There are several techniques for discovering exoplanets (planets orbiting other stars). The first exoplanets were detected by the gravitational effect that they exerted on their host stars. More recent techniques have been able to image the planets. Each method is sensitive to specific types of exoplanets, and when we piece the information together, we can begin to understand the diversity of exoplanets. Humans have long wondered whether other solar systems with planets like our own Earth might exist among the billions of stars in our galaxy and this moment will go down as the time when we figured this out.
The first failures and successes
The discovery of worlds around other stars has a long history with many false starts. In the 1960's, Peter van de Kamp (1901 - 1995) interpreted a small wobble in the position of Barnard's star as an exoplanet. Observations by other astronomers contradicted that result, although van de Kamp never admitted that his claim was in error. In 1991, Lyne and Bailes reported the discovery of a planet orbiting the pulsar star PSR 1829-10 in the prestigious Nature journal. They had measured frequency of pulse arrival times and used the Doppler effect to infer the presence of a planet, but later realized that they had not properly accounted for the velocity of the Earth around the Sun. When Lyne retracted the result at a meeting of the American Astronomical Society in January 1992 he received a standing ovation for his scientific integrity and courage.
There were also a few signals that were initially published with alternative interpretations that later turned out to be exoplanets. In 1988, Campbell, Walker and Yang observed a periodic radial velocity signal in the red giant star, Gamma Cephei. They tentatively interpreted this as photospheric variability in the star, but additional data by Hatzes and colleagues in 2002 confirmed that this was indeed an orbiting exoplanet. Another example occurred in 1989 when Latham and colleagues published the discovery of a companion to the star HD 114762; the team cautiously interpreted this as a possible brown dwarf. However, by 2012, this object was reclassified as a massive exoplanet.
The first confirmed exoplanets were very peculiar. Aleksander Wolszczan and Dale Frail measured periodic variations the frequency of pulse arrival times to detect two small planets orbiting the pulsar neutron star, PSR 1257+12 in 1992. In 1994, they found one more planet in this system. These discoveries were puzzling because this planetary system should not have survived the supernova explosion of the host star before it evolved to become a spinning neutron star. The planets likely formed in a debris disk around the pulsar. In retrospect, perhaps that discovery should have told us that planet formation was a ubiquitous process. If planets can form around an exploding supernova, then we should have expected that exoplanets were common.
In 1952, astronomer Otto Struve made the remarkable assertion that if a Jupiter-like planet resided very close to its host star, that the gravitational tug of the planet on the star would produce radial velocity variations that might be detected in the stellar spectra with high enough precision measurements.
Twentieth century astronomers worked to improve the precision of their techniques and in 1995, the first exoplanet was finally discovered around a sun-like star. Most astronomers consider the dawn of exoplanets to be November 1995 when Michel Mayor and Didier Queloz discovered a gas giant planet orbiting the sun-like star 51 Pegasi at the Observatoire de Haute-Provence using the Doppler technique. The interpretation of this radial velocity signal as an exoplanet remained controversial for a few years, but is no longer questioned. 51 Peg b was the first confirmed detection of an exoplanet around a main sequence star and the 2019 Nobel Prize was awarded to Mayor and Queloz.