Gliese 581 c

**Gliese 581 c**
Parent star
Extrasolar planet		List of extrasolar planets
Gliese 581 c and its star as rendered in Celestia. The Sun (marked 'Sol') is visible as a magnitude 3.8 star to the right of a slightly distorted Taurus constellation. The true orientation of the system is unknown.
Star		Gliese 581
Constellation		Libra
Right ascension	(α)	15^h 19^m 26^s
Declination	(δ)	−07° 43′ 20″
Spectral type		M2.5V
Orbital elements
Semimajor axis	(a)	0.073^[1] AU
Eccentricity	(e)	0.16±0.07^[1]
Orbital period	(P)	12.93^[1] d
Angular distance	(θ)	11.661 mas
Time of periastron	(T₀)	2,452,996.74±0.45 JD
Semi-Amplitude	(K)	3.01±0.16 m/s
Physical characteristics
Mass	(m)	> 5.03 M_E
Radius	(r)	~1.5^[2] R_E
Temperature	(T)	~290 K
Discovery information
Discovery date		2007-04-04, announced 2007-04-24
Discoverer(s)		Udry et al.
Detection method		Radial Velocity
Discovery status		published

Gliese 581 c (IPA: /ˈgliːzə/), also unofficially known as Ymir^[3], is a "super-earth" extrasolar planet orbiting the red dwarf star Gliese 581.^[4] It appears to be the first terrestrial extrasolar planet discovered in the hypothetical habitable zone surrounding its star, where surface temperatures might maintain liquid water^[4]^[5] and therefore be suitable for life as known on Earth. The planet is astronomically close, at 20.4 light years (193 trillion km or 119 trillion miles) from Earth in the direction of the constellation of Libra.^[2] Its star is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; with respect to Earth it is the 87th closest star system.^[6]

Gliese 581 c is the first extrasolar planet believed to possibly have a surface temperature similar to that of Earth. Assuming the planet's mass is close to the lower limit determined by radial velocity measurements (the true mass is unknown), it would be the smallest extrasolar planet known around a main sequence star to date.^[4]

Discovery

The discovery of the planet by the team of Stéphane Udry University of Geneva's Observatory in Switzerland was announced on April 24, 2007. The team used the HARPS instrument (an echelle spectrograph) on the European Southern Observatory 3.6 m Telescope in La Silla, Chile. The team employed the radial velocity technique. The Canadian-built MOST space telescope was used to conduct a follow-up study over the next six weeks. No transit was detected over this time, so a direct measurement of the planet has not yet been possible; however, the star's apparent magnitude changed very little, indicating that it provides a stable source of light and heat to Gliese 581c. ^[7]

The team released a paper dated April 27, 2007 which will be published in an edition of the journal Astronomy and Astrophysics.^[8] In the paper they also announced the discovery of another planet in the system, Gliese 581 d, 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 581 c and its mass have been measured by the Radial Velocity Method or the "wobble-effect" method of detecting exoplanets. The mass of a planet is calculated by the small periodical movements around a common centre of mass between the host star Gliese 581 and its planets. Because the "wobbling" of Gliese 581 is a result of all planets in its system, the calculation of the mass of Gliese 581 c depends on the presence of other planets in the Gliese 581 system and on the inclination of the orbital plane with respect to Earth. Using the known minimum mass of the previously detected Gliese 581 b, and assuming the existence of Gliese 581 d, Gliese 581 c has a mass at least 5.03 times that of Earth. The mass of the planet cannot be very much larger than this or the system would be dynamically unstable.^[8]

Radius

If it is a rocky planet with a large iron core, Gliese 581 c has a radius approximately 50% larger than that of Earth, according to Udry's team.^[8]^[9] Gravity on such a planet's surface would be approximately 2.24 times as strong as on Earth.

If Gliese 581 c 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.^[10] Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth.

With the Radial Velocity method of detection, it is not possible to measure the radius of an exoplanet. The real value may be anything between the two extremes calculated by density models outlined above.^[11] If the planet transits the star as seen from our direction, the radius should be measurable, although with some uncertainty. Udry's team intends to use the Canadian-built MOST space telescope to look for a transit of the planet before its host star. A transit measurement could very well determine whether Gliese 581 c is a primarily rocky or watery object.

Age

The Gliese 581 system is estimated to be around 4.3 billion years old.^[12] By comparison, the Solar System is estimated to be 4.57 billion years old.

Orbit

Gliese 581 c has an orbital period ("year") of 13 Earth days^[2] and its orbital radius is only about 7% that of the Earth, about 11 million km^[13], while the Earth is 150 million kilometres from the Sun^[14]. 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 ^[8] ^[9]. This proximity also means that the primary star appears several times larger for an observer standing on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.

Climate and habitability

Scale comparison of the relative sizes of the Earth and Gliese 581 c, assuming Gliese 581 c is a rocky body

Temperature

Using the measured stellar luminosity of Gliese 581 of 0.013 times that of our Sun, it is possible to calculate Gliese 581 c equilibrium surface temperature, which does not take into account a possible atmosphere. According to Udry's team, the equilibrium temperature for Gliese 581 c is −3° C / 26.6° F, assuming an albedo (reflectivity) such as Venus' (0.64) and 40°C / 104° F for an Earth-like albedo (0.35).^[8]^[2] The actual temperature on the surface also depends on the planet's atmosphere, which remains unknown. Xavier Delfosse of the research team expects that the actual surface temperatures will be hotter; for instance, the corresponding calculation for Earth yields an "effective surface temperature" of 256 K/−17 °C/− 28°F, yet Earth's true surface is 32 K warmer (an average of 288 K/15 °C/59°F) due to the greenhouse effect. Gliese 581c receives more heat from the star than Venus does from our sun.^{[citation needed]}

Derivation

The temperature estimate is arrived at by equating the power absorbed by the planet and the power radiated by it as a result of it being at a given temperature. This assumes the planet is in thermodynamic equilibrium.

To calculate the amount of power absorbed by the planet, consider that the star radiates a certain amount of power. The power radiated from the star is termed its luminosity, given the symbol L. Assuming the star radiates isotropically, at a given distance D from the star, this power is spread out over the surface of a sphere of radius D. This gives the flux F of energy at the planet:

$F=\frac{L}{4 \pi D^2}$

The power absorbed P_abs is the flux multiplied by the cross sectional area presented by the planet. For a spherical planet, the cross-sectional area is a disk with the same radius r as the planet. We also allow for the fact that the planet may reflect a certain fraction of the incident radiation by inserting a term called the albedo A. If the albedo is 1, then it reflects all incident radiation (absorbs none). If the albedo is 0, all incident radiation is absorbed. So:

$P_{abs} = \frac{L \pi r^2 (1-A)}{4 \pi D^2} = \frac {L r^2 (1-A)}{4 D^2}$

The next stage is to calculate the amount of power radiated by the planet. The planet is assumed to be a spherical black body of temperature T, and thus obeys the Stefan-Boltzmann law. The power radiated by the planet is thus:

$P r a d = 4π r 2 σ T 4$

The absorbed and radiated powers can then be equated and rearranged to solve for T, the temperature of the planet:

Failed to parse (unknown function\tfrac): T = \left (\frac{L (1-A)}{16 \pi \sigma D^2} \right )^{\tfrac{1}{4}}

Note that the planet's radius has cancelled out in equating the two quantites. Of the quantities in this equation, for Gliese 581 c, which orbits a star with 0.013 times the solar luminosity at a distance of 0.073 AU, the albedo is unknown. If it is set to an Earthlike value of 0.35, the temperature obtained is 40°C. If it is set to a Venuslike value of 0.64, the temperature predicted is −3°C. This would appear to suggest that the planet experiences temperatures suitable for liquid water to exist on its surface. However it should be noted that this is a very basic model. A major flaw is that planets are not perfect black bodies, and thus do not radiate as efficiently at certain frequencies as at others. A notable consequence of this is the greenhouse effect, which allows planets to be much warmer than the equilibrium temperature would predict: according to this formula, the Earth is predicted to be around −20°C.

Liquid water

Gliese 581 c is within the habitable zone where water—a necessary ingredient for life as we know it—could exist.^[4]^[15] However, no direct evidence has been found. Techniques like the one used to measure HD 209458 b could be applied to determine the existence of water vapor in an extrasolar planet's atmosphere, but only in the rare case of a planet with an orbit including transit directly between its star and our planet, which Gliese 581 c is not known to do.

Tidal forces

Because of its small separation from Gliese 581, the planet is quite likely to be tidally locked, with one hemisphere always day (facing the star) and the other always night (facing away).^[16]. Even then, the planet would undergo violent tidal flexing, because the orbital eccentricity is between 0.09 and 0.23. The planet might also be in a different tidal lock, like Mercury, with eccentricity of just 0.20, which is tidally locked 3:2, so that every point is in sunlight once over 2 orbital periods. The permanently lit hemisphere would be extremely hot and the dark hemisphere extremely cold, while the narrow terminator or "twilight zone" between them might have a moderate climate more suitable for life. In any case, even in case of 1:1 tidal lock, the planet would undergo libration and the terminator would be alternatively lit and darkened during libration.^[17]

Theoretical models predict that volatile compounds such as water and carbon dioxide, if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to form ice caps. Over time, the entire atmosphere might freeze into ice caps on the night side of the planet. Alternatively, an atmosphere large enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.^[18] For example, although Venus has a small axial inclination, very little sunlight reaches the surface at the poles. A slow rotation rate approximately 117 times slower than Earth's produces prolonged days and nights. Despite the uneven distribution of sunlight cast on Venus at any given time, polar areas and the night side of Venus are kept almost as hot as day by globally circulating winds. However, it remains unknown if water and/or carbon dioxide are even present on the surface of Gliese 581 c.

Greenhouse effect

It has been hypothesized that, due to its strong gravity and proximity to the hotter edge of the habitable zone, Gliese 581 c could be prone to a runaway greenhouse effect, and would not be habitable, thus mimicking what happened to Venus in our solar system.^[19]

Further study

Artist's Impression of Gliese 581 c, the smallest extrasolar planet discovered within its star's habitable zone

Gliese 581 c presents several challenges for study or exploration. It has not been directly observed, and the development of equipment sensitive enough to look for signs of life will take years.^[20] However, according to the research-team member Xavier Delfosse:

Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an X.^[20]^[9]

Several astronomers have suggested that the earthlike properties of Gliese 581 c and its relative proximity (20.4 light-years away) would make it a potential target for a future interstellar probe project.^[21]^[2]

References

^ ^a ^b ^c Planet : Gl 581 c. The Exoplanet Encyclopedia. Retrieved on 2007-05-10.
^ ^a ^b ^c ^d ^e "New 'super-Earth' found in space", BBC News, 25 April 2007. Retrieved on 2007-04-25.
^ The Neighbor: Gliese 581c. The Geochemical Society. Retrieved on 2007-10-16.
^ ^a ^b ^c ^d Than, Ker (2007-04-24). Major Discovery: New Planet Could Harbor Water and Life. space.com. Retrieved on 2007-04-29.
^ Than, Ker (2007-02-24). Planet Hunters Edge Closer to Their Holy Grail. space.com. Retrieved on 2007-04-29.
^ The 100 Nearest Stars. RECONS. Retrieved on 2007-05-10.
^ Boring Star May Mean Livelier Planet
^ ^a ^b ^c ^d ^e Udry et al. (2007). "The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M_⊕) in a 3-planet system". Astronomy and Astrophysics preprint: preprint.
^ ^a ^b ^c Astronomers Find First Earth-like Planet in Habitable Zone. ESO. Retrieved on 2007-05-10.
^ Valencia et al. (2006). "Radius and Structure Models of the First Super-Earth Planet".
^ Valencia and Sasselov (2007). "Detailed Models of super-Earths: How well can we infer bulk properties?". ArXiv: 0704.3454v1 preprint: preprint.
^ Star : Gl 581. Exoplanets Encyclopedia. Retrieved on 2007-04-25.
^ Overbye, Dennis. "20 light years away, the most Earthlike planet yet", International Herald Tribune, 2007-04-25. Retrieved on 2007-05-10.
^ The Earth Worldbook. NASA. Retrieved on 2007-05-10.
^ Dead Link "Earth-Like Planet Found", Associated Press. Retrieved on 2007-05-10.
^ "Out of our world: Earthlike planet", USA Today, 2007-04-25. Retrieved on 2007-05-10.
^ Perlman, David. "New planet found: It might hold life", San Francisco Chronicle, 2007-04-24. Retrieved on 2007-04-24.
^ Alpert, Mark (2005-11-07). Red Star Rising. Scientific American. Retrieved on 2007-04-25.
^ The Habitability of Super-Earths in Gliese 581. Retrieved on 2007-05-29.
^ ^a ^b Earth-like planet found that may support life. CTV News. Retrieved on 2007-04-25.
^ New Planet Could Be Earthlike, Scientists Say, New York Times.

Selected media articles

Hazel Muir (April 25 2007). 'Goldilocks' planet may be just right for life. NewScientistSpace.
Astronomers find first habitable Earth-like planet. Scientificblogging.com (April 24 2007).
Ian Sample (April 24 2007). 'Second Earth' may mean we're not alone. The Hindu.
J.R. Minkle (April 24 2007). All Wet? Astronomers Claim Discovery of Earth-like Planet. Scientific American.
Distant planet judged possibly habitable. World Science (April 23 2007).
ANI (April 23 2007). First habitable Earth like planet outside Solar System discovered. DailyIndia.com.
Joe Campbell (April 26 2007). Meet the neighbours. BBC Magazine.

External links

Gliese 581 System
Star	Gliese 581
Planets	b · c · d

zh-classical:葛利斯五八一丙 zh-yue:Gliese 581 c

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