Gliese 581d
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Gliese 581d (often shortened to Gl 581d or GJ 581d) is a doubtful, and frequently disputed,
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...
candidate
orbiting In celestial mechanics, an orbit (also known as orbital revolution) 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 obje ...
within the
Gliese 581 Gliese 581 () is a red dwarf star of spectral type M3V which hosts a planetary system, away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 101st closest known star system to ...
system, approximately 20.4
light-years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...
away in the Libra constellation. It was the third planet claimed in the system and the fourth (in a 4-planet model) or fifth (in a disproven 5- or 6-planet model) in order from the star. Multiple subsequent studies found that the planetary signal in fact originates from stellar activity, and thus the planet does not exist, but this remains disputed. Though significantly more massive than
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
(at a minimum mass of 6.98 Earth masses), this
super-Earth A super-Earth is a type of exoplanet with a mass higher than Earth, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to t ...
was the first exoplanet of relatively low
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
regarded as orbiting within the
habitable zone In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...
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 Abiogenesis, develop and sustain an environment hospitable to life. Life may be abiogenesis, generated directly on a planet or satellite endogenously. Res ...
.


History


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 (, ) is an astronomical observatory at Sauverny (CH) in the municipality of Versoix, Canton of Geneva, in Switzerland. It shares its buildings with the astronomy department of the École Polytechnique Fédérale de Lausann ...
used the
HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision Echelle grating, echelle planet-finding spectrograph installed in 2002 on the ESO 3.6 m Telescope, ESO's 3.6m telescope at La Silla Observatory in Chile. The First l ...
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 m ...
3.6 meter
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
in La Silla,
Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...
, to discover the planet in 2007. Udry's team employed the
radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...
technique, in which the
minimum mass In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars, binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets detected by the radial velocit ...
of a
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
is determined based on the small perturbations it induces in its parent star's
orbit In celestial mechanics, an orbit (also known as orbital revolution) 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 ...
via
gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
. This study estimated an orbital period of 83 days for the planet. 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 66.8 days. They concluded that the planet is within the habitable zone where liquid water could exist. A 2010 study of aliasing in radial velocity data found that the true period of Gliese 581d remained unclear, with even a 1-day period being a possibility. Later models of the system including planet d from 2010-2013 supported a 67-day period.


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, and thus is uncertain. Earlier that same year, however, S. S. Vogt (
USNO The United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the ...
), together with R. P. Butler and N. Haghighipour, published a study that supported 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, claiming 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. Another 2015 study added support to the conclusion that the radial velocity signal originates from stellar activity, and a 2016 study provided additional strong evidence for it. 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, half of it, if the standard value of 132 days is assumed); this aspect seriously complicates the verification of any such planets. Evidence based on a 2022 paper confirmed the results of 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. A 2024 research note argued that it is still possible that Gliese 581d might exist, on the basis of a new measurement of Gliese 581's rotation period at , as opposed to the previous value of from the 2014 study refuting the planet. This new rotation period is not a multiple of the planet's proposed period. However, no reanalysis of the radial velocity data was done, so further research will be needed to draw a conclusion.


Orbital characteristics

Gliese 581d was thought to orbit 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 were two models for its orbit, a circular one like Earth's, and an eccentric one like Mercury's. These were based on the six-planet and four-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.


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 France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...
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 or water world is a type of planet or natural satellite that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the planetary surface, surface, as subsurface ...
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 Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
. If the atmosphere of Gliese 581d produces a sufficiently large
greenhouse effect The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source (as in the case of Jupiter) or ...
, 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 quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
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 (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
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. If Gliese 581d exists, it would be the first super-Earth identified to be located in a habitable zone outside of the Solar System, according to work published in 2007.


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 English is a West Germanic language that developed in early medieval England and has since become a English as a lingua franca, global lingua franca. The namesake of the language is the Angles (tribe), Angles, one of the Germanic peoples th ...
. 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 196 ...
at Tidbinbilla,
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
, 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, created by Sydney Newman, C. E. Webber and Donald Wilson (writer and producer), Donald Wilson, depicts the adventures of an extraterre ...
'' 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 d ...
". It is also shown in ''
Into the Universe with Stephen Hawking ''Into the Universe with Stephen Hawking'' is a 2010 science documentary television mini-series written by British physicist Stephen Hawking. The series was created for Discovery Channel by Darlow Smithson Productions and features computer gener ...
s 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 science documentary television series that provides scientific explanations about the inner workings of the universe and everything it encompasses. With the use of computer-generated imagery (CGI) and visual effect ...
''.


See also

*
Earth analog An Earth analog, also called an Earth twin or second Earth, is a Exoplanet, planet or Exomoon, 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 terrestria ...
*
Gliese 667 Cc Gliese 667 Cc (also known as GJ 667 Cc, HR 6426 Cc, or HD 156384 Cc) 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 conste ...
* HD 85512 b *
Kepler-22b Kepler-22b (also known by its Kepler Object of Interest designation ''KOI-087.01'') is an exoplanet orbiting within the Circumstellar habitable zone, habitable zone of the Solar analog, Sun-like star Kepler-22. It is located about from Earth i ...
*
List of potentially habitable exoplanets The following list includes some of the potentially habitable exoplanets discovered so far. It is mostly based on estimates of habitability by the Habitable Worlds Catalog (HWC), and data from the NASA Exoplanet Archive. The HWC is maintained by ...


References


External links


National Science Week 2009 - Send A Message To Gliese 581d
* {{DEFAULTSORT:Gliese 581D Exoplanet candidates Gliese 581