1.	Basic facts

1.1.

What?

1.2.

Where?

1.3.	How do we find them?

1.4.

When?

1.5.

How many?

1.6.	Why are we looking for them?

1.7.	Are there any general characteristics of exoplanets?

1.8.	Who finds them?

2.	Methods used for detecting exoplanets

2.1.	Astrometry

2.2.	Radial velocity

2.3.	Pulsar timing

2.4.	Photometry

2.5.	Gravitational microlensing

2.6.	Circumstellar disks

2.7.	Direct imaging

3.	What is European Space Agency (ESA)?

3.1.	Does ESA research in exoplanets?

"Basic facts

What? The meaning of prefix "exo" is outside; therefore exoplanets are planets that are outside of our solar system. An extrasolar planet is another word of an exoplanet.
Where? Extrasolar planets are located near other stars than Sun.
How do we find them? There are various methods of finding the exoplanets. More information can be found in the article Methods used for detecting exoplanets.
When? Exoplanets' origin is usually billions years ago, we had discovered first of them in 1992.
How many? According to the server www.exoplanet.eu there are 271 candidates of exoplanets on the date 11 January 2008.
Why are we looking for them? Finding exoplanets brings us important information about our universe and helps us to understand requirements for existence of planets. There is also a possibility of existence life on them.
Are there any general characteristics of exoplanets? All but six exoplanets are more than ten times heavier than Earth, most of them are closer to their star than Mercury is to the Sun, eccentricity of extrasolar planets is relatively high, and most of the exoplanet are nearby a star that is similar to our Sun. The first two characteristics are caused by observational effect, because it is easier to detect planets with such characteristics.
Who finds them? There are a lot of projects searching for extrasolar planets. Ground projects that have found a candidate for an exoplanet are: AFOE, Anglo-Australian Planet Search Program, California & Carnegie Planet Search, Coralie at Leonard Euler Telescope, Elodie, ESO Coude Echelle Spectrometer, Exoplanet Tracker, High Accuracy Radial velocity Planetary Search, Hobby-Eberly Telescope, Hungarian Automated Telescope Network, Mc Donald Observatory, Microlensing Planet Search Project, NK2 Consortium, OGLE III, Pulsar Planet Detection, Sophie, Spectrashift.com, STARE (STellar Astrophysics & Research on Exoplanets), Super WASP, XO Project. Space projects that have found a candidate for an exoplanet are: COROT, HST Astrometry and Spitzer Space Telescope.

Methods used for detecting exoplanets

Astrometry
This is the oldest method of finding exoplanets. This method is based on precise measuring orbit of a star. A planet causes that the system of the star and the planet rotates around its center of mass. When the planet is heavy enough, the change in the star's orbit is measurable from the Earth. On the other hand not all changes in orbit are due to a planet. That is a reason why there were claimed discoveries of non-existing planets in the past. "