Did you know that over 5000 exoplanets have already been discovered? Exoplanets are planets outside our solar system. These exoplanets cannot be seen “normally” with a telescope but are discovered using special techniques. For instance, small variations in the brightness of the star are observed when the planet passes in front of the star, or light fluctuations are observed in the rotation of the star caused by the gravity of the exoplanet. These indirect methods are used to observe exoplanets because existing telescopes are not powerful enough to directly observe relatively small planets at such great distances.

Kepler
Meanwhile, several satellites have been placed in orbit around the Earth, all engaged in discovering new exoplanets. One of these satellites is Kepler. This telescope looks for light fluctuations from a star, which can be caused by a planet passing in front of the star. Kepler has already found nearly 9000 candidate exoplanets, 90% of which will likely be confirmed as exoplanets.

A second Earth
Scientists are naturally most interested in exoplanets that resemble our own Earth, as these are the most likely to harbor life. Enthusiasm often follows the discovery of a planet in the so-called habitable zone of a star. This habitable zone is at such a distance from the star that liquid water can exist. Closer to the star, it is too warm, and water evaporates; farther from the star, it is too cold, and water can only exist as ice. Liquid water is a prerequisite for the emergence of life. However, there are more criteria that a planet must meet for life to have a chance. For instance, it must be a rocky planet and have an atmosphere. There are more criteria a planet must meet to support life, but these were the most important.

The image accompanying this article is from ESO/M. Kornmesser and is created by: S. Guisard