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The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, [2] speculating that the gravity of a large unseen ninth planet could have ...
As of 2024, no large planet has been found beyond Neptune that would explain any alleged discrepancy, despite the discovery of trans-Neptunian objects (most notably, Pluto). While the astronomical community widely agrees that "Planet X", as originally envisioned, does not exist, the concept of an as-yet-unobserved planet has been revived by a ...
Planet V, a planet thought by John Chambers and Jack Lissauer to have once existed between Mars and the asteroid belt, based on computer simulations. Various planets beyond Neptune: Planet Nine, a planet proposed to explain apparent alignments in the orbits of a number of distant trans-Neptunian objects. Planet X, a hypothetical planet beyond ...
According to the IAU's explicit count, there are eight planets in the Solar System; four terrestrial planets (Mercury, Venus, Earth, and Mars) and four giant planets, which can be divided further into two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). When excluding the Sun, the four giant planets account for more than ...
Beyond Neptune, many sub-planet sized objects formed. Several thousand trans-Neptunian objects have been observed. Unlike the planets, these trans-Neptunian objects mostly move on eccentric orbits, inclined to the plane of the planets. The positions of the planets might have shifted due to gravitational interactions. [2]
Eventually, new moons were discovered also around Uranus starting in 1787 by Herschel, [23] around Neptune starting in 1846 by William Lassell [24] and around Mars in 1877 by Asaph Hall. [25] Further apparent discrepancies in the orbits of the outer planets led Percival Lowell to conclude that yet another planet, "Planet X", must lie
This is a list of trans-Neptunian objects (TNOs), which are minor planets in the Solar System that orbit the Sun at a greater distance on average than Neptune, that is, their orbit has a semi-major axis greater than 30.1 astronomical units (AU). The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of ...
The timeline of discovery of Solar System planets and their natural satellites charts the progress of the discovery of new bodies over history. Each object is listed in chronological order of its discovery (multiple dates occur when the moments of imaging, observation, and publication differ), identified through its various designations (including temporary and permanent schemes), and the ...