Gliese 876 b
Encyclopedia
Gliese 876 b is an extrasolar planet
orbit
ing the red dwarf
star
Gliese 876
. It completes one orbit in approximately 61 day
s. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf star.
(with data from Keck Observatory and Lick Observatory
) and the other by Xavier Delfosse (at Geneva Observatory). Like the majority of known extrasolar planets, it was discovered by detecting variations in its star's radial velocity
as a result of the planet's gravity. This was done by making sensitive measurements of the Doppler shift of the spectral line
s of Gliese 876. It was the first discovered of four known planets in the Gliese 876 system.
and the outer planet Gliese 876 e
: in the time it takes planet e to complete one orbit, planet b completes two and planet c completes four. This is the second known example of a Laplace resonance, the first being Jupiter
's moons Io
, Europa
and Ganymede
. As a result, the orbital elements
of the planets change fairly rapidly as they dynamically interact with one another. The planet's orbit has a low eccentricity, similar to the planets in our solar system
. The semimajor axis of the orbit is only 0.208 AU
, less than that of Mercury
in our solar system. However Gliese 876 is such a faint star that this puts it in the outer part of the habitable zone
.
A limitation of the radial velocity method used to detect Gliese 876 b is that only a lower limit on the planet's mass
can be obtained. This lower limit is around 1.93 times the mass of Jupiter
. The true mass
depends on the inclination
of the orbit, which in general is unknown. However because Gliese 876 is only 15 light years from earth Benedict et al. (2002) were able to use one of the Fine Guidance Sensor
s on the Hubble Space Telescope
to detect the astrometric
wobble created by Gliese 876 b. This constituted the first unambiguous astrometric detection of an extrasolar planet. Their analysis suggested that the orbital inclination is 84°±6° (close to edge-on). In the case of Gliese 876 b, modelling the planet-planet interactions from the Laplace resonance shows that the actual inclination of the orbit is 59° resulting in a true mass of 2.2756 times the mass of Jupiter.
with no solid
surface. Since the planet has only been detected indirectly through its gravitational effects on the star, properties such as its radius
, composition, and temperature
are unknown. Assuming a composition similar to Jupiter
and an environment close to chemical equilibrium
, it is predicted that the atmosphere of Gliese 876 b is cloud
less, though cooler regions of the planet may be able to form water
clouds.
Gliese 876 b lies within the habitable zone of Gliese 876 as defined by the ability of an Earth
-mass planet to retain liquid
water at its surface. While the prospects for life
on a gas giant are unknown, large moons
may be able to support a habitable
environment. Models of tidal
interactions between a hypothetical moon, the planet and the star suggest that large moons should be able to survive in orbit around Gliese 876 b for the lifetime of the system. On the other hand, it is unclear whether such moons could form in the first place.
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...
orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
ing the red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
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...
Gliese 876
Gliese 876
Gliese 876 is a red dwarf star approximately 15 light-years away from Earth in the constellation of Aquarius. As of 2011, it has been confirmed that four extrasolar planets orbit the star...
. It completes one orbit in approximately 61 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf star.
Discovery
Gliese 876 b was discovered independently by two different teams, one led by Geoffrey MarcyGeoffrey Marcy
Geoffrey W. Marcy is an American astronomer, who is currently Professor of Astronomy at the University of California, Berkeley, famous for discovering more extrasolar planets than anyone else, 70 out of the first 100 to be discovered, along with R...
(with data from Keck Observatory and Lick Observatory
Lick Observatory
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...
) and the other by Xavier Delfosse (at Geneva Observatory). Like the majority of known extrasolar planets, it was discovered by detecting variations in its star's 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...
as a result of the planet's gravity. This was done by making sensitive measurements of the Doppler shift of the spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s of Gliese 876. It was the first discovered of four known planets in the Gliese 876 system.
Orbit and mass
Gliese 876 b is in a 1:2:4 Laplace resonance with the inner planet Gliese 876 cGliese 876 c
Gliese 876 c is an extrasolar planet orbiting the red dwarf star Gliese 876, 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 :...
and the outer planet Gliese 876 e
Gliese 876 e
Gliese 876 e is an extrasolar planet orbiting the star Gliese 876 in the constellation of Aquarius. It is in a 1:2:4 Laplace resonance with the planets Gliese 876 c and Gliese 876 b: for each orbit of planet e, planet b completes two orbits and planet c completes four...
: in the time it takes planet e to complete one orbit, planet b completes two and planet c completes four. This is the second known example of a Laplace resonance, the first being 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 moons 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....
, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
and Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
. As a result, the orbital elements
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...
of the planets change fairly rapidly as they dynamically interact with one another. The planet's orbit has a low eccentricity, similar to the planets in our solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. The semimajor axis of the orbit is only 0.208 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, less than that of 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...
in our solar system. However Gliese 876 is such a faint star that this puts it in the outer part of 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...
.
A limitation of the radial velocity method used to detect Gliese 876 b is that only a lower limit on the planet's mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
can be obtained. This lower limit is around 1.93 times the mass of 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,...
. 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...
depends on the inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
of the orbit, which in general is unknown. However because Gliese 876 is only 15 light years from earth Benedict et al. (2002) were able to use one of the Fine Guidance Sensor
Fine Guidance Sensor
The Fine Guidance Sensor is an interferometric instrument on board the Hubble Space Telescope that provides high-precision pointing information as input to the observatory's attitude control systems....
s on the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
to detect the astrometric
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
wobble created by Gliese 876 b. This constituted the first unambiguous astrometric detection of an extrasolar planet. Their analysis suggested that the orbital inclination is 84°±6° (close to edge-on). In the case of Gliese 876 b, modelling the planet-planet interactions from the Laplace resonance shows that the actual inclination of the orbit is 59° resulting in a true mass of 2.2756 times the mass of Jupiter.
Physical characteristics
Given the planet's high mass, it is likely that Gliese 876 b is a gas giantGas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
with no solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
surface. Since the planet has only been detected indirectly through its gravitational effects on the star, properties such as its radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
, composition, and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
are unknown. Assuming a composition similar to 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,...
and an environment close to chemical equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
, it is predicted that the atmosphere of Gliese 876 b is cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
less, though cooler regions of the planet may be able to form 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...
clouds.
Gliese 876 b lies within the habitable zone of Gliese 876 as defined by the ability of an 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...
-mass planet to retain liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
water at its surface. While the prospects for life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
on a gas giant are unknown, large moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
may be able to support a habitable
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...
environment. Models of tidal
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
interactions between a hypothetical moon, the planet and the star suggest that large moons should be able to survive in orbit around Gliese 876 b for the lifetime of the system. On the other hand, it is unclear whether such moons could form in the first place.