Expert: Ed - 12/30/2007

Question
If mercury is the closest planet to the sun, one would assume it is the hottest. But why is Venus the hottest planet in the solar system?

Answer
Lindsy,

Basically, the answer is because Venus has an atmosphere, which creates a runaway greenhouse effect.  Temperatures on Venus are usually several hundred degrees.  Mercury has no atmosphere to trap the sun's heat, so it never gets as hot there as Venus.  Believe it or not, Mercury even has some ice in its polar regions (in areas which never get the sun's direct rays, in permanent shade).

This NASA link discusses a localized "greenhouse effect" on a small area of the Pacific Ocean, which appears to be comparable, on a smaller scale, to what happened to Venus.

http://www.nasa.gov/centers/ames/news/releases/2002/02_60AR.html

This NASA link is about ice on Mercury, if you're interested.
http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html

I hope this is helpful to you.  Keep Looking Up!
--Ed