Pluto
Pluto
is the smallest planet in the solar system (being only 2,300 km
in diameter it is even smaller than seven of our solar
system’s moons), usually is the farthest planet from the Sun
(from 1979-1999 Neptune was the farthest), and is very different
from all the other planets (2). It is really more like a moon
than it is a planet, so it might be an escaped moon of Neptune
(1). It also may be an asteroid or an object from the Kupier
belt, but it definitely doesn’t resemble any of the
terrestrial or Jovian planets (2). Its orbit is very different
from those of the other planets, being highly eccentric and
inclined, which is why for a short period of time it comes closer
to the Sun than Neptune does (2). Pluto is made mostly of ices,
primarily methane ice (1). Although always frozen on the dark
side, some methane ice may have evaporated on the side facing the
Sun and formed a small atmosphere (1). If this is true, it is the
only planet in our solar system that has an atmosphere on its
sunny side and none on its dark (1). Because of its huge distance
from the Sun, Pluto’s year is about 247.7 Earth years long,
and its average temperature is around -230º C (1). Its day in
Earth time would be around 6 days and 9.5 hours long (1).
Pluto has one moon, Charon, which is about one half the size of
Pluto making it the largest moon relative to its primary (2).
Because they are so close in size, some astronomers consider
Pluto and Charon to be a double planet (1). The two seemingly
rotate around each other always pointing in the same direction
relative to each other (3). Unlike Pluto, Charon’s surface
is made mostly of water ice, and it has no atmosphere (3).