The first exoplanet evidence sat unrecognised in a 1917 photograph for a century
In 1917 Walter Adams recorded the spectrum of a white dwarf called Van Maanen 2. Only in 2016 did astronomers realise its odd chemistry probably came from a shredded planet raining dust onto the star. Today the confirmed tally exceeds 6,300 worlds, and billions of habitable-zone planets may orbit Milky Way stars.
The idea is old. Giordano Bruno argued in the 16th century that the fixed stars are suns with planets of their own, and Isaac Newton floated the same possibility at the close of his Principia. In 1952, over four decades before anyone found one, Otto Struve suggested giant planets might circle very close to their stars and could be caught by Doppler shifts or by dimming as they cross in front. Mid-century claims around Barnard's Star and 61 Cygni did not survive scrutiny.
The accepted breakthroughs came in the 1990s. In 1992 Aleksander Wolszczan and Dale Frail announced two Earth-mass planets around the pulsar PSR B1257+12, a dead star. In 1995 a giant planet was found racing around the ordinary star 51 Pegasi in just four days. As of 20 August 2026 the count stands at 6,354 confirmed exoplanets in 4,756 systems. Most were found by transits or radial velocity, methods biased toward big planets hugging their stars.
The range is startling. Draugr weighs roughly twice the Moon, while HR 2562 b, around 30 Jupiter masses, may really be a brown dwarf. Orbits last from under an hour to thousands of years. The nearest known exoplanets orbit Proxima Centauri, 4.2 light-years away, and microlensing has hinted at planets in another galaxy. Estimates suggest about one in five Sun-like stars hosts an Earth-sized planet in its habitable zone, perhaps 11 billion in the Milky Way, or 40 billion counting red dwarfs.
Defining a planet is harder than it sounds. The IAU's official definition covers only the Solar System, so its working rule for exoplanets uses the deuterium-fusion limit of about 13 Jupiter masses as the ceiling. Critics call that line physically arbitrary; catalogues set their own limits of 24, 30 or even 60 Jupiter masses, and some prefer to separate planets from brown dwarfs by how they formed. Names follow the host star plus a letter, starting with b.
Source: Exoplanet
