Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf Gliese 581. It is
the second planet discovered in the system and the third in order from the
star. With a mass at least 5.5 times that of the Earth, it is classified as a
super-Earth. At the time of discovery, 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, with an approximate mass of 1.9 Earth masses,
was announced. Gliese 581 c 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 casts doubt upon the planet's habitability. It is tidally locked so if
life had a chance to emerge, the best hope of survival would be "the twilight
zone." In astronomical terms, the Gliese 581
system is relatively close to Earth, at 20.3 light years 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 89th closest known star system to the Sun.
Discovery 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 = Mass=
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 six 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. For Gliese 581 c, the upper bound is 10.4
Earth masses. = Radius=
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. Assuming that the true mass is the
minimum mass, the radius may be calculated using various models. For
example, if Gliese 581 c 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 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. 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 581 c 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 our direction, 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 our Solar System because they appear to hold on to
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 581 c has an orbital period 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. = Tidal lock=
Because of its small separation from Gliese 581, the planet has been
generally considered to always have one hemisphere facing the star, and the
other always facing away, or in other words being tidally locked. The most
recent orbital fit to the system, taking stellar activity into account indicates
a circular orbit, but older fits use an eccentricity between 0.10 and 0.22. If
the orbit of the planet were eccentric, it would undergo violent tidal flexing.
Because tidal forces are stronger when the planet is close to the star,
eccentric planets are expected to have a rotation period which is shorter than
its orbital period, also called pseudo-synchronization. An example of
this effect is seen in Mercury, which is tidally locked in a 3:2 resonance,
completing three rotations every two orbits. In any case, even in the case of
1:1 tidal lock, the planet would undergo libration and the terminator would be
alternatively lit and darkened during libration.
Models of the evolution of the planet's orbit over time suggest that heating
resulting from this tidal locking may play a major role in the planet's
geology. Models proposed by scientists predict that tidal heating could yield a
surface heat flux about three times greater than the Jupiter's moon Io's,
which could result in major geological activity such as volcanoes and plate
tectonics. 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 then
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, like Venus today.
Temperature speculations by other scientists were based on the temperature
of the parent star Gliese 581 and have been calculated without factoring in the
margin of error for the star's temperature of 3432 K to 3528 K, which
leads to a large irradiance range for the planet, even before eccentricity is
considered. = Effective temperatures=
Using the measured stellar luminosity of Gliese 581 of 0.013 times that of our
Sun, it is possible to calculate Gliese 581 c's effective temperature a.k.a.
black body temperature.. According to Udry's team, the effective temperature
for Gliese 581 c, assuming an albedo such as Venus', would be −3 °C, and
assuming an Earth-like albedo, then it would be 40 °C, a range of temperatures
which overlaps with the range that water would be liquid at a pressure of 1
atmosphere. However, the effective temperature and actual surface
temperature can be very different due to the greenhouse properties of the
planetary atmosphere: for example, Venus has an effective temperature of 34.25
°C, but a surface temperature of 463.85 °C, a difference of about 430 °C.
Studies of the habitability conclude that Gliese 581 c is likely to suffer
from a runaway greenhouse effect similar to that found on Venus, as such, is
highly unlikely to be habitable. Nevertheless, this runaway greenhouse
effect could be prevented by the presence of sufficient reflective cloud
cover on the planet's day side. Alternatively, if the surface were
covered in ice, it would have a high albedo, and thus could reflect enough of
the incident sunlight back into space to render the planet too cold for
habitability, although this situation is expected to be very unstable except for
very high albedos greater than about 0.95: release of carbon dioxide by
volcanic activity or of water vapor due to heating at the substellar point would
trigger a runaway greenhouse effect. = Liquid water=
Gliese 581 c is likely to lie outside the habitable zone. No direct evidence
has been found for water to be present, and it is probably not present in the
liquid state. Techniques like the one used to measure the extrasolar planet HD
209458 b may in the future be used to determine the presence of water in the
form of vapor in the planet's atmosphere, but only in the rare case of
a planet with an orbit aligned so as to transit its star, which Gliese 581 c is
not known to do. = Tidally-locked models=
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. However, it remains unknown if water and/or carbon dioxide are even
present on the surface of Gliese 581c. Alternatively, an atmosphere large
enough to be stable would circulate the heat more evenly, allowing for a wider
habitable area on the surface. 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 on the day side by globally circulating winds.
= List of Potentially Habitable Planets= On the list of "Potentially Habitable
Planets" Gliese 581 c is considered very low.
See also COROT-7b
Planetary habitability Habitable zone
Interstellar travel A Message From Earth
References Further reading
= News media reports= Dennis Overbye. "A Planet Is Too Hot for
Life, but Another May Be Just Right". New York Times. Retrieved 2009-07-11.
"Astronomers Find First Earth-like Planet in Habitable Zone". European
Southern Observatory. 2007-04-25. Retrieved 2008-06-20.
"New 'super-Earth' found in space". BBC News. 2007-04-25. Retrieved 2008-06-20.
Than, Ker. "Major Discovery: New Planet Could Harbor Water and Life". SPACE.com.
Retrieved 2008-06-20. Hazel Muir. "'Goldilocks' planet may be
just right for life". NewScientistSpace. "Astronomers find first habitable
Earth-like planet". Scientificblogging.com. April 24, 2007.
"Found 20 light years away:the new Earth". Daily Mail. April 26, 2007.
Ian Sample. "'Second Earth' may mean we're not alone". The Hindu.
J. R. Minkle. "All Wet? Astronomers Claim Discovery of Earth-like Planet.".
Scientific American. "Distant planet judged possibly
habitable". World Science. April 23, 2007.
ANI. "First habitable Earth like planet outside Solar System discovered".
DailyIndia.com. = Non-news media=
"Artist conceptions of extrasolar planet Gliese 581 c". Cosmographica. Retrieved
2008-06-20. "The Neighbor: Gliese 581c". The
Geochemical Society. Retrieved 2007-12-06.
"Red, Willing, and Able: 2001 New Scientist article on types of planets
likely to be around red dwarf stars". KenCroswell.com. Retrieved 2008-06-20.
"Sunrise from the Surface of Gliese 581c". NASA. Astronomy Picture of the
Day. 2007-05-02. Retrieved 2008-06-20. External links
Gliese 581 - The "Red Dwarf" and implications for its "earthlike" planet
Gliese 581c