Gliese 581

From Wikipedia, the free encyclopedia

**Gliese 581**
Observation data Epoch J2000
Constellation (pronunciation)	Libra
Right ascension	15^h 19^m 26.8250^s
Declination	-07° 43′ 20.209″
Apparent magnitude (V)	10.56
Characteristics
Spectral type	M3V
B-V color index	1.60
Variable type	BY Draconis variable
Astrometry
Radial velocity (R_v)	-9.5 km/s
Proper motion (μ)	RA: -1224.55 mas/yr Dec.: -99.51 mas/yr
Parallax (π)	159.52 ± 2.27 mas
Distance	20.4 ± 0.3 ly (6.27 ± 0.09 pc)
Absolute magnitude (M_V)	11.56
Details
Mass	0.31^[1] M_☉
Radius	0.38^[1] R_☉
Luminosity	0.013^[2] L_☉
Temperature	3,480^[3] K
Metallicity	[M/H] = -0.33^[3]
Age	4.3 × 10⁹^[1] years
Other designations
HO Librae, GJ 581, BD -07°4003, LHS 394, LTT 6112,TYC 5594-1093-1, HIP 74995, Wolf 562.
Database references
SIMBAD	data

Gliese 581 (pronounced /ˈgliːzə/) (also known as Wolf 562) is an M2.5V red dwarf star located 20.4 light years away from Earth. It is 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 with respect to Earth, is the 87th known closest star system.^[4] The star system recently 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^[5].

It has since been shown that Gliese 581 c will almost certainly have a runaway greenhouse effect, and would not be habitable. However, Gliese 581 d is near the outer edge of the habitable zone. ^[6] Observations suggest that the star has at least three planets.

1 Star
2 Planetary system
3 See also
4 References
5 External links

[edit] 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.

Gl 581 is wrongly classified as a BY Draconis variable in the General Catalogue of Variable Stars, 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^[7]. Both the ASAS-3 data and Hipparcos Epoch Photometry 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 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^[2]. 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.

[edit] Planetary system

At least three planets are believed to be orbiting this star. One, about Neptune-sized, was discovered in 2005.^[2] Another, having an estimated radius 1.5 times that of Earth, was discovered in 2007.^[8] The latter is notable as it is the smallest planet yet discovered in the habitable zone of another star, making it the most "earthlike" exoplanet found to date. Observations of the star also revealed the possibility of a third planet with a mass of roughly 7.7 Earths, or half a Uranus, and an orbit of 84 Earth days.

Gliese 581 b was the fifth planet to be discovered around a red dwarf star. This inner planet is 16 times as massive as Earth (similar to Neptune's mass) and completes a full orbit of Gliese 581 in only 5.4 days. 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 outside^[9] of the habitable zone of its parent star. 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),^[8] , however, the temperatures could be much higher (about 500 degrees Celsius) due to a runaway greenhouse effect akin to that of Venus. ^[6] Some 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.^{[citation needed]} Gliese 581 c completes a full orbit in just under 13 days. Gliese 581 d has a lower mass limit of about 7.7 times the Earth (about half the mass of Uranus) and orbits its star in 84 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.^[6]