Planetary Science

I

Introduction

Planetary Science, study of the forces and influences that determine the composition, structure, and evolution of planets and planetary systems, including moons, dwarf planets, asteroids, and comets. Planetary scientists also study how planetary systems form around other stars. In particular, planetary science includes a study of the properties of the Earth compared to the properties of other worlds, which helps explain some of the properties of Earth through the example of other planets.

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. See Extrasolar Planets.

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 (see Exobiology). Planetary science is therefore also concerned with life on other planets.

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II

Sources of Information Used by Planetary Scientists

Planetary science evolved from the study of the solar system—Earth’s planetary system. Astronomers have observed the other planets of the solar system with telescopes and through photographic images transmitted to Earth by interplanetary spacecraft (see Space Exploration). Planetary scientists have characterized the chemical signatures of the Moon and Mars by studying the chemical compositions of rocks brought back from the Moon by astronauts and robotic spacecraft, and soils that were analyzed on the surface of Mars by robotic spacecraft. This has allowed geologists to identify the origin of a small number of meteorites—fragments of interplanetary debris that landed on Earth—as rocks that came from the Moon or from Mars. Terrestrial, or Earth-based, geologists, atmospheric scientists, oceanographers, and other scientists who study Earth have accumulated a wealth of data and have constructed a detailed picture of the composition and structure of Earth. The recent telescopic discovery of planetary systems orbiting other stars promises to expand the information base of other planetary systems. Several schemes for a large-scale systematic search for planetary systems are currently under consideration. See also Extrasolar Planets.

A

Telescopic Observations

Astronomers have used telescopes for centuries to make exact measurements of the positions of planets and their satellites over time. Such information was critical to acceptance of the law of universal gravitation proposed by English physicist Sir Isaac Newton. Modern telescopes allow astronomers to make enlarged images of other planets and to collect the light emitted from them so that it can be analyzed by spectroscopy. The enlarged images enable scientists to study their larger surface features, and spectroscopic analysis, which separates light into its component colors, or spectra, gives information about the chemical composition of the light source. In the case of light reflected from a planet, spectroscopy reveals the compositions of its atmosphere and surface materials. Astronomers have exploited recent advances in spectroscopy and in charge-coupled devices—instruments that measure the intensity of weak light sources—to detect evidence of planets orbiting stars other than the Sun. The new techniques make use of the Doppler effect—subtle shifts in the spectra of moving light sources—to detect wobbles in the motions of stars that are orbited by smaller bodies such as planets.

In the 1960s radio telescopes were built to gather electromagnetic radiation in the radio portion of the electromagnetic spectrum (see Radio Astronomy). Radio telescopes have provided valuable information about other stars and about the magnetic fields of other planets in our solar system, especially Jupiter’s. In 1990 the National Aeronautics and Space Administration (NASA) launched into orbit the Hubble Space Telescope (HST). The HST is an optical telescope that orbits high above the distorting effects of Earth’s atmosphere. For this reason, it can see objects that are ten times smaller than the smallest object that can be seen by any Earth-based telescope. Space telescopes designed to search for extrasolar planets include ESA’s COROT and NASA’s Kepler. Kepler can detect planets the size of Earth.

B

Interplanetary Space Missions

Interplanetary space missions allow close-up observation of other planets. On some missions, robotic landing craft actually landed on the surface of a planet to measure seismic activity and to chemically analyze soil, rock, and atmospheric samples (see Seismology). On other missions, spacecraft orbiting distant planets and their moons have taken photographs, measured magnetic fields, taken samples of atmospheres for chemical analysis, sampled solar winds and other forms of radiation in space, and spectroscopically analyzed light transmitted through planetary atmospheres.

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