For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the
Solar System. Various detection claims made in the nineteenth century were rejected by astronomers. The first evidence of a possible exoplanet, orbiting
Van Maanen 2, was recorded in 1917, but was not recognized as such until 2016. The astronomer
Walter Sydney Adams produced a spectrum of the star using
Mount Wilson's 60-inch telescope which he interpreted the spectrum to be of an
F-type main-sequence star. This spectrum was reexamined during studies of
white dwarf stars with unpredicted compositions. It is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star. Numerous other claims of discovery took place in the mid 20th century, involving 61 Cygnus, Lalande 21185, and Barnard's Star, which were not discredited until the mid to late 1970s (see Discredited claims below). Another
suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first detection that is currently accepted came in 1992 when
Aleksander Wolszczan and
Dale Frail announced the discovery of two terrestrial-mass planets orbiting the
millisecond pulsar PSR B1257+12. In February 2018, researchers using the
Chandra X-ray Observatory, combined with a planet detection technique called
microlensing, found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion."
Early speculations In the sixteenth century, the Italian philosopher
Giordano Bruno, an early supporter of the
Copernican theory that Earth and other planets orbit the Sun (
heliocentrism), put forward the view that fixed stars are similar to the Sun and are likewise accompanied by planets. In the eighteenth century, the same possibility was mentioned by
Isaac Newton in the "
General Scholium" that concludes his
Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of
One." In 1938, D.Belorizky demonstrated that it was realistic to search for exo-Jupiters by using
transit photometry. In 1952, more than 40 years before the first
hot Jupiter was discovered,
Otto Struve wrote that there is no compelling reason that planets could not be much closer to their parent star than is the case in the Solar System, and proposed that
Doppler spectroscopy and the
transit method could detect
super-Jupiters in short orbits.
Discredited claims Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the
binary star 70 Ophiuchi. In 1855,
William Stephen Jacob at the
East India Company's
Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s,
Thomas J. J. See of the
University of Chicago and the
United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year
period around one of the stars. However,
Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable. Multiple claims have been made that
61 Cygni might have a planetary system.
Kaj Strand of the Sproul Observatory in 1942 observed tiny but systematic variations in the orbital motions of 61 Cygni A and B, suggesting that a third body of about 16 Jupiter masses must be orbiting 61 Cygni A. Multiple further claims were made, but more recent observations have yet to find confirmation. More information at
61 Cygni § Claims of a planetary system. Around the same time that 61 Cygni was being investigated, similar claims about the presence of exoplanets were made about
Lalande 21185:
Lalande 21185 § Past claims of planets. During the 1950s and 1960s,
Peter van de Kamp of
Swarthmore College made another prominent series of detection claims, this time for planets orbiting
Barnard's Star. Astronomers now generally regard all early reports of detection as erroneous. In 1991,
Andrew Lyne,
M. Bailes and S. L. Shemar claimed to have discovered a
pulsar planet in orbit around
PSR 1829-10, using
pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.
Confirmed discoveries ic image of
AB Pictoris showing a companion (bottom left), which is either a brown dwarf or a massive planet. The data were obtained on 16 March 2003 with
NACO on the
VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris. As of , a total of confirmed exoplanets are listed in the
NASA Exoplanet Archive, including a few that were confirmations of controversial claims from the late 1980s. The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the
University of Victoria and the
University of British Columbia. Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star
Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been
brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed. On 9 January 1992, radio astronomers
Aleksander Wolszczan and
Dale Frail announced the discovery of two planets orbiting the
millisecond pulsar PSR 1257+12 based on the variability of timing of the pulses. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the
supernova that produced the pulsar, in a second round of planet formation, or else to be the
remaining rocky cores of
gas giants that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that a planet could form in their orbit. In the early 1990s, a group of astronomers led by
Donald Backer, who were studying what they thought was a binary pulsar (
PSR B1620−26 b), determined that a third object was needed to explain the observed
Doppler shifts. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and
white dwarf had been measured, giving an estimate of the mass of the third object that was too small to be a star. The conclusion that the third object was a planet was announced by
Stephen Thorsett and his collaborators in 1993. On 6 October 1995,
Michel Mayor and
Didier Queloz of the
University of Geneva announced the first definitive detection of
an exoplanet orbiting a
main-sequence star, nearby
G-type star 51 Pegasi. This discovery, made at the
Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019
Nobel Prize in Physics. Technological advances, most notably in high-resolution
spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their
gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it. In 1999,
Upsilon Andromedae became the first main-sequence star known to have multiple planets.
Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system. On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the
Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity". Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they were more easily detected, but the
Kepler planets are mostly between the size of Neptune and the size of Earth. On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo. This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called
small planet radius gap. The gap, sometimes called the Fulton gap, is the observation that it is unusual to find exoplanets with sizes between 1.5 and 2 times the radius of the Earth.
Candidate discoveries As of January 2020, NASA's
Kepler and
TESS missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars. In September 2020, astronomers reported evidence, for the first time, of an
extragalactic planet,
M51-ULS-1b, detected by eclipsing a bright
X-ray source (XRS), in the
Whirlpool Galaxy (M51a). == Detection methods ==