- Pluto has never been visited by a spacecraft. We know little
about it relative to other objects in the Solar System.
- Pluto/Charon is truly a ``double planet".
- Charon, its satellite, is 1/2 the size of Pluto
- Pluto's orbit is quite elliptical and even crosses
inside Neptune's orbit. Between 1989 and 1999 it was
the 8th most distant planet from the Sun.
- Pluto has a thin methane atmosphere.
- The atmosphere is quite variable because it depends
on gases which can be frozen onto Pluto's surface.
- The atmosphere is significant when Pluto is close to
the Sun and almost non-existent when far away.
- Pluto was discovered during an intensive search for the next
major planet beyond Neptune and thus its status as a major
planet was not seriously questioned,.... until recently.
- Should Pluto really be considered as a major planet???
- Note the distinction between ``major" planets (Mercury,
Venus,...) and ``minor" planets (i.e. asteroids, comets, etc.).
- Pluto orbits the Sun
- Pluto has a satellite, Charon (but we know know of a
number of asteroids with satellites!)
- Pluto has an atmosphere (albeit a thin one)
- Pluto is comparable in size to Mercury (being generous)
- Pluto is pathetically small.
- It is really half the size of Mercury.
- It is smaller than the Earth's Moon and
any of the Galilean Satellites of Jupiter.
- Mercury is 20 times more massive than Pluto. The
Earth's Moon is 5 times more massive than Pluto.
- Pluto more resembles one of the large icy satellites of
the outer Solar System rather than one of the Jovian Planets.
- Specifically, it is likely to be a twin of Neptune's
largest moon, Triton.
- Hundreds of icy minor planets are being discovered in
the outer parts of the Solar System.
- Pluto/Charon may be just a
large member of this class of object, which as a whole resembles a distant
version of the asteroid belt.
- This outer asteroid belt, considered to be a reservoir
for short period comets, is called the Kuiper Belt
- We are just beginning our exploration of this region of the Solar System.
- Pluto is now not even be the largest of this group.
- Many of these objects share orbital characteristics
with Pluto. Specifically, like Pluto they orbit the Sun
twice for every three orbits of Neptune. This group of objects
are known as "Plutinos" and Pluto is the biggest known one.
Revised December 1, 2006