Super-Earths
Exoplanets more massive than Earth but lighter than ice giants.
Super-Earths are a class of exoplanets defined by their mass, which is higher than Earth's but substantially below that of Uranus and Neptune (14.5 and 17.1 Earth masses, respectively). The term refers only to mass and does not imply anything about surface conditions, composition, or habitability. Some astronomers also use the term for planets with radii between about 1.25 and 2 Earth radii, a definition made by the Kepler space telescope personnel. The first super-Earths were discovered in 1992 orbiting a pulsar, with masses roughly four times that of Earth. The first such planet around a main-sequence star was found in 2005, orbiting the red dwarf Gliese 876 with an estimated mass of 7.5 Earth masses and a very short orbital period of about two days, making it too hot for liquid water. In 2007, two super-Earths were announced within the Gliese 581 system, both on the edge of the habitable zone where liquid water might be possible. One of these, Gliese 581c, has a mass of at least five Earth masses and an estimated mean temperature ranging from -3 to 40 degrees Celsius depending on albedo, though later research suggested it likely suffered a runaway greenhouse effect like Venus. Other notable discoveries include a 2006 planet found by gravitational microlensing with a mass of 5.5 Earth masses, and three super-Earths around HD 40307 in 2008 with minimum masses of 4.2, 6.7, and 9.4 Earth masses. The first super-Earth orbiting a G-class or larger main-sequence star, COROT-7b, was announced in 2009 with a mass of 4.8 Earth masses and a density suggesting a rocky composition similar to the Solar System's inner planets. That same year, Gliese 581e was announced with a minimum mass of 1.9 Earth masses, then the smallest known extrasolar planet.
- Definition
- Mass higher than Earth's, below ~10 Earth masses; radius 1.25–2 Earth radii (Kepler definition)
- First discovered
- 1992, around pulsar PSR B1257+12 by Aleksander Wolszczan and Dale Frail
- First in habitable zone
- 2007, Gliese 581c (at least 5 Earth masses)
- Smallest confirmed by radial velocity (a
- Gliese 581e (1.9 Earth masses)
- Notable detection methods
- Radial velocity, gravitational microlensing, transit (Kepler)
Lore & Background
Super-Earths are a class of exoplanet defined primarily by mass, ranging from roughly one to ten times that of Earth, placing them between Earth and the Solar System's ice giants, Uranus and Neptune. The term refers only to mass and does not imply any specific surface conditions, composition, or habitability. Some astronomers further restrict the definition to planets with solid surfaces or oceans, excluding those with thick, gas-dominated atmospheres, which are often called mini-Neptunes. The first super-Earths were discovered in 1992 orbiting the pulsar PSR B1257+12, with two outer planets each about four Earth masses. The first found around a main-sequence star was Gliese 876 d in 2005, a 7.5-Earth-mass planet with a two-day orbit and surface temperatures too high for liquid water. In 2007, Gliese 581c, at least five Earth masses, was announced on the warm edge of its star's habitable zone, though later studies indicated it likely experienced a runaway greenhouse effect. Gliese 581e, with a minimum mass of 1.9 Earth masses, was the smallest exoplanet around a normal star at its 2009 discovery. Other notable examples include COROT-7b, a rocky super-Earth with a density suggesting silicate composition, and HD 40307's three-planet system with masses between 4.2 and 9.4 Earth masses. Super-Earths are among the most common exoplanet types detected.
Reader's Guide
Super-Earths represent a critical category in exoplanet science because they bridge the gap between terrestrial planets and gas giants. Their study has revealed that planets with masses between Earth and Neptune are common in the galaxy, challenging earlier assumptions about planetary system architectures. The term's focus on mass rather than composition or habitability has allowed astronomers to classify a wide range of worlds, from rocky planets like COROT-7b to those that may be 'mini-Neptunes' with thick atmospheres. The Kepler mission's radius-based definition further refined the category, enabling statistical studies of planet occurrence rates. Disputes over specific planets, such as Gliese 581g (questioned by another team and listed as unconfirmed), highlight the challenges of detection and confirmation. The discovery of super-Earths in habitable zones, like Gliese 581c and Kepler-22b, has fueled interest in the potential for liquid water and life, though many such planets may be too hot or have runaway greenhouse effects. Overall, super-Earths have reshaped our understanding of planetary formation and the diversity of worlds beyond the Solar System.
Did You Know?
- The first super-Earths were discovered in 1992 around the pulsar PSR B1257+12 by Aleksander Wolszczan and Dale Frail.
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