87. Hot Planets
Jupiter, Saturn, and Neptune each radiate away more than twice the heat energy they receive from the Sun.a Uranusb and Venusc also radiate too much heat. Calculations show that it is very unlikely that this energy comes from nuclear fusion,d radioactive decay, gravitational contraction, or phase changese within those planets. This suggests that these planets have not existed long enough to cool off.f
Figure 34: Small Comets. The Dynamic Explorer satellite took this picture in ultraviolet light showing small comets (the dark spots) colliding with Earth’s upper atmosphere. The comets begin to break up 800 miles above the Earth’s surface, then frictional heating vaporizes the pieces and their descent stops at an elevation of about 35 miles. The water vapor, which soon dissipates, blocks ultraviolet light from Earth, producing the dark spots. The northern lights are shown by the halo.