GJ 3021
Encyclopedia
GJ 3021, formally cataloged as Gliese 3021, is a binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 system approximately 57 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 of Hydrus
Hydrus
Hydrus is a small constellation in the southern sky, created in the sixteenth century. Its name means "male water snake", and it should not be confused with Hydra, a much larger constellation which represents a female water snake.-History:...

 (the Water Snake).

The visible star in the system, A, is considered to be a Solar twin. As of 2000, it has been confirmed that an 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...

 orbits the star. It is of note for being a relatively Sun-like star not very far from the Sun that is home to an extrasolar planet.

Stellar components

As a nearby Sun-like star, the last decade has seen GJ 3021A being studied carefully for the first time, especially after its substellar companion was discovered. It is currently believed that it is 800 million years old, though age estimates range from 150 million to 8.8 billion years old depending on the method used for the determination. The star is more enriched with 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...

 than the Sun, is chromospherically active, and rotates around its axis more quickly than the Sun.
The secondary star GJ 3021B is a M4 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 at a projected physical separation of 68 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

.

Planetary system

Announced in 2000, the Jovian planet GJ 3021 b
GJ 3021 b
GJ 3021 b is an extrasolar planet approximately 57 light-years away in the constellation of Hydrus. The planet is a Jupiter-like planet orbiting about 0.5 AU from its star with a minimum mass 3.37 times that of Jupiter, as determined by measuring variations in the radial velocity of the star...

 orbits about 0.5 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s from GJ 3021 A with a minimum mass 3.37 times that of Jupiter, as determined by measuring variations in the 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...

 of the star. A study published in 2001 suggested that the usual inability to determine the orbital inclination of an extrasolar planet through radial velocity measurement had caused this mass to be severely underestimated. 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...

 orbit gives an orbital inclination of 11.8° and a mass of 16 Jupiter masses, which would make the object a brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

. However later analysis showed that Hipparcos was not sensitive enough to accurately determine astrometric orbits for substellar companions, which means the inclination (and hence the true mass) of the planet are still unknown.
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