An exoplanet is a planet orbiting a star other than the Sun, belonging to its planetary system. This group can also include the so-called wandering planets, which are planets that do not orbit any star. Exoplanets form in protoplanetary disks.
The very first discovered exoplanet was the object orbiting pulsar B1257 + 12 in 1992. The first planets orbiting the stars of the main sequence were not discovered until the 1990s.
Extrasolar planets have a very low brightness of the order of a billionth of the brightness of their star. Extrasolar planets cannot be observed directly by current telescopes, and only in recent years have the first photographic observations of these objects appeared.
The methods of their discovery are based almost exclusively on the observation of the parent star. It is also much easier to visually observe protoplanetary disks.
Methods of discovering extrasolar planets.
Transit- This method occurs when a (large) planet passes in front of a star, causing a slight decrease in its brightness. It can be measured and the existence of an exoplanet can be deduced from it. However, it is necessary that the plane of the planet’s orbit lies exactly in the plane of the observer’s angle of view.
Gravitational lens – This method is valid when the exoplanet passes in front of a star, but can increase its brightness by bending its light with its gravitational field.
Gravitational influence on the orbit of a star- This method is valid only for the discovery of sufficiently massive planets, which by their gravity cause a visible orbit of the star around its center of gravity (change in the orbit of the parent star or Doppler phenomenon).
Changes in the spectrum of a star – If the spectral lines of molecules unexpectedly appear in the spectrum of a star at regular intervals, it is almost certain that it is orbited by an invisible colder companion. Stars, especially hotter ones, have almost no molecules in their spectra.
Heat emission detection – This method is based on the detection of infrared radiation emanating from a planet.
In June 2005, the first extrasolar planet of Earth-sized size and composition was discovered near Gliese 876. Until then, all the planets observed were predominantly gas giants of the Jupiter type.
Towards the end of 2008, the Hubble Space Telescope discovered an exoplanet with a mass 3 times that of Jupiter by the star Fomalhaut.
The discovery of extrasolar planets has led to the question of whether extraterrestrial life can exist on them.
As of 14 April 2017, 3,472 exoplanets were discovered, of which 42 in the habitable zone, of which 0 habitable.