So, astronomers use other ways to detect and study these distant planets. They search for exoplanets by looking at the effects these planets have on the stars they orbit. More than 4,000 exoplanets are known, and about 6,000 await further confirmation.
How to detect exoplanets
Planets are much fainter than the stars they orbit. Hence, exoplanets are extremely difficult to detect. By far, the most successful technique for finding and studying exoplanets has been the radial velocity method, which measures the motion of host stars in response to gravitational tugs by their planets. Swiss astronomers Michel Mayor and Didier Queloz discovered the first planet using this technique, 51 Pegasi b, in the 1990s. Other techniques that have detected exoplanets are - pulsation timing, microlensing, and direct imaging.
The nearest exoplanets are located 4.2 light-years from Earth and orbit Proxima Centauri, the closest star to the Sun.
Future research
NASA's Kepler mission, launched on March 6, 2009, found that about 1 in 4 Sun-like stars had a planet analogous to Earth. In 2010 the Kepler team announced its first discoveries: four gas giant planets somewhat larger than Jupiter and one planet slightly larger than Neptune that is more enriched in heavy elements; all five orbit very close to their stars. In 2011 the Kepler said that they had discovered a planet, Kepler-22b, that was the first to be found in the habitable zone of a star like the Sun. They also discovered the first Earth-sized exoplanets, Kepler-20e and Kepler-20f. By the end of its mission in 2018, Kepler had discovered 2,741 planets, about two-thirds of all known exoplanets.
Other research carried out by Trappist telescope on Earth and the Spitzer Space Telescope were used to discover seven Earth-sized planets in this system, three of which are in the habitable zone. TESS (Transiting Exoplanet Survey Satellite) is another mission by NASA's Explorer's programme, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission.
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