Artist concept of exoplanet 51 Pegasi b. Discovered on October 6, 1995, 51 Pegasi b showed that planets orbit stars similar to our own outside of the solar system. Image Credit: NASA / JPL-Caltech

Celebrating 20 Years Of Exoplanet Discoveries

20 years ago, astronomers discovered the first planet orbiting a sun-like star outside the solar system.

The groundbreaking discovery of the first planet to be found orbiting a star similar to the sun occurred 20 years ago with the discovery of 51 Pegasi b on Oct. 6, 1995. The discovery led to an entirely new field of study and started the search for planets similar to our own beyond the solar system.

51 Pegasi b resides approximately 50 light-years away in the constellation Pegasus. It is approximately half the size of Jupiter and orbits its host star once about every 4 days. It belongs to a category of planets known today as exoplanets. Exoplanets are a classification for planets that exist beyond the solar system which orbit stars, stellar remnants, or brown dwarfs.

The discovery of 51 Pegasi b proved that planets outside the solar system orbit sun-like stars. For centuries it has been suggested that planets may exist beyond our solar system, however, it wasn’t until recently that it became possible to detect them. 51 Pegasi b also altered theories on planet formation. It was previously thought that planets of its size could not orbit so close to their parent stars.

Infographic highlighting the characteristics of 51 Pegasi b, the first exoplanet discovered orbiting a sun-like star. Image Credit: NASA / JPL-Caltech

Infographic highlighting the characteristics of 51 Pegasi b, the first exoplanet discovered orbiting a sun-like star. Image Credit: NASA / JPL-Caltech

The confirmation of 51 Pegasi b as a planet on Oct. 12, 1995 marked a breakthrough that ended decades of searching for the first extrasolar planet and propelled the field of study forward with a sense of legitimacy. However, it actually was not the first planet to be discovered beyond the solar system.

The first confirmed detections of extrasolar planets came in 1992 with the discovery of two planets orbiting a pulsar known as PSR B1257+12, with a third confirmed orbiting it in 1994. However, because these planets orbit a pulsar, the stellar remnant of a supernova explosion, and not a main-sequence star like the sun, many consider 51 Pegasi b to be the first exoplanet discovery.

Since the discovery of 51 Pegasi b, more than 1,800 exoplanets have been confirmed, the majority of which were discovered using NASA’s Kepler Space Telescope. Kepler was launched in 2009 on a mission to discover planets similar to Earth in orbit around other stars.

Earlier this summer, NASA announced the discovery of the first Earth-sized planet to be found orbiting a sun-like star. The exoplanet, known as Kepler-452b, is approximately 60 percent larger than Earth and orbits a star that is only 10 percent larger and 20 percent brighter than the sun. Its orbital period is 385 days, less than 20 days longer than a year on Earth. [Read more: NASA Discovers First Earth-Sized Planet Orbiting Sun-Like Star]

Infographic highlighting the similarities between Earth (left) and Kepler-452b (right), as well as their respective host stars. Image Credit: NASA / JPL-Caltech / T. Pyle

Infographic highlighting the similarities between Earth (left) and Kepler-452b (right), as well as their respective host stars. Image Credit: NASA / JPL-Caltech / T. Pyle

Kepler-452b resides in the habitable zone of its host star and is only five percent further away from its star than Earth is from the sun. The habitable zone, sometimes referred to as the Goldilocks zone, is the region around a star where the conditions are “just right” for liquid water to be found on the surface of planets. The Slooh Community Observatory provided the public with a first glimpse at Kepler-452, the parent star of what the media has dubbed “Earth 2.0.” [Watch Slooh's Broadcast Of Kepler-452, Home To "Earth 2.0"]

Earlier this year, the Slooh Community Observatory made history providing the first live broadcast of an exoplanet observation to the public. Slooh observed exoplanet TrES-2b, also known as “Dark Knight,” which reflects only one percent of the light it receives from its parent star, making it the darkest exoplanet yet to be discovered. [Watch The First Live Broadcast Of An Exoplanet Observation]

Artist conception of TrES-2b orbiting its host star. Image Credit: Harvard-Smithsonian Center For Astrophysics

Artist conception of TrES-2b orbiting its host star. Image Credit: Harvard-Smithsonian Center For Astrophysics

In 20 years of since the first discovery, more than 5,000 candidate planets have been discovered, with more than 3,000 yet to be confirmed. The pace of discovery has accelerated rapidly thanks to spacecraft like NASA’s Kepler observatory devoted to the search for alien worlds.

Exoplanet observations will be assisted by the James Webb Space Telescope, the successor to Hubble, which will be able to identify additional characteristics of exoplanets, such as their color, seasonal differences, weather, rotation and even vegetation. The James Webb Space Telescope is scheduled for launch in 2018.