GJ 1214 b
Encyclopedia
GJ 1214 b is an extrasolar planet
that orbits the star GJ 1214
, in the constellation Ophiuchus
at a distance of 13 parsecs (approximately 40 light-years) from the Sun
. The planet was discovered in December 2009. It is classified as a super-Earth
because it is larger than Earth
but has a mass and radius significantly less than those of the gas giants
in the Solar System
. After COROT-7b, it is the second such planet to be known and is the first of a new class of planets with small size and relatively low density. GJ 1214 b is also significant because its parent star is relatively near the Sun
and because it transits
(crosses in front of) that parent star, which allows the planet's atmosphere to be studied using current technologies.
, which searches for the small drops in brightness that can occur when an orbiting planet briefly passes in front of its parent star. In early 2009, the astronomers running the project noticed that the star GJ 1214 appeared to show drops in brightness of that sort. They then observed the star more closely and confirmed that it dimmed by roughly 1.5% every 1.58 days. Follow-up radial-velocity measurements were then made with the HARPS spectrograph on the ESO
's 3.6-meter telescope at La Silla, Chile; those measurements succeeded in providing independent evidence for the reality of the planet. A paper was then published in Nature
announcing the planet and giving estimates of its mass, radius, and orbital parameters.
. Given the planet's mass and radius, its density can be calculated. Through a comparison with theoretical models, the density in turn provides limited but highly useful information about the composition and structure of the planet.
On that basis, it has been suggested that GJ 1214 b has a relatively thick gaseous envelope. It is possible to propose structures by assuming different compositions, guided by scenarios for the formation and evolution of the planet. GJ 1214 b could be a rocky planet with an outgassed hydrogen-rich atmosphere, a mini-Neptune
, or an ocean planet
. If it is a water world it could possibly be thought of as a bigger and hotter version of Jupiter's Galilean moons
. If GJ 1214 b is assumed to be an ocean planet, i.e. the interior is assumed to be composed primarily of a rocky
core surrounded by water
, proportions of the total mass consistent with the mass and radius are ~25% rock and ~75% water, covered by a thick envelope of gases such as hydrogen
and helium
(~0.05%). Water planets could result from inward planetary migration
and originate as protoplanet
s that formed from volatile
ice-rich material beyond the snow-line
but that never attained masses sufficient to accrete large amounts of H/He nebular gas. Because of the varying pressure at depth, models of a water world include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water. Some of the solid-phase water could be in the form of ice VII
.
Due to the relatively small size of GJ 1214 b's parent star, it is feasible to perform spectroscopic observations during planetary transits. By comparing the observed spectrum before and during transits, the spectrum of the planetary atmosphere can be inferred. In December 2010, a study was published showing that spectrum to be largely featureless over the wavelength range of 750–1000 nm. Since a thick and cloud-free hydrogen-rich atmosphere would have produced detectable spectral features, such an atmosphere appears to be ruled out. Although no clear signs were observed of water vapor or any other molecule, the authors of the study believe the planet may have an atmosphere composed mainly of water vapor. Another possibility is that there may be a thick layer of high clouds, which absorbs the starlight. Further observations are necessary to determine the composition of its atmosphere.
Though a planetary atmosphere has not yet been directly confirmed, the relative close proximity of the planet should allow existing space-based telescopes, such as the Hubble Space Telescope
, to detect and characterize one if it exists. Because of the estimated old age of the planetary system and the calculated hydrodynamic escape
rate of 9×105 kg s−1, scientists conclude that there has been a significant atmospheric loss during the lifetime of the planet and any current atmosphere cannot be primordial.
GJ 1214 b may be cooler than any other known transiting planet prior to the discovery of Kepler-16b
in 2011 by the Kepler mission
. Its equilibrium temperature could be between approximately 393–555 K
(120–282°C or 248–540°F), depending on how much of the star's radiation is reflected back into space
.
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...
that orbits the star GJ 1214
GJ 1214
GJ 1214 is a dim M4.5 red dwarf in the constellation Ophiuchus with an apparent magnitude of 14.7. It is located at a distance of approximately 40 light years from Earth. It is about one-fifth as large as the Sun with a surface temperature estimated to be...
, in the constellation Ophiuchus
Ophiuchus
Ophiuchus is a large constellation located around the celestial equator. Its name is from the Greek "serpent-bearer", and it is commonly represented as a man grasping the snake that is represented by the constellation Serpens. Ophiuchus was one of the 48 constellations listed by the 2nd-century...
at a distance of 13 parsecs (approximately 40 light-years) from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. The planet was discovered in December 2009. It is classified as a super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...
because it is larger than 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...
but has a mass and radius significantly less than those of the gas giants
Gas Giants
Gas Giants were a pop rock band from Tempe, Arizona, formed as a successor project to the Gin Blossoms. The group was known as The Pharaohs when they formed in 1997, but changed their name after their label, A&M Records, merged with Universal Records and the band changed hands, re-signing with...
in the 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...
. After COROT-7b, it is the second such planet to be known and is the first of a new class of planets with small size and relatively low density. GJ 1214 b is also significant because its parent star is relatively near the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and because it transits
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
(crosses in front of) that parent star, which allows the planet's atmosphere to be studied using current technologies.
Detection
GJ 1214 b was first detected by the MEarth ProjectMEarth Project
The MEarth Project is a United States NSF-funded, robotic observatory that is part of Fred Lawrence Whipple Observatory on Mt. Hopkins, Arizona, USA. The project monitors the brightness of thousands of red dwarf stars with the goal of finding transiting planets...
, which searches for the small drops in brightness that can occur when an orbiting planet briefly passes in front of its parent star. In early 2009, the astronomers running the project noticed that the star GJ 1214 appeared to show drops in brightness of that sort. They then observed the star more closely and confirmed that it dimmed by roughly 1.5% every 1.58 days. Follow-up radial-velocity measurements were then made with the HARPS spectrograph on the ESO
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
's 3.6-meter telescope at La Silla, Chile; those measurements succeeded in providing independent evidence for the reality of the planet. A paper was then published in Nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
announcing the planet and giving estimates of its mass, radius, and orbital parameters.
Features
The radius of GJ 1214 b can be inferred from the amount of dimming seen when the planet crosses in front of its parent star as viewed from Earth. The mass of the planet can be inferred from sensitive observations of the parent star's radial velocity, measured through small shifts in stellar spectral lines due to the Doppler effectDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. Given the planet's mass and radius, its density can be calculated. Through a comparison with theoretical models, the density in turn provides limited but highly useful information about the composition and structure of the planet.
On that basis, it has been suggested that GJ 1214 b has a relatively thick gaseous envelope. It is possible to propose structures by assuming different compositions, guided by scenarios for the formation and evolution of the planet. GJ 1214 b could be a rocky planet with an outgassed hydrogen-rich atmosphere, a mini-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...
, or an ocean planet
Ocean planet
An ocean planet is a hypothetical type of planet whose surface is completely covered with an ocean of water.Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly half water and half rock by mass...
. If it is a water world it could possibly be thought of as a bigger and hotter version of Jupiter's Galilean moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...
. If GJ 1214 b is assumed to be an ocean planet, i.e. the interior is assumed to be composed primarily of a rocky
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
core surrounded by 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...
, proportions of the total mass consistent with the mass and radius are ~25% rock and ~75% water, covered by a thick envelope of gases such as 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...
(~0.05%). Water planets could result from inward planetary migration
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
and originate as protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
s that formed from volatile
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...
ice-rich material beyond the snow-line
Frost line (astrophysics)
In astronomy or planetary science, the frost line, also known as the snow line or ice line, refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. Depending...
but that never attained masses sufficient to accrete large amounts of H/He nebular gas. Because of the varying pressure at depth, models of a water world include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water. Some of the solid-phase water could be in the form of ice VII
Ice VII
Ice VII is a cubic crystalline form of ice. It has a triple point with liquid water and Ice VI at 355 K and 2.216 GPa, with the melt line extending to at least 715 K and 10 GPa. It can also be reached in the solid state by increasing the pressure on ice VI at ambient temperature. Like the majority...
.
Due to the relatively small size of GJ 1214 b's parent star, it is feasible to perform spectroscopic observations during planetary transits. By comparing the observed spectrum before and during transits, the spectrum of the planetary atmosphere can be inferred. In December 2010, a study was published showing that spectrum to be largely featureless over the wavelength range of 750–1000 nm. Since a thick and cloud-free hydrogen-rich atmosphere would have produced detectable spectral features, such an atmosphere appears to be ruled out. Although no clear signs were observed of water vapor or any other molecule, the authors of the study believe the planet may have an atmosphere composed mainly of water vapor. Another possibility is that there may be a thick layer of high clouds, which absorbs the starlight. Further observations are necessary to determine the composition of its atmosphere.
Though a planetary atmosphere has not yet been directly confirmed, the relative close proximity of the planet should allow existing space-based telescopes, such as 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 and characterize one if it exists. Because of the estimated old age of the planetary system and the calculated hydrodynamic escape
Hydrodynamic escape
Hydrodynamic escape refers to a thermal atmospheric escape mechanism that can lead to the escape of heavier atoms of a planetary atmosphere through numerous collisions with lighter atoms....
rate of 9×105 kg s−1, scientists conclude that there has been a significant atmospheric loss during the lifetime of the planet and any current atmosphere cannot be primordial.
GJ 1214 b may be cooler than any other known transiting planet prior to the discovery of Kepler-16b
Kepler-16b
Kepler-16b is an extrasolar planet. It is a Saturn-mass planet consisting of half rock and half gas, and it orbits a binary star, Kepler-16, with a period of 229 days...
in 2011 by the Kepler mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
. Its equilibrium temperature could be between approximately 393–555 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(120–282°C or 248–540°F), depending on how much of the star's radiation is reflected back into space
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...
.
See also
- COROT-7b
- HD 149026 bHD 149026 bHD 149026 b is an extrasolar planet approximately 257 light-years away in the constellation of Hercules. The planet was discovered after it transited its parent star, HD 149026...
- Gliese 581 cGliese 581 cGliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...
- Gliese 581 dGliese 581 dGliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....
- MOA-2007-BLG-192LbMOA-2007-BLG-192LbMOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b, is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of...
- Gliese 876 dGliese 876 dGliese 876 d is an extrasolar planet approximately 15 light-years away in the constellation of Aquarius . The planet was the third planet discovered orbiting the red dwarf star Gliese 876. At the time of its discovery, the planet had the lowest mass of any known extrasolar planet apart from the...