity Also known as the "Doppler method" or "wobble
       method". Variations in the speed with which the star moves towards
       or away from Earth — i.e. variations in the radial velocity of the
       star with respect to Earth — can be deduced from the displacement
       in the parent star's spectral lines due to the Doppler effect. This
       has been by far the most productive technique used by planet
       hunters.

     * Pulsar timing Pulsars (the small, ultradense remnant of a star that
       has exploded as a supernova) emit radio waves extremely regularly
       as they rotate. Slight anomalies in the timing of its observed
       radio pulses can be used to track changes in the pulsar's motion
       caused by the presence of planets.

     * Transit method If a planet crosses ( transits) in front of its
       parent star's disk, then the observed visual brightness of the star
       drops a small amount. The amount the star dims depends on its size
       and on the size of the planet.

     * Gravitational microlensing Occurs when the gravitational field of a
       star acts like a lens, magnifying the light of a distant background
       star. If the foreground lensing star has a planet, then that
       planet's own gravitational field can make a detectable contribution
       to the lensing effect.

     * Circumstellar disks Disks of space dust surround many stars, which
       can be detected because it absorbs ordinary starlight and re-emits
       it as infrared radiation. Features in dust disks sometimes suggest
       the presence of full-sized planets.

   For the future, several space missions are planned that will employ
   already proven planet-detection methods. Astronomical measurements done
   from space can be more sensitive than measurements done from the
   ground, since the distorting effect of the Earth's atmosphere is
   removed, and the instruments can view in infrared wavelengths that do
   not penetrate the atmosphere. Some of these space probes should be
   capable of detecting planets similar to our own Earth. Huge proposed
   ground telescopes may also be able to directly image extrasolar
   planets.

Nomenclature

   A lower case letter is placed after the star name, starting with "b"
   for the first planet found in the system (e.g. 51 Pegasi b), with the
   next planet being for example "51 Pegasi c", then "51 Pegasi d"... (The
   letter "a" is not used because it might be interpreted as referring to
   the star itself.)

   Planet naming conventions are based on discovery date - for example,
   the first planet detected will be designated with the letter "b." Any
   additional planets will be given additional letters regardless of
   position. A real world example is the Gliese 876 system: that latest
   discovered planet is Gliese876d, which is the closest orbiting planet.

   Before the discovery of 51 Pegasi b in 1995, extrasolar planets were
   named differently. The first extrasolar planets found around pulsar PSR
   1257+12 were named with capital letters: PSR 1257+12 B and PSR 1257+12
   C. When a new, closer-in exoplanet was found around the pulsar, it was
   named PSR 1257+12 A, not D.

   Several extrasolar planets also have unofficial nicknames. For example,
   HD 209458 b is unofficially called "Osiris", and 51 Pegasi b is called
   "Bellerophon."

General properties of exoplanets

   All extrasolar planets discovered by radial velocity (blue dots),
   transit (red) and microlensing (yellow) to 31 August 2004. Also shows
   detection limits of forthcoming space- and ground-based instruments.
   Enlarge
   All extrasolar planets discovered by radial velocity (blue dots),
   transit (red) and microlensing (yellow) to 31 August 2004. Also shows
   detection limits of forthcoming space- and ground-based instruments.

   Most known exoplanets orbit stars roughly similar to our own Sun—that
   is, main-sequence stars of spectral categories F, G, or K. One reason
   is simply that planet search programs have tended to concentrate on
   such stars. But even after taking that into account, statistical
   analysis suggests that lower-mass stars (red dwarfs, of spectral
   category M) are either less likely to have planets or have planets that
   are themselves of lower mass and hence harder to detect. Recent
   observations by the Spitzer Space Telescope indicate that planetary
   formation does not occur within the vicinity of an O class star due to
   the photo-evaporation effect.

   All stars are composed mainly of the light elements hydrogen and
   helium. They also contain a small fraction of heavier elements such as
   iron; astronomers refer to that fraction 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.

   The vast majority of exoplanets found so far have high masses. Ninety
   percent of them have more than 10 times the mass of Earth. Many are
   considerably more massive than Jupiter, our own Solar System's largest
   planet. However, these high masses are in large part an observational
   selection effect: All detection methods are much more likely to
   discover massive planets. That observational selection effect 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. Also, 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.

   It is believed that the vast majority of known exoplanets are in
   substantial part gaseous, like the giant planets of our own Solar
   System. That has only been confirmed, however, for the exoplanets that
   have been studied with the transit method. A few of the smallest known
   exoplanets are suspected to be rocky, like Earth and the other inner
   planets of our Solar System.

   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. 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.
   This planetary habitability chart shows where life might exist on
   extrasolar planets based on our own solar system and life on Earth.
   Enlarge
   This planetary habitability chart shows where life might exist on
   extrasolar planets based on our own solar system and life on Earth.

   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 how eccentric its orbit is. The
   prevalence of elliptical orbits is a major puzzle, since current
   theories of planetary formation strongly suggest planets should form
   with circular (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 do follow basically circular orbits.

   Many unanswered questions remain about the properties of exoplanets,
   such as details of their composition and how likely they are to have
   moons. One of the most intriguing questions about them 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. All of those planets are giant planets more
   similar to Jupiter than to Earth, so if they have large moons perhaps
   those would be the most 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.

Notable extrasolar planets

   Artist's impression from 2005 of the planet HD 69830 d, with the star
   HD 69830's asteroid belt in the background
   Enlarge
   Artist's impression from 2005 of the planet HD 69830 d, with the star
   HD 69830's asteroid belt in the background
   Artist's impression of the pulsar planet PSR B1620-26c (discovered in
   2003); it is over 12.5 billion years old, making it the oldest known
   extrasolar planet
   Enlarge
   Artist's impression of the pulsar planet PSR B1620-26c (discovered in
   2003); it is over 12.5 billion years old, making it the oldest known
   extrasolar planet
   Artist's impression of a triple sunset on a conjectural moon orbiting
   HD 188753 Ab.
   Enlarge
   Artist's impression of a triple sunset on a conjectural moon orbiting
   HD 188753 Ab.
   Artist's conception of the planet OGLE-2005-BLG-390Lb (with surface
   temperature of −220°C), orbiting its star 20,000 light years (117.5
   quadrillion miles) from Earth; this planet was discovered with
   gravitational microlensing
   Enlarge
   Artist's conception of the planet OGLE-2005-BLG-390Lb (with surface
   temperature of −220°C), orbiting its star 20,000 light years (117.5
   quadrillion miles) from Earth; this planet was discovered with
   gravitational microlensing

   There have been a number of milestones in the discovery of extrasolar
   planets, beginning in 1992, when Wolszczan and Frail published results
   in Natureindicating that pulsar planets existed around PSR B1257+12.
   Wolszczan had discovered the millisecond pulsar in question in 1990 at
   the Arecibo radio observatory. These were the first exoplanets ever
   verified, and they are still considered highly unusual in that they
   orbit a pulsar.

   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.

   Since that time, other notable discoveries have included:

   1999, HD 209458 b
          This exoplanet, originally discovered with the radial-velocity
          method, became the first exoplanet to be seen transiting its
          parent star. The transit detection conclusively showed that the
          radial velocity measurements suspected to be planets actually
          were planets.

   2001, HD 209458 b
          Astronomers using the Hubble Space Telescope announced that they
          had detected the atmosphere of HD 209458 b. They found the
          spectroscopic signature of sodium in the atmosphere, but at a
          smaller intensity than expected, suggesting that high clouds
          obscure the lower atmospheric layers.

   2003, PSR B1620-26c
          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 d and TrES-1
          In August, 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 third 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. Further, 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.

   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 must almost certainly be
          rocky in composition. It orbits at 0.021 AU with a period of
          1.94 days.

   2005, HD 149026 b
          In July a planet with the largest core ever was announced. The
          planet, HD 149026 b orbits the star HD 149026, has a core that
          is estimated to be 70 Earth masses, accounting for two thirds of
          the planet's mass.

   2005, HD 188753 Ab
          In July, astronomers announced the discovery of a planet in a
          relatively tight triple star system, a finding that challenges
          current theories of planetary formation. The planet, a gas giant
          slightly larger than Jupiter, orbits the main star of the HD
          188753 system, in the constellation Cygnus, and is hence known
          as HD 188753 Ab. The stellar trio (yellow, orange, and red) is
          about 149 light years away from Earth. The planet orbits the
          main star (HD 188753 A) about once every 3.3 days, at a distance
          of about a twentieth the distance between Earth and the Sun. The
          other two stars whirl tightly around each other in 156 days, and
          circle the main star every 25.7 years at a distance from the
          main star that would put them between Saturn and Uranus in our
          own Solar system. The latter stars call into question the
          leading hot Jupiter formation theory, which holds that these
          planets form at "normal" distances and then migrate inward
          through some debatable mechanism. Such migration could not have
          occurred here, since the outer star pair would have disrupted
          outer planet formation.

   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 yet found. It is believed to orbit a red dwarf star
          around 21,500 light years away, towards the centre of our
          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, HAT-P-1b
          Using a network of small automated telescopes known as HAT,
          Smithsonian astronomers discovered a planet, since designated
          HAT-P-1b, that orbits one member of a pair of distant stars 450
          light-years away in the constellation Lacerta. The planet has a
          radius 1.38 times that of Jupiter, but one-half the density,
          making it the least dense planet on record. It remains unclear
          how such a planet could evolve, and it is believed this object
          and HD 209458 b (also a low-density giant planet) could
          ultimately provide insight on how planets form. According to
          Robert Noyes of the Harvard-Smithsonian Centre for Astrophysics
          (CfA), "We can't dismiss HD 209458 b as a fluke. This new
          discovery suggests something could be missing in our theories of
          how planets form."

   2006, SWEEPS-10
          The planet candidate with the shortest orbital period yet found,
          named SWEEPS-10, completes a full orbit of its star in just 10
          hours. Located only 1.2 million kilometres from its star
          (roughly three times the distance between the Earth and the
          Moon), the planet is among the hottest ever detected, with an
          estimated temperature of about 1650 degrees C. "This
          star-hugging planet must be at least 1.6 times the mass of
          Jupiter, otherwise the star’s gravitational muscle would pull
          the planet apart," said team leader Kailash Sahu of the Space
          Telescope Science Institute in Baltimore, USA. Such ultra-short
          period planets (USPPs) seem to occur only around dwarf stars.
          The smaller star’s relatively lower temperature allows the
          planet to exist. "USPPs occur preferentially around normal red
          dwarf stars that are smaller and cooler than our Sun," Sahu
          said.

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