Planetary system

Planetary system

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A planetary system consists of the various non-stellar objects 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 a 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...

 such as planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s (and their moon
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....

s), asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...

s, comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, and cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

. 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...

 together with its planetary system, which includes 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...

, is known as 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...

.

Before the 16th Century and Copernican heliocentrism
Copernican heliocentrism
Copernican heliocentrism is the name given to the astronomical model developed by Nicolaus Copernicus and published in 1543. It positioned the Sun near the center of the Universe, motionless, with Earth and the other planets rotating around it in circular paths modified by epicycles and at uniform...

, human knowledge of planetary systems was limited to heliocentrism
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 and our own Solar System. Despite the discovery and exploration of the Solar System
Discovery and exploration of the Solar System
For many thousands of years, civilizations, with a few notable exceptions, did not recognize the existence of the Solar System. It was believed the Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky...

 and centuries of conjecture, it remained this way until the first confirmed detection of 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 during the 1990s
1990s
File:1990s decade montage.png|From left, clockwise: The Hubble Space Telescope floats in space after it was taken up in 1990; American F-16s and F-15s fly over burning oil fields and the USA Lexie in Operation Desert Storm, also known as the 1991 Gulf War; The signing of the Oslo Accords on...

, after which the nature of planetary systems including the formation of our own Solar System has increased considerably.

As of 2011, a total of 573 planetary systems and 81 multi-planet systems have been confirmed with 53 consisting of two or more planets. Hundreds more systems are unconfirmed.

Of particular interest to astrobiology
Astrobiology
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...

 is the 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...

, believed to be the region of planetary systems with the most potential to develop and sustain extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

.

Heliocentrism


Historically, heliocentrism (that the sun was the centre of the universe) was opposed to geocentrism, which placed the Earth at the center of the universe. The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos
Aristarchus of Samos
Aristarchus, or more correctly Aristarchos , was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...

, but had received no support from most other ancient astronomers.

Discovery of the Solar System



De revolutionibus orbium coelestium
De revolutionibus orbium coelestium
De revolutionibus orbium coelestium is the seminal work on the heliocentric theory of the Renaissance astronomer Nicolaus Copernicus...

 by Nicolaus Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

 and published in 1543 was the first mathematically predictive heliocentric model of a planetary system. 17th-century successors Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

, Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

, and Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 developed an understanding of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth.

Speculation on extrasolar planetary systems


In the sixteenth century the Italian philosopher Giordano Bruno
Giordano Bruno
Giordano Bruno , born Filippo Bruno, was an Italian Dominican friar, philosopher, mathematician and astronomer. His cosmological theories went beyond the Copernican model in proposing that the Sun was essentially a star, and moreover, that the universe contained an infinite number of inhabited...

, an early supporter of the Copernican
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

 theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. He was burned at the stake for his ideas by the Roman Inquisition
Roman Inquisition
The Roman Inquisition was a system of tribunals developed by the Holy See during the second half of the 16th century, responsible for prosecuting individuals accused of a wide array of crimes related to heresy, including Protestantism, sorcery, immorality, blasphemy, Judaizing and witchcraft, as...

.

In the eighteenth century the same possibility was mentioned by Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 in the "General Scholium
General Scholium
The General Scholium is an essay written by Isaac Newton, appended to his work of Philosophiæ Naturalis Principia Mathematica, known as the Principia. General Scholium was first published with the second edition of the Principia and reappeared with some additions and modifications on the third ...

" that concludes his Principia
Philosophiae Naturalis Principia Mathematica
Philosophiæ Naturalis Principia Mathematica, Latin for "Mathematical Principles of Natural Philosophy", often referred to as simply the Principia, is a work in three books by Sir Isaac Newton, first published 5 July 1687. Newton also published two further editions, in 1713 and 1726...

. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."

His theories gained traction through the 19th and 20th Centuries despite a lack of supporting evidence.

Detection of Exoplanets


The first confirmed detection of a planetary system was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 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 :...

. The first confirmed detection of planetary system of a main-sequence star was made in 1995, when a giant planet, 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...

, was found in a four-day orbit around the nearby G-type star 51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

. The frequency of detections has increased since then.

Origin and evolution



Early theories of planetary systems were based on knowledge of the formation and evolution of the Solar System
Formation and evolution of the Solar System
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...

. Planetary systems are generally believed to form as part of the same process which results in star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

. Some early theories involved another star passing extremely close to the star, drawing material out from it which then coalesced to form the planets. However, the probability of such a near collision is now known to be far too low to make this a viable model. Accepted theories today argue that 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...

 forms by gravitational collapse of a molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....

 and then evolves into a planetary system by collisions and gravitational capture.

Some planetary systems may form differently, however. Planets orbiting pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

s—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...

s which emit periodic bursts of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

—have been discovered by the slight variations they cause in the timing of these bursts. Pulsars are formed in violent supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 explosions, and a normal planetary system could not possibly survive such a blast—planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star. One theory is that existing stellar companions were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may somehow form in the accretion disk surrounding pulsars.

The formation of planetary systems has also been found to be linked to stellar classification
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

. Stars are composed mainly of the light elements 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...

. They also contain a small proportion of heavier elements such as 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...

, and this fraction is referred to as a star's metallicity
Metallicity
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...

. Stars of higher metallicity are much more likely to form and retain planetary systems, and the planets within them have tend to be more massive than those of lower-metallicity stars. It has also been shown that stars with planets are more likely to be deficient in lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

. Recent observations by the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 indicate that planetary formation does not occur around other stars in the vicinity of an O class star due to the photoevaporation effect.

Types and Attributes


With the increasing discovery of planetary systems, astronomers have begun to classify them by type. Two common types of planetary systems have been classified. Hot Jupiter systems with a gas giant very close in to the star are one of the most common found so far. Dusty disks with a wide ring of dust and comets are another common type of system. Protoplanetary disks, still in the process of forming have also been discovered. At present, few systems have been found to analogues of our own with terrestrial planets close in to the parent star. More commonly, systems consisting of multiple Super-Earths have been detected.

Orbital properties


Unlike the solar system, the majority of known systems display high much higher orbital eccentricity
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...

 and some consist of planets in relatively eccentric orbits while some consist of planets on inconsistent orbital inclination unlike the solar system which is coplanar and has mainly circular orbits.

Although no planets in the Solar System are completely tidally locked, planets in close to their primary stars typically experience tidal locking
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

.

Habitable Zone



The habitable zone is the region in the planetary system where an Earth-like planet could maintain liquid water on its surface and potentially therefore Earth-like life. The distance of this zone from the parent star can vary greatly from system to system.

Notable planetary systems


  • 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 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 its planetary system, the first such system discovered
  • 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 :...

     – the first extrasolar planetary system discovered, the first pulsar
    Pulsar
    A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

     planetary system discovered, the first multi exoplanet system discovered
  • 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 first multiplanet extrasolar planetary system discovered around a 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...

     star, found to be so in April 1999
  • PSR B1620-26
    PSR B1620-26
    PSR B1620-26 is a binary star system located at a distance of 3,800 parsecs in the globular cluster of Messier 4 in the constellation of Scorpius. The system is composed of a pulsar and a white dwarf...

     – the first multistar planetary system discovered.
  • 55 Cancri
    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...

     – the largest known extrasolar planetary system prior to the discovery of the 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...

     system (5 known planets as of November 6, 2007, along with a distant stellar companion.)
  • Gliese 876
    Gliese 876
    Gliese 876 is a red dwarf star approximately 15 light-years away from Earth in the constellation of Aquarius. As of 2011, it has been confirmed that four extrasolar planets orbit the star...

     – the first system around a 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 and the first discovered to have an orbital 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...

     (subsequently shown to be a 3-body Laplace resonance).
  • HD 69830
    HD 69830
    HD 69830 is an orange dwarf star approximately 41 light-years away in the constellation of Puppis. It has the Gould designation 285 G. Puppis, though this is infrequently used. In 2005, the Spitzer Space Telescope discovered a debris disk orbiting the star. The disk contains substantially more...

     – found to have three Neptune-mass planets and an asteroid belt, all within 1 AU
  • 2M1207
    2M1207
    2M1207, 2M1207A or 2MASS J12073346-3932539 is a brown dwarf located in the constellation Centaurus; a companion object, 2M1207b, may be the first extrasolar planetary mass companion to be directly imaged, and is the first discovered orbiting a brown dwarf.2M1207 was discovered during the course of...

     – the first imaged system and the first brown dwarf
    Brown dwarf
    Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

     system with a planet discovered
  • 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:...

     - first extrasolar system discovered with 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...

     planet located within the 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...

     (Gliese 581 d
    Gliese 581 d
    Gliese 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....

    )
  • 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...

     - the system's innermost planet was the first "hot Neptune" to be discovered
  • 16 Cygni
    16 Cygni
    16 Cygni or 16 Cyg is a triple star system approximately 70 light-years away from Earth in the constellation of Cygnus. It consists of two Sun-like yellow dwarf stars, 16 Cygni A and 16 Cygni B, together with a red dwarf, 16 Cygni C...

     - the first triple star planetary system discovered
  • HD 37124
    HD 37124
    HD 37124 is a yellow dwarf star approximately 110 light-years away in the constellation of Taurus . As of 2005, three extrasolar planets have been confirmed to orbit the star. This system is currently one of the only that contains multiple planet that are not hot Jupiters and are either near or...

     - all three known planets are in or near the habitable zone where liquid water can exist
  • Epsilon Eridani
    Epsilon Eridani
    Epsilon Eridani is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years , it has an apparent magnitude of 3.73...

     - at 10.5 light years away, it is currently the closest known planetary system
  • 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...

     – system with 5 confirmed and 2 currently unconfirmed planets.
  • Kepler-9
    Kepler-9
    Kepler-9 is a sunlike star in the constellation Lyra. It is situated in the Kepler Mission's field of view. Kepler-9 has three confirmed extrasolar planets, all discovered by Kepler using the transit method. The discovery of the first two planets was announced on August 26, 2010, and of the third...

     - the first pair of transiting exoplanets found to be in a mean motion resonance
  • 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...

     - system with 6 confirmed planets.
  • Kepler-16
    Kepler-16
    Kepler-16 is a binary star system in the constellation of Cygnus that was targeted by the Kepler spacecraft. Both stars are smaller than the Sun; the primary, Kepler-16A, is a class K red dwarf, and the secondary, Kepler-16B, is a class M red dwarf...

     - first 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...

     planetary system discovered.

See also


  • List of planetary systems
  • Stars and planetary systems in fiction