55 Cancri

55 Cancri

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55 Cancri also cataloged Rho1 Cancri or abbreviated 55 Cnc, 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...

 approximately 41 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 from Earth 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 Cancer
Cancer (constellation)
Cancer is one of the twelve constellations of the zodiac. Its name is Latin for crab and it is commonly represented as such. Its symbol is . Cancer is small and its stars are faint...

. The system consists of a yellow dwarf
Yellow dwarf
A G-type main-sequence star , often called a yellow dwarf, is a main-sequence star of spectral type G and luminosity class V. Such a star has about 0.8 to 1.2 solar masses and surface temperature of between 5,300 and 6,000 K., Tables VII, VIII...

 star and a smaller 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, separated by over 1,000 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s (one thousand times the distance from the Earth to the Sun).

As of 2011, five 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...

s have been confirmed to be orbiting the primary, 55 Cancri A (the yellow dwarf). The 55 Cancri system is one of only six 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...

s (the others being our own 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...

, Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

, HD 10180
HD 10180
HD 10180 is a solar-type star. A research team led by Christophe Lovis of the University of Geneva discovered that the star has at least five planets, and possibly as many as seven. It is 127 light-years away, in the southern constellation of Hydrus...

, Kepler-11
Kepler-11
Kepler-11 is a sun-like star slightly larger than the Sun in the constellation Cygnus, located some 2,000 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission uses to detect planets that may be transiting their stars...

 and KOI-500) known to have at least five planets, and may possibly have more. The innermost planet, 55 Cancri e, transits 55 Cancri A as viewed from Earth. The innermost planet is thought to be a terrestrial
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...

 "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...

" planet, with a mass similar to 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...

 but without the massive gas atmosphere that Neptune has, although signs of a smaller atmosphere may have been detected. The outermost planets in the system are thought to be Jovian planets with masses 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,...

. 55 Cancri A is ranked 63rd in the list of top 100 target stars for the NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 Terrestrial Planet Finder
Terrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...

 mission.

Distance and visibility


The 55 Cancri system is located fairly close to 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 Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

 astrometry
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...

 satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 measured the parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

 of 55 Cancri A as 81.03 milliarcsecond
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

s, corresponding to a distance of 12.3 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s (40.3 light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....

). 55 Cancri A has an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of 5.95, making it visible through binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

. It is just visible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...

 under very dark skies. The red dwarf 55 Cancri B is of the 13th magnitude and only visible through a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

.

System components


The primary star 55 Cancri A is a yellow dwarf star of main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 spectral type G8V. It is smaller in radius and slightly less massive than our Sun, and so is cooler and less luminous
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

. The star has little or no variability and only low emission from its chromosphere.

55 Cancri A is more enriched than our sun in elements heavier than 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...

, with 186% the solar abundance of 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...

; it is therefore classified as a rare "super metal-rich" (SMR) star. This abundance of metal makes estimating the star's age and mass difficult, as evolutionary
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

 models are less well defined for such stars. One estimate based on chromospheric
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....

 activity suggests an age of around 5.5 billion years. Other studies yield age estimates of 7.4 and 8.7 billion years.

A hypothesis for the high metal content in SMR dwarf stars is that material enriched in heavy elements fell into the atmosphere from a protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

. This would pollute the star's external layers, resulting in a higher than normal metallicity. The lack of a deep convection zone
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...

 would mean that the outer layers would retain higher abundance ratios of these heavy elements.

Observations of 55 Cancri A in the submillimeter
Submillimetre astronomy
Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a...

 region of the spectrum have thus far failed to detect any associated dust. The upper limit on emissions within 100 AU of this star is about 850 mJy
Jansky
The flux unit or jansky is a non-SI unit of spectral flux density equivalent to 10−26 watts per square metre per hertz...

, at a wavelength of 850 μm. This limits the total mass of fine dust around the star to less than 0.01% of the Earth's mass. However, this does not exclude the presence of an asteroid belt or a Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 equivalent.

55 Cancri B is a red dwarf star located at an estimated distance of 1065 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 from the primary star, and is much less massive and luminous than our Sun. Despite their wide separation, the two stars appear to be gravitationally bound, as they share a common proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

. There are indications that component B may itself be a double star, though this is uncertain.

Planetary system


In 1997, the discovery of a 51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

-like planet 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 55 Cancri A was announced, together with the planet of Tau Boötis
Tau Boötis
Tau Boötis is a yellow-white dwarf approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf...

 and the inner planet of Upsilon Andromedae
Upsilon Andromedae
Upsilon Andromedae is a binary star located approximately 44 light-years away from Earth in the constellation Andromeda. The primary star is a yellow-white dwarf star that is somewhat younger than the Sun...

. The planet was discovered by measuring the 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...

, which showed a periodicity of around 14.7 days corresponding to a planet at least 78% of 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,...

. These radial velocity measurements still showed a drift unaccounted-for by this planet, which could be explained by the gravitational influence of a more distant object.

This planet was designated HR 3522b by its discoverers (HR 3522 is an alternative designation for 55 Cancri), though it is more commonly referred to as 55 Cancri b
55 Cancri b
55 Cancri b is an extrasolar planet orbiting the Sun-like star 55 Cancri A every 14.65 days. It is the second planet in order of distance from its star, and is an example of a hot Jupiter. Discovered in 1996 by Geoffrey Marcy and R...

. Under the rules for naming objects in binary star systems it should be named 55 Cancri Ab and this more formal form is occasionally used to avoid confusion with the secondary star 55 Cancri B.

In 1998 the discovery of a possible dust disk around 55 Cancri A was announced. Calculations gave the disk radius at least 40 AU, similar to the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 in our solar system, with an inclination of 25° with respect to the plane of the sky. However, the discovery could not be verified and was later deemed to be spurious, caused instead by background radiation.


After making further radial velocity measurements, a planet orbiting at a distance of around 5 AU was announced in 2002. This planet received the designation 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...

. At the time of discovery, the planet was thought to be in an orbit of mild eccentricity (close to 0.1), but this value was increased by later measurements. Even after accounting for these two planets, a periodicity at 43 days remained, possibly due to a third planet. Measurements of the star suggested that this was close to the star's rotation period, which raised the possibility that the 43-day signal was caused by stellar activity. This possible planet received the designation 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...

.
A planet designated 55 Cancri e
55 Cancri e
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...

 orbits 55 Cnc A with a 0.78-day period (while the 2004 discovery claimed a 2.8-day orbit), observations of e transiting show a 0.78-day period and indicate that the initial period measurement was aliased
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...

.) At 8.3 Earth masses (1/2 Neptune mass), it is 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...

. The measurements that led to the discovery of this planet also confirmed the existence of 55 Cancri c.

Jack Wisdom has suggested that there is a planet with a mass similar to that of Neptune in a 261-day orbit (corresponding to 0.77 AU in distance). This analysis has been partially confirmed in November 2007 – a planet 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...

 with half the mass of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 was announced in a 260-day orbit, in 55 Cancri A's 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...

. The planet itself is not thought to be conducive to life, but hypothetical moons
Extrasolar moon
An extrasolar moon, or exomoon, is a natural satellite that orbits an extrasolar planet or other extrasolar body. Although no extrasolar moons have yet been observed, it can be inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of...

 in principle could maintain at least microbial life.

With five planets, the system cannot deviate far from 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....

 in order to maintain stability. Assuming that the astrometric determination of the orbit (with an inclination of 53°) of the outer planet made by 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...

 is correct, 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...

es of the planets are around 25% greater than the lower limits measured by the radial velocity method. However, this conflicts with the observed transits of e, which show that its orbit is inclined to within 9º of normal to the line-of-light, placing its mass only slightly above the minimum. Assuming coplanarity, the same applies for the rest of the planets.

The approximate ratios of periods of adjacent orbits are (proceeding outward): 1:20, 1:3, 1:6, 1:20. The nearly 1:3 ratio between 55 Cancri b and c is apparently a near resonance, rather than a genuine mean motion resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

.

More planets are possible within the stable zone, between f and d at 0.9 to 3.8 AU with eccentricities below 0.4. Given hypothetical planet g of up to 50 Earth masses, stable mean motion resonance regions lie at 3f:2g, 2g:1d, and 3g:2d. As for the space outside d's orbit, its stability zone begins beyond 10 AU.

A METI message was sent to 55 Cancri. It was transmitted from Eurasia
Eurasia
Eurasia is a continent or supercontinent comprising the traditional continents of Europe and Asia ; covering about 52,990,000 km2 or about 10.6% of the Earth's surface located primarily in the eastern and northern hemispheres...

's largest radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 – 70-meter (230-foot) Eupatoria
Eupatoria
Yevpatoria or Eupatoria is a city in Crimea, Ukraine.-History:The first recorded settlement in the area, called Kerkinitis , was built by Greek colonists around 500 BC...

 Planetary Radar. The message was named Cosmic Call 2, it was sent on July 6, 2003, and it will arrive at 55 Cancri in May 2044.

See also


  • 51 Pegasi
    51 Pegasi
    51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

  • Mu Arae
    Mu Arae
    Mu Arae , often referred to by its designation in the Henry Draper catalogue HD 160691, is a main sequence G-type star around 50 light-years away from Earth in the constellation of Ara. The star has a planetary system with four known planets, three of them with masses comparable to that of Jupiter...

  • PSR 1257+12
  • Upsilon Andromedae
    Upsilon Andromedae
    Upsilon Andromedae is a binary star located approximately 44 light-years away from Earth in the constellation Andromeda. The primary star is a yellow-white dwarf star that is somewhat younger than the Sun...

  • List of extrasolar planets

External links