SUPER-EARTHS

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants.^[1] The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability. An alternative term "Gas Dwarf" may be more accurate for some examples, especially higher mass ones as suggested by MIT professor Sara Seager. Alternatively, Super-Venus or Super-Pluto might be used for super-hot or cold rocky exoplanets to give a less potentially misleading label.

Definition

In general, Super-Earths are defined exclusively by their mass, and the term does not imply temperatures, compositions, orbital properties, or environments similar to Earth's. A variety of specific mass values are cited in definitions of super-Earths. While sources generally agree on an upper bound of 10 Earth masses,^[1][2][3] (~69% the mass of the smallest Solar System gas giant Uranus), the lower bound varies from 1^[1] or 1.9^[3] to 5^[2], with various other definitions appearing in the popular media.^[4][5][6] Some authors further suggest that the term be limited to planets without a significant atmosphere.^[7] Planets above 10 Earth masses are termed giant planets^[8].

Discoveries

The Solar System does not contain examples of this category of planets, as the largest terrestrial planet in the Solar System is the Earth, and all larger planets have at least 14 times Earth's mass.

First Super-Earth Found

Mass and radius values for transiting super-Earths in context of other detected exoplanets and selected composition models.

The first super-Earths were discovered by Aleksander Wolszczan and Dale Frail around the pulsar PSR B1257+12 in 1991. The two outer planets of the system have masses approximately 4 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 (two Jupiter sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf), it may have a surface temperature of 430–650 kelvin^[9] and may support liquid water.^[10]

Other Super-Earths Discovered in 2006

Two further 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.^[1]

First Super-Earth in Habitable Zone

In April 2007, a team headed by Stephane Udry based in Switzerland announced the discovery of two new super-Earths around Gliese 581,^[11] both on the edge of the habitable zone around the star where liquid water may be possible on the surface. With Gliese 581 c having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical units (AU; 6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without taking into consideration effects from an atmosphere) of −3 degrees Celsius with an albedo comparable to Venus and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggests Gliese 581 c has likely suffered a runaway greenhouse effect like Venus, but that its sister planet, Gliese 581 d, does in fact lie within the star's habitable zone, with an orbit at 0.22 AU and a mass of 7.7 Earths.

Super-Earths Found in 2008

The smallest Super-Earth found to date is MOA-2007-BLG-192Lb was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2, 2008.^[12][13] 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 our Sun. The 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 (High Accuracy Radial Velocity Planet Searcher) in Chile.^[14]

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 every three years.^[15]

Super-Earths Found in 2009

Illustration of the inferred size of the super-Earth COROT-7b (center) in comparison with Earth and Neptune.

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 the four inner planets of Earth's solar system, a new and significant discovery.^[16] 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.^[17]

The discovery of Gliese 581 e with a minimum mass of 1.9 Earth masses was announced on April 21, 2009. It is 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,^[18] and may have 100 times more tidal heating than Jupiter’s volcanic satellite Io.^[19]

Additionally, Gliese 581 d, at 0.2 AU with a 67 day orbital period, has been confirmed to be within the habitable zone of the red dwarf star, making it the first exoplanet where the existence of liquid water is a real possibility.^[18]

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 our 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.^[20]

By November 2009, a total of 30 Super-Earths have been discovered, 24 of which were first observed by HARPS.^[21]

Super-Earths Found in 2010

Discovered on January 5, 2010, a planet HD 156668 b with a minimum mass of 4.15 Earth masses, is the second least massive planet detected by the radial velocity method.^[22] The only confirmed radial velocity planet smaller than this planet is Gliese 581 e at 1.9 Earth masses (see above). On August 24 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.^[23] Although unconfirmed, there is 98.6% probability that this planet does exist.^[24]

The National Science Foundation announced on September 29 the discovery of a fourth super-Earth orbiting the M dwarf Gliese 581. Planet g 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.^[25][26][27]

Characteristics

Comparison of sizes of planets with different compositions

Due to the larger mass of super-Earths, their physical characteristics differ from Earth's. A study on Gliese 876 d by a team around Diana Valencia^[1] revealed it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition of a relevant super-Earth is. For Gliese 876 calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9g and 3.3g (19 and 32 m/s²). High surface gravity (generally higher than Neptune-, Uranus-, and Saturn-class planets, and in certain circumstances higher than Jupiter-class planets) is one of the predominant known characteristics of super-Earths.

Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics.^[28] However, other studies determine that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibits plate tectonics. The planet's surface would be too strong for driving forces of magma to break the crust into plates.^[29]

Temperatures

Since the atmospheres and greenhouse effects of Super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black body temperature of the Earth is 254.3 K (−19 °C or −2 °F ).^[30] It is the greenhouse gases that keep the Earth warmer. Venus has a black-body temperature of only 231.7 K (−41 °C or −43 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ).^[31] Though the atmosphere of Venus traps more heat than Earth, NASA lists the black body temperature of Venus based on the fact Venus has an extremely high albedo, giving it a lower black body temperature than the more absorbent (lower albedo) Earth.