Super-Earth
A super-Earth is a type of exoplanet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's mass respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.
Definition
In general, super-Earths are defined by their masses. The term does not imply temperatures, compositions, orbital properties, habitability, or environments. While sources generally agree on an upper bound of 10 Earth masses, the lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. The term "super-Earth" is also used by astronomers to refer to planets bigger than Earth-like planets, but smaller than mini-Neptunes.This definition was made by the Kepler space telescope personnel.
Some authors further suggest that the term super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in the Solar System do not have.
Planets above 10 Earth masses are termed massive solid planets, mega-Earths, or gas giant planets, depending on whether they are mostly made of rock and ice or mostly gas.
History and discoveries
First
The first super-Earths were discovered by Aleksander Wolszczan and Dale Frail around the pulsar PSR B1257+12 in 1992. The two outer planets of the system have masses approximately four times Earth—too small to be gas giants.The first super-Earth around a main-sequence star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876 and received the designation Gliese 876 d. It has an estimated mass of 7.5 Earth masses and a very short orbital period of about 2 days. Due to the proximity of Gliese 876 d to its host star, it may have a surface temperature of 430–650 kelvin and be too hot to support liquid water.
First in habitable zone
In April 2007, a team headed by Stéphane Udry based in Switzerland announced the discovery of two new super-Earths within the Gliese 581 planetary system, both on the edge of the habitable zone around the star where liquid water may be possible on the surface. With Gliese 581c having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical units, it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature of −3 degrees Celsius with an albedo comparable to Venus and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggested Gliese 581c had likely suffered a runaway greenhouse effect like Venus.Others by year
2006
Two further possible super-Earths were discovered in 2006: OGLE-2005-BLG-390Lb with a mass of 5.5 Earth masses, which was found by gravitational microlensing, and HD 69830 b with a mass of 10 Earth masses.2008
The smallest super-Earth found as of 2008 was MOA-2007-BLG-192Lb. The planet was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2, 2008. This planet has approximately 3.3 Earth masses and orbits a brown dwarf. It was detected by gravitational microlensing.In June 2008, European researchers announced the discovery of three super-Earths around the star HD 40307, a star that is only slightly less massive than the Sun. Planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by the radial velocity method by the HARPS in Chile.
In addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD 181433. This star also has a Jupiter-like planet that orbits it every three years.
2009
Planet COROT-7b, with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals similar to that of the Solar System's four inner planets, a new and significant discovery. COROT-7b, discovered right after HD 7924 b, is the first super-Earth discovered that orbits a main sequence star that is G class or larger.The discovery of Gliese 581e with a minimum mass of 1.9 Earth masses was announced on 21 April 2009. It was at the time the smallest extrasolar planet discovered around a normal star and the closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone, and may have 100 times more tidal heating than Jupiter's volcanic satellite Io.
A planet found in December 2009, GJ 1214 b, is 2.7 times as large as Earth and orbits a star much smaller and less luminous than the Sun. "This planet probably does have liquid water," said David Charbonneau, a Harvard professor of astronomy and lead author of an article on the discovery. However, interior models of this planet suggest that under most conditions it does not have liquid water.
By November 2009, a total of 30 super-Earths had been discovered, 24 of which were first observed by HARPS.
2010
Discovered on 5 January 2010, a planet HD 156668 b with a minimum mass of 4.15 Earth masses, is the least massive planet detected by the radial velocity method. The only confirmed radial velocity planet smaller than this planet is Gliese 581e at 1.9 Earth masses. On 24 August, astronomers using ESO's HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star, HD 10180, one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be the lowest mass of any exoplanet found to date orbiting a main-sequence star. Although unconfirmed, there is a 98.6% probability that this planet does exist.The National Science Foundation announced on 29 September the discovery of a fourth super-Earth orbiting within the Gliese 581 planetary system. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington. However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed at The Extrasolar Planets Encyclopaedia.
2011
On 2 February, the Kepler Space Observatory mission team released a list of 1235 extrasolar planet candidates, including 68 candidates of approximately "Earth-size" and 288 candidates of "super-Earth-size". In addition, 54 planet candidates were detected in the "habitable zone." Six candidates in this zone were less than twice the size of the Earth A more recent study found that one of these candidates is in fact much larger and hotter than first reported. Based on the latest Kepler findings, astronomer Seth Shostak estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds." Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.On 17 August, a potentially habitable super-Earth HD 85512 b was found using the HARPS as well as a three super-Earth system 82 G. Eridani. On HD 85512 b, it would be habitable if it exhibits more than 50% cloud cover. Then less than a month later, a flood of 41 new exoplanets, including 10 super-Earths, were announced.
On 5 December 2011, the Kepler space telescope discovered its first planet within the habitable zone or "Goldilocks region" of its Sun-like star. Kepler-22b is 2.4 times the radius of the Earth and occupies an orbit 15% closer to its star than the Earth to the Sun. This is compensated for, however, as the star, with a spectral type G5V, is slightly dimmer than the Sun. Thus, surface temperatures would still allow liquid water on its surface.
On 5 December 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data.
In 2011, a density of 55 Cancri Ae was calculated which turned out to be similar to Earth's. At the size of about 2 Earth radii, it was the largest planet until 2014, which was determined to lack a significant hydrogen atmosphere.
On 20 December 2011, the Kepler team announced the discovery of the first Earth-size exoplanets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20.
Planet Gliese 667 Cb was announced by HARPS on 19 October 2009, together with 29 other planets, while Gliese 667 Cc was included in a paper published on 21 November 2011. More detailed data on Gliese 667 Cc were published in early February 2012.