Gliese 876 c is an
exoplanet 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 ...
ing the
red dwarf Gliese 876
Gliese 876 is a red dwarf approximately 15 light-years away from Earth in the constellation of Aquarius. It is one of the closest known stars to the Sun confirmed to possess a planetary system with more than two planets, after Gliese 1061 ...
, taking about 30
days to complete an orbit. The planet was discovered in April 2001 and is the second planet in order of increasing distance from its star.
Discovery
At the time of discovery, Gliese 876 was already known to host an extrasolar planet designated
Gliese 876 b. On January 9, 2001, it was announced that further analysis of the star's
radial velocity had revealed the existence of a second planet in the system, which was designated Gliese 876 c.
The
orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
of Gliese 876 c was found to be exactly half that of the outer planet, which meant that the radial velocity signature of the second planet was initially interpreted as a higher
eccentricity
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-Centre (geometry), center, in geometry
* Eccentricity (g ...
of the orbit of Gliese 876 b.
Orbit and mass
Gliese 876 c is in a 1:2:4
Laplace resonance with the outer planets Gliese 876 b and
Gliese 876 e: for every orbit of planet e, planet b completes two orbits and planet c completes four.
This leads to strong
gravitational
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 stron ...
interactions between the planets,
causing the
orbital elements to change rapidly as the orbits
precess
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...
.
This is the second known example of a Laplace resonance, the first being
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
's
moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
Io,
Europa and
Ganymede.
The orbital
semimajor axis
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the lo ...
is only 0.13
AU, around a third of the average distance between
Mercury and the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, and is more
eccentric than the orbit of any of the major
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 you ...
s of the Sun's
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
.
Despite this, it is located in the inner regions of the system's
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. Kast ...
, since Gliese 876 is such an intrinsically faint star.
A limitation of the radial velocity method used to detect Gliese 876 c is that only a lower limit on the planet's
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 ...
can be obtained. This is because the measured mass value depends on the
inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.
For a satellite orbiting the Eart ...
of the orbit, which is not determined by the radial velocity measurements. However, in a resonant system such as Gliese 876, gravitational interactions between the planets can be used to determine the true masses. Using this method, the inclination of the orbit can be determined, revealing the planet's true mass to be 0.72 times that of
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
.
Characteristics
Based on its high mass, Gliese 876 c is likely to be a
gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
with no
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural ...
surface. Since it was detected indirectly through its gravitational effects on the star, properties such as its
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
, composition, and
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
are unknown. Assuming a composition similar to Jupiter and an environment close to
chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...
, the planet is predicted to have a
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
less upper
atmosphere.
Gliese 876 c lies at the inner edge of the system's habitable zone. While the prospects for
life
Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...
on gas giants are unknown, it might be possible for a large
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 of ...
of the planet to provide a
habitable
Habitability refers to the adequacy of an environment for human living. Where housing is concerned, there are generally local ordinances which define habitability. If a residence complies with those laws it is said to be habitable. In extreme e ...
environment. Unfortunately
tidal
Tidal is the adjectival form of tide.
Tidal may also refer to:
* ''Tidal'' (album), a 1996 album by Fiona Apple
* Tidal (king), a king involved in the Battle of the Vale of Siddim
* TidalCycles, a live coding environment for music
* Tidal (servic ...
interactions between a hypothetical moon, the planet, and the star could destroy moons massive enough to be habitable over the lifetime of the system.
In addition it is unclear whether such moons could form in the first place.
This planet, like b and e, has likely migrated inward.
See also
*
Appearance of extrasolar planets
*
Eccentric Jupiter
An eccentric Jupiter is a Jovian planet that orbits its star in an eccentric orbit. Note: this study treats eccentric Jupiters as giant planets having an orbital eccentricity of 0.1 or greater. Eccentric Jupiters may probably disqualify a plane ...
*
Gliese 581
Gliese 581 () is a red dwarf star of spectral type M3V at the center of the Gliese 581 planetary system, about 20 light years away from Earth in the Libra constellation. Its estimated mass is about a third of that of the Sun, and it i ...
*
List of nearest stars
This list covers all known stars, brown dwarfs, and sub-brown dwarfs within of the Sun. So far, 131 such objects have been found, of which only 22 are bright enough to be visible without a telescope. The visible light needs to reach or exce ...
Notes
References
External links
*
*
{{DEFAULTSORT:Gliese 876 c
Gliese 876
Aquarius (constellation)
Exoplanets discovered in 2001
Exoplanets detected by radial velocity
Giant planets in the habitable zone
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