Gliese 581c (; Gl 581c or GJ 581c) is an exoplanet orbiting within the Gliese 581 system. It is the second planet discovered in the system and the third in order from the star. With a mass about 6.8 times that of the Earth, it is classified as a super-Earth (a category of planets with masses greater than Earth's up to ten Earth masses). At the time of its discovery in 2007, Gliese 581c gained interest from astronomers because it was reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and, by extension, potentially capable of supporting extremophile forms of Earth-like life. However, further research cast doubt upon the planet's habitability. Based on newer models of the habitable zone, the planet is likely too hot to be potentially habitable. In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.55 light-years (194 trillion kilometres; 121 trillion miles) in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give its exact location in the Milky Way.
Discovery The team released a paper of their findings dated 27 April 2007, published in the July 2007 journal Astronomy & Astrophysics. At the time of discovery, it was reported to be the first potentially Earth-like planet in the habitable zone of its star and the smallest-known exoplanet around a main-sequence star, but on 21 April 2009, another planet orbiting Gliese 581, Gliese 581e, with an approximate mass of 1.9 Earth masses, was announced. In the paper, they also announced the discovery of another planet in the system, Gliese 581d, with a minimum mass of 7.7 Earth masses and a semi-major axis of 0.25 astronomical units.
Physical characteristics
Mass The existence of Gliese 581c and its mass have been measured by the radial velocity method of detecting exoplanets. The mass of a planet is calculated by the small periodic movements around a common centre of mass between the host star Gliese 581 and its planets. When all planets are fitted with a Keplerian solution, the minimum mass of the planet is determined to be 5.5 Earth masses. The radial velocity method cannot by itself determine the true mass, but it cannot be very much larger than this or the system would be dynamically unstable. Dynamical simulations of the Gliese 581 system which assume the orbits of the planets are coplanar indicate that the planets cannot exceed approximately 1.6 to 2 times their minimum masses or the planetary system would be unstable (this is primarily due to the interaction between planets e and b). For Gliese 581c, the upper bound is 10.4 Earth masses. A 2024 study determined the inclination of the planet, allowing its true mass to be determined, which is about 30% greater than the minimum mass at about 6.8 Earth masses.
Radius Since Gliese 581c has not been detected in transit, there are no measurements of its radius. Furthermore, the radial velocity method used to detect it only puts a lower limit on the planet's mass, which means theoretical models of planetary radius and structure can only be of limited use. However, assuming a random orientation of the planet's orbit, the true mass is likely to be close to the measured minimum mass. Assuming that the true mass is the minimum mass, the radius may be calculated using various models. For example, if Gliese 581c is a rocky planet with a large iron core, it should have a radius approximately 50% larger than that of Earth, according to Udry's team. Gravity on such a planet's surface would be approximately 2.24 times as strong as on Earth. However, if Gliese 581c is an icy and/or watery planet, its radius would be less than 2 times that of Earth, even with a very large outer hydrosphere, according to density models compiled by Diana Valencia and her team for Gliese 876 d. Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth. They claim the real value of the radius may be anything between the two extremes calculated by density models outlined above. Other scientists' views differ. Sara Seager at MIT has speculated that Gliese 581c and other five-Earth-mass planets could be:
"rock giants" mostly of silicate; "cannonball" planets of solid iron; "gas dwarfs" mostly of helium and hydrogen; carbon-rich "diamond worlds"; purely hot "ice VII worlds"; purely "carbon monoxide worlds". If the planet transits the star as seen from the direction of the Earth, the radius should be measurable, albeit with some uncertainty. Unfortunately, measurements made with the Canadian-built MOST space telescope indicate that transits do not occur. The new research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold onto their large atmospheres. Rather than evolving to a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope.
Orbit
Gliese 581c has an orbital period ("year") of 13 Earth days and its orbital radius is only about 7% that of the Earth, about 11 million km, while the Earth is 150 million km from the Sun. Since the host star is smaller and colder than the Sun—and thus less luminous—this distance places the planet on the "warm" edge of the habitable zone around the star according to Udry's team. Note that in astrophysics, the "habitable zone" is defined as the range of distances from the star at which a planet could support liquid water on its surface: it should not be taken to mean that the planet's environment would be suitable for humans, a situation which requires a more restrictive range of parameters. In any case, based on newer models of the habitable zone, the planet is likely too hot to be potentially habitable. A typical radius for an M0 star of Gliese 581's age and metallicity is 0.00128 AU, against the Sun's 0.00465 AU. This proximity means that the primary star should appear 3.75 times wider and 14 times larger in area for an observer on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.
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