Gliese 581 c

Gliese 581 c

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Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, it is classified as a super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...

 (a planet of 1 to 10 Earth masses). Assuming that the planet's mass is close to the lower limit determined by radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

 measurements (the true mass
True mass
The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...

 is unknown), it was the smallest known extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 around a main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 star, but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581 e
Gliese 581 e
Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra...

, 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 initially generated interest because it was originally reported to be the first potentially Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

-like planet in the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 of its star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...

 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 g
Gliese 581 g
Gliese 581 g , also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet proven nonexistent by the Geneva Team, orbiting the red dwarf star Gliese 581, 20.5 light-years from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth...

, is a better candidate for habitability. In astronomical terms, the Gliese 581 system is relatively close to Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, at 20.3 light years
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

 (192 trillion km or 119 trillion miles) in the direction of the constellation of Libra
Libra (constellation)
Libra is a constellation of the zodiac. Its name is Latin for weighing scales, and its symbol is . It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east.-Notable features:]...

. This distance, along with the declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

 and right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...

 coordinates, give its exact location in our galaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars
Gliese Catalogue of Nearby Stars
The Gliese Catalogue of Nearby Stars is a frequently referenced, modern star catalogue of stars located within 25 parsecs of the Earth.-First edition and supplements:...

; it is the 87th 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
Astronomy and Astrophysics
* Astronomy and Astrophysics has a 2010 impact factor of 4.410.-See also:*The Astronomy and Astrophysics Review‎*Advanced Satellite for Cosmology and Astrophysics*Astronomy & Geophysics*Astronomical Journal*Astrophysical Journal...

. 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 mass
Earth mass
Earth mass is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9722 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets....

es and a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 of 0.25 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s. (A reanalysis of the radial velocity data has since reduced the minimum possible mass of Gliese 581 d to 5.6 Earth masses.)

Mass


The existence of Gliese 581 c and its mass have been measured by the radial velocity method of detecting extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s. 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 fit with a Keplerian
Kepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....

 solution, the minimum mass of the planet is determined to be 5.6 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
Coplanarity
In geometry, a set of points in space is coplanar if all the points lie in the same geometric plane. For example, three distinct points are always coplanar; but a fourth point or more added in space can exist in another plane, incoplanarly....

 indicate that the planets cannot exceed approximately 1.6 – 2 times their minimum masses or the planetary system becomes unstable (this is primarily due to the interaction between planets e and b). 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
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 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
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 and/or water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

y planet, its radius would be less than 2 times that of Earth, even with a very large outer hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

, according to density models compiled by Diana Valencia and her team for Gliese 876 d
Gliese 876 d
Gliese 876 d is an extrasolar planet approximately 15 light-years away in the constellation of Aquarius . The planet was the third planet discovered orbiting the red dwarf star Gliese 876. At the time of its discovery, the planet had the lowest mass of any known extrasolar planet apart from the...

. 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
Sara Seager
Sara Seager is a Canadian-American astronomer who is currently a professor at the Massachusetts Institute of Technology and known for her work on extrasolar planets. She was born in Toronto, Canada. In 1994, she earned the degree of Bachelor of Science in Mathematics and Physics from the...

 at MIT has speculated that Gliese 581 c and other five-Earth-mass planets could be:
  • "rock giants" mostly of silicate
    Silicate
    A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

    .
  • "cannonball" planets of solid iron
    Iron
    Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

    .
  • "gas dwarfs" mostly of helium
    Helium
    Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

     and hydrogen
    Hydrogen
    Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

    .
  • carbon-rich "diamond
    Diamond
    In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

     worlds"
  • purely hot "ice VII worlds".
  • purely "carbon monoxide
    Carbon monoxide
    Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

     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
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

-built MOST
Microvariability and Oscillations of STars telescope
The Microvariability and Oscillations of STars telescope, better known simply as MOST, is Canada's first and only space telescope. It is also the smallest space telescope in the world...

 space telescope indicate that transits do not occur.

For comparison, the smallest confirmed diameter of a planet around a main-sequence star is that of COROT-7b, which is about 70% larger than Earth.

Orbit



Gliese 581 c has an orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...

 ("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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. 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
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

 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
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 facing the star (only day), and the other always facing away (only night), or in other words being tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

. Although a recent orbital fit to the Gliese 581 system uses a circular orbit for Gliese 581 c, 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
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, 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
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar 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
Tidal heating
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...

 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
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's moon Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

'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 (430 to 730 °C), like Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 today. Temperature speculations by other scientists were based on the temperature of (and heat from) the parent star Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

 and have been calculated without factoring in the margin of error (96 °C/K) for the star's temperature of 3432 K to 3528 K, which leads to a large irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

 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
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

 a.k.a. black body
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

 temperature. (note: this probably differs from its surface temperature). According to Udry's team, the effective temperature for Gliese 581 c, assuming an albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 (reflectivity) such as Venus' (0.64), would be -3 °C, and assuming an Earth-like albedo (0.296), then it would be 40 °C (104 °F), 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 (307.4 K; 93.7 °F), but a surface temperature of 463.85 °C (737 K; 866.9 °F) (mainly due to a 96.5% carbon dioxide atmosphere
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

), a difference of about 430 C-change. Studies of the habitability (i.e. liquid water for extremophile forms of life) conclude that Gliese 581 c is likely to suffer from a runaway greenhouse effect
Runaway greenhouse effect
A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away...

 similar to that found on Venus
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

, 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
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 (reflectivity), 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 (i.e. ice): release of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 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
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

. No direct evidence has been found for water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 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
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....

 may in the future be used to determine the presence of water in the form of vapor in the planet's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

, but only in the rare case of a planet with an orbit aligned so as to transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

 its star, which Gliese 581 c is not known to do.

Tidally-locked models


Theoretical models predict that volatile compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 such as water and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to form ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

s. 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
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 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.

See also


  • COROT-7b
  • Planetary habitability
    Planetary habitability
    Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

  • Habitable zone
    Habitable zone
    In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

     (Goldilocks phenomenon)
  • Interstellar travel
    Interstellar travel
    Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...

  • A Message From Earth
    A Message From Earth
    A Message from Earth is a high-powered digital radio signal that was sent on 9 October 2008 towards Gliese 581 c, a large terrestrial extrasolar planet orbiting the red dwarf star Gliese 581. The signal is a digital time capsule containing 501 messages that were selected through a competition on...


External links