Entry: Space
Focus: Solar System

In physics, Planck's Law describes how a perfect radiator (a blackbody) radiates light at all colors of light as determined by its temperature. While Neptune, and the distant planets (and stars) are not perfect blackbodies, they are close enough that we can make fairly accurate measurements of their temperatures by looking at the light emitted by them in the near infrared, beyond those wavelengths where reflected light from the Sun is significant. The age of the planets, on the other hand, is something that is estimated on the basis of theory. We have measured the age of rocks from Earth, the Moon, and Mars, and the oldest rocks all show a maximum estimated age of several billion years. Our theories lead us to believe that all the planets formed at essentially the same time, and that the age of the Earth and the other solar system planets is about 4.5 to 4.6 billion years. The Solar System Exploration Home Page is part of NASA's Office of Space Science and describes NASA's program to explore the solar system.