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Forced by gravitational attraction, resonances between two or more celestial bodies occur whenever the ratio of one period of motion makes a simple fraction with another period. Several structures and patterns in the solar system are caused by resonances, but the details of these patterns require sophisticated analysis that may help in understanding the structure of galaxies.

To many, science and religion are perceived as polar opposites. Science was a product of the sixteenth and seventeenth centuries. Nicholaus Copernicus’s solar system work using detailed mathematics was the beginning of the Scientific Revolution. Up to that time, the study of astronomy and astrology supported the church’s position on planetary motion and influence. Copernicus had a theory (heliocentrism) that the Earth and other planets revolved around the Sun, which was inconsistent with the theory of his time. Theologians generally opposed the manuscript of his work as contradicting the scriptures. Using the scientific method rather than proposing explanations based on theological assumptions has often placed scientists and their work in opposition to some religious worldviews. In practice, however, science and religion coexist, with mutually beneficial contributions by practitioners in both arenas.

Planets in the solar system move around the sun in elliptical orbits. Those whose orbits are closest to the sun move more rapidly than those that are farther away. These simple, universally accepted observations form the basis of the knowledge of planetary motions. Gravity—the force that causes apples to fall from trees and keeps humans firmly planted on Earth’s surface—plays the central role in the mechanics of planetary motions.

Orbital or rotational (spin) motions of objects are said to be “coupled” or “in resonance” when the relationships between the periods of such motions can be expressed as ratios of small integers such as 1:1, 1:2, 1:3, 2:3, or 3:4. This usually occurs as the result of the gravitational interaction of the objects. Many examples of couplings and resonances occur in the orbital and rotational motions of solar system objects.

Far beyond the orbit of Neptune lies a vast, spherical region of space that surrounds the solar system. This area, called the Oort cloud , is located at the very edge of the solar system. Most experts place the Oort cloud at a distance about 50,000 to 100,000 AU from the sun. The Oort cloud is more distant in space than even the Kuiper Belt, a disk-shaped region of space just beyond the orbit of Neptune. The Oort cloud is named for famed Dutch astronomer Jan Hendrik Oort (1900–92), who first hypothesized its existence.

German astronomer Johannes Kepler (1571–1630) used observations collected by Danish astronomer Tycho Brahe (1546–1601) to develop his three laws of orbital motion. Kepler’s first law states that planets move in elliptical orbits with the sun at one focus point. More generally, the law says that one object moves around another object in an elliptical orbit with the system’s center of mass located at one focus. An orbit can be described by its semimajor axis and eccentricity. In an ellipse, the long axis is referred to as the "major axis" and the short is referred to as the "minor axis." The semimajor axis is half the length of the major axis. Eccentricity describes the shape of an object’s orbit, with an eccentricity of zero referring to a circular orbit and an eccentricity of 1 being very flat orbit. The eccentricities of the orbits of the planets in the sun’s solar system are small (almost circular).

The discovery of extrasolar planets orbiting sunlike stars has excited the imaginations of astronomers and laypersons alike. If it can be demonstrated that planetary systems are a common occurrence among the billions of stars within the Milky Way galaxy, the possibility of extraterrestrial life in the universe takes on greater credibility. The idea that intelligent civilizations may exist on other planets could become more compelling.

The habitable zone is a popular concept describing the range within a star's orbit in which a planet can support liquid water on its surface, and therefore have a chance of supporting Earth-like life. It is also known as the circumstellar habitable zone (CHZ) or the Goldilocks zone (after the fairy tale in which a girl chooses things that are "just right" rather than those on either extreme). Since the concept emerged in the mid-twentieth century, it has proved useful in shaping astronomical observation and astrobiological speculation, and many planets within habitable zones have been identified.

For Planetary orbits Astronomy millennia, the shape of the paths in which the planets orbited was dominated by metaphysical concerns and assumed, almost without question, to be circular. It was not until the seventeenth century that science discovered the actual shape of planetary orbits, the ellipse.

Since ancient times, no formal scientific definition of the word “planet” existed. In 2006, professional astronomers, for the first time, developed a formal definition of a planet. The new definition removed Pluto, previously labeled a planet for seven decades, from the list of planets. Pluto, however, was later reinstated as a dwarf planet, though debate rages on as to whether Pluto is a major planet in the solar system.