Extrasolar planets are planets orbiting stars outside our solar system.The first extrasolar planet discovery was confirmed in 1992, although they had been predicted long before. Planets are difficult to detect directly because they are so much dimmer than the stars they orbit. The Sun is a billion (109) times brighter than Jupiter and 10 billion (1010) times brighter than the Earth. 708 extrasolar planets have been discovered as of 7 December 2011. Astronomers use 5 main methods to find extrasolar planets.
Radial Velocity Method:
This method uses the fact that if a star has a planet (or planets) around it, it is not strictly correct to say that the planet orbits the star. Instead, the planet and the star orbit their common center of mass. Because the star is so much more massive than the planets, the center of mass is within the star and the star appears to wobble slightly as the planet travels around it. Astronomers can measure this wobble by using spectroscopy. If a star is traveling towards us, its light will appear blueshifted, and if it is traveling away the light will be redshifted. Spectroscopy can show this change in color from a star as it moves towards and away from us, orbiting the center of mass of the star-planet system.
Many of the planets that have been discovered by this method are very large and very close to their stars. These planets, called super-Jupiters, are much larger than Jupiter, and orbit their stars in a matter of days. Such a large planet causes a large wobble, and this, as well as the short orbit time, makes these planets easier to detect than ones that are smaller or farther from their stars. Over 650 planets have been discovered (as of 7 December 2011) by this method.
This method only works for star-planet systems that have orbits aligned in such a way that, as seen from Earth, the planet travels between us and the star and blocks some of the light from the star.
A planet does not block much light from a star, (only 1% or less) but this can be detected. Like the radial velocity method, this method has a bias towards discovering large planets orbiting close to their stars, because larger planets block more light and transit more frequently so they are easier to detect. This method will not work for all systems, however, because only about 10% of super-Jupiters are aligned in such a way that we see them transit. Smaller planets in larger orbits are even less likely to be aligned in such a way that we can observe transits. For planets that do transit, astronomers can get valuable information about the planet's atmosphere, surface temperatures and size. Over 180 planets have been discovered by this method (as of 7 December 2011).
Gravitational Microlensing:
Gravitational microlensing relies on chance events where from our viewpoint, one star passes in front of another star. The farther star is usually a bright star, and the near one is normally one we couldn't ordinarily see from Earth. When it passes in front of the farther star, however, its gravity causes the light from the farther star to bend and the star is magnified from our point of view. If, during the event, the background star appears to be magnified even more for a short time, that means a planet orbiting the smaller star is increasing the effect of the magnification. Click on the diagram below to see a larger version.
For more, please see our Gravitational Microlensing Observing Program. At least 13 planets have been discovered by this method.
Astrometry:
This method consists of making precise measurements to a star's position over time to detect the wobble mentioned above. Until recently telescopes did not have the resolution needed to detect this type of motion. Astrometry will be more sensitive to planets with large orbits, which is an advantage it has over some of the other methods. The Gaia mission, with a planned launch in 2012, will advance the field of astrometry much further. No extrasolar planet discoveries by this method have been confirmed by other methods, but the field is promising.
Direct Imaging:
Direct imaging has used infrared wavelengths to observe planets. This works because at infrared wavelengths a star like the Sun is only 100 times brighter than Jupiter, compared to a billion (109) times brighter at visual wavelengths. This works for planets that are very far from their stars, so an orbit might take hundreds or thousands of years for a planet discovered by this method. Astronomers have to verify that the planet and the star move together through space to prove that the planet orbits the star since observing an entire orbit will take so long. This method does not allow astronomers to measure the mass of a planet directly, but they can use the spectrum and brightness to get information about its surface temperature and diameter. At least 29 planets have been discovered by this method (as of 7 December 2011).
