Exoplanets are planets in solar systems other than our own. The first definitive confirmation of an exoplanet was in 1992, and the first exoplanet orbiting a main sequence star was discovered in 1995. Hot Jupiters and super-Earths are two classes of exoplanets that have been discovered, with Gliese 581 c being the first discovered in the habitable zone.
An extrasolar planet, or exoplanet, is a planet in a solar system other than our own. We always assumed they existed, but it wasn’t until 1992 that the presence of an extrasolar planet was definitively confirmed. Another discovery was made slightly earlier, in 1988, but would not be confirmed until 2003. The 1992 discovery, made by radio astronomers Aleksander Wolszczan and Dale Frail, uncovered a planetary system around the rapidly rotating pulsar PSR 1257+ 12. These unusual planets are thought to have formed from the remnants of the supernova explosion that created the pulsar, or to be the rocky cores of gas giants whose atmospheres were stripped away by the supernova.
Another milestone in exoplanet astronomy came on October 6, 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced that they had discovered an exoplanet orbiting a main sequence (Sun-like star), 51 Pegasi, about 50 light years away. The planet was discovered using the radial velocity method, which uses a sensitive spectroscope to detect regular velocity changes in the spectral lines of a star with magnitudes up to 70 meters per second. The planet was named 51 Pegasi b, a naming convention used for all exoplanets. The first extrasolar planet orbiting a star is designated “b”, the second “c”, the third “d”, and so on.
51 Pegasi b was an interesting example of an exoplanet because it was very massive, about half the mass of Jupiter – about 150 times that of Earth – but orbited extremely close to its parent star, just 0.05 AU, about eighth as small as distance between Mercury and the Sun. 51 Pegasi b orbits its parent star every four days. This discovery forced astronomers to recognize that such a massive, small-orbiting planet could exist. Before that, massive planets were thought to form further out in solar systems, as we observe in our own.
This class of planets became known as “Hot Jupiters” due to their great heat and mass. In the years since, many more Hot Jupiters have been discovered, forcing scientists to investigate how so many planets could engage in orbital migration, forming in the outer reaches of the star system, then migrating inward. The question still does not have a complete answer.
Another class of discovered exoplanets is known as super-Earths. Due to the sensitivity of our current detection methods, these are the smallest extrasolar planets we have discovered: planets with between 2 and 10 Earth masses (although the definition varies). The first two exoplanets discovered, PSR B1257+12 b and PSR B1257+12 c, are super-Earths, each with about 4 times the mass of Earth. Since then many more Super-Earths have been discovered. Most notable is Gliese 581 c, discovered in April 2007 by a team led by Switzerland-based Stephane Udry.
This was the first Super-Earth discovered in the star’s habitable zone, meaning its surface temperature is likely between 0 and 40 degrees Celsius, without factoring in any possible atmospheres. This is the first exoplanet discovered that could be habitable for humans, and it won’t be the last.
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