Gliese 581 System

Gliese 581 (pronounced /ˈɡliːzə/) is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggested that the star has six planets: Gliese 581 e, b, c, g, d and f. Significantly, planet g would be close to the middle of the star's habitable zone.

History
The star system first gained attention after Gliese 581 c, the first low mass extrasolar planet found to be near its star's habitable zone, was discovered in April 2007. It has since been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, and hence is probably too hot to be habitable, analogous to Venus. A subsequently discovered planet Gliese 581 d, may be just inside or just outside the outer boundary of the habitable zone (depending in part on the greenhouse properties of its atmosphere), analogous to Mars. The discovery of exoplanet Gliese 581 e, the lightest-known planet around a normal star, was announced in April 2009. Excitement spiked again in September 2010 with the claimed discovery of Gliese 581 g, orbiting between c and d, believed to be the planet with the greatest likelihood of having conditions suitable for liquid water at its surface found to date because it is within the middle of the habitable zone.

Star
The name Gliese 581 refers to the catalog number from the 1957 survey Gliese Catalogue of Nearby Stars of 965 stars located within 20 parsecs of the Earth. Other names of this star include BD-07° 4003 (BD catalogue, first known publication) and HO Librae (variable star designation). It does not have an individual name such as Sirius or Procyon. The star is a red dwarf with spectral type M3V, located 20.3 light-years away from Earth. It is located about two degrees north of Beta Librae, the brightest star in the constellation Libra. Its mass is estimated to be approximately a third that of the Sun, and it is the 89th closest known star system to the Sun. An M-class dwarf star such as Gliese 581 has a much lower mass than the Sun, causing the core region of the star to fuse hydrogen at a significantly lower rate. From the apparent magnitude and distance, we can estimate an effective temperature of 3200 kelvins and a visual luminosity of 0.2% of that of the Sun. However, a red dwarf such as Gliese 581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly 830 nanometres (estimated using Wien's displacement law, which assumes the star radiates as a black body), so such an estimate will underestimate the star's total luminosity.

Gliese 581 E (Vulcan)
Gliese 581 E during the 2027 NASA Libra Interstellar Mission was one of three planets in which research priority was put to a minimum. Being the closest planet to the star Gliese 581, it is too hot to sustain water or lifeforms. Three satellites orbit 581 E which have performed various tests, mapping operations and measurements. 581 E was named after the Roman god of the forge, fire and blacksmiths, Vulcan.

Gliese 581 B (Mephitis)
Gliese 581 B is the second furthest planet from the Gliese 581 star and a gas giant similar to Neptune. Three satellites orbit 581 B which have performed tests, measurements and mapping operations. 581 B was named after the Roman god of poisonous gases and volcanic vapors, Mephitis.

Gliese 581 C (Cacus)
Gliese 581 C is a planet on the outside edge of the habitable zone, closer to the sun with temperatures too hot to sustain life. Similar to Venus in the sense it possesses a runaway greenhouse effect, the surface temperature has been recorded at a maximum of 540 degrees Fahrenheit. 581 C has three satellites in orbit which performed tests, mapping of the surface and measurements of the atmosphere, magnetic and gravitational fields and rovers have been deployed to the surface to test rock samples. 581 C can theoretically be terraformed through atmosphere replacement and the introduction of water sources from the 581 system's further ice planet, 581 D. Such a process for the planet, classified as a terrestrial super Earth, could take decades. 581 C was named after the Roman ancient god of fire, Cacus.

Gliese 581 G (Flora)
Gliese 581 G is the primary focus of current NASA and Union of Everett military exploration and research. 581 G is the only Earth-like planet discovered so far at slightly larger in size to Earth. 581 G has an average cooler temperature capable of sustaining plant and animal life. The 2027 Libra Interstellar Mission focused on 581 G, deploying the EVSS Discovery Scientific Cruiser. Satellites were initially deployed into orbit to take measurements and perform test while a series of unmanned vehicles were deployed to the surface including aerial vehicles, submersibles for the oceans and ground rovers. Following confirmation of an oxygen atmosphere capable of sustaining life, the EVSS Discovery sent astronauts to the surface. Over the next three years into 2030, a large research base and small scientific community colony had been established, containing the largest non-Earth population of Everetti citizens at nearly 35,000 residents. The Everetti interstellar military installation SPCCOM Libra orbits Gliese 581 G. During 2029 and 2030, NASA invited the European Space Agency to join in Gliese 581 G's exploration, research and colonization. Currently, the Union of Everett only authorizes scientific researchers to live on 581 G. 581 G was named after the Roman goddess of flowers, Flora.

Gliese 581 D (Venelia)
Gliese 581 D lies on the outer edge of the habitable zone, making it an ice planet with open freezing oceans and an average temperature equal to that of Earth's Antarctica. Following deployment and establishment of bases on 581 G, the EVSS Discovery deployed satellites into 581 D's orbit and sent unmanned vehicles to the surface including ground rovers, aerial vehicles and submersibles. 581 D's atmosphere consists of less oxygen content than neigboring 581 G, forcing astronauts to use respirators when exploring the surface. A base has been established on the surface near the equator, containing almost 500 researchers. 581 D contains sea life and oceanic plant life. No plant or animal life have been found yet on the ground surface. 581 D can theoretically be terraformed through atmospheric replacement which would create a stronger greenhouse effect to warm global temperatures and melt away the ice surface. Additionally, water and ice from 581 D can be transported to 581 C during its own terraforming to provide water to the barren planet. The heating process is estimated to take a decade. The melted ice would force the establishment of a floating colony as most of the planet's surface would be covered in water. 581 D was named after the Roman sea goddess wife of Neptune, Venelia.

Gliese 581 F (Salacia)
Gliese 581 F is the furthest planet from its star and a large terrestrial planet, known as a super Earth. 581 F is outside of the habitable zone making it too cold to sustain life but possesses a significant amount of water ice. Three satellites orbit 581 F and several rovers have been deployed to the surface. 581 F was named after the Roman goddess of seawater, Salacia.