Ten Planets: Stories

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The IAU discourages the use of planetary symbols in modern journal articles in favour of one-letter or (to disambiguate Mercury and Mars) two-letter abbreviations for the major planets. Villarreal D'Angelo, Carolina; Esquivel, Alejandro; Schneiter, Matías; Sgró, Mario Agustín (21 September 2018).

The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. There are many smaller dwarf planet candidates, such as Salacia, that have not been included in the tables because astronomers disagree on whether or not they are dwarf planets. The largest known trans-Neptunian objects with their moons; the Earth and Moon have been added for comparison. In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars.Since that time, more than 100 planets have been identified that are approximately the same size as Earth, 20 of which orbit in the habitable zone of their star – the range of orbits where a terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure. In the 6th and 5th centuries BC, the Pythagoreans 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. The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node.

This may even include objects that are not round but happen to be larger than Mimas, like Neptune's moon Proteus.The naming of planets differs between planets of the Solar System and exoplanets (planets of other planetary systems).

The other dwarf-planet symbols are mostly initialisms (except Haumea) in the native scripts of the cultures they come from; they also represent something associated with the corresponding deity or culture, e. With the development of the telescope, the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt; and Pluto, later found to be the largest member of the collection of icy bodies known as the Kuiper belt. The astronomical symbol for Pluto is a P-L monogram (♇), [124] though it has become less common since the IAU definition reclassified Pluto. a] The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.

One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12 light-years distance from Earth. The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition, regarding where exactly to draw the line between a planet and a star.

The symbols for the Sun and Earth are nonetheless common, as solar mass, Earth mass, and similar units are common in astronomy. In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. It soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (the asteroid belt), with sometimes overlapping orbits.At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation. The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs, or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/M central< 2/(25+ √ 621) are "planets" (no matter how they formed).