The universe is composed of many galaxies, planets, an abundant amount of stars, and matter. There are different types of planets and one of the types is extrasolar. An extrasolar planet is a planet around other stars, and these planets are Mercury, Venus, Earth and Mars. One can see these planets through powerful telescopes. A technique used to search for a planet is by watching for small gravitational tugs that it exerts on its star, which can measure a star's velocity. The Doppler-shift method had been used to identify more than fifty planets around stars. Some recent discoveries are a "planetary system" orbiting the star "Upsilon Andromedae, and of three planets; the smallest would be the size of Saturn. What were also discovered were a dusk disk and a planet orbiting the star Epsilon Eridani. .
Some astronomers have discovered planet-sized objects floating between stars. Their mass is like that of Jupiter's, but because they do not orbit stars, astronomers still are not sure whether or not they should be called planets. .
The Doppler-shift technique is not always the most reliable one. It depends both on the mass of the orbiting object and on the tilt of its orbit. An observed Doppler-shift could be produced either by a low mass planet or by a bigger planet. Even with precise estimates, many observers still would not know whether the planet could have life or not. .
In the fall of 1999, a planet orbiting in space, the mass and size vanished when two teams of astronomers exposed a drop in brightness of the star. The drop occurred at the time that a newly discovered planet was expected to pass on the side nearest to Earth. What was thought was that the planet passed directly on the line between Earth and the star, which blocked some light from the star. .
Because of the discovery of the extrasolar planets, opportunities come to mind, which tests the theory of solar system formation. The challenge that is brought forth is that from the fact that most of the recently discovered planets are different from those of solar system formation.