Planets

(adapted from Elementary Astronomy by Pierce (2000))

In the grand scheme of the universe, planets are really quite small, and thus they have been left until the end of our discussion. The planets we know best are found orbiting about our Sun.

A Model Solar System

We will introduce the Sun’s planets by making a model of the solar system. We will assemble a collection of spheres of different sizes and assign each of them to one of the celestial bodies in our model. To gain the most insight from our model, we should construct it to scale, such that each planet has the proper size relative to all the others. We can choose any scale we want – the first model planet can be any size – but once the scale is chosen, the sizes of all the other model planets will be set.

The following table shows one such model; it includes the Sun, the planets, and the Moon, along with their scale sizes and corresponding scale distances. (Pluto – reclassified in 2006 from planet to dwarf planet – is included for comparison.) The scale has been chosen by selecting a blue marble to be the Earth: this results in model planets that range in size from Jupiter, a 6-inch foam ball of the type found in hobby and craft stores, to Pluto, a tiny ball even smaller than the BB that represents the Moon. The Sun is quite large in comparison.

This table also includes the nearest star to the Sun – Alpha Centauri. Even in our scale model, this star is a tremendous distance from the Sun – much farther away than Pluto, which appears quite local in comparison.

Planetary Groups

Some of the planets share similar properties. This allows us to group them together and make some general statements about each group. The two main groups are the terrestrial planets and the Jovian planets. The terrestrial (or Earth-like) planets are Mercury, Venus, Earth, and Mars; the Jovian (or Jupiter-like) planets are Jupiter, Saturn, Uranus, and Neptune. (Pluto is not a good match for either group, a fact that has led to its reclassification.) The following table shows the differences in several basic properties for the two groups. In each case the division is absolute: every terrestrial planet is more dense than any Jovian; every Jovian planet has more moons than any terrestrial; and so on.

Distinctive Features of Planets

In addition to these general planetary properties, we should learn a few specifics. What distinguishes each planet from the others?

Mercury

Mercury, the smallest terrestrial planet, is very Moon-like in appearance as shown by the images from the Mariner 10 flybys in 1974-5. It has essentially no atmosphere, and its clearly visible surface has lots of craters but relatively few of the maria found on the Moon. Its proximity to the Sun makes it difficult to study from the Earth, even though it is visible to the unaided eye. Being the closest planet to the Sun, Mercury has the shortest orbital period – only 88 days. Mercury has an interesting relation between its rotation and its revolution: the two motions are linked in a 3:2 resonance, such that Mercury rotates three times in every two revolutions about the Sun. The solar day length depends on both these motions, with the result that the interval from one noon to the next on Mercury is equal to two revolutions, or 176 days. Thus we find that Mercury residents will experience two Mercury years per Mercury day. But with long, hot days and equally long, cold nights, Mercury is not apt to attract many visitors in the near future. There are no moons around Mercury.

Venus

Venus is very similar is size to the Earth and has often been called Earth's twin or sister planet. Early observers knew that Venus was closer to the Sun – and therefore hotter – and covered with a layer of dense clouds. By assuming the clouds were similar to those found on Earth, they envisioned a tropical climate for Venus, with good chances for a variety of life. Discovery of carbon dioxide in the atmosphere made the possibility of plant life on Venus even more likely. However, we now know that the temperatures on Venus are far too high for liquid water to exist, due to the greenhouse effect. Light from the Sun filters through the clouds and warms the surface of the planet, which then reradiates the energy as infrared rays. However, the dense carbon dioxide atmosphere absorbs the infrared rays, trapping the energy below the cloud layers and keeping the surface temperature abnormally high – nearly 900° F.

With its high temperatures and dense atmosphere, the surface of Venus is quite unlike that of the Earth, and certainly is unsuitable for life as we know it. Venus appears to be geologically active: radar mapping of the cloud-covered surface by the Magellan spacecraft in 1991 has revealed a variety of terrain, including impact craters and features produced by volcanic activity. The clouds are also unique: unlike our clouds of water droplets, those on Venus are made of sulfuric acid droplets.

Like Mercury, Venus has a very slow rotation; its 243-day sidereal rotation period is the longest of any planet. In addition, its rotation is retrograde, or backwards from the normal counterclockwise motion of the planets. This motion couples with the planet's orbital motion to produce a solar day on Venus of 117 days. Thus, even though Venus has the longest sidereal day, its solar day is second to Mercury's in length. And while Mercury calendars have two years per day, those on Venus have approximately two days per year.

And like Mercury, Venus is moonless.

Earth

The Earth, our home, is the largest of the terrestrial planets and has the highest density of any planet in the solar system. It is the only planet with substantial amounts of liquid water on its surface and the only planet known to have life. This life has had a part in establishing the present atmosphere: the large fraction (nearly 21%) of highly reactive free oxygen in our atmosphere was produced by plant life over the last few billion years. No other planet in our solar system has such a high abundance of molecular oxygen.

Earth is one of only two terrestrial planets to have a moon.

Mars

The 'red planet', Mars, has been studied with great interest for many years, due in part to its similarities to the Earth. Mars has a mostly transparent atmosphere, which allowed early astronomers to view seasonal changes in the polar ice caps and the blue-green markings on its otherwise orange surface. Such observations suggested the presence of water and perhaps plant life on Mars. Later, observations of straight-line markings and their interpretation as canals caused some to assume the presence of Martians, who supposedly constructed the canals to bring water from the polar regions to irrigate the equatorial deserts. Numerous books and movies supported this idea, but the Mariner and Viking missions to Mars in the 1960s and 70s did not. They depicted a less hospitable world with a very thin atmosphere of carbon dioxide and temperatures too low for liquid water to exist. The Martian surface is certainly interesting, with occasional craters, inactive volcanoes, dust storms, dry riverbeds, and polar ice caps of water ice and frozen carbon dioxide (dry ice), but there is no indication of life on Mars. Although it appears that in the distant past the atmosphere was denser and liquid water flowed on the Martian surface, the Viking spacecraft that landed there in 1976 found no conclusive evidence of any type of life. More recent landings on Mars have confirmed the previous existence of liquid water, but whether life ever evolved there is still in question.

Mars owns two of the three satellites that orbit the terrestrial planets: Phobos and Deimos are tiny, irregularly shaped moons that may be captured asteroids. Because of Phobos' very rapid orbital motion about Mars, Martian observers would see Phobos rise in the west and set in the east while Deimos follows a more normal east-to-west route.

Jupiter

Jupiter is completely different from the terrestrial planets previously described; in fact, it is the prototype of the Jovian planets. Jupiter is the largest planet in the solar system and also the most massive, containing more mass than all the other planets put together. But Jupiter's density – about one quarter the value of the Earth's density – is not extremely high because Jupiter is a gas giant, composed largely of gaseous hydrogen, helium, and other lightweight elements. The planet probably does have a small core of iron and rock at the center that is surrounded by hydrogen and helium compressed to a liquid form by the tremendous pressures of Jupiter's interior. The surface of Jupiter is not solid but fluid, making it quite unlike the terrestrial planets. We cannot see this surface because it is covered by layers of clouds; the patterns we see on Jupiter – colored bands, belts, and oval spots – are all features in the cloud layers. Jupiter's rotation period is the shortest of any planet, and this rapid rotation causes considerable motion in the cloud features, with rotating spots, and adjacent bands flowing in opposite directions. The most notable of these features is the Great Red Spot, which has been observed for over three centuries.

Jupiter has the greatest number of satellites – currently 63 – with the four largest (Io, Europa, Ganymede, and Callisto) easily visible in binoculars. The orbital periods of these four range from two days to about two weeks, and they can be seen to change position from night to night. Io and Europa are about the size of our Moon while Ganymede and Callisto are about the size of Mercury. The Voyager flyby missions in 1979 discovered several new moons and a set of thin rings orbiting the planet. More recent closeup images were obtained by the Galileo spacecraft, which arrived at Jupiter in 1995.

Saturn

Saturn is the most distant planet to be known to the ancient astronomers; its brightness is sufficient to make it easily visible to the naked eye. In a small telescope, Saturn is a special sight because of its system of bright rings. These rings are made of millions of small particles orbiting the planet in its equatorial plane. Unlike the rings of the other Jovian planets, which are thin and relatively distinct, Saturn's rings appear to be broad and fairly continuous, without significant gaps between them.

In other respects, Saturn is another gas giant planet, smaller than Jupiter. It too is composed chiefly of hydrogen and helium, but due to its smaller mass, it is compressed less than Jupiter. As a result, Saturn has the lowest density of any of the planets – so low that Saturn could float in water if a large enough bathtub could be found. Saturn's clouds are not as colorful as Jupiter's; the bands and spots found here are much subtler. Saturn's rapid rotation has made it the most oblate planet, with a noticeable difference between its polar and equatorial diameters.

Saturn has the second most moons of any of the planets, with a current total of 34. The majority of these appear to be solid, with surfaces heavily covered with craters. The largest moon – Titan – has its own atmosphere, which has prevented direct viewing of its surface and promoted speculation on the possibility of life having developed there. Saturn was visited by the Voyager spacecraft during flyby missions in 1980 and 1981, and more recently by the Cassini mission in 2004.

Uranus

Uranus was not studied by the early astronomers because it is barely bright enough to be seen by the unaided eye and thus easily mistaken for a faint star. It was discovered in 1781 by William Herschel, who noted that its telescopic image appeared somewhat different from that of the stars in the same field. Further observations showed that it was moving slowly among the stars – orbiting the Sun.

Uranus is one of the smaller Jovian planets, but it still has a diameter four times that of the Earth. Uranus is covered with a thick cloud cover and lacks a solid surface; its clouds have proved to be especially featureless, without significant spots or bands. The most distinctive feature of Uranus has always been its obliquity of 82°, which means that the rotational axis lies nearly in the orbital plane. (To see this effect, tip a globe until its axis is almost horizontal. Such an orientation for the Earth would result in rather dramatic seasonal changes.) At the solstices, Uranus' poles point almost directly toward the Sun, rather than just tilting toward it as the Earth's do. The land of the midnight sun extends from the pole to within 8° of the equator on Uranus, and the Sun reaches the zenith as far north as 8° from the pole.

Uranus has an 82° obliquity and retrograde (CW) rotation; some sources will list a 98° obliquity and normal (CCW) rotation. Both are valid -- they result from using the two different poles to measure the angle and view the rotation.

In 1977 Uranus became the second planet known to have rings, but they are not easily seen from Earth. The passage of Voyager 2 in 1986 increased the number of moons of Uranus from five to 15 (12 more have since been added, for a total of 27). Both the rings and the moons orbit in Uranus' equatorial plane, which is tilted by 82° from its orbital plane. At some times in its orbit, Uranus presents itself as a huge target, with the planet as the bull's-eye and the rings and moon orbits forming the surrounding rings of the target. It was during such an alignment that the rings were discovered, during an occultation event. As Uranus passed in front of a distant star, each ring briefly dimmed the star’s light before the planet finally covered it; these dimmings were recorded and the rings were discovered.

Neptune

After the discovery of Uranus in 1781, astronomers monitored its motion to determine its orbit. After several years, sufficient observations had been made to allow its orbit to be predicted, but in the years that followed it became apparent that Uranus was not following the predictions, moving too rapidly at first and then too slowly. Several mathematicians and astronomers suspected the presence of another planet, whose gravitational tugs would have altered Uranus' orbital speed. Their calculations resulted in the prediction of a new planet beyond Uranus. When the search was finally made in 1846, Neptune was discovered, right where the predictions had placed it.

Neptune is often confused with Uranus because both planets are similar, in both size and location in the solar system. Another Jovian planet, Neptune is a bit smaller, slightly more massive, and considerably more colorful than Uranus. Voyager images from the 1989 flyby show Neptune to be quite blue, due to absorption of red light by methane in its atmosphere. Several interesting features were seen, including small white spots and one large blue oval called the Great Dark Spot. The Voyager confirmed that Neptune has a set of thin rings and also raised its total of moons from two up to eight; more recent observations have upped this figure to 13.

Pluto

Following Neptune's discovery, its position was monitored to determine its orbit. Calculation of the orbit followed by further monitoring showed that it too was not behaving as predicted. Again several astronomers made predictions of another planet and began making plans for a search. In 1930, Clyde Tombaugh discovered a new planet, which was given the name Pluto.

Pluto is a planet of many extremes: its orbit is the largest, most eccentric, and most highly inclined; its average orbital speed is the slowest, and its orbital period is the longest, at 248 years. Pluto itself turns out to be the smallest, least massive planet – a body even smaller than our Moon. Despite its small size, Pluto has three moons of its own; the largest, discovered in 1978, finally enabled accurate determinations of Pluto's size and mass. By then it had become apparent that tiny Pluto is far too small to have been responsible for the perturbations in Neptune's orbit that originally led astronomers to search for a ninth planet. It would seem that Pluto’s discovery was really a matter of luck.

Pluto's icy composition and frozen atmosphere make it neither a terrestrial nor a Jovian body, and Pluto appears to have more in common with comets or some of the Jovian moons than with the other planets. All of these characteristics led to Pluto’s reclassification as a dwarf planet in 2006, reducing the solar system’s official planet count from nine to eight. To date, there have been no visits to Pluto by terrestrial spacecraft, but the New Horizons spacecraft is due to arrive in 2015.

Other Solar System Bodies

Asteroids

Besides the planets, there are a number of smaller bodies orbiting the Sun; some of these are called asteroids, or minor planets. Most of the asteroids seem to have orbits that lie between the orbits of Mars and Jupiter, a region known as the asteroid belt. Although there are thousands of asteroids known and probably many more yet undiscovered, the spacing between asteroids is still quite large; our space probes to the outer planets have all passed through the asteroid belt without colliding with anything large enough to damage them.

The typical asteroid is not spherical but irregular in shape, a few kilometers across. The largest asteroids are a few hundred kilometers across, but there are not many of these; the total asteroid mass, if combined, would form a body smaller than the Moon. (Ceres, the largest asteroid, is spherical in shape, and it is now classed along with Pluto as a dwarf planet.) Most asteroids are composed of rocky material, believed to be left over from the time of the formation of the solar system. Whereas most such rocky planetesimals collided with each other and formed into planets, the asteroids were apparently kept stirred up by the tidal forces of Jupiter and never had a chance to form a single body.

Asteroids do occasionally collide and break apart, providing fresh debris in the solar system. Some of these small chunks of rock and iron eventually collide with the Earth and produce meteors as they burn up in our atmosphere or meteorites when they land on the Earth's surface. A collision between the Earth and a whole asteroid could be disastrous, even if the asteroid is fairly small. Such collisions were common billions of years ago as the Earth was forming, but they are quite rare now. There is some evidence that such a collision might have led to the extinction of the dinosaurs about 65 million years ago. Let us hope that we do not suffer a similar fate, as there is little we could do to avoid such a catastrophe today.

Comets

There are other bodies orbiting the Sun that are neither planets nor asteroids; these are the comets, which can provide spectacular shows in the night sky. Comets were observed (and completely misunderstood) by early sky watchers, who thought these strange lights were part of the Earth's atmosphere. They seemed to appear rather suddenly, remain visible for a few weeks, and then disappear. They did not resemble any of the normal celestial objects – Moon, planets, or stars – instead being rather fuzzy and often displaying a long tail. They were generally regarded as bad omens, responsible for all sorts of human misery – of which there was plenty – on Earth.

We now know that the comets' strange appearance results from their composition. Whereas the asteroids tend to be mostly rocky, the comets seem to be composed of frozen gases of lightweight elements. The standard model of a comet nucleus is a 'dirty snowball', a mass of ices a few kilometers across with bits of rock and dust imbedded in it.

These icy planetesimals orbit the Sun in highly elongated orbits. Most of their time is spent far from the Sun, but periodically they head for the inner solar system, swoop around the Sun, and leave after only a few weeks or months of visibility. Their presence is made even more spectacular by the growth of a tail. As the comet nears the Sun, it is warmed by the Sun's radiation. Some of the ices vaporize and stream out behind the comet forming a tail, which always points away from the Sun. Some of the snowball's dirt is released as the ice melts, providing additional material for the tail and eventually more debris for the solar system collection.

While there are usually a few comets passing through the inner solar system at any given time, only rarely do they become bright enough to view without a telescope. Although they can be found in any part of the sky, the brighter ones are often seen best when they are closest to the Sun and have the longest, brightest tails. As such, they will be seen either in the eastern sky, before sunrise, or in the western sky after sunset. Comets do not streak across the sky; instead they hang among the stars, gradually changing their position from night to night over several weeks or months. Those that pass close to the Earth will seem to change most rapidly. Some comets are periodic, making repeated appearances in our skies; most notable of these is Comet Halley, with a period of 75 years; Comet Hale-Bopp, which became quite bright in 1997, has a 2500-year period.

Like asteroids, comets may occasionally collide with planets, including the Earth. In 1994, astronomers witnessed the collision of a stream of comet fragments with Jupiter. Although the planet was not seriously damaged by the event, the impacts did produce large discolorations in the Jovian atmosphere that persisted for several weeks and were visible from Earth. Comet impacts with our planet several billion years ago may have delivered much of water for the Earth’s oceans.

Meteors

The solar system is strewn with debris, small bits of rock that have been produced by collisions between asteroids or released by the evaporation of comets. Those bits of space debris with orbits about the Sun that carry them into the Earth's path are called meteoroids. When a meteoroid encounters the Earth, it plunges through the atmosphere at a high rate of speed. The meteoroid's passage through the atmosphere creates friction with the air molecules, heating them enough to cause them to radiate light. The luminous trail of heated air produced in this fashion is called a meteor. Other common names for this phenomenon include shooting star and falling star; but clearly, meteors have nothing to do with stars.

Because most meteoroids are quite small, they tend to burn up or vaporize as they pass through the atmosphere. However, the larger ones are not destroyed in this manner but manage to land on the Earth; these rocks are called meteorites. Most meteorites are made of stony materials which may be difficult to distinguish from ordinary terrestrial rocks. Some of the meteorites are made of nickel and iron and thus are easier to identify as visitors from space.

Meteors are not rare; in fact they can be seen on any clear, moonless night at a location where the sky is relatively dark. The meteor's streak will be brief, lasting only a second or so before it fades from view. Meteors can occur in any part of the sky at any time, but they are more abundant after midnight when the observer is on the front half of the Earth as it orbits the Sun, plowing through the meteoroids.

As comets evaporate, they release debris, which becomes strewn along their orbits. If the Earth passes through the orbit of a comet, it will collide with an increased number of meteoroids, causing a greater abundance of meteors. Such an event, called a meteor shower, happens at the same point in the Earth's orbit – and thus the same date – every year. Furthermore, because these meteoroids are all traveling in about the same direction in space, the meteors they create seem to all radiate from the same point in the sky. Meteor showers are named for the constellation in which this radiant is located. One of the most famous showers is the Perseids; these meteors occur around August 11 each year and have a radiant in the constellation Perseus.