Mercury, our innermost planet, orbits very close to the Sun. A point on the daylight side of Mercury receives sunlight for so long, and from such a close distance to the sun, that the maximum temperature reaches over 400 degrees centigrade (800* F.). Mercury has practically no atmosphere to spread the energy around the planet. Traces of sodium making a very thin atmosphere around Mercury were discovered fairly recently. But for the most part, sunlight gives particles near Mercury so much energy that they escape from Mercury's weak gravity. Photographs from NASA's Mariner 10, showed that Mercury is covered with craters and fine dust that has a small magnetic field. Mercury has no moon.
Venus comes nearer to Earth than any other planet. It most closely matches Earth in size, mass and density. The clouds in Venus's atmosphere allow sunlight to pass through. The sunlight heats Venus's surface, which gives off mainly infrared rays. Unlike sunlight, these infrared rays cannot pass up and out through the clouds. The atmosphere of Venus traps the energy of the infrared radiation and the planet heats up. The process is known as the "greenhouse effect". The "greenhouse effect" found on Venus is very different from that found on Earth. Venus has over 90% of it's atmosphere made up of carbon dioxide, giving it a very large "greenhouse effect", with temperatures hot enough to melt lead. On Earth, carbon dioxide is only about 300 parts per million.
Venus's surface appears to have giant volcanoes. Studies of Venus's atmosphere have also shown changes in the level of certain gases that seem to show that volcanoes are currently active on Venus. Although, no actual eruptions have been seen. Venus rotates much slower than the Earth, and has no moon. It has no magnetic field or drifting continental plates.
Mars is smaller than Earth; its diameter is only about one-half of the Earth's diameter and has only one-tenth the Earth's mass. Mars rotates at almost the same rate as Earth, and its rotation axis is inclined to the ecliptic at almost the same angle as the Earth's axis. Because of its axis tilt, Mars has daily and seasonal cycles similar to those on Earth.
Mars has polar ice caps that grow and diminish as the seasons change. One Martian year lasts 780 Earth days, or two Earth years and two months. The appearance of the planet changes because of seasonal dust storms that obscure its surface. Dust storms on Mars can be studied from Earth. Amateur astronomers keep track of them. Studies of the weather on Mars are helping us understand our own weather system on Earth.
Evidence reveals that liquid water once existed in great quantity on the surface of Mars. Two types of flow features are seen, the run off channels and the outflow channels. They bear a strong resemblance to river systems on Earth, and geologists believe that this is just what they are. There is no evidence of liquid water today, and in fact it could not exist on its surface, since the melting temperature coincides with the boiling temperature. All of the water on Mars today is locked in the permafrost layer under the surface, and perhaps in the polar caps. Mars has no magnetic field. Since the planet rotates rapidly, this implies that its core is nonmetallic, nonliquid or both.
Mars has two tiny moons, which are probably asteroids that were captured, by the planet's gravitational field early in its history.
Jupiter is the largest planet in the solar system. Its mass is more than twice the mass of all the other planets combined, although it is still about 1000 times less massive than the Sun. It is composed primarily of hydrogen and helium gas.
Jupiter rotates very fast, producing a pronounced equatorial flattening. The amount of flattening allows astronomers to infer the presence of a large core in its interior.
Jupiter's atmosphere consists of three main cloud layers. The colors that we see are the result of chemical reactions, fueled by the planet's interior heat, solar ultraviolet radiation, auroral phenomena, and lightening, at varying depths below the cloud tops, seen through "holes" in the overlying clouds. The main weather pattern on Jupiter is the Great Red Spot, an Earth-size hurricane that has been raging for at least three centuries. Other, smaller, weather systems-the white and brown ovals-are also observed. They can persist for decades.
Jupiter's atmosphere becomes hotter and denser with depth, eventually becoming liquid. Interior pressures are so high that the hydrogen is "metallic" in nature near the center. The planet has a large "terrestrial" core 10-20 times the mass of the Earth.
The magnetosphere of Jupiter is about a million times more voluminous than Earth's magnetosphere, and the planet has a long magnetic "tail" extending away from the Sun to at least the distance of Saturn's orbit.
Jupiter and its system of moons resemble a small solar system. Sixteen moons have been discovered so far. The innermost Galilean moon, Io, has active volcanoes. Europa has a cracked, icy surface that may possibly conceal an ocean of liquid water. Ganymede, the largest moon in the solar system, shows some evidence for past geological activity, but it is now unmoving rock and ice.
Saturn is smaller than Jupiter, but still much larger than any of the inner planets. Like all jovian planets, Saturn has many moons and a planetary ring system around it.
Weather systems are seen on Saturn, as on Jupiter, although they are less distinct. Saturn has weaker gravity and a more extended atmosphere than Jupiter. The planet's overall butterscotch hue is due to cloud chemistry similar to that occurring in Jupiter's atmosphere.
Saturn's rings are made up of trillions of icy particles ranging in size from dust grains to boulders, all orbiting Saturn like so many tiny moons. Their total mass is comparable to that of a small moon. Many scientists believe that planetary rings have a lifetime of only a few tens of millions of years. The fact that we see rings around all four jovian planets means that they must constantly be reformed or replenished, perhaps by material chipped off moons by meteoritic impact or by the tidal destruction of entire moons.
Saturn has the most extensive system of natural satellites of all the planets. It has at least 19. The reflected light from Saturn's moons suggests that most are covered with snow and ice. Saturn's largest moon Titan is the second-largest moon in the solar system. Titan is the size of the planet Mercury.
Uranus, the seventh planet from the Sun is smaller than Saturn, but still four times the diameter of the Earth. Uranus orbits about twice as far out from the Sun as Saturn. Uranus takes 84 of our years to orbit the Sun.
Uranus itself shows very little structure on its surface. It appears blue-green because the methane gas in its atmosphere absorbs all the other colors in the sunlight that hits it. Uranus has atmospheric clouds and flow patterns that move around the planet in the same sense as the planet's rotation, with wind speeds in the range of 200-500 km/h. Voyager 2 found that Uranus and Neptune both have magnetospheres comparable in strength to Earth's field. Theoretical models indicate that Uranus and Neptune have rocky cores similar to those found in Jupiter and Saturn.
Uranus is known to have 17 moons. The frequency of large craters on the outer moons suggests that destructive impacts may once have been common in the Uranus system.
Neptune orbits the Sun once every 164.8 Earth years. It has not completed one revolution since its discovery. Neptune is so distant that surface features are impossible to discern.
In 1989, Voyager 2 revealed a Great Dark Spot on Neptune, a spiral storm the size of Earth. (Later, in 1994 the Hubble Space Telescope viewed Neptune and found the Spot had disappeared.) Neptune has several storm systems similar to those found on Jupiter.
Spectroscopic studies of sunlight reflected from Uranus and Neptune's dense clouds indicate that the two planets' outer atmospheres are similar to those of Jupiter and Saturn. The most abundant element being hydrogen, followed by helium and methane, which is more abundant on Neptune than on Uranus.
Neptune has 8 known moons. Its largest moon, Triton, has an atmosphere, which is made up of mostly nitrogen, like Earth's. But Triton is much colder. Triton's surface temperature is 37K, making it the coldest place in our solar system. Triton's surface consists primarily of water ice. The cameras on Voyager 2 detected two great jets of nitrogen gas erupting from below the surface, rising several kilometers into the sky. Scientists conjecture that nitrogen geysers may be very common on Triton and are perhaps responsible for much of the moon's thin atmosphere.
Pluto is a small planet with a solid surface just past large gaseous planets. Pluto's orbit is quite elongated. Because of its substantial orbital eccentricity, Pluto's distance from the Sun varies considerably. At perihelion, it lies 29.7 AU. (4.4billion km) from the Sun, inside the orbit of Neptune. At aphelion, the distance is 49.3 AU. (7.4 billion km), well outside Neptune's orbit. At 40 AU. from the Sun it is never visible by the naked eye. It takes Pluto 247.7 Earth years to make one revolution around the Sun.
Pluto's great distance from Earth makes it very difficult to study its physical nature. In July 1978 astronomers at the U.S. Naval Observatory discovered that Pluto has a satellite. It is now named Charon. Knowing the moon's orbital period of 6.4 days, astronomers could determine the mass of Pluto accurately. It is 0.0025 Earth masses, more similar to the mass of a moon than a planet. Because Pluto does not possess characteristics of either terrestrial or jovian planets and because of its similarity to the ice moons of the outer planets, some researchers suspect that Pluto is not a "true" planet. Pluto may be an escaped planetary moon or a large icy chunk of debris left over from the formation of the solar system.
Our Sun is a star. It is a fairly average star, but unique in its location to Earth. It is some 300,000 times closer than out next nearest neighbor Alpha Centauri. While Alpha Centauri is at a distance of 4.3 light years distance, the Sun is only 8 light minutes away from us.
A star is a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center. The main interior region of the Sun is the core, where nuclear reactions generate energy. The Sun generates this energy by "burning" hydrogen into helium in its core by the process of nuclear fusion. When four protons are converted into a helium nucleus in the proton-proton chain, some mass is lost. The law of conservation of mass and energy requires that this mass appear as energy, eventually resulting in the light that we see.
The sun is 870,000 miles across. A million Earth could fit inside it. The everyday surface of the Sun that lights up our daytime sky is called the photosphere. It is too bright to stare at safely. Scientists and amateur astronomers either project an image of the Sun onto a screen or use special filters that cut out all but about one-millionth of the Sun's light.