An extrasolar planet is a planet which orbits a star other than the Sun, i.e. which belongs to a planetary system other than our solar system.

Extrasolar planets were 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. 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. 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 Michael 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 two main 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.

The second involves catching the planet as it passes in front of the star's tiny disk which will cause the light of the star to "dip" in a distinctive way, and do so periodically as the planet completes multiple orbits. The second method is theoretically more sensitive, but is newer and has scored fewer successes. 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.

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.

Extrasolar Planets

Fomalhaut b is an extrasolar planet orbiting the star Fomalhaut. It is the first exoplanet observed directly in visible light, after researchers spent eight years trying to pinpoint its position. The planet's existence was theoretically predicted. NASA released the photograph, taken by the Hubble Space Telescope's ACS, on November 13, 2008. In the image, the bright outer oval band is the dust ring, while the features inside of this band represent noise from scattered starlight.

The Fomalhaut system is located 25 light-years away from Earth, in the constellation Piscis Austrinus. Fomalhaut b is believed to be the coolest, lowest-mass object ever seen outside our own solar neighbourhood. The existence of the planet was inferred in 2005 due to its influence on the Fomalhaut dust belt; the belt is not centered on the star, and has a sharper inner boundary than would normally be expected. However, the planet was only located in May 2008 after Paul Kalas singled it out of Hubble photographs taken in 2004 and 2006.

Kalas remarked, "ItÕs a profound and overwhelming experience to lay eyes on a planet never before seen. I nearly had a heart attack at the end of May when I confirmed that Fomalhaut b orbits its parent star." The planet is estimated to be approximately the same size as Jupiter, with a maximum mass of three Jupiters and a most probable mass of 0.5 to two. It is 115 AU (17 billion km, 11 billion mi, about 20% greater than the aphelion distance of Eris and 3.8 times the semi-major axis of Neptune) from its sun, giving it an orbital period of 872 earth years. Fomalhaut has about 16 times the Sun's luminosity, so Neptune and Fomalhaut b are in regions of similar temperature (due to the inverse square law).

Fomalhaut b is suspected, on the basis of its brightness in visible light and dimness in infrared, to have planetary rings far larger than those of Saturn.

Find Fomalhaut in the Celestial Sea   MSNBC - November 14, 2008

In ancient Persia, star announced the arrival of autumn weather

Discovering Fomalhaut b (sounds like "foam-a-lot")    NASA - November 14, 2008

First-ever images taken of extrasolar planets   MSNBC - November 13, 2008

Major Breakthrough: First Photos of Planets Around Other Stars    Live Science - November 13, 2008

Exoplanets finally come into view    BBC - November 13, 2008
Two studies show the first direct images of planets outside our Solar System, including a three-planet system.

First Pictures of Alien Planet System Revealed   National Geographic - November 14, 2008

Distant 'waterworld' is confirmed   BBC - February 20, 2012

GJ 1214b is one of the most interesting exoplanets yet detected. Astronomers have confirmed the existence of a new class of planet: a waterworld with a thick, steamy atmosphere. Exoplanet GJ 1214b is a so-called "Super Earth" - bigger than our planet, but smaller than gas giants such as Jupiter. Observations using the Hubble telescope now seem to confirm that a large fraction of its mass is water. The planet's high temperatures suggest exotic materials might exist there. The planet was discovered in 2009 by ground-based telescopes. It is about 2.7 times the Earth's diameter, but weighs almost seven times as much. It orbits its red-dwarf star at a distance of just two million km, meaning temperatures on GJ 1214b probably reach above 200C.

  Re-thinking an alien world   PhysOrg - January 16, 2012

Forty light years from Earth, a rocky world named "55 Cancri e" circles perilously close to a stellar inferno. Completing one orbit in only 18 hours, the alien planet is 26 times closer to its parent star than Mercury is to the Sun. If Earth were in the same position, the soil beneath our feet would heat up to about 3200 F. Researchers have long thought that 55 Cancri e must be a wasteland of parched rock.

Kepler 22-b: Earth-like planet confirmed   BBC - December 6, 2011

Astronomers have confirmed the existence of an Earth-like planet in the "habitable zone" around a star not unlike our own. The planet, Kepler 22-b, lies about 600 light-years away and is about 2.4 times the size of Earth, and has a temperature of about 22C. It is the closest confirmed planet yet to one like ours - an "Earth 2.0". However, the team does not yet know if Kepler 22-b is made mostly of rock, gas or liquid.

Alien Planet Is Rolling Over, Forcing 4 Others to Do Same   Live Science - December 1, 2011
A huge alien planet turns super-slow somersaults as it hurtles through space, dragging its four sibling planets along for the topsy-turvy ride, a new study suggests. The giant exoplanet, known as 55 Cancri d, gets tugged by a faraway companion star as it orbits its own parent star. As a result, the planet performs a flip over the course of millions of years, and the other four planets in the system follow suit, researchers said.

NASA Finds Smallest Earthlike Planet Outside Solar System   National Geographic - January 10, 2011
Rocky world 1.4 times Earth's size is "missing link," astronomer says. NASA's Kepler spacecraft has confirmed the discovery of a rocky world just 1.4 times the size of Earth circling a sunlike star.

Rocky exoplanet milestone in hunt for Earth-like worlds   BBC - January 10, 2011
Astronomers have discovered the smallest planet outside our solar system, and the first that is undoubtedly rocky like Earth. Measurements of unprecedented precision have shown that the planet, Kepler 10b, has a diameter slightly lower than Earth's, and a mass 4.6 times higher. However, because it orbits its host star so closely, the planet could not harbor life. The discovery has been hailed as "among the most profound in human history".

  Astronomers find first planet from another galaxy   PhysOrg - November 18, 2010
An exoplanet orbiting a star that entered our Milky Way from another galaxy has been detected by a European team of astronomers using the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile. The Jupiter-like planet is particularly unusual, as it is orbiting a star nearing the end of its life and could be about to be engulfed by it, giving tantalizing clues about the fate of our own planetary system in the distant future.

Discovery of an extrasolar earth-sized planet   PhysOrg - October 8, 2010
There are now over 490 confirmed extrasolar planets. The vast majority are gas giants like Jupiter, but they are much stranger because many orbit close to their stars and so are much hotter than Jupiter (some are even closer to their star than Mercury is to the sun).

Water-Ice Super-Earths   PhysOrg - August 27, 2010
A "super-Earth" is a planet around another star (an "exoplanet") whose mass is less than about ten times that of the Earth. Of the 480 or so extrasolar planets now known, most have masses larger than the mass of Jupiter, which is 318 times more massive than the Earth. About two dozen, though, appear to have masses that put them into the super-Earth category. About 70 exoplanets have orbits that happen to be aligned such that the planet passes directly between the star and the Earth (a transit).

New Planet System Found May Have Hidden "Super Earth"   National Geographic - August 26, 2010
A newly discovered planetary system orbiting a sunlike star may conceal a rare super-Earth, according to data from NASA's Kepler space telescope. Launched last March, Kepler was designed to look for extrasolar planets, aka exoplanets, via transits the periodic dimming of light from stars due to planets passing in front them, as seen from the telescope's vantage point. (Read about Kepler's first planet discoveries.) After analyzing seven months' worth of data from Kepler, a team led by Matt Holman of the Harvard-Smithsonian Center for Astrophysics found two transiting exoplanets orbiting the star Kepler-9, which lies about 2,300 light-years from Earth.

Rich exoplanet system discovered   BBC - August 24, 2010
The star is 127 light years away, in the southern constellation of Hydrus. They say this is the "richest" system of exoplanets - planets outside our own Solar System - ever found.

  Richest planetary system discovered around Sun-like star HD 10180   PhysOrg - August 24, 2010

'Superstorm' rages on exoplanet   BBC - June 23, 2010

Astronomers have measured high-speed winds in the atmosphere of a planet orbiting a distant star. Data on carbon monoxide gas in the atmosphere show that it is streaming at fierce speeds from the planet's hot day side to its cool night side. Writing in Nature, a team detected longitudinal winds of roughly 2km/s (7,000km/h) in the atmosphere of a "hot Jupiter" planet. Hot Jupiters are gas giants that orbit very close to their parent stars.

'Super-Earths' orbit nearby stars   BBC - December 15, 2009

Planet-hunters have discovered two "super-Earths" orbiting two nearby Sun-like stars. These rocky planets are larger than the Earth but much smaller than ice giants such as Uranus and Neptune. Scientists say the discoveries are a step towards finding potentially habitable planets - smaller planets that are comparable to the Earth. Details of the new planets are described in two papers in the Astrophysical Journal.

Nearby "Super Earth" May Have Oceans, Thick Atmosphere   National Geographic - December 16, 2009

Most Earth-Like Extrasolar Planet Found Right Next Door   Wired - December 16, 2009

Discovery of a Retrograde or Highly Tilted Extrasolar Planet   PhysOrg - November 18, 2009

  32 New Exoplanets Found   PhysOrg - October 19, 2009

32 New Planets Found Outside Our Solar System   National Geographic - October 19, 2009

  First Solid Evidence for a Rocky Exoplanet   PhysOrg - September 16, 2009

Sun-like star's 'oddball' planet   BBC - April 29, 2009
Astronomers have discovered a strange Jupiter-sized world circling a star similar to our own Sun.

First Exoplanet With CO2 Heats Up Hunt for Other Earths    National Geographic - December 8, 2008
The recent discovery of carbon dioxide in the atmosphere of a Jupiter-like planet 63 light-years away has some researchers excited that we may soon find habitable exoplanetsÑworlds circling other stars.

First Picture of Alien Planet Orbiting Sunlike Star? National Geographic - September 16, 2008

An image released today of a distant star and its potential planetary companion could go down in history as the first picture of a planet outside our solar system orbiting a sunlike star. The possible planet - a hot, young body (upper left) about eight times more massive than Jupiter - sits roughly 330 times as far from its host star as Earth is from the sun. The pair lies about 500 light-years from Earth.

Three "Super-Earths" Found Orbiting Sun-Like Star National Geographic - June 17, 2008

A trio of "super-Earths" have been found near a sun-like star, a team of European astronomers announced today. The planets orbiting the star HD 40307 which is 42 light-years away were found using an advanced "planet searcher" instrument at the European Southern Observatory in La Silla, Chile, the French and Swiss astronomers said.

Methane Detected on Distant Planet for First Time National Geographic - March 19, 2008

Methane has been detected in the atmosphere of a planet outside our solar system, the first time that an organic molecule has been found on a distant world. Studies of this carbon-bearing compound could shed light on the planet's formation and evolution.

Largest Known Planet Found, Has Density of Cork National Geographic - August 8, 2007
The biggest alien planet found so far is baffling scientists with properties that defy current scientific explanation. By all rights, TrES-4, a gas giant recently discovered about 1,400 light-years away in the constellation Hercules, shouldn't exist.

Distant Planets Could Have Plants of "Alien" Colors National Geographic - April 12, 2007
Scientists may be able to determine the color of extraterrestrial plant life while studying distant planets, according to a pair of new studies. Researchers have developed a way to analyze the light emitted by a given planet's parent star and determine how that light interacts with various chemicals in the planet's atmosphere.

First Sign of Water on Planet Outside Our System National Geographic - April 11, 2007
For the first time, astronomers have detected water in the atmosphere of a planet outside our solar system. Astronomer Travis Barman announced today that he has discovered water around planet HD209458b by combining theoretical models with observations from the Hubble Space Telescope. Scientists have predicted water vapor in the atmospheres of most planets orbiting other stars.

Future Space Telescopes Could Detect Earth Twin Science Daily - April 12, 2007
For the first time ever, NASA researchers have successfully demonstrated in the laboratory that a space telescope rigged with special masks and mirrors could snap a photo of an Earth-like planet orbiting a nearby star. This accomplishment marks a dramatic step forward for missions like the proposed Terrestrial Planet Finder, designed to hunt for an Earth twin that might harbor life.

Smallest extrasolar planet found BBC - June 13, 2005

Astronomers have detected the smallest extrasolar planet yet: a world about seven and a half times as massive as Earth, orbiting a star much like ours. All of the 150 or so exoplanets found orbiting normal stars are larger than Uranus, itself 15 times Earth's mass. The new find may be the first rocky world found around a star like our Sun. The newly discovered "super-Earth" orbits the star Gliese 876, located 15 light-years away in the direction of the constellation Aquarius.

First extrasolar planet seen BBC - March 2005

A European team claims to have obtained the first direct image of a planet beyond our own Solar System. The "extrasolar planet" is said to orbit a star called GQ Lup - thought to be like a young version of our Sun. Similar claims have been made in the past, but skeptical scientists believe the pictures merely show objects that share the same view in the sky.