Encyclopedia
An
extrasolar planet, or
exoplanet, is a
planet that is beyond the
Solar System. As of September 2006, over 200 extrasolar planets have been discovered, all of them orbiting
stars other than the
Sun .
Known exoplanets are members of
planetary systems and thus belong to
star systems. There have also been unconfirmed reports of free-floating planetary-mass objects . Since the
International Astronomical Union has currently left open the question as to whether such objects fall within the
definition of "planet", and none are known, 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 fraction 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.
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 . 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. In 1993, the
Polish astronomer
Aleksander Wolszczan 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 . 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 September 15, 2006, 206 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
? 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.
Current methods of detection
Any planet is an extremely faint light source compared to its parent star. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. Therefore astronomers have generally had to resort to indirect methods to detect exoplanets.
At the present time, six different indirect methods have yielded success.
Astrometry
Astrometry is the oldest search method for extrasolar planets. It 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. Effectively, star and planet each orbit around their mutual center of mass , as explained by solutions to the two-body problem. Since the star is much more massive, its orbit will be much smaller.
During the fifties and sixties, claims were made for the discovery of planets around more than ten stars using this method. Astronomers now generally regard those claims as erroneous. Unfortunately, the changes in stellar position are so small that even the best ground-based telescopes cannot produce precise enough measurements. In 2002, however, the
Hubble Space Telescope did succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876.
Future space-based observatories such as NASA's
Space Interferometry Mission may succeed in uncovering large numbers of new planets via astrometry, but for the time being it remains a minor method of planetary detection.
One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades.
Radial velocity
Like the astrometric method, the radial-velocity method uses the fact that a star with a planet will move in its own small orbit in response to the planet's gravity. The goal now is to measure variations in the speed with which the star moves towards or away from Earth. In other words, the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's
spectral lines due to the
Doppler effect.
The velocity of the star around the barycenter is much smaller than that of the planet because the radius of its orbit around the center of mass is so small. Velocity variations down to 1 m/s can be detected with modern spectrometers, such as the HARPS spectrometer at the ESO 3.6 meter telescope in La Silla Observatory, Chile, or the HIRES spectrometer at the
Keck telescopes.
This has been by far the most productive technique used by planet hunters. It is also known as the "Doppler method" or "wobble method". The method is distance independent, but requires high
signal-to-noise ratios to achieve high precision, and so is generally only used for relatively nearby stars out to about 160 light-years from Earth. It easily finds massive planets that are close to stars, but detection of those orbiting at great distances requires many years of observation. Planets with orbits perpendicular to the line of sight from Earth produce smaller wobbles, and are thus more difficult to detect. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass. Usually the true mass will be within 20% of this minimum value, but if the planet's orbit is almost perpendicular to the line of sight, then the true mass will be much higher.
The radial-velocity method can be used to confirm findings made by using the transit method. When both methods are used in combination, then the planet's true mass can be estimated.
Pulsar timing
A
pulsar is a neutron star: the small, ultradense remnant of a star that has exploded as a
supernova. Pulsars emit radio waves extremely regularly as they rotate. Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit.
This method was not originally designed for the detection of planets. But it is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system, thereby revealing further information about those planets and their orbital parameters.
The main drawback of the pulsar-timing method is that pulsars are relatively rare, so it is unlikely that a large number of planets will be found this way. Also, life as we know it could not survive on planets orbiting pulsars since high-energy radiation there is extremely intense.
In 1992,
Aleksander Wolszczan used this method to discover planets around the pulsar
PSR 1257+12. Wolszczan's discovery was quickly confirmed. This was the first confirmation of planets outside our
Solar System.
The main advantage of the transit method is that when combined with the radial velocity method, one can determine the density of the planet, and hence learn something about the planet's physical structure. The nine planets that have been studied by both methods are by far the best-characterized of all known exoplanets.
The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the stellar spectrum carefully, one can detect elements present in the planet's atmosphere. Additionally, the secondary eclipse allows direct measurement of the planet's radiation. If the star's photometric intensity during the secondary eclipse is subtracted from its intensity before or after, only the signal caused by the planet remains. It is then possible to measure the planet's temperature and even to detect possible signs of cloud formations on it. In March 2005, two groups of scientists carried out measurements using this technique with the
Spitzer Space Telescope. 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
TrES-1 and
HD 209458b respectively. The measurements revealed the planets' temperatures: 1,060
K for TrES-1 and about 1,130 K for HD 209458b.
Gravitational microlensing
Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.
If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions 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.
In 1991, Polish astronomer Bohdan Paczynski of
Princeton University first proposed using gravitational microlensing to look for exoplanets. Successes with the method date back to 2002, when a group of Polish astronomers during project OGLE developed a workable technique. During one month they found several possible planets, though limitations in the observations prevented clear confirmation. Since then, four confirmed extrasolar planets have been detected using microlensing.
As of 2006 this is the only method capable of detecting planets of Earthlike mass around ordinary
main-sequence stars.
A notable disadvantage of the method 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 with other methods are usually impossible. However, if enough background stars can be observed with enough accuracy then the method should eventually reveal how common earth-like planets are in the galaxy.
Observations are usually performed using networks of robotic telescopes. In addition to the
NASA/
National Science Foundation-funded OGLE, the Microlensing Observations in Astrophysics group is working to perfect this approach.
The PLANET /RoboNet project is even more ambitious. It allows nearly continuous round-the-clock coverage by a world-spanning telescope network, providing the opportunity to pick up microlensing contributions from planets with masses as low as Earth. This strategy was successful in detecting the first low-mass planet on a wide orbit, designated
OGLE-2005-BLG-390Lb.
Circumstellar disks
Disks of space dust surround many stars. The dust can be detected because it absorbs ordinary starlight and re-emits it as
infrared radiation. Even if the dust particles have a total mass well less than that of Earth, they can still have a large enough total surface area that they outshine their parent star in infrared wavelengths.
The
Hubble Space Telescope is capable of observing dust disks with its NICMOS instrument. Even better images have now been taken by its sister instrument, the
Spitzer Space Telescope, which can see far deeper into
infrared wavelengths than the Hubble can. Dust disks have now been found around more than 15% of nearby sunlike stars.
The dust is believed to be generated by collisions among comets and asteroids. Radiation pressure from the star will push the dust particles away into interstellar space over a relatively short timescale. Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies like comets and
asteroids that orbit the parent star. For example, the dust disk around the star tau Ceti indicates that that star has a population of objects analogous to our own Solar System's
Kuiper Belt, but at least ten times thicker.
More speculatively, features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around epsilon Eridani, hinting at the presence of a planet with an orbital radius of around 40 AU .
Direct imaging
As mentioned previously, planets are extremely faint light sources compared to stars and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is impossible to detect them directly. In a few unusual cases, however, current telescopes may be capable of directly imaging planets. Specifically, this may be possible when the planet is especially large , widely separated from its parent star, and young .
In July 2004, a group of astronomers used the
European Southern Observatory's
Very Large Telescope array in Chile to produce an image of
2M1207b, a companion to the
brown dwarf 2M1207. In December 2005, the planetary status of the companion was confirmed.
The planet is believed to be several times more massive than
Jupiter and to have an orbital radius greater than 40 AU.
Three other possible exoplanets have now been directly imaged: GQ Lupi b, AB Pictoris b, and SCR 1845 b. As of March 2006 none have been confirmed as planets; instead, they might themselves be small
brown dwarfs.
Future methods of detection
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.
The
European Space Agency's COROT satellite and NASA's
Kepler Space Observatory will both use the transit method. COROT will be sensitive enough to detect planets slightly larger than Earth, while Kepler should be capable of detecting planets even smaller than Earth.
Kepler should also be able to detect the reflected light from giant planets in close orbits, even though it will not be able to resolve that light into an image. The amount of light reflected from such a planet will vary over time because, like the
Moon, it goes through phases from full to new and back again. The variation, although small, will be the signature of a planet. The phase function of the giant planet may be constrained, which will lead to constraints on the actual particle size distribution of its atmospheric particles. This reflected-light method may actually provide the greatest number of planets to be discovered by the Kepler satellite.
NASA's
Space Interferometry Mission, currently scheduled for launch in 2014, will use astrometry. It may be able to detect Earth-like planets around several nearby stars. The
European Space Agency's Darwin probe and
NASA's
Terrestrial Planet Finder probes will attempt to image planets directly. A recently proposed idea is the
New Worlds Imager, which will use an occulter to block a star's light, alllowing astronomers to directly observe the dimmer orbiting planets.
Nomenclature
A lower case letter is placed after the star name, starting with "b" for the first planet found in the system , with the next planet being for example "51 Pegasi c", then "51 Pegasi d"...
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."
General properties of exoplanets
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 are either less likely to have planets or have planets that are themselves of lower mass.
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 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 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.
The
eccentricity of an orbit is a measure of how elliptical it is. Most known exoplanets have quite eccentric orbits. This is
not an observational selection effect, since a planet can be detected about 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 orbits. 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 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
There have been a number of milestones in the discovery of extrasolar planets, beginning in 1992, when Wolszczan and Frail published results in
Nature;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 certain 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 is about 149 light years away. The planet orbits the main star 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 could ultimately provide insight on how planets form. According to Robert Noyes of the Harvard-Smithsonian Center for Astrophysics , "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."
See also
References
External links
;Search projects:
;Resources:
- Andrew Collier Cameron, Extrasolar planets, Physics World .
- - D. Montes, UCM
;News:
- from space.com
- from space.com
- from space.com
- from news.bbc.co.uk
