This artist’s impression shows the super-Earth exoplanet  55 Cancri e in front of its parent star. 55 Cancri e is about 40 light-years away and orbits a star slightly smaller, cooler, and less bright than our Sun. As the planet is so close to its parent star, one year lasts only 18 hours, and temperatures on the surface are thought to reach around 2000 degrees Celsius. Credit: ESA/Hubble, M. Kornmesser

While there is currently no formal agreement on what exactly defines an exoplanet, the word is used to refer to planet-sized bodies that are located beyond our Solar System. By convention, exoplanets have sufficient mass to maintain a roughly spherical shape, but not so much mass that nuclear fusion is triggered in them (as is the case for stars). They might orbit one star, a pair of stars, or multiple stars — or not orbit a star at all. The categorization of exoplanets is also not formalized, but certain categories are typically used, including Super Earths, Hot Jupiters, and Mini Neptunes. The discovery of the first exoplanet was only confirmed in 1992, when two exoplanets were discovered orbiting a pulsar.

In 2001, astronomers using Hubble announced the first direct detection of the atmosphere of a planet orbiting a star outside our Solar System. The planet orbits a yellow, Sun-like star called HD 209458, located 150 light-years away. The planet was not seen directly by Hubble. Rather, the presence of sodium, as well as evaporating hydrogen, oxygen, and carbon, was detected in light filtered through the planet’s atmosphere when it passed in front of its star as seen from Earth. These observations demonstrated that with Hubble and other telescopes it is possible to measure the chemical makeup of alien planet atmospheres and to search for the chemical markers of life beyond Earth.