Gliese 436
Encyclopedia
Gliese 436 is a red dwarf
star approximately 33 light-year
s away in the constellation
of Leo
. In 2004, the existence of an extrasolar planet
orbiting the star was confirmed. There are indications that there may be an additional low-mass planet in the system, but this is not yet confirmed.
(with masses greater than helium-4) equal to 44% of the Sun's. The projected rotation velocity is less than 3 km/s, and the chromosphere
has only a low level of magnetic activity.
This star is a member of the "old-disk population" with velocities in the galactic coordinate system
of U=+44, V=−20 and W=+20 km/s.
. The planet has an orbital period of 2.6 Earth Days and transits the star as viewed from Earth. It has a mass of 22.2 times Earth's Mass
and is roughly 55,000 km in diameter, giving it a mass and radius similar to the ice giant
planets Uranus
and Neptune
in our solar system. In general, Doppler spectroscopy measurements do not measure the true mass of the planet, but instead measure the product m sin
i, where m is the true mass and i is the inclination of the orbit (the angle between the line-of-sight and the normal to the planet's orbital plane), a quantity that is generally unknown. However, for Gliese 436 b, the transits enable the determination of the inclination, as they show that the planet's orbital plane is very nearly in the line of sight (i.e. that the inclination is close to 90 degrees
). Hence the mass quoted is the actual mass. The planet is thought to be largely composed of hot ices
with an outer envelope of hydrogen
and helium
, and is termed a "hot Neptune". The planet's orbit is eccentric
. Because tidal force
s would tend to circularise the orbit of the planet on short timescales, this suggested that Gliese 436 b is being perturbed by an additional planet orbiting the star.
es and have a radius about 1.5 times larger than the Earth's. Despite being announced in the media as being the smallest known extrasolar planet, it would still have been more massive than the three planets orbiting the pulsar PSR B1257+12
, which have been known since 1992. Due to its size, the planet was thought to be a rocky, terrestrial planet
. Gliese 436 c was announced by Spanish scientists in April 2008 by analyzing its influence on the orbit of Gliese 436 b. Further analysis showed that the transit length of the inner planet is not changing, a situation which rules out most possible configurations for this system. Also, if it did orbit at these parameters, the system would be the only "unstable" orbit on UA's Extrasolar Planet Interactions chart. The existence of this "Gliese 436 c" was thus regarded as unlikely, and the discovery was eventually retracted at the Transiting Planets conference in Boston, 2008.
Despite the retraction, studies have concluded that the possibility that there is an additional planet orbiting Gliese 436 remains plausible. With the aid of an unnoticed transit automatically recorded at NMSU on January 11, 2005, and observations by amateur astronomers, it has been suggested that there is a trend of increasing inclination of the orbit of Gliese 436 b, though this trend remains unconfirmed. This trend is compatible with a perturbation by a planet of less than 12 Earth masses on an orbit within about 0.08 AU of the star. This is still unconfirmed.
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 approximately 33 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 Leo
Leo (constellation)
Leo is one of the constellations of the zodiac. Its name is Latin for lion. Its symbol is . Leo lies between dim Cancer to the west and Virgo to the east.-Stars:...
. In 2004, the existence of 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...
orbiting the star was confirmed. There are indications that there may be an additional low-mass planet in the system, but this is not yet confirmed.
Star
Stellar models for this star give an estimated size of about 42% of the Sun's radius. The same model predicts a surface temperature of about 3318 K. Gl 436 is older than the Sun by several billion years but it only has an abundance of heavy elementsMetallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
(with masses greater than helium-4) equal to 44% of the Sun's. The projected rotation velocity is less than 3 km/s, and the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
has only a low level of magnetic activity.
This star is a member of the "old-disk population" with velocities in the galactic coordinate system
Galactic coordinate system
The galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...
of U=+44, V=−20 and W=+20 km/s.
Planetary system
The star is orbited by one known planet, designated Gliese 436 bGliese 436 b
Gliese 436 b is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436. It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.-Discovery:...
. The planet has an orbital period of 2.6 Earth Days and transits the star as viewed from Earth. It has a mass of 22.2 times Earth's Mass
Earth mass
Earth mass is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9722 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets....
and is roughly 55,000 km in diameter, giving it a mass and radius similar to the ice giant
Ice giant
Ice Giant or Ice giants may refer to:* A type of gas giant composed largely of materials less volatile than hydrogen and helium* The frost giants of Norse mythology, see also Hrimthurs...
planets Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
in our solar system. In general, Doppler spectroscopy measurements do not measure the true mass of the planet, but instead measure the product m sin
Sine
In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....
i, where m is the true mass and i is the inclination of the orbit (the angle between the line-of-sight and the normal to the planet's orbital plane), a quantity that is generally unknown. However, for Gliese 436 b, the transits enable the determination of the inclination, as they show that the planet's orbital plane is very nearly in the line of sight (i.e. that the inclination is close to 90 degrees
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
). Hence the mass quoted is the actual mass. The planet is thought to be largely composed of hot ices
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
with an outer envelope of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, and is termed a "hot Neptune". The planet's orbit is eccentric
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
. Because tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
s would tend to circularise the orbit of the planet on short timescales, this suggested that Gliese 436 b is being perturbed by an additional planet orbiting the star.
Possible second planet
In 2008, a second planet, designated "Gliese 436 c" was claimed to have been discovered, with an orbital period of 5.2 days and an orbital semimajor axis of 0.045 AU. The planet was thought to have a mass of roughly 5 Earth massEarth mass
Earth mass is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9722 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets....
es and have a radius about 1.5 times larger than the Earth's. Despite being announced in the media as being the smallest known extrasolar planet, it would still have been more massive than the three planets orbiting the pulsar PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...
, which have been known since 1992. Due to its size, the planet was thought to be a rocky, terrestrial planet
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 are the inner planets closest to the Sun...
. Gliese 436 c was announced by Spanish scientists in April 2008 by analyzing its influence on the orbit of Gliese 436 b. Further analysis showed that the transit length of the inner planet is not changing, a situation which rules out most possible configurations for this system. Also, if it did orbit at these parameters, the system would be the only "unstable" orbit on UA's Extrasolar Planet Interactions chart. The existence of this "Gliese 436 c" was thus regarded as unlikely, and the discovery was eventually retracted at the Transiting Planets conference in Boston, 2008.
Despite the retraction, studies have concluded that the possibility that there is an additional planet orbiting Gliese 436 remains plausible. With the aid of an unnoticed transit automatically recorded at NMSU on January 11, 2005, and observations by amateur astronomers, it has been suggested that there is a trend of increasing inclination of the orbit of Gliese 436 b, though this trend remains unconfirmed. This trend is compatible with a perturbation by a planet of less than 12 Earth masses on an orbit within about 0.08 AU of the star. This is still unconfirmed.