55 Cancri b
Encyclopedia
55 Cancri b is 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 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 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...

 every 14.65 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

s. It is the second planet in order of distance from its star, and is an example of a hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...

. Discovered in 1996 by Geoffrey Marcy
Geoffrey Marcy
Geoffrey W. Marcy is an American astronomer, who is currently Professor of Astronomy at the University of California, Berkeley, famous for discovering more extrasolar planets than anyone else, 70 out of the first 100 to be discovered, along with R...

 and R. Paul Butler
R. Paul Butler
R. Paul Butler is an astronomer who searches for extrasolar planets.He received a BA and an MS from San Francisco State University, completing a Master's thesis with Geoffrey Marcy, and then completed his doctoral studies at the University of Maryland, College Park in 1993...

, 55 Cancri b was the fourth known extrasolar planet, excluding pulsar planet
Pulsar planet
Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars. The first such planet to be discovered was around a millisecond pulsar and was the first extrasolar planet to be confirmed as discovered.-Pulsar planets:...

s.

Discovery

Like the majority of known extrasolar planets, 55 Cancri b was discovered by detecting variations in its 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...

 caused by the planet's gravity. 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, a 15-day periodicity was detected. The planet was announced in 1996, 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 innermost 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...

.

Even when this inner planet, with a 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:...

 at least 78% times that 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,...

 was accounted for, the star still showed a drift in its radial velocity. This eventually led to the discovery of 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...

 in 2002.

Orbit and mass

55 Cancri b is in a short-period orbit, though not so extreme as that of the previously detected hot Jupiter 51 Pegasi b
51 Pegasi b
51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...

. The orbital period indicates that the planet is located close to a 1:3 mean motion
Mean motion
Mean motion, n\,\!, is a measure of how fast a satellite progresses around its elliptical orbit. Unless the orbit is circular, the mean motion is only an average value, and does not represent the instantaneous angular rate....

 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...

 with 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...

, however investigations of the planetary parameters in a Newtonian simulation indicate that while the orbital periods are close to this ratio, the planets are not actually in the resonance.

A limitation of the radial velocity method used to discover the planet is that only a lower limit on the mass can be determined. 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...

 measurements 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 is inclined
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 at around 53° with respect to the plane of the sky. The system is further expected to be coplanar. Assuming all this is confirmed, the planet's 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...

 is around 1.03 Jupiter masses.

Characteristics

Given the planet's high mass, it is likely that 55 Cancri b is a gas giant
Gas 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...

 with no solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

 surface. Since the planet has only been detected indirectly, properties such as its radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

, composition, and temperature
Temperature
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...

 are unknown. Assuming a composition similar to that of Jupiter and that its environment is close to chemical equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

, 55 Cancri b is predicted to have a cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

less upper atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 with a 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...

 dominated by alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

 absorption.

The planet is unlikely to have large moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

, since tidal forces
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

would either eject them from orbit or destroy them on short timescales relative to the age of the system.
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