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Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune [161] knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit. [162]
The average distance between Neptune and the Sun is 4.5 billion km (about 30.1 astronomical units (AU), the mean distance from the Earth to the Sun), and it completes an orbit on average every 164.79 years, subject to a variability of around ±0.1 years. The perihelion distance is 29.81 AU, and the aphelion distance is 30.33 AU.
In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the largest member as well as the namesake of this group.
There are only a few known mean-motion resonances (MMR) in the Solar System involving planets, dwarf planets or larger satellites (a much greater number involve asteroids, planetary rings, moonlets and smaller Kuiper belt objects, including many possible dwarf planets). 2:3 Pluto–Neptune (also Orcus and other plutinos) 2:4 Tethys–Mimas ...
2015 – Dawn spacecraft enters orbit around the dwarf planet Ceres making detailed measurements. [246] 2015 – New Horizons spacecraft flies by Pluto, providing the first ever sharp images of its surface, and its largest moon Charon. [247] 2017 – 'Oumuamua, the first known interstellar object crossing the Solar System, is identified. [248]
A total of five planets are going retrograde between May and September: Mercury, Saturn, Uranus, Neptune and Pluto. "Retrograde" is a term used to describe when a planet's orbit appears to slow.
The planetary orbit is not a circle with epicycles, but an ellipse. The Sun is not at the center but at a focal point of the elliptical orbit. Neither the linear speed nor the angular speed of the planet in the orbit is constant, but the area speed (closely linked historically with the concept of angular momentum) is constant.
Earth accretes or ejects near-Earth asteroids on million-year time scales, thereby clearing its orbit. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called plutinos, into a 3:2 resonance (i.e., they orbit the Sun twice for every three Neptune orbits). Since the orbits ...