section_path: History and etymology/Exoplanets
section_name: Exoplanets
paragraph_text: Exoplanets
  
  Even before the discovery of exoplanets, there were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium. Complicating the matter even further, bodies too small to generate energy by fusing deuterium can form by gas-cloud collapse just like stars and brown dwarfs, even down to the mass of Jupiter: there was thus disagreement about whether how a body formed should be taken into account.
  
  In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on 6 October 1995, Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).
  
  The discovery of exoplanets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known exoplanets are many times the mass of Jupiter, approaching that of stellar objects known as brown dwarfs. Brown dwarfs are generally considered stars due to their theoretical ability to fuse deuterium, a heavier isotope of hydrogen. Although objects more massive than 75 times that of Jupiter fuse simple hydrogen, objects of 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, constituting less than 0.0026% of the hydrogen in the galaxy, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.