55 Cancri e
Encyclopedia
55 Cancri e is an extrasolar planet
with half the mass
of Neptune
orbiting the Sun
-like star
55 Cancri A
. Its mass is about 8.3 Earth masses, thus classifying it as the first Super-Earth
discovered around a main sequence star, predating Gliese 876 d
by a year. It takes less than 18 hours to complete an orbit
and is the innermost known planet in its planetary system
. 55 Cancri e was discovered on August 30, 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.
. This was achieved by making sensitive measurements of the Doppler shift of the spectrum
of 55 Cancri A. At the time of its discovery, three other planets were known orbiting the star. After accounting for these planets, a signal at around 2.8 days remained, which could be explained by a planet of at least 14.2 Earth
masses in a very close orbit. The same measurements were used to confirm the existence of the uncertain planet 55 Cancri c
.
55 Cancri e was one of the first extrasolar planets with a mass comparable to that of Neptune to be discovered. It was announced at the same time as another "hot Neptune" orbiting the red dwarf
star Gliese 436 named Gliese 436 b
.
and colleagues at San Francisco State University
published a new analysis indicating that both planets existed; the planet in the 260-day orbit was accordingly designated 55 Cancri f
.
can be obtained, in this case around 8.3 times that of Earth, or 48% of the mass of Neptune. Astrometric
observations made with the Hubble Space Telescope
suggest that the outer planet 55 Cancri d
is inclined at 53° with respect to the plane of the sky. If these measurements are confirmed and the system is assumed to be coplanar
, the true mass
of the planet would therefore be about 25% greater than this lower limit, at around 0.6 Neptune mass. However, a transit has been confirmed, thus its inclination is about 83.4 ± 1.7.
like Neptune or a large rocky terrestrial planet
. In 2011, a transit
of the planet was confirmed, allowing to calculate its density. The high density calculated suggests that the planet has a "rock-iron composition supplemented by a significant mass of water, gas, or other light elements".
55 Cancri e receives more radiation than Gliese 436 b
.
were unknown. The formation models proposed for Mu Arae c
do not apply to a planet this close to its star. Large terrestrial planets may be formed from material compacted by the inward migration
of the system's gas giants.
Alternatively 55 Cancri e may be the core
of a gas giant pushed inwards before it had time to accumulate a significant gaseous envelope. This was dismissed in 2004, as gas giants can survive for long timescales in the inner regions of a planetary system. However the star could have blown off a volatile layer via coronal mass ejection
. In this model (also proposed for Gliese 876 d
), the original composition was richer in volatile
substances, such as water
. As it arrived in range, the planet boiled away to a pressurised
ocean
of water (in the form of a supercritical fluid
) separated from the silicate
core
by a layer of ice
kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor
and free oxygen
produced by the breakdown of water by ultraviolet
radiation.
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...
with half the 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:...
of 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...
orbiting 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...
-like 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...
55 Cancri A
55 Cancri
55 Cancri , also cataloged Rho1 Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away from Earth in the constellation of Cancer...
. Its mass is about 8.3 Earth masses, thus classifying it as the first 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...
discovered around a main sequence star, predating Gliese 876 d
Gliese 876 d
Gliese 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...
by a year. It takes less than 18 hours to complete an 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...
and is the innermost known planet in its planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
. 55 Cancri e was discovered on August 30, 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.
Discovery
Like the majority of known extrasolar planets (found previous to the Kepler mission), 55 Cancri e was discovered by detecting variations in its star's radial velocityRadial 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...
. This was achieved by making sensitive measurements of the Doppler shift of the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of 55 Cancri A. At the time of its discovery, three other planets were known orbiting the star. After accounting for these planets, a signal at around 2.8 days remained, which could be explained by a planet of at least 14.2 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...
masses in a very close orbit. The same measurements were used to confirm the existence of the uncertain planet 55 Cancri c
55 Cancri c
55 Cancri c is an extrasolar planet in an eccentric orbit around the Sun-like star 55 Cancri A, making one revolution every 44.34 days. It is the third known planet in order of distance from its star...
.
55 Cancri e was one of the first extrasolar planets with a mass comparable to that of Neptune to be discovered. It was announced at the same time as another "hot Neptune" orbiting 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 Gliese 436 named Gliese 436 b
Gliese 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:...
.
Planet challenged
In 2005 the existence of planet e was questioned by Jack Wisdom in a reanalysis of the data. According to him, instead of the 2.8-day planet there is a planet with a mass similar to that of Neptune in a 261-day orbit around 55 Cancri A. In 2007, Debra FischerDebra Fischer
Debra A. Fischer is a professor of astronomy at Yale University. Fischer has co-authored over 100 papers on dwarf and sub-stellar mass objects in the galactic neighborhood, including many on extrasolar planets. She is a principal investigator with the N2K Consortium searching for exoplanets...
and colleagues at San Francisco State University
San Francisco State University
San Francisco State University is a public university located in San Francisco, California. As part of the 23-campus California State University system, the university offers over 100 areas of study from nine academic colleges...
published a new analysis indicating that both planets existed; the planet in the 260-day orbit was accordingly designated 55 Cancri f
55 Cancri f
55 Cancri f is an extrasolar planet approximately 41 light-years away from Earth in the constellation of Cancer...
.
Transit
The planet's transit of its primary was announced on April 27, 2011. This is one of the few planetary transits to be confirmed around a well-known star, and allowed assumptions about the planet's composition to be made.Orbit and mass
55 Cancri e is located in a very close orbit around the star which takes less than 18 hours to complete and falls into the category of "hot Super-Earths". A limitation of the radial velocity method used to detect 55 Cancri e is that only a minimum massMinimum mass
In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets...
can be obtained, in this case around 8.3 times that of Earth, or 48% of the mass of Neptune. 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...
observations made with 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...
suggest that the outer planet 55 Cancri d
55 Cancri d
55 Cancri d is an extrasolar planet in a long-period orbit around the Sun-like star 55 Cancri A. Located at a similar distance from its star as Jupiter is from our Sun, it is the fifth and outermost known planet in its planetary system...
is inclined at 53° with respect to the plane of the sky. If these measurements are confirmed and the system is assumed to be coplanar
Coplanarity
In geometry, a set of points in space is coplanar if all the points lie in the same geometric plane. For example, three distinct points are always coplanar; but a fourth point or more added in space can exist in another plane, incoplanarly....
, 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...
of the planet would therefore be about 25% greater than this lower limit, at around 0.6 Neptune mass. However, a transit has been confirmed, thus its inclination is about 83.4 ± 1.7.
Characteristics
It was initially unknown whether 55 Cancri e was a small 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...
like Neptune or a large 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...
. In 2011, a transit
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...
of the planet was confirmed, allowing to calculate its density. The high density calculated suggests that the planet has a "rock-iron composition supplemented by a significant mass of water, gas, or other light elements".
55 Cancri e receives more radiation than Gliese 436 b
Gliese 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:...
.
Formation theories
Prior to the transit observations in April 2011, composition and temperatureTemperature
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...
were unknown. The formation models proposed for Mu Arae c
Mu Arae c
Mu Arae e is one of the four extrasolar planets orbiting the star Mu Arae. Its discovery was announced on June 13, 2002. Mu Arae e is a gas giant at least 1.8 times as massive as Jupiter. The planet orbits at Jupiter-like distance at 5.235 AU.-References:...
do not apply to a planet this close to its star. Large terrestrial planets may be formed from material compacted by the inward 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...
of the system's gas giants.
Alternatively 55 Cancri e may be the core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
of a gas giant pushed inwards before it had time to accumulate a significant gaseous envelope. This was dismissed in 2004, as gas giants can survive for long timescales in the inner regions of a planetary system. However the star could have blown off a volatile layer via coronal mass ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
. In this model (also proposed for Gliese 876 d
Gliese 876 d
Gliese 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...
), the original composition was richer in volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
substances, such as 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...
. As it arrived in range, the planet boiled away to a pressurised
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
of water (in the form of a supercritical fluid
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...
) separated from the silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
by a layer of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
and free oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
produced by the breakdown of water by ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation.