Saturday, June 27, 2009

Extrasolar planet

Gliese 581 c

Gliese 581 c (pronounced /ˈɡliːzə/) or Gl 581 c is a "Super-Earth", a large extrasolar terrestrial planet, orbiting the red dwarf star Gliese 581. Assuming that the planet's mass is close to the lower limit determined by radial velocity measurements (the true mass is unknown), it was the smallest known extrasolar planet around a main sequence star, but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581 e, was announced with an approximate mass of 1.9 earth masses, which is now the smallest known extrasolar planet around a main sequence star. Gliese 581 c generated interest because it was initially 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 on the potential effects of the planetary atmosphere casts doubt upon the habitability of Gliese 581 c and indicates that the fourth planet in the system, Gliese 581 d, is a better candidate for habitability. In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.3 light years (192 trillion km or 119 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 our galaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 87th closest known star system to the Sun.

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, and employed the radial velocity technique to identify the planet's influence on the star. 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 581 c.
The team released a paper of their findings dated April 27, 2007, published in the July, 2007 journal Astronomy and Astrophysics. 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

The existence of Gliese 581 c and its mass have been measured by the radial velocity method of detecting extrasolar planets. 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 four planets are fit with a Keplerian solution, the minimum mass of the planet is determined to be 5.36 Earth masses.

Since Gliese 581 c has not been detected directly, 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.

Scale comparison of the relative sizes of the Earth and Gliese 581c, assuming Gliese 581c is a rocky body with a mass close to the minimum mass determined by the radial velocity method.

Gliese 581 c 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 kilometres 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. 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.

Habitability and Climate
The study of Gliese 581 c by the von Bloh et al. team has been quoted as concluding "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. "The study by Selsis et al. claims even "a planet in the habitable zone is not necessarily habitable" itself, and this planet "is outside what can be considered the conservative habitable zone" of the parent star, and further that if there was any water there and it was lost when the red dwarf was a strong X-ray and EUV emitter, it could have surface temperatures ranging from 700 K to 1000 K (427 to 727 °C). Temperature speculations by other scientists are based on the temperature of (and heat from) the parent star Gliese 581 and have been calculated without factoring in the wide margin of error (96 °C/K) for the star's temperature of 3432 K to 3528 K.

Planetary habitable zones of the Solar System and the Gliese 581 system compared.

Source from Wikipedia

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