Mercury is about one-third the size of Earth. It is the closest to our Sun, orbiting it in just 88 days. Because it is so close to the Sun, its surface temperatures are extreme, ranging from 427ºC (800ºF) on the sunward side to –183ºC (–297ºF) on the side facing away from the Sun. Mercury has no atmosphere and no surface water; the high temperatures prevent these from forming. The surface of Mercury looks like our Moon. It is covered by craters, indicating its long history of bombardment by asteroids and other impactors. Other than impact events, Mercury's surface has not been modified in billions of years; Mercury is not geologically active. While there is evidence of ancient lava flows, Mercury has no active volcanos. Because it lacks an atmosphere and flowing water it has no active weathering processes.

Mariner 10 mosaic of Mercury images, courtesy of JPL/NASA.

Venus is almost the size of Earth. Its rotation is very slow. Venus turns once on its axis every 243 Earth days and it spins backward relative to the other planets. The time it takes to rotate is very close to the time it takes to orbit the Sun. Surface temperatures on Venus range from 377ºC to 487ºC (710º to 908ºF) — even hotter than Mercury! The reason that Venus is hotter, even though it is farther from the Sun, is that it has a thick atmosphere composed of carbon dioxide and traces of water and sulfuric acid. This atmosphere — about 90 times the pressure of Earth's atmosphere — creates an intense greenhouse effect; heat is trapped in the atmosphere. Venus has many volcanos, some of which may still be active.

Mosaic of Magellan images of Venus, color coded to represent elevation.
Image courtesy of USGS/JPL/NASA.

Earth is a dynamic planet. It also is the only planet we know that has life. It spins on its axis once a day and orbits the Sun once a year (other planet's years and days often are presented relative to Earth's). The rotation axis is tilted, giving Earth its seasons. Surface temperatures range from –73º to 48ºC (–100º to 120ºF) and liquid water is abundant. Earth's atmosphere traps energy from sunlight, creating a greenhouse effect that warms the surface. It also moderates the climate and protects the surface from damaging components of solar radiation. Earth has water, rock, and tectonic cycles, which are important for renewing nutrients. It has one natural satellite — the Moon.

Earth from Space.
Image courtesy of NASA.

Mars is about half the size of Earth. Its period of rotation is very close to Earth's, but it takes about two Earth years to orbit the Sun. Mars is tilted on its axis, so it experiences seasons. Surface temperatures are cold — –83º to –33ºC (–117º to –27ºF) — and the planet is very dry. The atmosphere is thin and composed mostly of carbon dioxide. There is no liquid water present at the surface. There may be frozen water in the subsurface, and Mars has ice caps in its polar regions. The ice is a combination of carbon dioxide and water ice. There is evidence that Mars had flowing water and oceans at its surface during its early history, perhaps until about three and a half billion years ago. Mars has the tallest volcano in our solar system — it is about 22 kilometers tall (almost 14 miles high). [Compare this to Hawaii's Mauna Loa at 9 kilometers (5.5 miles) tall measured from the sea floor.] Some of the volcanos on Mars have been recently active.

Hubble Space Telescope image of Mars as it made its closest approach to Earth in August 2003.
Image courtesy of NASA.

The asteroid belt resides between the inner planets and the outer planets. Actually, there are several distinct bands within the belt. Asteroids are rocky remnants from our early solar system. They range in size from a 1000 kilometers (620 miles) across to the size of sand grains. Asteroids are materials that have not accreted into a planet because of Jupiter's powerful gravitational field. Asteroids occasionally reach Earth's surface as meteorites, providing scientists with information about when our solar system formed and the processes that occurred.

Photo mosaic of images taken by Galileo spacecraft of asteroid Ida.
Image courtesy of JPL/NASA.

Jupiter is the largest planet in our solar system; about 1000 Earths could fit inside a hollow Jupiter. It contains more mass than all the other planets combined. Jupiter spins on its axis once every 10 hours and orbits the Sun once every 12 years. It is about 90% hydrogen and 10% helium with some methane, water, and ammonia. Temperatures reach –200ºC (–325ºF) at the top of the atmosphere. The atmosphere is tumultuous, divided into distinct bands. Wind speeds are high, up to 400 kilometers per hour (250 miles per hour) and lightening is frequent. The Giant Red Spot is a massive storm system larger than the diameter of Earth that has been raging for at least 400 years. Jupiter has at least 60 moons. Ganymede, the largest of Jupiter's moons, is larger than the planet Mercury.

Voyager 1 image of the Giant Red Spot of Jupiter.
Image courtesy of NASA.

Saturn is the second-largest planet. Its day is 11 hours long and its orbit around the Sun takes about 30 years. Its composition and atmosphere are similar to Jupiter's. Winds reach 1770 kilometers per hour (1100 miles an hour). Saturn is best known for its beautiful rings. The ring system stretches to a diameter of 250,000 kilometers (155,000 miles) but is only 1 kilometer thick (a little over half a mile). Saturn has at least 30 moons; more are being discovered by the Cassini mission.

Color image of Saturn taken by Cassini spacecraft. Image courtesy of JPL/Space Science Institute/NASA.

Uranus was the first planet discovered by telescope. Like the other gas giants, its atmosphere is mostly hydrogen and helium. It has a little methane in its atmosphere, which absorbs red light, giving Uranus its blue-green color. The interior of Uranus contains more rock and ice than Jupiter and Saturn. It rotates on its axis once every 17 hours and orbits the Sun once every 84 years. Unlike the other planets, Uranus' axis is tilted so that the planet rotates on its side. Given Uranus' long period of orbit, this translates into a 20-year winter or summer! Uranus has 22 known satellites.

Infrared composite of the two hemispheres of Uranus showing the rings. Image courtesy of Lawrence Sromovsky, University of Wisconsin-Madison/W. M. Keck Observatory.

Neptune is the most distant gas giant planet. Neptune spins on its axis once every 16 hours, and an orbit around the Sun takes 165 years. Like Uranus, it has methane in its atmosphere, which gives it its blue hue. Neptune has the fastest winds in the solar system (2000 kilometers or 1250 miles per hour), and some massive storm systems that move within its atmosphere. It has eight known moons and four rings.

Color image of Neptune from Voyager 2. Image courtesy of JPL/NASA.

Pluto is the smallest planet in our solar system and usually is the farthest planet from our Sun. However, Pluto has a highly elliptical orbit and sometimes is located inside Neptune's orbit. Pluto rotates on its axis once every 6 days, and its journey around the Sun takes 240 Earth years. It is a small icy, rocky body, with a single moon, Charon, that is half Pluto's size. Pluto may be a large-sized member of the Kuiper belt.

Hubble Space Telescope image of Pluto and its moon Charon. Image courtesy of Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility/NASA.