after the Roman god of the underworld, Pluto discovered
on February 18,
1930, and originally considered the ninth "planet,"  was demoted to
the status of "dwarf planet"  by the IAU on August 24, 2006, and
further redefined as a "plutoid"  on June 11, 2008. 
It's also official that Pluto
is the largest of the dwarf planets (at least for now), its
confirmed diameter of 2,370 kilometers (1,473 miles) making it slightly larger than Eris a distant relative and at one time the supposed title holder.
has a thin atmosphere, which due to an eccentric orbit 29.66 AU at
perihelion and 48.87 AU at aphelion (1 AU being the mean distance between
approximately 150,000,000 kilometers) forms when frozen methane,
nitrogen and carbon monoxide on its surface sublimates into gas upon
solar approach, refreezing again as it retreats outward. The mean
surface temperature is -229 °Celsius.
composition is still largely unknown, the best guess being a rocky silicate
and water ice core surrounded by a layer of water ice in turn covered
by a layer of frozen methane.
Horizons, the first mission in NASA's New Frontiers Program, launched
January 19, 2006, reached its closest approach to Pluto, some 12,500 kilometers
(7,750 miles) on July 14, 2015, the first Earth vessel to do so. Armed
with a remote-sensing package, its task at Pluto, was to image
and examine the atmosphere, geology and
the dwarf planet and its largest moon as well as grab glimpses of
else of interest (smaller moons, rings etc.). Due to certain
considerations (power etc.) the information gathered was trickled back to
Earth over an extended period of time. The last of said data (Pluto
flyby) was received by mission control, Laurel, Maryland, on October
Pluto is indeed the god of the underworld then this dwarf planet/plutoid is well
named, for the chances of their being indigenous life on this frigid
orb are slim indeed.
As an interesting
aside some of the ashes of Clyde Tombaugh, Pluto’s discoverer, are on board the
New Horizons spacecraft.
date Pluto has five known moons: Charon, Nix, Hydra,
Kerberos, (formerly S/2011 (134340) 1 or P4) and Styx (formerly S/2012 (134340) 1 or
A "planet" is a celestial body that (a) is in orbit around the Sun, (b)
has sufficient mass for its self-gravity to overcome rigid body forces
so that it assumes a hydrostatic equilibrium (nearly round shape) and
(c) has cleared the neighborhood around its orbit.
A "dwarf planet" is a celestial body that (a) is in orbit around the
Sun, (b) has sufficient mass for its self-gravity to overcome rigid
body forces so that it assumes a hydrostatic equilibrium (nearly round
shape), (c) has not cleared the neighborhood around its orbit and (d)
is not a satellite.
 A "plutoid"
is a trans-Neptunian dwarf planet.
 Due to its
position in the Kuiper belt  (sometimes called the Edgeworth-Kuiper
belt) Pluto along with Haumea, Makemake and Eris
is also classified as a Kuiper belt object or KBO.
 The Kuiper
belt is a region of the outer Solar System populated by
billions of rock-ice objects. It reaches from the orbit of Neptune
(30 AU) outward to approximately 55 AU.
New Horizons is now, with fuel to spare, far beyond Pluto, its
next objective a rendezvous with Kuiper belt object (KBO) 2014 MU69.
Discovered in June 2014 by astronomers using the Hubble Space
Telescope, 2014 MU69 (also known as 1110113Y) has an estimated diameter
of 30 miles (48km)—more
than ten times that of a typical comet 0.5 to 1% that of
Pluto. Combined with an orbit that has likely not undergone any
significant perturbations this classical KBO (also called a cubewano)
thought to be akin to the building blocks of the Kuiper belt's dwarf
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