by Dominic Ford, Editor

Pluto will soon pass behind the Sun at solar conjunction. From Ashburn, it is not observable – it will reach its highest point in the sky during daytime and is no higher than 6° above the horizon at dusk.


Pluto, as seen by the Hubble Space Telescope

Pluto is a dwarf planet which orbits in an outer region of the solar system known as the Edgeworth-Kuiper belt. Its formal minor-planet designation is 134340 Pluto.

From its discovery in 1930 until 2006, Pluto was considered to be the solar system's outermost known planet. However, it is much smaller than the other planets, measuring 18% of the diameter of the Earth, and less than half the diameter of the next smallest planet, Mercury.

In the early 2000s, its status as a planet became increasingly untenable due to theoretical predictions of large numbers of other similarly-sized objects in the Edgeworth-Kuiper belt.

Pluto was at last downgraded from its status as a planet in 2006, and given the newly-generated title of dwarf planet. This was prompted by the discovery of another Edgeworth-Kuiper belt object, which was 27% more massive than Pluto. In view of its role in history, that object was named Eris, after the goddess of strife of discord in Greek mythology; Eris was also classified as a dwarf planet.

Finding Pluto

Pluto comes to opposition once every 367 days – its synodic period – almost exactly once a year. The date it comes to opposition moves only one day later each year.

Pluto is a faint and distant object. At around magnitude 14, it is beyond the reach of all but the largest amateur telescopes. Pluto's largest moon, Charon, is not much smaller than Pluto itself, measuring a little under 10% of the diameter of the Earth, and appears 2–3 magnitudes fainter. Two other much smaller moons, Nix and Hydra, were discovered in 2005.

A chart of the time of day when Pluto rises and sets on any given day of the year can be found here. A chart of Pluto's path relative to the background stars can be found here.

Pluto has only ever been visited once by one spacecraft: the NASA New Horizons probe flew past it in 2015 before going on to study other Edgeworth-Kuiper belt objects.




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