Astronomical Society of the Pacific
The Astronomical Society of the Pacific is an American scientific and educational organization, founded in San Francisco on February 7, 1889. Its name derives from its origins on the Pacific Coast, but today it has members all over the country and the world, it has the legal status of a nonprofit organization. It is the largest general astronomy education society in the world, with members from over 40 countries; the ASP's goal is to promote public interest in and awareness of astronomy through its publications, web site, many educational and outreach programs. These include: Project ASTRO - a national program that improves the teaching of astronomy and physical science by pairing amateur and professional astronomers with 4th through 9th grade teachers and classes. Family ASTRO - a project that develops kits and games to help families enjoy astronomy in their leisure time and trains astronomers and community leaders Astronomy from the Ground Up - a national program to help educators at smaller science museums, nature centers and environmental education organizations create or enhance astronomy education programs.
The Night Sky Network - a program with the Jet Propulsion Laboratory that supports a community of over 450 amateur astronomy clubs around the U. S. in public outreach activities, providing them with kits and training. The clubs have sky gazing events, solar viewings, give presentations for the public. Classroom materials and resources in astronomy sold through their online AstroShop or made available free through their web siteThe ASP assists with astronomy education and outreach by partnering with other organizations both in the United States and internationally, organizes an annual meeting to promote the appreciation and understanding of astronomy. Presidents of the ASP have included such notable astronomers as Edwin Hubble, George O. Abell, Frank Drake. George Pardee, who became Governor of the State of California, served as president in 1899; the society promotes astronomy education through several publications. The Universe in the Classroom, a free electronic educational newsletter for teachers and other educators around the world who help students of all ages learn more about the wonders of the universe through astronomy.
Mercury, the ASP's quarterly on-line membership magazine, covers a wide range of astronomy topics, from history and archaeoastronomy to cutting-edge developments. First published in 1925 as the Leaflets of the ASP, Mercury is now disseminated to thousands of ASP members and schools, libraries and institutions around the world; the ASP publishes the journal Publications of the Astronomical Society of the Pacific aimed at professional astronomers. The PASP is a technical journal of refereed papers on astronomical research covering all wavelengths and distance scales as well as papers on the latest innovations in astronomical instrumentation and software, has been publishing journals since 1889; the Astronomical Society of the Pacific Conference Series is a series of over 400 volumes of professional astronomy conference proceedings. Started in 1988, the Conference Series has grown to become a prominent publication series in the world of professional astronomy publications, now publishes an average of 20-25 volumes per year.
Volumes are sold to the attendees of the conferences of which the proceedings are published, as well as being offered through the Astronomical Society of the Pacific's AstroShop, can be found in the libraries of major universities and research institutions worldwide. In 2004, the ASPCS stepped into electronic publishing, offering electronic access subscriptions for libraries and institutions, as well as individual access to volumes which they have purchased in hard copy form. Astronomy Beat is an on-line column, which comes out every other week, features a behind-the-scenes report on some aspect of astronomical discovery, astronomy education, or astronomy as a hobby, written by a key participant. Authors have included: Clyde Tombaugh, retelling the story of his discovery of the planet Pluto Michael E. Brown, discussing the naming of the dwarf planet Makemake Noted astronomical photographer David Malin describing the transition from chemical to digital photography Virginia Louise Trimble explaining how she selected her list of the top ten astronomical discoveries of the last thousand years.
The ASP makes several different awards annually: The Bruce Medal for lifetime contribution to astronomy research. The medal is named after Catherine Wolfe Bruce; this award is arguably the most prestigious award given in astronomy. The Klumpke-Roberts Award for outstanding contributions to the public understanding and appreciation of astronomy, named for Dorothea Klumpke-Roberts; the Amateur Achievement Award in recognition of significant contributions to astronomy by one not employed in the field of astronomy in a professional capacity. The Bart Bok Award, named in honor of astronomer Bart Bok, awarded jointly with the American Astronomical Society to outstanding student projects in astronomy at the International Science and Engineering Fair; the Thomas Brennan Award for exceptional achievement related to the teaching of astronomy at the high school level. The Maria and Eric Muhlmann Award for recent significant observational results made possible by innovative advances in astronomical instrumentation, software, or observational infrastructure.
The Robert J. Trumpler Award, named in honor of astronomer Robert J. Trumpler, given to a recent recipient of a Ph. D degree with a notable thesis; the Richard Emmons Award is given for a lifetime of contributions to the teaching of astronomy to college non-s
A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
A dwarf planet is a planetary-mass object, neither a true planet nor a natural satellite. That is, it is in direct orbit of a star, is massive enough for its gravity to compress it into a hydrostatically equilibrious shape, but has not cleared the neighborhood of other material around its orbit; the term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, precipitated by the discovery of an more massive object, Eris. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; as of July 2008 the International Astronomical Union recognizes five dwarf planets: Ceres in the asteroid belt, Pluto, Haumea and Eris in the outer Solar System. Only two of these bodies and Pluto, have been observed in enough detail to demonstrate that they fit the IAU's definition; the IAU accepted Eris as a dwarf planet.
They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. At the time, the only additional bodies to meet this secondary criterion were Makemake. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for decades considered to be planets. Between and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and stopped naming or classifying them as planets. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of smaller bodies. For 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass and to determine that it was much smaller than initial estimates, it was one-twentieth the mass of Mercury, which made Pluto by far the smallest planet.
Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets. In the 1990s, astronomers began to find objects in the same region of space as Pluto, some farther away. Many of these shared several of Pluto's key orbital characteristics, Pluto started being seen as the largest member of a new class of objects, plutinos; this led. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported, it became clear that either they would have to be classified as planets, or Pluto would have to be reclassified. Astronomers were confident that more objects as large as Pluto would be discovered, the number of planets would start growing if Pluto were to remain a planet.
Eris was discovered in January 2005. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006; the IAU's initial draft proposal included Charon and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi: they proposed an intermediate category for objects large enough to be round but which had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal removed Pluto and Eris, because they have not cleared their orbits; the IAU's final Resolution 5A preserved this three-category system for the celestial bodies orbiting the Sun. It reads: Although concerns were raised about the classification of planets orbiting other stars, the issue was not resolved; the term dwarf planet has itself been somewhat controversial, as it could imply that these bodies are planets, much as dwarf stars are stars.
This is the conception of the Solar System. The older word planetoid has no such connotation, is used by astronomers for bodies that fit the IAU definition. Brown states that planetoid is "a good word", used for these bodies for years, that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution. Indeed, the draft of Resolution 5A had called these median bodies planetoids, but the plenary session voted unanimously to change the name to dwarf planet; the second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session; because of the semantic inconsistenc
Harold F. Levison
Harold F. Levison is a planetary scientist specializing in planetary dynamics, he works at the Southwest Research Institute, Boulder, CO, studies planetary orbits and their evolution through solar system history. Levison is the Principal Investigator of the Lucy mission to tour multiple Jupiter trojans, selected as the thirteenth mission in NASA's Discovery Program in January 2017. Among other achievements, Levison is the co-author of SWIFT, a used symplectic integrator that solves planetary equations of motion for periods of billions of years. Levison argued for a distinction between what are now called dwarf planets and the other eight planets based on their inability to "clear the neighborhood around their orbits", although his proposal suggested the terms "unterplanet" and "überplanet" and used the word "dwarf" to mean something else
Sol Alan Stern is an American engineer and planetary scientist. He is the principal investigator of the New Horizons mission to Pluto and the Chief Scientist at Moon Express. Stern has been involved in 24 suborbital and planetary space missions, including eight for which he was the mission principal investigator. One of his projects was the Southwest Ultraviolet Imaging System, an instrument which flew on two space shuttle missions, STS-85 in 1997 and STS-93 in 1999. Stern has developed eight scientific instruments for planetary and near-space research missions and has been a guest observer on numerous NASA satellite observatories, including the International Ultraviolet Explorer, the Hubble Space Telescope, the International Infrared Observer and the Extreme Ultraviolet Observer. Stern was Executive Director of the Southwest Research Institute's Space Science and Engineering Division until becoming Associate Administrator of NASA's Science Mission Directorate in 2007, he resigned from that position after nearly a year.
In early 2009 Stern's name was mentioned as a potential contender for the position of NASA administrator under President Obama's administration. Stern has stated, that he is not interested in the position at this time given his desire to spend time with his family. Stern was born in New Orleans, the son of Joel and Leonard Stern, he graduated from St. Mark's School of Texas in 1975, he attended the University of Texas, where he received his bachelor's degrees in physics & astronomy and his master's degrees in aerospace engineering and planetary atmospheres. He earned a doctorate in astrophysics and planetary science from the University of Colorado, Boulder. From 1983 to 1991, Stern held positions at the University of Colorado in the Center for Space and Geoscience Policy, the office of the Vice President for Research, the Center for Astrophysics and Space Astronomy, he received his doctorate in 1989. From 1991 to 1994 he was the leader of Southwest Research Institute's Astrophysical and Planetary Sciences group and was Chair of NASA's Outer Planets Science Working Group.
From 1994 to 1998 he was the leader of the Geophysical and Planetary Science section in Southwest Research Institute's Space Sciences Department, from 1998 to 2005 he was the Director of the Department of Space Studies at Southwest Research Institute. In 1995 he was selected to be a Space Shuttle mission specialist finalist and in 1996 he was a candidate Space Shuttle payload specialist but did not have the opportunity to fly on the Space Shuttle, his research has focused on studies of our solar system's Kuiper belt and Oort cloud, the satellites of the outer planets and the search for evidence of planetary systems around other stars. He has worked on spacecraft rendezvous theory, terrestrial polar mesospheric clouds, galactic astrophysics, studies of tenuous satellite atmospheres, including the atmosphere of the Moon. In 2007, Stern was listed among Time magazine's 100 Most Influential People in The World. On August 27, 2008 Stern was elected to the Board of Directors of the Challenger Center for Space Science Education.
In 2015, Stern was the recipient of Smithsonian Magazine's American Ingenuity Award in the Physical Sciences category. On October 7, 2016, Stern was inducted into the Colorado Space Hall of Fame. On June 14, 2007, in an address to the Smithsonian Institution for their "Exploring the Solar System Lecture Series", Stern commented on the New Horizons mission: I recall going to JPL, the Jet Propulsion Lab, the summer of 1989 when I was in graduate school to take a summer course in planetary exploration at Caltech and this was the summer of the Voyager fly-by of Neptune and Triton, it was amazing to get to be a part of some first-time exploration like that! Within a matter of months, a small group of us had formed a team, an advocacy group, Why don't we get a mission together for Pluto? After completing a master's degree in aerospace engineering Stern spent seven years as an aerospace systems engineer, concentrating on spacecraft and payload systems at the NASA Johnson Space Center, Martin Marietta Aerospace, the Laboratory for Atmospheric and Space Physics at the University of Colorado.
Stern is active as a consultant for private sector space efforts and has stated: I am a fan of public-private partnerships and building bridges to new markets, I believe we are on the verge of a whole new era of space exploration and that the private sector can provide reliable cost effective services that can increase the value and leverage government space budgets. On June 18, 2008, Stern joined Odyssey Moon Limited, a private industry effort, as a part-time Science Mission Director/consultant in their efforts to launch a robotic mission to the Earth's Moon by participating in the $30 Million Google Lunar X-Prize competition. In December 2008, Stern joined Blue Origin, a company, founded by Amazon.com's Jeff Bezos as an independent representative for research and education Missions. The company has stated that its objective is to develop a new vertical-take-off, vertical-landing vehicle known as New Shepard, designed to take a small number of astronauts on a sub-orbital journey into space and reduce the cost of space transportation.
The company is located in Kent and has flight tested some hardware. In 2012, Stern co-founded Uwingu. Stern has extensive experience in instrument development, with a strong concentration in ultraviolet technologies. Stern is a principal investigator in NASA's UV sounding rocket program, was the project scientist on a Shuttle-deployable SPARTAN astronomical satellite, he was the PI of the advanced, miniaturized HIPPS Pluto breadboard camera/IR spectrom
Asteroids are minor planets of the inner Solar System. Larger asteroids have been called planetoids; these terms have been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were found to have volatile-rich surfaces similar to comets; as a result, they were distinguished from objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There exist millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets; the vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, S-type. These were named after and are identified with carbon-rich and silicate compositions, respectively; the sizes of asteroids varies greatly. Asteroids are differentiated from meteoroids. In the case of comets, the difference is one of composition: while asteroids are composed of mineral and rock, comets are composed of dust and ice. Furthermore, asteroids formed closer to the sun; the difference between asteroids and meteoroids is one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Meteoroids can be composed of either cometary or asteroidal materials. Only one asteroid, 4 Vesta, which has a reflective surface, is visible to the naked eye, this only in dark skies when it is favorably positioned. Small asteroids passing close to Earth may be visible to the naked eye for a short time; as of October 2017, the Minor Planet Center had data on 745,000 objects in the inner and outer Solar System, of which 504,000 had enough information to be given numbered designations.
The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit, but we're not 100 percent sure when." In 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched; the first asteroid to be discovered, was considered to be a new planet.
This was followed by the discovery of other similar bodies, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής, or asteroeidēs, meaning'star-like, star-shaped', derived from the Ancient Greek ἀστήρ astēr'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" were still used interchangeably. Overview of discovery timeline: 10 by 1849 1 Ceres, 1801 2 Pallas – 1802 3 Juno – 1804 4 Vesta – 1807 5 Astraea – 1845 in 1846, planet Neptune was discovered 6 Hebe – July 1847 7 Iris – August 1847 8 Flora – October 1847 9 Metis – 25 April 1848 10 Hygiea – 12 April 1849 tenth asteroid discovered 100 asteroids by 1868 1,000 by 1921 10,000 by 1989 100,000 by 2005 ~700,000 by 2015 Asteroid discovery methods have improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would be spotted; the expected motion of the missing planet was about 30 seconds of arc per hour discernible by observers. The first object, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily, he discovered a new star-like object in Taurus and followed the displacement of this object during several nights. That year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, found to be between the planets Mars and Jupiter.
Piazzi named it after Ceres, the Roman goddess of agriculture. Three other asteroids (2 Pallas, 3 Juno, 4 Ves
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy; the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t