Exoplanets

One of the central questions of ESA’s Cosmic Vision is: “What are the conditions for planet formation and the emergence of life?” ESA’s past, present and future exoplanet missions are designed to tackle this question.

An exoplanet is a planet outside our own Solar System, sometimes referred to as an extrasolar planet.

Humans have long speculated on the existence of other worlds, with the idea first mooted by the Ancient Greek philosophers and recurring through the Middle Ages and Renaissance. Searches for exoplanets began in earnest in the mid-20th Century. The first unambiguous discovery of an exoplanet orbiting a star like our Sun, in 1995, completely changed our perspective on the Solar System. A giant planet with a mass of about half that of Jupiter, subsequently named 51 Pegasi b, was found orbiting its host star in just over four days. The presence of such a massive planet in such a short orbit – much closer to its star than Mercury is to our Sun – was completely unexpected and did not fit with our then understanding of planet formation.

Imagine a gaseous exoplanet five times the size of Jupiter but much closer to its star as Mercury is to our Sun. This planet orbits its star in just a couple of days and always shows the same face towards it. Now, imagine a tiny rocky planet, only a third of the size of Earth orbiting its star in only 4.5 hours. These kinds of worlds really exist. Exoplanets vary in size, orbit, composition and more. But how can we know all these aspects?Different characterisation techniques have been developed and are adopted by ESA’s exoplanet missions. In this endeavor, thousands of exoplanets will be subject to a study of mass, size, density, composition and age. Below you can find explanations of the various characterisation techniques.