Extrasolar planet
Even though extrasolar planets were long proposed, they were not found before 1990, when new space telescopes good enough were constructed. One issue connected to the exoplanets is extraterrestrial life.
Extrasolar planets were first discovered during the 1990s as a result of improved telescope technology, such as CCD and computer-based image processing along with the Hubble Space Telescope. Such advances allowed for more accurate measurements of stellar motion, allowing astronomers to detect planets, not visually (the luminosity of a planet being too low for such detection), but by measuring gravitational influences upon stars (see astrometrics). In addition, extrasolar planets can be detected by measuring the variance in a star's apparent luminosity, as a planet passes in front of it (see eclipse).
Besides the detection of at least 80 planets (mostly gas giants), many observations point to the existence of millions of comets also in extrasolar systems.
The Polish astronomer Aleksander Wolszczan claimed to have found the first extrasolar planets in 1993, orbiting the pulsar PSR 1257+12. Subsequent investigation has determined that these objects are not "true" planets in that they are technically "sub-brown dwarf masses orbiting an object that is or once was a star"; it is believed that they are unusual remnants of the supernova that produced the pulsar, and did not form as conventional planets do.
The first "true" extrasolar planet was announced on October 6, 1995 by Michel Mayor and Didier Queloz; the primary star was 51 Pegasi. Since then dozens of planets have been detected, many 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 Upsilon Andromedae. The majority of the detected planets have highly elliptical orbits.
There are currently three methods of detecting extrasolar planets, which are too faint to be detected by present conventional optical means.
The first involves measuring the displacement in the parent star's spectral lines due to the Doppler effect induced by the planet orbiting the star and moving it through mutual gravitation. This is the first and by far most successful technique used by planet hunters. It is known as the "Doppler method" or "Wobble method". But it works only for relatively nearby stars out to about 160 light-years. It easily finds planets that are close to stars, but struggles to detect those orbiting at great distances.
The second method, known as the gravitational microlensing method involves catching the planet as it passes in front of the star's tiny disk which will cause the light of the star to be displaced in a distinctive way, and do so periodically as the planet completes multiple orbits. This method uses an effect predicted by Albert Einstein. Gravity can bend a beam of light as it tends to follow the planet's gravity well. It also depends on the plane of the planet's orbit being aligned with the line of sight between the star and the Earth. As a result, any number of stars with planets that are not so aligned will be missed. This method is most fruitful for planets between earth and the center of the galaxy, where stars are more concentrated.
Gravitational microlensing has a checkered past. In 1986, Bohdan Paczynski of Princeton University first proposed using it to look for mysterious dark matter, the unseen material that is thought to dominate the universe. In 1991 he suggested it might be used to find planets. Sucesses with the Gravity Lensing method date back to 2002, when a group of Polish astronomers (Professors Andrzej Udalski and Marcin Kubiak and Dr. Michal Szymanski from Warsaw, and Polish-American Professor Bohdan Paczynski from Princeton) during project OGLE (the Optical Gravitational Lensing Experiment) perfected a workable method. During one month they claimed to find 46 objects, many of which could be planets.
Lensing events are brief, lasting for weeks or days as the two stars and Earth are all moving in relation to each other. More than 1,000 stars have been detected in microlensing relationships over the past ten years.
A notable disadvantage is that the lensing cannot be repeated because the chance alignment never occurs again.
In addition to the NASA/National Science Foundation-funded OGLE, the Microlensing Observations in Astrophysics (MOA) group is working to perfect this technique. Astronomers expect that it may be possible to detect an earth-sized world within five years.
The third, and most recently developed method, detects a planet's shadow when it transits in front of its host star. This "Transit method", as it is called, works only for the small percentage of planets whose orbits happen to be perfectly aligned from our vantagepoint. It also can be used, however, on very distant stars. The transit method is expected to lead to the first detection of an Earth-size planet, when it is employed by NASA's space-based Kepler observatory, set to launch in 3 years.
Most of the planets found are of relatively high mass (at least 40 times that of the Earth); however, a couple seem to be approximately the size of the Earth. This reflects the current telescope technology, which is not able to detect smaller planets. The mass distribution should not be taken as a reference for a general estimate, since it is likely that many more planets with smaller mass, even in nearby solar systems, are still undetected.
The Kepler Space Mission will be launched in the next few years. It is a space-based telescope designed specifically to search large numbers of stars for Earth-sized terrestrial planets.
One question raised by the detection of extrasolar planets is why so many of the detected planets are gas giants which, in comparison to Earth's solar system, are unexpectedly close to the orbited star. For example, Tau BoÃÂötis has a planet 4.1 times Jupiter's mass, which is less than a quarter of an astronomical unit (AU) from the orbited star; HD 114762 has a planet 11 times Jupiter's mass, which is less than half an AU from the orbited star. One possible answer to these unexpected planetary orbits is that since astrometrics detects the extrasolar planets due to their gravitational influences and partially-ecliptic interference, perhaps current technology only permits the detection of systems where a large planet is close to the orbited star, rather than such systems being the norm.
The frequency of extrasolar planets is one of the parameters in the Drake equation, which attempts to estimate the probability of communications with extraterrestrial intelligence.
On November 27, 2001, astronomers using the Hubble Space Telescope announced that they had detected the atmosphere of the planet orbiting HD 209458 (provisionally dubbed "Osiris"). Also during that year, a star was located which had the remnants of one or more planets within the stellar atmosphere - apparently the planet was mostly vaporized. It has been suggested that there may be planets that orbit so closely to their suns that most of their mass has been stripped away by the heat, provisionally referred to as Cthonian planets.
On July 10, 2003, using information obtained from the Hubble Space Telescope, scientists discovered the oldest extrasolar planet yet. Dubbed Methuselah after the biblical figure, the planet is about 5,600 light years from Earth, has a mass twice that of Jupiter, and is estimated to be 13 billion years old. It is located in the globular star cluster M4, approximately 7200 light years from Earth in the constellation Scorpius.
On April 15, 2004, separate teams announced the discoveries of three planets outside our solar system, including one that is 17,000 light years away, more than three times farther away than the previous record holder. The background star that was used in the gravitational lensing is 24,000 light-years away.
The newly-discovered exoplanet is estimated to be about 1.5 times the mass of Jupiter and presumed to be similarly gaseous. It orbits the star about 3 astronomical units (AU). Jupiter is 5.2 AU from the Sun.
The same day, a European team of planet hunters based at the Geneva Observatory found two giant planets using the transit method.
Both planets are called "hot Jupiters," close to one Jupiter-mass but orbiting its star so closely that it completes an orbit in less than two earth days.
The planned Space Interferometry Mission and Terrestrial Planet Finder would both have to examine more nearby systems.
See the list of stars with confirmed extrasolar planets for a list of confirmed observations.
History of detection
Methods of detection
Solar system formation processes
Notable extrasolar planets
External links and references