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An extrasolar planet, or exoplanet, is a planet which orbits a star other than the Sun, and therefore belongs to a planetary system other than the solar system. As of June 2006, nearly 200 extrasolar planets have been discovered (see List of extrasolar planets).

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 our own 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 possess a planet, 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. One of the earliest was made by Thomas Jefferson Jackson See at the United States Naval Observatory in the 1890s. See did not explicitly state he had found a planet in the 70 Ophiuchi system, a well known binary star. He instead submitted to the Astronomical Journal his measurements of orbital anomalies which suggested an object of planetary mass around one of the stars, with a 36 year period. Forest Ray Moulton published a paper proving that such a three-body system would be highly unstable. During the 1950s and 1960s, another prominent series of detection claims, this time around Barnard's Star, were made by astronomer Peter van de Kamp. However, all these early "detections" are now generally regarded 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 the presence of a planet orbiting the star Gamma Cephei (also known as Alrai). They remained extremely 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 1989, observations were published that described radial-velocity variations in the star HD 114762. At the time, it was unclear whether those variations were due to a planet or a brown dwarf. It is now thought that a brown dwarf is responsible, or perhaps even an ordinary low-mass stellar companion.

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 (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 discoveries. In the years immediately following, exoplanets began to be discovered in large numbers. Improvements in telescope technology, such as CCD and computer-based image processing, made the search much more feasible . These advances allowed for more accurate measurements of stellar motion, allowing astronomers to detect planets, not visually (the luminosity of a planet is generally too low for such detection), but by measuring gravitational influences upon stars (see astrometrics and radial velocity). Extrasolar planets were also be detected by measuring the variation in a star's apparent luminosity as a planet passes in front of it (see eclipse).

As of June 21, 2006, 194 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 4 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.

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 orbital motion. Whenever any object orbits a pulsar, the pulsar itself will move in its own orbit in response, and calculations based on pulse-timing observations can reveal the parameters of both orbits.

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 own solar system.

The same method led to the discovery of the oldest known planet, by Steinn Sigurdsson's team, around PSR B1620-26's binary stellar core. This is the only known planet to orbit two stars.

Astrometry

] Astrometry is the oldest search method for extrasolar planets, used as early as 1943. 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 (barycenter), 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.

To date, no planets have been discovered using this method. While astrometry is able to detect smaller mass planets at greater distance from the host star, this requires very long observation time - 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 component along the line of sight of 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 (High Accuracy Radial Velocity Planet Searcher) 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 occasionally the true mass may 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.

Gravitational microlensing

The gravitational microlensing effect occurs when the gravitational field of a planet and its parent star act to magnify the light of a distant background star. For the effect to work the planet and star must pass almost directly between the observer and the distant star. Since such events are rare, a very large number of distant stars must be continuously monitored in order to detect planets at a reasonable rate. This method is most fruitful for planets between earth and the center of the galaxy, as the galactic center provides a large number of background stars.

In 1991, Polish astronomer Bohdan Paczyński 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 (Andrzej Udalski, Marcin Kubiak and Michał Szymański from Warsaw, and Bohdan Paczyński) during project OGLE (the Optical Gravitational Lensing Experiment) perfected a workable technique. During one month they claimed to find objects, many of which could be planets. Since then, four extrasolar planets have been detected using microlensing, and this technique is viewed as one of the most promising methods for finding Earth-mass planets around sun-like stars.

Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than 1,000 stars have been detected in microlensing relationships over the past ten years. Observations are usually performed using networks of robotic telescopes.

The key advantage of gravitational microlensing is that it allows low mass (i.e. Earth-mass) planets to be detected using available technology. A notable disadvantage is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. However, if enough background stars can be observed with enough accuracy then the method can be used to determine how common earth-like planets are in the galaxy.

In addition to the NASA/National Science Foundation-funded OGLE, the Microlensing Observations in Astrophysics (MOA) group is working to perfect this technique.

Even more ambitious, microlensing observations with a world-spanning telscope network as carried out by the PLANET (Probing Lensing Anomalies NETwork)/RoboNet campaign allow nearly-continuous round-the-clock coverage providing the opportunity to pick up and follow signals 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. As of 2006 this is the only technique capable of detecting Earth-sized planets.

Transit method

While the aforementioned methods allow the determination of a planet's mass, this method can be used to measure the radius of a planet. 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. For example, in the case of HD 209458, the star dims 1.7%.

This method has two major disadvantages. First of all, planetary transits are only observable for the small percentage of planets whose orbits happen to be perfectly aligned from astronomers' vantage point. Such alignment is especially unlikely for planets with large orbits.

Secondly, the method suffers from a high rate of false detections. At least at present, a transit detection requires confirmation from some other method.

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 (when the planet is blocked by its star) 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 HD 209458b and TrES-1. The measurements revealed the planets' temperatures: 1,060 K (1,450ºF) for TrES-1 and about 1,130 K (1,570ºF) for HD 209458b.

Circumstellar disks

An even newer approach is the study of circumstellar disks. Many solar systems contain a significant amount of space dust that is present due to frequent dust generation activity such as comets, asteroid and planetary collisions. This dust forms as a disc around a star and absorbs regular star light and re-emits it as infrared radiation. These dust clouds can provide invaluable information through studies of their density and distortion, caused either by an orbiting planet "catching" the dust, or distortion due to gravitational influences of orbiting planets.

Unfortunately this method can only be employed by space-based observations because our atmosphere absorbs most infrared radiation, making ground based observation impossible. Our own solar system contains enough dust to make up about 1/10th the mass of our moon. Although its mass is negligible, its surface area is so great that at a distance, its infrared emissions would outshine all our planets by a factor of 100.

The Hubble Space Telescope is capable of these observations using its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument, but was unable to do so due to a cooling unit malfunction that left NICMOS inoperative between 1999 and 2002. Even better images were then taken by its sister instrument, the Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility), in 2003. The Spitzer Telescope was designed specifically for use in the infrared range and probes far deeper into the spectrum than the Hubble Space Telescope can.

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 (considerably larger than Jupiter), widely separated from its parent star, and young (so that it is hot and emits intense infrared radiation).

In early 2005, two groups, both using the European Southern Observatory's Very Large Telescope array in Chile announced direct infrared images of extrasolar planets: GQ Lupi b and 2M1207b. Both planets are believed to be several times the mass of Jupiter and orbit at distances greater than 50 AU from their primary star. As of May 2005, their status as planetary objects (as opposed to being small brown dwarfs) has not been firmly established.

Possible 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. NASA's Kepler Space Observatory and the European Space Agency's COROT satellite will both use the transit method. NASA's Space Interferometry Mission will use astrometry. The European Space Agency's Darwin probe and NASA's Terrestrial Planet Finder* probes will attempt to image planets directly.

(On 2006-02-06 NASA announced an indefinite suspension of work on the Terrestrial Planet Finder due to budget problems. Then in June 2006, the Appropriations Committe of the U.S. House of Representatives partially restored funding, permitting development work on the project to continue at least through 2007. COROT remains on track for a launch in October 2006 and Kepler's launch is scheduled for 2008.)

Eclipsing binary minima timing

When a double star system is aligned such that the stars pass in front of each other in their orbits, the system is called an "eclipsing binary" star system. The time of minimum light, when the star with the brigher surface area is at least partially obscured by the disc of the other star, is called the primary eclipse, and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. These times of minimum light, or central eclipse, constitute a time stamp on the system, much like the pulses from a pulsar (except that rather than a flash, they are a dip in the brightness). If there is a planet in circum-binary orbit around the binary stars, the stars will be offset around a binary-planet barycenter. As the stars in the binary are displaced by the planet back and forth, the times of the eclipse minima will vary; they will be too late, on time, too early, on time, too late, etc.. The periodicity of this offset may be the most reliable way to detect extrasolar planets around close binary systems. "Bioastronomy 2002: Life Among the Stars" IAU Symposium 213, R.P Norris and F.H. Stootman (eds), A.S.P., San Francisco, CA, 80-84.Doyle, Laurance R., Hans-Jorg Deeg, J.M. Jenkins, J. Schneider, Z. Ninkov, R. P.S. Stone, J.E. Blue, H. Götzger, B, Friedman, and M.F. Doyle (1998). "Detectability of Jupiter-to-brown-dwarf-mass companions around small eclipsing binary systems". Brown Dwarfs and Extrasolar Planets, A.S.P. Conference Proceedings, in Brown Dwarfs and Extrasolar Planets, R. Rebolo, E. L. Martin, and M.R.Z. Osorio (eds.), A.S.P. Conference Series 134, San Francisco, California, 224-231..

Orbital phase reflected light variations

Short period giant planets in close orbits around their stars will undergo reflected light variations changes because, like the Moon, they will go through phases from full to new and back again. Although the effect is small — the photometric precision required is about the same as to detect an Earth-sized planet in transit acorss a solar-type star — such Jupiter-sized planets should be detectable by space telescopes such as the Kepler Space Observatory. This method may actually constitute the most planets that will be discovered by that mission because the reflected light variation with orbital phase is largely independent of orbital inclination of the planet's orbit. In addition, the phase function of the giant planet may be constrained which will, in turn, lead to constraints on the actual particle size distribution of the atmospheric particles.

Naming

A lower case letter is placed after the star name, starting with "b" instead of "a" (which usually stands for the star) 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"...

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.

General properties of exoplanets

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 plausible that in most exoplanetary systems, the largest planets are comparable in size to Jupiter or Saturn. 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.

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. 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 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. This is also an indication that our own Solar System may be unusual, since almost all of its planets do all follow basically circular orbits. (The only two planets in our own Solar System that do have substantial eccentricities, Pluto and 2003 UB313, are sometimes considered to be Kuiper-belt objects rather than genuine planets.)

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 accomplished for many years.

Notable extrasolar planets

  • In 1992, 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.

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Extrasolar planet".

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