The Astrophysics Spectator

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The Structure of Our Universe

Planets

The eight planets of our Solar System are the smallest and best-studied astronomical bodies. All of the planets within the Solar System have been visited by our spacecraft. Our spacecraft have landed onto the surfaces of Mars, Venus, and Saturn's largest moon, Titan. One of our spacecraft dropped into Jupiter's atmosphere to measure its composition. Other spacecraft have orbited for many years Venus, Mars, Jupiter, and Saturn. The vast amount of data these missions generated have fractured the study of the planets into a dozen specialties that are more strongly allied with geology and meteorology than with traditional astrophysics. Contemporary astrophysics now principally concerns itself with planets around other stars, on Jupiter-like planets moving freely through the Galaxy, and on the creation and evolution of the Solar System.

Earth is on the small-end of the planetary scale. It is large enough to retain both an atmosphere and any hydrogen temporarily liberated from water, allowing Earth to hold onto its oceans for billions of years. Venus is slight smaller at 81% of Earth's mass; while it retains an atmosphere, its closeness to the Sun keeps Venus too hot to retain hydrogen, making it a dry planet. Mars, at only 11% of Earth's mass, has only a tenuous atmosphere. The two smallest planets, Mercury, at 6% of Earth's mass, and Pluto, at 0.2% of Earth's mass, have no atmosphere.

The four inner planets, Mercury, Venus, Earth, and Mars, which are the four terrestrial planets of the Solar System, have similar compositions of iron, nickel, silicon, and other elements with high melting temperatures. Mercury has a circumference that is 38% of Earth's, while Mars and Venus have circumferences that are 53% and 95% of Earth's.

The large planets in our Solar System are many times the size of Earth. The largest, Jupiter, is 318 time Earth's mass, while Saturn, Uranus, and Neptune are respectively 95, 15, and 17 times Earth's mass. In terms circumference, these four planets are 11.2, 9.5, 4.0, and 3.9 times Earth's circumference. These four giants neatly fall into two pairs: the giant gaseous planets of Jupiter and Saturn, and the giant ice planets of Uranus and Neptune. This separation is largely a consequence of size. Neptune and Uranus are small enough to have lost all of the energy liberated at their cores through gravitational collapse since their formation four and a half billion years ago. Jupiter, on the other hand, is large enough that it remains hot as it continues a slow gravitational collapse. Saturn is just at the stage where its gravitational collapse ceases and it core cools to a low temperature. All of these planets are composed of light elements, principally hydrogen and helium, with smaller amounts of carbon, nitrogen, and oxygen. The compositions of Jupiter and Saturn, in fact, are similar to the Sun.

Many smaller bodies orbit the Sun. The asteroids close to the Sun resemble the inner planets in composition. The largest of these asteroids is Ceres, which because of its size is considered a dwarf planet. The asteroids far from the Sun, including the Kuiper Belt objects, are composed of lighter elements, such as hydrogen, oxygen, and nitrogen, that are chemically bound in ices of water, methane, and ammonia. The Kuiper Belt, which lies outside of the orbit of Neptune, contains numerous dwarf planets, including Pluto and its companion Charon; these dwarf planets are called plutonian objects. The comets are though to be Kuiper Belt objects that were scattered by the giant planets into highly-elliptical orbits that extend far beyond the Kuiper Belt.

Planets have been found orbiting other stars. They are found through their gravitational pull on the star they orbit, which causes the star to move back and forth on the sky. This effect is very subtle, so the extrasolar planets found through this effect are inevitably large, many of them much larger than Jupiter.

There is some speculation that Jupiter-like bodies are traveling freely in our Galaxy like the stars that we see. These Jupiters produce no light, so they are invisible at Earth, detectable only through the effect their gravitational fields have on passing starlight. These objects may account for part of the invisible mass of our Galaxy.

As large as Jupiter is, it is far below the mass a planet can have without driving the thermonuclear fusion of hydrogen. This limit is 25,000 time Earth's mass, or 7.5% of the Sun's mass. Anything above this limit is a star. Objects just below this limit are called brown dwarfs, because, while they cannot fuse hydrogen, they can fuse deuterium, the two-nucleon isotope of hydrogen. This type of fusion produces only a small amount of energy, because deuterium is only present in trace amounts in any star or planet. The energy released through deuterium fusion can compensate for the energy lost as surface radiation for a relatively short time—anywhere between 100 million and 1 billion years—after which the deuterium is exhausted and the brown dwarf cools to a low temperature like a planet. The minimum mass for which deuterium burning occurs is 4,100 time the mass of Earth, or 1.2% of the Sun's mass.

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