Extra Solar Planets
The word planet, which comes from the greek word planetes which means wanderer, was used by ancient astronomers to describe those ‘stars’ that were observed to be moving, relative to millions of other ‘stars’ that appeared to be static. It is now known that the planets are actually worlds that orbit around the Sun, some of them rocky, like Earth or Mars, and others made of gas, like Jupiter and Saturn. On the other hand, the ‘static stars’ are in fact objects similar to our Sun, appearing so dim because of the fact that they are at huge distances from the Earth.
Unlike stars, planets do not generate their own light and they are only visible by the light reflected off their surface, light produced by their parent stars. For this reason, planets are extremely dim, and therefore, very difficult to directly observe with a telescope. So it is not surprising that, since their original discovery more than 2000 years ago, the only planets known to exist were the ones orbiting around the Sun. It was not until recently (early 90′s) that the first planets orbiting other stars (‘Extra-solar Planets’) were discovered. Since then, the list of Extra-solar planets has been ever increasing, with more than 150 planets currently discovered (as of March 2011 this is now more than 500).
As already mentioned, planets are very difficult to be directly observed due to their inherent dimness. Even if a planet reflects enough light to be observed by a telescope, this light is swamped by the brightness of the parent star. To work around this problem, astronomers use ‘indirect methods’ of detection, where the presence of a planet, as well as its mass and distance from the parent star are inferred, by studying the properties of the star.
One method of inferring the presence of planet is to look for evidence of the gravitational influence exerted on the star by the planet. Even though we usually refer to planets as orbiting around a star, in reality both the planets and the star orbit around their common centre of gravity, which, although very close, is offset relative to the star’s centre. As the star orbits around the centre of gravity it appears to ‘wobble’ periodically and this periodicity can provide clues on the presence of a planet, its mass and its distance from the star. Generally, the more massive planets exert a stronger gravitational pull on their stars resulting to a stronger ‘wobble’ which is easier to detect.
Another way of detecting planets is by looking for evidence of periodic dimming in a star’s brightness, resulting by the passing of a planet in front of the star (transit). The amount of decrease in the star’s brightness depends on the size of the planet, with bigger planets causing a higher decrease. The decrease in brightness is usually very small (approximately 0.1%) and therefore very difficult to detect. However, with the constant development in technology, imaging sensors (CCDs) are becoming sensitive enough to detect such miniscule changes in brightness.
Extra-solar planets, since their first discovery, have captured the interest not only of astronomers but also the general public, since they conjure up images of distant worlds similar to Earth, and the possibility of alien life inhabiting them. The search for extra-solar planets is nowadays very popular, with a large number of astronomy teams working on the subject all around the world, employing new and innovative methods to detect them. To date, almost all the planets detected are gas giants, similar to Jupiter, rather than Earth, however, telescopes are currently being built, designed for the specific purpose of detecting Earth-like planets.
