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Basic Astronomy Concepts

Planetary Science

icy planetesimals, solar nebula, Copernican revolution, biological perspective, earth satellite

Deeper web pages:

>  Sources of Information Used By Planetary Scientists

>  Origins and Compositions of Planets

>  Structures and Features of the Terrestrial Planets

>  Structures and Features of the Jovian Planets

>  Structure of Icy Moons and Dwarf Planets

An orbit is perturbed when the forces are more complex than those between two spherical bodies. (Kepler's laws are exact only for unperturbed orbits.) The attraction between planets causes their elliptical orbits to change with time. The sun, for example, perturbs the lunar orbit by several thousand kilometers. Atmospheric drag causes the orbit of an earth satellite to shrink, and the oblate shape of the earth causes the direction of its node and perigee to change. The theory of relativity developed by German-born American physicist Albert Einstein explains an observed perturbation in the perihelion of the planet Mercury.

The origins of modern planetary science can be traced to the Copernican revolution of the 16th and 17th centuries, which led to overturning the old idea that Earth is unique and central in creation. Polish astronomer Nicolaus Copernicus, Italian astronomer and philosopher Galileo, and others showed that the Sun is the central body in Earth’s solar system and that Earth is only one planet among several that orbit the Sun. Continued advances in astronomy have revealed that the Sun is an average star in a universe filled with billions of stars. Recent observations indicate that a significant fraction of the stars in the universe could be encircled by planetary systems—some of which may be similar to Earth’s solar system, and many that are probably quite different.

Scientists have debated what kind of object should be called a planet, but the problem can be seen as more one of terminology than of science. In 2006 the International Astronomical Union (IAU) voted on a formal definition of planet for bodies in our solar system. The term “classical planet” is used for a body that orbits the Sun, that has settled into a rounded shape from effects of its own gravitation, and that is massive enough to have cleared the neighborhood of its orbit of primordial asteroid-size bodies called planetesimals as it formed in the solar nebula. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all fit this definition. The term dwarf planet is used for an object that, like a classical planet, orbits the Sun and has a rounded shape from its own gravitation but, unlike a classical planet, is not massive enough to have cleared planetesimals from the neighborhood of its orbit. Dwarf planets orbit through regions such as the asteroid belt (a zone filled with small, rocky planetesimals) and the Kuiper Belt (a zone filled with small, icy planetesimals). Currently, Ceres in the asteroid belt and Pluto and Eris in the region of the Kuiper Belt are recognized as dwarf planets. For now, the IAU’s definition of a planet does not officially apply to extrasolar planets.

Modern planetary science draws from many fields of science, including astronomy, physics, chemistry, atmospheric science, and geology. To some degree, the study of planets also requires a biological perspective, for it is now clear that the evolution of the atmosphere and surface environment of at least one planet—Earth—has been radically influenced by the presence of life. Many scientists believe that life may not be limited to Earth and may, in fact, be fairly common throughout the universe. Planetary science is therefore also concerned with life on other planets.

Directions for Future Research

Study of Earth’s planetary system has revealed much about the origin, evolution, and essential processes of planetary systems in general. In turn, knowledge of the general principles governing planetary systems has shed new light on Earth. The immediate practical applications of planetary science concern the preservation of Earth’s environment in a state that supports life. The long-term applications of planetary science focus on the evolution of the physical structures of planetary systems and on the search for planets surrounding stars other than the Sun. Scientific agencies in several countries are currently considering proposals for several projects designed to send more robotic probes to other planets, satellites, asteroids, and comets, and to detect planetary systems orbiting stars other than the Sun, particularly planets like Earth.


Hartmann, William K., B.S., M.S., Ph.D.

Senior Scientist, Planetary Science Institute. Coauthor of "The Grand Tour: A Traveller's Guide to the Solar System".

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