Extrasolar planet

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An extrasolar planet, or exoplanet, is a planet beyond the Solar System. As of April 2007, the count of known exoplanets stands at 218. The vast majority have been detected through various indirect methods rather than actual imaging.^[1] Most of them are giant planets likely to resemble Jupiter more than Earth.

Known exoplanets are members of planetary systems that orbit a star. There have also been unconfirmed reports of free-floating planetary-mass objects (sometimes called "rogue planets"): 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.^[2] For more information, see interstellar planet.

False-color infrared image of the brown dwarf 2M1207 (blue) and its planetary companion 2M1207b (red), as viewed by the Very Large Telescope. As of September 2006 this is the only confirmed extrasolar planet to have been directly imaged.

Extrasolar planets became a subject of scientific investigation in the mid-nineteenth century. 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.^[3] The discovery of extrasolar planets further raises the question of whether some might support extraterrestrial life.^[4]

[edit] History of detection

Claims about the detection of exoplanets have been made for over a 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.

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.^[5] 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. It was mainly 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 that are intermediate in mass between planets and stars.

The following year, additional observations were published that supported the reality of the planet orbiting Gamma Cephei,^[6] though subsequent work in 1992 raised serious doubts.^[7] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.^[8]

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.^[9] The claim briefly received intense attention, but Lyne and his team soon retracted it.^[10]

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)

In early 1992, radio astronomers Aleksander Wolszczan (Polish) and Dale Frail (Canadian) announced the discovery of planets around another pulsar, PSR 1257+12.^[11] 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).^[12] 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, 212 exoplanets have been found,^[1] 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 and R. Paul Butler 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.

[edit] Detection methods

Main article: Methods of detecting extrasolar planets

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 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 indirect methods have yielded success:

Diagram showing how a smaller object orbiting a larger could produce changes in position and velocity of the latter as they orbit their common center of mass.

• Astrometry: Astrometry consists of precisely measuring a star's position in the sky and observing the ways in which 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 about their common center of mass.

• Radial velocity: This is also known as the "Doppler method". Variations in the speed with which the star moves towards or away from Earth — that is, 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: A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. 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 (or transits) in front of its parent star's disk, then the observed visual brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet.

• Gravitational microlensing: 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, and this dust 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.

In 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.

[edit] Nomenclature

A lower-case letter is placed after the star name, starting with "b" for the first planet found in the system (for example, 51 Pegasi b). The next planet is labeled, for example, as "51 Pegasi c", the one following that "51 Pegasi d", and so on. (The letter "a" is not used because it might be interpreted as referring to the star itself.)

Note that the letters assigned are based on the order in which the planets are discovered, and not on their position. 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 or Gliese 876 c.

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 have unofficial nicknames, as well. For example, HD 209458 b is unofficially called "Osiris", and 51 Pegasi b is called "Bellerophon". The IAU currently has no plans to officially name extrasolar planets, considering it impractical.^[13]

[edit] 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.

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 this 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.^[14] Recent observations by the Spitzer Space Telescope indicate that stars of spectral category O, which are much hotter than our Sun, produce a photo-evaporation effect that inhibits planetary formation.^[15]

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.^[3]

The vast majority of exoplanets found so far have high masses. All but two of them have more than ten 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. 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.^[3]

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.^[16] A few of the smallest known exoplanets are suspected to be rocky, like the Earth and the other inner planets of our Solar System.^[17]

Large planets form soon after star formation. A gas giant such as Saturn or Jupiter typically takes 3 to 30 million years to become a fully fledged planet. Small planets such as the Earth take hundreds of millions of years to form.^[18]

A planetary magnetosphere provides protection from stellar wind. In order for a planet to have a magnetosphere it must have a liquid metal core and be rotating. Planets without a magnetosphere steadily loose their atmosphere to stellar wind. On smaller planets such as Mars the liquid core quickly freezes. Larger planets retain their liquid cores for a longer period. Moons can be protected from the stellar wind by the magnetosphere of the planet they orbit. Titan for example is partially protected by Saturns magnetosphere. Additionally despite its size Titan still retains a liquid core due to gravitational compression provided by its proximity to Saturn. ^[19] Observations from the Hubble Space Telescope reveal what is most likely to be a magnetic field around an extrasolar planet. The planet is a gas giant orbitting the star HD 179949. It is currently thought that most gas giants will possess a magnetic field.^[20]

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.

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.^[21] This is also an indication that our own Solar System may be unusual, since all of its planets do follow basically circular orbits.^[3]

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.^[22] 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. All 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.

[edit] 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

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 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.

The first milestone in the discovery of extrasolar planets was in 1992, when Wolszczan and Frail published results in the journal Nature indicating that pulsar planets existed around PSR B1257+12.^[11] 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.^[12] 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.^[23]

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.^[24]

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.^[25]

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 European Southern Observatory's 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.^[17] Furthermore, a planet was discovered using the transit method with the smallest aperture telescope to date of four 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.^[26]

2005, HD 149026 b

In July, a planet with the largest core ever discovered was announced. The planet, HD 149026 b, orbits the star HD 149026, and has a core that is estimated to be 70 Earth masses, accounting for two-thirds of the planet's mass.^[27]

2005, HD 188753 Ab

In July, the astronomer Maciej Konacki claimed to have discovered of a roughly Jupiter-mass planet in a relatively tight triple star system. The stellar trio is about 149 light years away from Earth. This discovery was thought to raise a challenge to theories about planetary formation, since the presence of so many stars so close together would probably have disrupted the sort of protoplanetary disk that is believed to give rise to giant planets.^[28][29] However, in 2007 a team of astronomers strongly challenged Konacki's conclusion, saying that they saw no sign of the planet even though their instruments should have been easily capable of detecting it.^[30] Konacki has defended his work and intends to carry out further observations.^[31]

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.^[32][33]

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 mass, making it the least dense planet on record (about one quarter that of water). 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 Center 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."^[34]

2006, SWEEPS-10

The planet candidate with the shortest orbital period yet found, named SWEEPS-10 (SWEEPS stands for Sagittarius Window Eclipsing Extrasolar Planet Search), completes an orbit of its star in just 10 hours. Located only 1.2 million kilometers from its star (roughly three times the distance between the Earth and the Moon), the planet is among the hottest ever detected; its estimated temperature is approximately 1650 degrees Celsius. "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, Maryland. Such ultra-short period planets (USPPs) seem to occur only around dwarf stars. The small star's relatively low temperature allows the planet to exist. "USPPs occur preferentially around normal red dwarf stars that are smaller and cooler than our Sun," Sahu said.^[35]

2007, HD 209458 b and HD 189733b

On February 21, 2007, NASA and Nature released news that HD 209458 b and HD 189733 b were the first two extrasolar planets to have their spectra directly observed.[1][2] This was long seen as the first mechanism by which extrasolar but non-sentient life forms could be searched for, by way of influence on a planet's atmosphere. A group of investigators led by Dr. Jeremy Richardson of NASA's Goddard Space Flight Center were first to publication, in the February 22 issue of Nature. Richardson et al. spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2 microns. The results defied theoretical expectations in several ways. The spectrum had been predicted to have a peak at 10 microns which would have indicated water vapor in the atmosphere, but such a peak was absent, indicating no detectable water vapor. Another, unpredicted peak was observed at 9.65 microns, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another unpredicted peak occurred at 7.78 microns, which the investigators did not have an explanation for. A separate team led by Mark Swain of the Jet Propulsion Laboratory also observed HD 209458 b's spectrum, and indicated that their findings were similar. They had submitted their results to Astrophysical Journal Letters. A team led by Carl Grillmair of NASA's Spitzer Science Center made the observations of HD 189733 b, and their results were pending publication in Astrophysical Journal Letters at the time of the news release.

[edit] See also

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At Wikiversity, you can learn about:

Observational astronomy/Extrasolar planet

• Appearance of extrasolar planets
• Binary star
• COROT — current ESA mission to detect extrasolar planets
• Geoffrey Marcy
• R. Paul Butler — co-discoverer with Geoffrey Marcy of more exoplanets than anyone else
• Distant Wanderers
• Debra Fischer — co-discoverer with Geoffrey Marcy and R. Paul Butler of more exoplanets than anyone else
• Hypothetical planet
• Interstellar planet
• List of stars with confirmed extrasolar planets
• List of extrasolar planet extremes
• List of unconfirmed exoplanets
• New Worlds Imager
• Planetary habitability
• Planetary system
• Pulsar planet
• Systemic (amateur extrasolar planet search project)
• Extrasolar moon

[edit] References

[edit] External links

Search projects

• University of California Planet Search Project
• The Geneva Extrasolar Planet Search Programmes
• PlanetQuest distributed computing project
• SuperWASP Wide Angle Search for Planets

Resources

• NASA's PlanetQuest
  • PlanetQuest 3D Atlas of extrasolar planets within 400 light years of our Solar System
  • Planetquest Flash
• German Center for Exo-Planet Research Jena/Tautenburg
• Astrophysical Institute & University Observatory Jena (AIU)
• The Extrasolar Planets Encyclopedia
• Table of known planetary systems
• Extrasolar Planet XML Database
• Andrew Collier Cameron, Extrasolar planets, Physics World (January 2001). (See the online version.)
• searchable dynamic database of extrasolar planets and their parent stars
• List of important exoplanets
• Extrasolar Planets - D. Montes, UCM
• Extrasolar Visions

News

• Exoplanets Exhibit at the American Museum of Natural History in New York City
• 6-8 Earth-Mass Planet Discovered orbiting Gliese 876
• Newfound World Shatters Distance Record from space.com
• Oldest Known World from space.com
• Earth Sized Planets Confirmed from space.com
• Sunshade to Look for Distant Life from news.bbc.co.uk