A comparison of models from different eras canreveal the gradual shift from an Earth-centereduniverse to a sun-centered solar system, thediscovery of new planets and moons orbiting otherplanets, and eventually the understanding that oursolar system is just one of many in our galaxy.These sources clearly show that ideas changegradually--even once new discoveries are made.Older models of the universe persisted for centurieseven after new evidence contradicted them.Seemingly contradictory ideas about the cosmosstill exist today.
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 multitudes of other extrasolar systems. The consensus definition as to what counts as a planet vs. other objects orbiting the Sun has changed several times, previously encompassing asteroids, moons, and dwarf planets like Pluto,[5][6][7] and there continues to be some disagreement today.[7]
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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 probably dates as early as the second millennium BC.[17] The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.[18] Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the exact sciences.[19] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[20] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[21][22] Venus, Mercury, and the outer planets Mars, Jupiter, 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.[23]
The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star (Hesperos) and morning star (Phosphoros) as one and the same (Aphrodite, Greek corresponding to Latin Venus),[24] though this had long been known in Mesopotamia.[25][26] In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution.[16]
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[46] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on 6 October 1995, Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[47]
The IAU noted that this definition could be expected to evolve as knowledge improves.[70] A 2022 review article discussing the history and rationale of this definition suggested that the words "in young star clusters" should be deleted in clause 3, as such objects have now been found elsewhere, and that the term "sub-brown dwarfs" should be replaced by the more current "free-floating planetary mass objects".[69]
Geoscientists often reject the IAU definition, preferring to consider round moons and dwarf planets as also being planets. Some scientists who accept the IAU definition of "planet" use other terms for bodies satisfying geophysical planet definitions, such as "world".[7] The term "planetary mass object" has also been used to refer to ambiguous situations concerning exoplanets, such as objects with mass typical for a planet that are free-floating or orbit a brown dwarf instead of a star.[69]
Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya 'Sun', Chandra 'Moon', Budha for Mercury, Shukra ('bright') for Venus, Mangala (the god of war) for Mars, Bṛhaspati (councilor of the gods) for Jupiter, and Shani (symbolic of time) for Saturn) and the ascending and descending lunar nodes Rahu and Ketu.[82]
An exoplanet (extrasolar planet) is a planet outside the Solar System. As of 1 February 2023, there are 5,307 confirmed exoplanets in 3,910 planetary systems, with 853 systems having more than one planet.[146] Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. Analysis of gravitational microlensing data suggests a minimum average of 1.6 bound planets for every star in the Milky Way.[147]
The defining dynamic characteristic of a planet, according to the IAU definition, is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. As described above, this characteristic was mandated as part of the IAU's official definition of a planet in August 2006.[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[190]
All of the Solar System planets except Mercury[204] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth;[205] Neptune's largest moon Triton[206] and the dwarf planet Pluto have more tenuous atmospheres.[207] The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space.[208] The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.[209]
Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).[175] Weather patterns detected on exoplanets include a hot region on HD 189733 b twice the size of the Great Red Spot,[210] as well as clouds on the hot Jupiter Kepler-7b,[211] the super-Earth Gliese 1214 b and others.[212][213]
Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, Europa, and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus).[223][224][225][226][227]
Scientists believe the solar system was formed some 4.6 billion years ago when a cloud of gas and dust collapsed under gravity possibly triggered by a cataclysmic explosion from a nearby massive star or supernova. As this cloud collapsed, it formed a spinning disk with the sun in the center. Since then scientists have been able to establish the formation of the solar system piece by piece. Now, new research has enabled scientists from The University of New Mexico, Arizona State University and NASA's Johnson Space Center to add another piece to that puzzle with the discovery of the oldest-ever dated igneous meteorite.
Most meteorites are formed through the collision of asteroids orbiting the sun in a region called the asteroid belt. Asteroids are the remains from the formation of the solar system formation some 4.6 billion years ago. The chemical composition ranges of ancient igneous meteorites, or achondrites, are key to understanding the diversity and geochemical evolution of planetary building blocks. Achondrite meteorites record the first episodes of volcanism and crust formation, the majority of which are basaltic.
130. This implies taking certain indispensable steps, especially in response to those who are fleeing grave humanitarian crises. As examples, we may cite: increasing and simplifying the granting of visas; adopting programmes of individual and community sponsorship; opening humanitarian corridors for the most vulnerable refugees; providing suitable and dignified housing; guaranteeing personal security and access to basic services; ensuring adequate consular assistance and the right to retain personal identity documents; equitable access to the justice system; the possibility of opening bank accounts and the guarantee of the minimum needed to survive; freedom of movement and the possibility of employment; protecting minors and ensuring their regular access to education; providing for programmes of temporary guardianship or shelter; guaranteeing religious freedom; promoting integration into society; supporting the reuniting of families; and preparing local communities for the process of integration.[111] 2ff7e9595c
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