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Gliese 581
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Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth. Its mass is estimated to be approximately a third that of the Sun, and it is the 87th closest known star system to the Sun. Observations suggest that the star has at least three planets.
The star system 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 not habitable.
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Encyclopedia
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth. Its mass is estimated to be approximately a third that of the Sun, and it is the 87th closest known star system to the Sun. Observations suggest that the star has at least three planets.
The star system 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 not habitable. However, Gliese 581 d is near the outer edge of the habitable zone.
Star
The name Gliese 581 refers to the Star catalogue of nearby stars by Wilhelm Gliese. 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 87th closest known star system to the Sun.
Gl 581 is wrongly classified as a BY Draconis variable in the General Catalogue of Variable Stars (hence the assignment of the variable star designation HO Librae), where the reference to the study of the star that led to its inclusion in the General Catalogue clearly shows no evidence of variability, least of all data indicative of BY Draconis variability. The data is well within the expected noise level of data discovered in this way. Astronomical catalogues show the star to be stable to at least 0.1 magnitudes (the approximate noise limit of these respective surveys for this object) both in amplitude and in mean magnitude over time, and no real evidence exists for classical variability having ever been seen in this star, least of all formal evidence of BY Draconis variability (such as a periodic sinusoidal light curve representing the motion of any starspot as it moves due to the stellar rotation).
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 burn hydrogen at a significantly lower rate. From the apparent magnitude and distance, we can estimate an effective temperature of 3200 Kelvin 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 blackbody), so such an estimate will underestimate the star's total luminosity. (For comparison, the peak emission of the Sun is roughly 530 nanometres, in the middle of the visible part of the spectrum). When radiation over the entire spectrum is taken into account (not just the part that humans are able to see), something known as the bolometric correction, this star has a bolometric luminosity 1.3% of the Sun's total luminosity. A planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth. The region of space around a star where a planet would receive roughly the same energy as the Earth is sometimes termed the "Goldilocks Zone", or more prosaically, the habitable zone. The extent of such a zone is not fixed and is highly specific for each planet.
Planetary system
At least three planets are believed to be orbiting Gliese 581. Gliese 581 b, approximately Neptune-sized, was discovered in August 2005 and was the fifth planet to be discovered around a red dwarf star. This inner planet is at least 16 times as massive as Earth (similar to Neptune's mass) and completes a full orbit of Gliese 581 in only 5.4 days.
Another planet, Gliese 581 c, was discovered in April 2007. Gliese 581 c is believed by some to be a rocky planet with a radius 1.5 times that of Earth. A direct measurement of the radius cannot be taken because the planet is not a transiting object. With a minimum mass of roughly five times Earth - or one third that of Neptune - Gliese 581 c orbits just inside of the habitable zone of its parent star. It is notable as it is the planet with lowest minimum mass yet discovered in the habitable zone of another star, making it the most "earthlike" exoplanet found to date. The mean blackbody surface temperature has been estimated to lie between -3 °C (for a Venus-like albedo) and 40 °C (for an Earth-like albedo), however, the temperatures could be much higher (about 500 degrees Celsius) due to a runaway greenhouse effect akin to that of Venus. Some astronomers believe the system may have undergone planetary migration and Gliese 581 c may have formed beyond the frost line, with a composition similar to icy bodies like Ganymede. Gliese 581 c completes a full orbit in just under 13 days.
Observations of the star also revealed the possibility of a third planet, Gliese 581 d, with a minimum mass of roughly 7.7 Earths, or half a Uranus, and an orbit of 84 Earth days. It orbits on the outer edge of the habitable zone of its star, which makes it a potential candidate for being able to support life.
Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets are coplanar show that the system becomes unstable if its component masses are so high as to correspond to an overall inclination angle of 10° or less. In this way, the maximum mass for Gliese 581 b is around 100 Earth masses, comparable to the mass of Saturn.
In October 2008, members of the networking website Bebo beamed a high-power transmission at Gliese 581 c, using the RT-70 radio telescope belonging to the National Space Agency of Ukraine. This transmission is due to arrive in the Gliese 581 system's vicinity by the year 2029; the earliest possible arrival for a response, should there be one, would be in 2049.
See also
External links
- , Scientific American
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- by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona
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