The term extrasolar planets or exoplanets stands for planets outside our solar system—that is, planets that orbit other stars, not our sun. Planets in our solar system are defined as objects with enough mass to be spherical and round by their own gravity and to be alone on their orbit around the sun; in other words, to be the dominant object in a particular orbit and not to be a moon or asteroid
Most exoplanets are discovered by observing the stellar motion around the common center of mass of the combined star-and-planet system, that is, by observing somehow the motion of the objects in orbit around each other. This is typically done by measuring precisely the periodic variation of certain values, such as radial velocity or brightness, with time. For example, the first extrasolar planets were found with this timing technique around a pulsating neutron star.
The recent definition of “planets of our solar system” by the International Astronomical Union deals mainly with the question of the minimum mass for an object to qualify as planet and excludes Pluto. This matter was raised by the fact that more and more objects similar to Pluto were discovered by larger and larger telescopes. The questions of maximum mass and formation of planets were left out in this new definition, possibly partly because there is not yet a consensus in the international community. For a discussion of extrasolar planets, however, the maximum mass is very important in order to classify an object as planet or nonplanet and to distinguish between planets and brown dwarfs.
Both planets and brown dwarfs are substellar objects in the sense that they are less massive than stars so that they cannot fuse normal hydrogen (as stars do to produce energy and to shine for a long time). Brown dwarfs, while they cannot fuse normal hydrogen (which has an atomic nucleus of just one proton), can burn deuterium (heavy hydrogen, which has an atomic nucleus of a proton plus a neutron) so that they are self-luminous for a few millions of years until the original deuterium content is depleted.
The upper mass limit of planets can be defined either through the lower mass limit for deuterium fusion, which is around 13 times the mass of Jupiter (depending slightly on the chemical composition) or by the mass range of the so-called brown dwarf desert (as discussed in the following paragraphs).
We will next discuss the different exoplanet discovery techniques by chronological order of success and thereby also discuss the properties of objects found so far.
For a few thousand years, speculations have existed as to whether other stars can have their own planets. Both Giordano Bruno and Nicholas of Cusa answered this question positively a few hundred years ago. However, not until 1989 was the first object discovered that could really be an extrasolar planet and that is today still regarded as planet candidate. This first extrasolar planet candidate was discovered serendipitously by the so-called radial velocity technique: The velocity of the motion of an object directly toward us or away from us (in one dimension) is called radial velocity and can be measured by the so-called Doppler shift of spectral lines. Atoms in the atmosphere of stars can absorb light coming from the interior of the star at a certain energy, frequency, or wavelength for each kind of atom or ion, producing absorption lines in the spectrum of the star. If the star moves away from us, such lines are said to be red-shifted; if the star is approaching us, they are called blue-shifted; in either case, their wavelength is different from the normal wavelength (larger for red-shifted).
When a second object, like a planet, orbits around a star, actually both objects orbit around their common center of mass. Hence, also, the star wobbles: It sometimes approaches us, sometimes flies away from us. This can be observed as periodically changing radial velocity. The period of the variation gives the orbital period, and the amplitude of the change in radial velocity yields the mass of the companion. However, because the inclination between the orbital plane and our line of sight is normally not known, only a lower mass limit is known. Therefore, such low-mass companions detected by the radial velocity technique are to be seen as planet candidates; they could have a mass above the maximum mass for planets, making them perhaps brown dwarfs or even low-mass stars. The first such case was published in 1989 by Latham, Stefanik, Mazeh, Mayor, and Burki, namely a companion with minimum mass of 11 Jupiter masses around the star HD 114762. This planet is called HD 114762 b, following the convention that the first planet found around a star is called by the name of the star plus a lowercase b behind the star name (thus, a lowercase c for the next planet, etc.). This planet may very well have a true mass above 13 Jupiter masses, in which case it could be regarded as brown dwarf.
The first object discovered around a sunlike normal star, which is almost certainly a planet, is called 51 Peg b and is a planet with about half the mass of Jupiter as minimum mass found by Mayor and Queloz around the star 51 Peg in 1995.
The radial velocity can nowadays be measured with an accuracy of about 1 meter per second, so that planets with minimum mass of a few Earth masses can be detected by the wobble they produce on a low-mass star. In the time of one Jupiter orbit, that is, 12 years, since the important discovery of 51 Peg b, about 250 planet candidates have been discovered. In some cases, several planet candidates are orbiting a single star, in some other cases, individual planet candidates are found in binary stars. Because the high precision of the radial velocity technique has been available only since about the early 1990s, planets with more than 20 years of orbital period have not yet been discovered; one needs to observe at least one orbital period. Many planet candidates found so far orbit their stars within only a few days, which is quite different from our solar system, where the innermost planet Mercury needs several months to orbit the sun. The high number of planet candidates with short orbital period, however, can be seen as observational bias, because such planets also introduce a larger wobble on their stars due to Johannes Kepler's and Isaac Newton's laws of gravity.
The mass range of all these planet candidates shows a strong peak at about one Jupiter mass and a strong dip at around 20 to 30 Jupiter masses, even though this method would be biased toward more massive companions, because they have a stronger effect on their central star. Across the range of planet candidates, there are about 250 planet candidates all with masses below about 20 Jupiter masses, then almost no objects with minimum mass between about 20 and 70 Jupiter masses (that is, there are only a few brown dwarfs), and then again a large number of stellar companions with minimum mass above 70 Jupiter masses. This paucity of brown dwarfs identified with the radial velocity technique is called the brown dwarf desert, and the dip in the mass spectrum is deepest at around 20 to 30 Jupiter masses. Either the lower mass range of the brown dwarf desert or the minimum mass for deuterium burning can be used as an upper mass limit for planets, if one would define the upper mass limit for planets.
At the end of the lifetime of a massive star, after most of the material is burned by fusion, the star collapses due to its own gravity, then forms a very dense and compact object made up mostly by neutrons, called a neutron star, while the rest explodes due to a rebound as supernova. A neutron star typically has about 1.4 times the mass of our sun but a diameter of only 20 to 30 kilometers. Such neutron stars rotate very fast, sometimes even about 100 times per second, sometimes once in few seconds. Most known neutron stars emit strong radio emission along their rotation axes (beams), which appear pulsed due to the fast rotation. Such objects are called pulsars. We should keep in mind that so-called pulsars are not pulsating, but rotating fast. One can measure the rotation period with both high precision and high accuracy. In the case of the pulsar called PSR1257, Wolszczan and Frail discovered sinusoidal variations of the millisecond pulses in 1992 and interpreted these variations to be caused by low-mass objects in orbit around the neutron star, each with a mass equivalent only to about that of Earth. This discovery of pulsar planets (by pulsar timing) came as a big surprise, because planets were not expected around neutron stars; it is still dubious as to whether planets can survive the supernova explosion, and it is unknown whether the objects found around PSR1257 are remnants of the explosion or were formed afterwards.
In the case of planets or planet candidates discovered by the radial velocity technique, the inclination of the orbit of the planet around the star in not known. One way to determine this inclination would be to use a transit. A transit occurs when the planet orbits around the star into our line of sight; the planet moves in front of the star once per orbit (and behind the star also once per orbit). When the planet is in front of the star (that is, in front of the spatially unresolved stellar disk), a small part of the stellar light is blocked by the planet. This event is called transit or eclipse. Such events also happen in our solar system; for example, as seen from Earth, the inner planets Mercury and Venus can follow a path directly in front of the sun, which can even be observed as spatially resolved. The transit light curve enables observers on Earth to measure the inclination of the orbit and also the radii of stars and planets. Then, one can determine not only the true mass of the companion (planet or brown dwarf), but also, from mass and radius, its density.
The first case for which this was successfully observed was HD 209458 in the year 2000, the first radial-velocity planet candidate confirmed to be a true planet (and found to be a gas giant planet with low density like Jupiter). About one Jupiter orbit after the discovery of 51 Peg b, about 33 transiting planets are known (as of November 2007), most of which have also been discovered first by the transit, then confirmed as planets by radial velocity. In a few cases, also the secondary transit is detected; this is a small decrease in the total combined brightness of star plus planet (one should keep in mind that in all such cases, the planet is not seen direcdy) when the planet is behind the star. From the difference in brightness between the time of secondary transit and the time immediately before and/or after the transit, one can indirecdy determine the brightness of the planet.
Whereas the radial velocity technique measures the wobble of the star due to the orbiting planet in just one dimension (radial), one can measure the wobble in the two other dimensions by astrometry, very accurate and/or precise determination of the position of a star on the sky. Our sun as seen from about 30 light years' distance also moves slightly in the sky due to Jupiter orbiting it, but this is a very small effect, less than .001 of an arc second (the moon has a diameter of 1,800 arc seconds). The star GJ 876 was the first for which this wobble was detected, using the fine guidance sensor of the Hubble Space Telescope, confirming the radial velocity planet candidate GJ 876 b to be a real planet with just about two Jupiter masses. In the meantime, a few more planet candidates were confirmed by astrometry, and also one radial velocity planet candidate was found to be a low-mass star. Other observing programs have started, using both ground-based and space-borne telescopes, wherein one tries to discover such a wobble in stars where no planets or candidates have been found by other techniques.
All previous techniques—radial velocity, astrometry, and transits—cannot determine which photons are coming from the stars and which photons from the planet; that is, they cannot directly detect (or see) the planet. While stars are bright and self-luminous due to fusion, planets are very faint, mostly shining only due to reflected light, and they are also very close to their respective stars so that they cannot be detected or seen next to the much brighter star. Young planets, which are still contracting and/or accreting matter, are self-luminous and, hence, not that faint, so that it could be less difficult to directly detect a young planet next to a young star. Several observational campaigns were started around the turn of the millennium in 2000 with the Hubble Space Telescope and ground-based 8- to 10-meter telescopes.
In the case of the ground-based observations, the earth's atmosphere is another problem, leading to the twinkling of stars, so that we obtain images lacking the best possible image quality. With the new technique of so-called adaptive optics, one can de-twinkle the stars: flatten the disturbed wave front with a deformable mirror in the telescope. With such a technique used at the 8-meter Very Large Telescope of the European Southern Observatory in Chile, a few companions to young stars have been found since 2004 that could really be young planets detected directly. The first such case was the star GQ Lupi with its companion GQ Lupi b detected by Neuhäuser, Guenther, Wuchterl, Mugrauer, Bedalov, and Hauschildt. In such cases, it is more difficult to be sure about the exact mass of the companion, because the orbital period is several hundreds of years, so that these few objects could also be low-mass brown dwarfs.
According to Albert Einstein's theory of general relativity, mass or matter deforms space, so that a light ray moving close to matter would be diverted. One would see a ring of light (Einstein ring) around the object. If such an Einstein ring is not resolved spatially due to small mass and/or small angular resolution, one would still see the background object being brightened by the foreground object, the gravitational lens. Such an event is called microlensing. If a binary lens (a star plus a planet) were to move—as seen from Earth—direcdy in front of a background single star, one would observe a double-peaked light curve, one brightening due to the star and one brightening due to the planet. This way, one also can detect planets at great distances— thousands of light years away. There are a few cases where such a double-peaked light curve has been observed that could possibly be due to a planet. However, in all such cases, due to the large distance and hence small brightness, the nature, mass, and distance of neither the lens (or the primary object in the double lens, the star) nor of the lensed background object are known, so that the mass of the secondary object in the lens (possibly a planet) cannot be determined without great uncertainty. The mass of the companion is determined from the mass of the primary (unknown) and the brightness ratio of the two peaks. Practically, such events can never be observed again nor confirmed.
Some stars toward the end of their normal lifespan (i.e., after most of the light material is burned) are pulsating: They periodically increase and decrease their volume and, hence, brightness. Such a pulsation is observable as periodic brightness change. With precise observations, one can detect a small periodic variation in the pulsations, which can be explained by a wobble of the star due to an orbiting low-mass object. This is very similar to the pulsar timing and radial velocity technique. In the case of the pulsating star V391 Peg, such a variation was detected recently that can best be explained by a planet candidate with three Jupiter masses as minimum mass. This star has burned all its hydrogen already, has expanded to the red giant phase, has lost large amounts of its material, and is now again contracted to become a so-called white dwarf. This is the first time that a planet candidate has been detected in a star after the red giant phase (except for the pulsar planets). This case shows that planets can survive the red giant phase. Our sun will undergo this red giant phase in a few billion years, when it will then expand enough to swallow Mercury and Venus. It is not yet clear what effects this will have on the earth, but it is likely that all life will be extinguished.
All these different techniques to discover planets have resulted in several hundred planets and planet candidates, including some planetary systems, where several planets orbit the same star. (Updates on planet discoveries can be found on www.exoplanets.org.) Planetary systems consist not only of the planets and their host star but also of minor bodies like asteroids, comets, and moons, and often if not always also of a circumstellar disk with dust remaining from the formation phase. This is also the case in our solar system, where dust in the so-called zodiacal disk can be observed on dark moonless nights due to reflection of sunlight on dust particles; such dust debris disks can also observed around other stars, with or without planets.
All the planets discovered so far have masses of at least several Earth masses and are much different from Earth. It is not yet possible to detect earthlike planets. Such discoveries may be possible in the future, either by the use of larger telescopes or by ground- or space-based interferometry, using a combination of several telesopes.
Another eminent question is the habitability of exoplanets. So far, no signs of life have been found on other planets, neither in our solar system nor elsewhere. It is difficult not only to define life, but also to detect earthlike planets—to say nothing of earthlike or even nonearthlike life on distant planets. However, it may well be that life could form either on some already detected planets or on their moons, if these exist.
Bruno, Giordano, Laplace, Marques Pierre-Simon de, Nebular Hypothesis, Nicholas of Cusa (Cusanus), Planets, Extrasolar, Planets, Motion of, Pulsars and Quasars, Telescopes, Time, Planetary
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