QUESTION:
Why is Neptune so hot if it is the ninth planet from the sun and Pluto so
cold if it is the eighth planet from the sun?

ANSWER from Heidi Hammel on April 24, 1996:
The distance of the planets from the Sun is only one part of what
makes their temperature.

All planets also have some heat stored inside them.  This heat comes
from several different causes, but one of the most important is
left-over heat from when the planet formed.  Bigger planets have
more heat, and it takes longer for bigger planets to cool down.
Since Neptune is much much larger than Pluto, it has a lot more of
this heat stored up inside it.

The outermost parts of both planets are really very cold - colder than
-200 F!  But if you were able to send a space probe into Neptune's
atmosphere, you would find that it got hotter and hotter as you went
deeper and deeper, due to the left-over heat from its formation.