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Gliese 581d (often shortened to Gl 581d or GJ 581d) is a proposed
extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
orbiting In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
within the
Gliese 581 system Gliese 581 () is a red dwarf star of stellar classification, spectral type M3V at the center of the Gliese 581 planetary system, about 20 Light-years, light years away from Earth in the Libra constellation. Its estimated mass is about a ...
, approximately 20.4
light-years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
away in the
Libra constellation Libra is a constellation of the zodiac and is located in the Southern celestial hemisphere. Its name is Latin for weighing scales. Its old astronomical symbol is (♎︎). It is fairly faint, with no first magnitude stars, and lies between Vi ...
. It is the third planet claimed in the system and (assuming a five-planet model) the fifth in order from the star. Though not confirmed to be a
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, V ...
and significantly more massive than
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
(at 6.98 Earth masses), the
Super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
is the first exoplanet of terrestrial
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
regarded as orbiting within the
habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
of its parent star. Assuming its existence, computer climate simulations have confirmed the possibility of the existence of surface water and these factors combine to a relatively high measure of
planetary habitability Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from a ...
.


Discovery

A team of astronomers led by
Stéphane Udry Stéphane Udry (born 1961 in Sion, Switzerland) is an astronomer at the Geneva Observatory in Switzerland, whose current work is primarily the search for extra-solar planets. He and his team, in 2007, discovered a possibly terrestrial planet in ...
of the
Geneva Observatory The Geneva Observatory (french: Observatoire de Genève, german: Observatorium von Genf) is an astronomical observatory at Sauverny (CH) in the municipality of Versoix, Canton of Geneva, in Switzerland. It shares its buildings with the astronomy ...
used the
HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
instrument on the
European Southern Observatory The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 mem ...
3.6 meter
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
in La Silla,
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
, to discover the planet in 2007. Udry's team employed the
radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection o ...
technique, in which the
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
of a
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a yo ...
is determined based on the small perturbations it induces in its parent star's
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a ...
via
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
.


Disputed existence

In September 2012, Roman Baluev filtered out the "red noise" from the Keck data and concluded that this planet's existence is probable only to 2.2 standard deviations. That same year, however, S. S. Vogt ( USNO), together with R. P. Butler and N. Haghighipour, confirmed the existence of the planet with a much higher probability; they also pursued a dynamical analysis of the system. Additional work on Gliese 581 as a four-planet system (thus, including planet d), demonstrating its long-term orbital stability, was given by Makarov and coauthors. A study in 2014 concluded that Gliese 581d is "an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g." In 2015, a study by Guillem Anglada-Escudé and Mikko Tuomi questioned the 2014 work owing to a significant shortcoming in the adopted statistical method; however, this study was published along with a rebuttal by the team that published the 2014 refutation. A 2016 study provided additional strong evidence that the radial velocity signal originates from the star. In 2016, E. R. Newton and collaborators pointed out that for early M dwarfs, planets in their habitable zones may have orbital periods coinciding with the stellar rotation period (or in rare cases, such as Gliese 581d, multiples of it); this aspect seriously complicates the verification of any such planets. Evidence based on a 2022 paper supports previous studies suggesting that the announcement of Gliese 581d stems from a false detection due to stellar activity. This work uses an updated technique correlating stellar activity to RV signals. On the other hand, if the planet does exist, Gliese 581d will constitute the first relatively small planet identified to be located in a stellar habitable zone outside of the Solar System, based on studies of 2007.


Orbital characteristics

Gliese 581d orbits Gliese 581 at 0.21847 AU, approximately a fifth of the distance that the Earth orbits the Sun, though its orbital eccentricity has not been confirmed. There are currently two models for its orbit, a circular one like Earth's, and an eccentric one like Mercury's. These are based on the four planet and six planet model for the Gliese 581 system, respectively. Under the four planet model Gliese 581d would most probably be in a spin-orbit resonance of 2:1, rotating twice for each orbit of its parent star. Therefore, the day on Gliese 581d should approximately be 67 Earth days long. The orbital distance places it at the outer limits of the habitable zone, the distance at which it is believed possible for water to exist on the surface of a planetary body. At the time of its discovery, the planet's orbit was originally thought to be farther out. However, in late April 2009 the original discovery team revised its original estimate of the planet's orbital parameters, finding that it orbits closer to its star than originally determined with an orbital period of 66.87 days. They concluded that the planet is within the habitable zone where liquid water could exist, thus confirming previous studies. Moreover, the data also suggested that the proposed exoplanet could have at least one or more large oceans.


Physical characteristics

The motion of the parent star indicates a minimum mass for Gliese 581d of 5.6 Earth masses (earlier analyses gave higher values). Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets are
coplanar In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique. How ...
show that the system becomes unstable if the masses of the planets exceed 1.6–2 times the minimum values. Using earlier minimum mass values for Gliese 581d, this implies an upper mass limit for Gliese 581d of 13.8 Earth masses. The composition of the planet, however, is not known.


Climate and habitability

As the planet is not known to transit from Earth and atmospheric conditions are not observable with current technology, no atmosphere for the planet has been confirmed to date. As such, all climate predictions for the planet are based on predicted orbits and computer modelling of theoretical atmospheric conditions. Because Gliese 581d was believed to orbit outside the habitable zone of its star it was originally thought to be too cold for liquid water to be present. With the 2009 revised orbit, climate simulations conducted by researchers in France in 2011 indicated possible temperatures suitable for surface water at sufficient atmospheric pressure. According to
Stéphane Udry Stéphane Udry (born 1961 in Sion, Switzerland) is an astronomer at the Geneva Observatory in Switzerland, whose current work is primarily the search for extra-solar planets. He and his team, in 2007, discovered a possibly terrestrial planet in ...
, "It could be covered by a 'large and deep ocean'; it is the first serious
ocean planet An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the sur ...
candidate." On average, the light that Gliese 581d receives from its star has about 30% of the intensity of light the Earth receives from the Sun. By comparison, sunlight on Mars has about 40% of the intensity of that on Earth. That might seem to suggest that Gliese 581d is too cold to support liquid water and hence is inhospitable to life. However, an atmospheric greenhouse effect can significantly raise planetary temperatures. For example, Earth's own mean temperature would be about −18 °C without any greenhouse gases, ranging from around 100 °C on the day side to −150 °C at night, much like that found on the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
. If the atmosphere of Gliese 581d produces a sufficiently large
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
, and the planet's geophysics stabilize the CO2 levels (as Earth's does via plate tectonics), then the surface
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
might permit a liquid water cycle, conceivably allowing the planet to support life. Calculations by Barnes et al. suggest, however, that
tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...
is too low to keep
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
active on the planet, unless radiogenic heating is somewhat higher than expected. Gliese 581d is probably too massive to be made only of rocky material. It may have originally formed on a more distant orbit as an icy planet that then migrated closer to its star.


Hello from Earth

As part of the 2009
National Science Week Science Week (sometimes National Science Week) refers to series of science-related events for the general public which are held in a specific countries during a designated week of the year. The aim of such science weeks is to engage and inspire pe ...
celebrations in Australia, ''Cosmos'' magazine launched a website called "Hello from Earth" to collect messages for transmission to Gliese 581d. The maximum length of the messages was 160 characters, and they were restricted to the English language. In total, 25,880 messages were collected from 195 countries around the world. The messages were transmitted from the DSS-43 70 m radio telescope at the
Canberra Deep Space Communication Complex The Canberra Deep Space Communication Complex (CDSCC) is a satellite communication station, part of the Deep Space Network of NASA's Jet Propulsion Laboratory (JPL), located at Tidbinbilla in the Australian Capital Territory. Opened in 19 ...
at Tidbinbilla, Australia, on 28 August 2009.


In popular culture

Gliese 581d is the setting for the ''
Doctor Who ''Doctor Who'' is a British science fiction television series broadcast by the BBC since 1963. The series depicts the adventures of a Time Lord called the Doctor, an extraterrestrial being who appears to be human. The Doctor explores the ...
'' episode "
Smile A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth. Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a Duchenne smile. Among humans, a smile expresses ...
". It is also shown in '' Into the Universe with Stephen Hawkings Episode 3: The Story of Everything, and in Episode 3 and 8 of Season 2 of ''
How the Universe Works ''How the Universe Works'' is a documentary science television series that originally aired on the Discovery Channel in 2010. All but the second and eighth seasons were narrated by Mike Rowe. The second and eighth seasons, as well as episodes of ...
''.


See also

*
Earth analog An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a planet or moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial p ...
*
Gliese 667 Cc Gliese 667 Cc (also known as GJ 667Cc, HR 6426Cc, or HD 156384Cc) is an exoplanet orbiting within the habitable zone of the red dwarf star Gliese 667 C, which is a member of the Gliese 667 triple star system, approximately away in the constella ...
*
HD 85512 b HD 85512 b is an exoplanet orbiting HD 85512, a K-type main-sequence star approximately 37 light-years from Earth in the constellation of Vela. Due to its mass of at least 3.6 times the mass of Earth, HD 85512 b is classified as a rocky Eart ...
*
Kepler 22 b Kepler-22b (also known by its Kepler object of interest designation KOI-087.01) is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about from Earth in the constellation of Cygnus. It was discovere ...
*
List of potentially habitable exoplanets This is a list of potentially habitable exoplanets. The list is mostly based on estimates of habitability by the Habitable Exoplanets Catalog (HEC), and data from the NASA Exoplanet Archive. The HEC is maintained by the Planetary Habitability Labo ...


References


External links


National Science Week 2009 - Send A Message To Gliese 581d
* {{DEFAULTSORT:Gliese 581D Exoplanet candidates Exoplanets in the habitable zone Exoplanets in the Gliese Catalog Gliese 581 Exoplanets discovered in 2007 Super-Earths in the habitable zone