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Planets beyond our solar system. For millennia, humankind has known about most of the major planets in our solar system because these worlds are visible to the naked eye. Exoplanets, however, were not indisputably known until 1995, when astronomers…

Any interaction between objects that causes motion. Fundamentally, objects interact through the four known physical forces: gravity, electromagnetism, and the strong and weak nuclear interactions. In practice, however, many of the forces worked with…

The highest-energy form of electromagnetic radiation, or light. The energies of gamma rays range from a few kiloelectronvolts [(keV), where the prefix kilo (k) means 103] to several hundreds of megaelectronvolts [(MeV), where the prefix mega (M)…

Electromagnetic radiation whose wavelengths are in the range from about 0.75 micrometer (μm) to 1 millimeter (mm). Infrared radiation (Fig. 1) is outside the range of visible light and cannot be seen with the unaided eye. Infrared radiation was…

A space telescope mission dedicated to detecting exoplanets. The tenth mission launched under the National Aeronautics and Space Administration's (NASA's) Discovery Program, Kepler was the agency’s first mission dedicated to the search for planets…

The planet fourth outward from the Sun and the planet most like Earth in terms of geology, climate, and suitability for life. Mars is nicknamed the "Red Planet" for its reddish hue (Fig. 1). Besides Earth, Mars is the most-studied world in the solar…

The change in the position of an object in a physical space over time. A familiarly encountered phenomenon, motion (see illustration) occurs on all levels of reality, from the constant movements of subatomic particles (conveyed by probabilities of…

The branch of acoustics that deals with the generation of sound by musical instruments, the transmission of sound to the listener, and the perception of musical sound. Musical acoustics has fascinated scientists at least since the time of the Greek…