You may think that in order to find a planet orbiting remote stars, you do need to have a huge telescopes that will be so powerful that can detect even the most dim and small parts. Why in that case not to build a truly huge telescope that could have made countless discoveries?

To detect such alien worlds quite difficult, since they are very close to the bright stars which light can't see them. Also, if the exoplanet is quite far from its parent star, a small light that it reflects, is too weak to make it possible to detect even with the huge telescope. Therefore, in order really to find the planet that you want to apply an incredibly inventive indirect techniques. Only 7 percent of exoplanets have been opened through direct observations.
Methods for discovery of new planets: Doppler

Distant stars are on the planet for their gravitational forces, thus keeping them on orbit, but the gravity of the planets themselves also causes them to be attracted to the star. If the gravity is equal, two heavenly bodies revolve around the same point. This Central point will be in a certain place, depending on the mass of the two objects.

The star will move away from its center of mass quite a bit, just as deviates from the centre of discus-thrower, when does the turnover before throwing the hammer. By studying the light emitted by the star, we may notice light movements and to determine changes in the positions of spectral lines. The measurement of these changes allows to determine the approximate mass of the planet. This method of detecting exoplanets is called the radial velocity, or Doppler Method.

Astrometry is the addition of this method when gravitational fluctuations too large for telescopes, which does not allow to see the change of position of the stars at the time, when its orbit is exoplanet. However, changing the position of the stars usually are so minor that the use of this method is problematic.

In the case of "super-Earth" HD40307g, this exoplanet was opened with a tool called HARPS (eng. Europe's High Accuracy Radial velocity Planet Searcher), a spectrograph, established in the European La Silla Observatory in Chile, which is able to feel the weakest changes in the stellar spectrum, thus allowing to prove the existence of extrasolar planets in the habitable zone.
Neutron stars, pulsars and planets

In some special cases similar method is used for determining the existence of planets close to the pulsars - rapidly rotating extremely dense neutron stars. During their rotation of stars emit intense radiation in the form of rays that are similar to the rays of light of the lighthouse. If the Land is in a position which falls on it, this ray of light, earth observers can notice a surge of energy. Thanks to such ripple these stars got its name - pulsars.

The presence of a planet orbiting the star of the pulsar causes fluctuations her light due to the gravitational forces of the planet that affects schedule" ripple. Measuring the variability of heart rate, you can determine the orbital characteristics and the mass of the planet.
Methods for discovery of new planets: the Transit method

In other cases, the exoplanet's orbit is built so well that it can be seen from the vantage point on Earth. When a planet transits in front of its parent star, she covers her light, and the terrestrial observer, seeing the star, may also at this point, to detect and planet. Measuring change the luminosity of a star during transit of the planet, you can determine the physical size of this planet and even its physical properties. This technique is called Transit method.

The space telescope NASA "Kepler" was designed to detect the slight variations in the luminosity of the stars, when exoplanets go ahead of them in transit. Now with this telescope, it was possible to detect nearly 2300 exoplanet candidates (although these signals have to check) in a small area of the milky Way.

In systems with many planets researchers use the Method of variation of the time transits (TTV). Small variations in the orbital periods show the presence nearby of another planet, which can be invisible to the observer.

Very rarely used a technique called Gravitational microlensing, when the star passes in front of the other, more distant star. The gravitational field is closer to us star makes light of more distant stars as if to round it, like a magnifying lens. In this case, you can define the bursts luminosity near stars. If near the nearest stars have exoplanet, its gravity would affect lensing effect.

There are other indirect methods for detecting exoplanets near distant stars, but probably the most faithful is direct observation, which still remains a complicated method. With the development of technology in the future will be possible to observe the exoplanets directly, but until these worlds we can only be imagined.

Source: news.discovery.com