What is the definition of a planet?
The word "planet" means "wanderer" -- the earliest stargazers noticed star-like objects that changed position with respect to the stars. Over the centuries as we learned more about the different objects in the sky, the definition of what a planet is has changed. For much of that time, there was not a clear definition to distinguish asteroids and comets from planets; they all orbit the Sun. Generally, it was accepted that planets were much larger than asteroids and comets.
In 2005, after astronomers discovered a comet-like object (Eris) that was larger than the then-accepted planet, Pluto, it became clear that a formal definition would be needed. In 2006, the International Astronomical Union defined a planet as an object that orbits the Sun, is large enough for its own gravity to make it round, and has "cleared its neighborhood" of smaller objects. Under this new definition, Pluto, along with Eris and other small icy bodies, does not qualify as a planet. This definition remains somewhat controversial.
Resource Video: What is a Planet? (new window)
Why are the inner and outer planets so different?
The rocky, terrestrial planets -- Mercury, Venus, Earth, and Mars -- all formed in the inner, hotter part of our solar system. It was so hot that volatile materials -- materials that evaporate easily at normal temperatures and pressures -- could not condense. Much of the gas and ice in the solar system could not exist as solids at the high temperatures in the inner region. However, metals and silicates could withstand the high temperatures and these materials became concentrated in the inner solar system. It was from these heavier materials that the rocky inner planets were made.
In the outer, cooler portion of the solar system more volatile materials such as water ice, other ices, and gases were able to accumulate onto the giant planets. Our outer gas giant planets -- Jupiter, Saturn, Uranus, and Neptune formed from these materials. The giants all may have small cores of rock and solid "gases," surrounded by large volumes of liquids and gas -- mostly hydrogen and helium. They have ring systems and several small moons. Like Earth, they each have a magnetic field produced by internal processes.
For more information about the formation of the solar system, visit the Birth of Worlds section.
Little sibling Mercury may only be about one-third the size of Earth, but its story is fascinating, and as yet, untold. NASA's MESSENGER spacecraft has passed by the planet as it in maneuvers to enter Mercury orbit in 2011, revealing new surprises with each pass. Because it is so close to the Sun, its surface temperatures are extreme, ranging from 450°C (840°F) on the sunward side to -170°C (-275°F) at night. Its cratered surface records a long history of bombardment by asteroids and other impactors, and new observations from MESSENGER suggest volcanic activity far longer than thought possible.
Despite continual bombardment by the solar wind, Mercury manages to hold onto a very thin atmosphere of scattered atoms captured from the solar wind and released from the planet itself. Mercury's unusually large liquid core produces a magnetic field for the tiny inner planet.
Venus is Earth's twin in size, but its clouds shroud a darker personality. Its thick atmosphere is 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. Surface temperatures on Venus range from 377°C to 487°C (710° to 908°F) -- even hotter than Mercury!
Venus has many volcanoes, some of which may still be active. 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 actually longer than the time it takes to orbit the Sun.
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 -73° to 48°C (-100° to 120°F) 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.
Mars takes after Earth in many ways. It is only about half the size of Earth, but its similar geology, thin atmosphere, and the presence of water make it seem more like home. Its day is almost as long as Earth's, but it takes about two Earth years to orbit the Sun. Mars is tilted on its axis, so it experiences seasons.
Mars has the tallest volcano in our solar system -- 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. However, its surface temperatures are cold -- -87° to -5°C (-125° to -23°F) -- 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, but robotic explorers have discovered frozen water in the subsurface and in its polar ice caps, which are comprised of frozen 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.
Jupiter has more than twice the mass of all the other planets combined. It grew large enough to capture and hold onto the materials of the solar nebula, so its mixture of about 90% hydrogen and 10% helium by percent volume (with some methane, water, and ammonia mixed in) reflects the composition of the primordial mixture that produced all the planets. Yet, its composition is not exactly like the primordial mixture, leaving scientists uncertain about how exactly Jupiter, and by extension, the solar system, formed. Better understanding of Jupiter's traces of methane, water, and ammonia will help scientists piece together exactly how a collection of gas and dust came to form the planets we see today.
Saturn's composition and atmosphere are similar to Jupiter's, but it is much less dense. This giant would float if there were an ocean large enough to hold it.
Because Uranus and Neptune are so much farther from the Sun, the cold temperatures lend them unique personalities among the giants. Both are huge -- four times as wide as Earth -- but both did not grow large enough to accumulate the massive amounts of the embryonic solar nebula that Jupiter and Saturn did. They are made of a higher percentage of water, ammonia and methane than their larger siblings. The methane absorbs the reds and oranges from sunlight, reflecting blue and green light back to our eyes to give them their unique blue shades. Deeper inside, hot and dense slushy layers may cover cores of rock and solid "gases," like ammonia.
Atmospheres and Weather
Jupiter's clouds shroud a very turbulent place. The immense pressure of the planet's bulk crushed the interior as it formed (and possibly still does as Jupiter continues to contract) and the resulting heat is still leaking from the planet. Jupiter is far from the Sun, so this internal heat warms the planet and plays a major role in its weather. Jupiter radiates twice as much infrared energy as it receives from the Sun! Its core temperature may be about 24,000°C (43,000°F) -- that is hotter than the surface of the Sun! This heat leaks up through the liquid metallic hydrogen and liquid hydrogen layers to supply energy to the atmosphere. Like a pot of soup on a hot stove, atmospheric gases boil up from the warm bottom layers to the cooler upper layers; temperatures are -163°C (-261°F) at the top of the atmosphere. Juno's microwave radiometer will map the atmosphere's temperature at different depths.
While it orbits the Sun only once every 12 years, Jupiter spins on its axis once every 10 hours. The rapidly spinning planet generates five jet streams in each hemisphere that produce Jupiter's unique banded appearance. Earth has only about four dynamic jet streams, two -- sometimes three -- in each hemisphere, which all travel from west to east. Wind speeds are high on Jupiter, up to 330 miles per hour (530 kilometers) per hour, and alternate direction from eastward to westward with latitude. Lightning is frequent and is produced as ice particles within storms rub past each other. The Great Red Spot is a massive storm system larger than the diameter of Earth that has been raging for at least several hundred years.
Like Jupiter, Saturn is a large, tumultuous ball of hydrogen and helium shrouded by a magnetic field, but this second-largest planet is adorned with beautiful rings. Its winds are whipped to a ferocious 1100 miles an hour (1770 kilometers per hour)! Haze high in its atmosphere masks its bands of jet streams that are similar to Jupiter's.
A giant impact may have knocked infant Uranus off-kilter and contributed to the planet's unique personality. Unlike the other planets, Uranus' axis is tilted so that the planet rotates on its side once every 17 hours. Given Uranus' long period of orbit (84 years), this translates during parts of the year into a 20-year day or night! As the parts of the northern hemisphere move out of the long winter night, astronomers are beginning to see action in its dull-looking atmosphere as these super-cold regions begin to warm. Uranus's upper atmosphere is particularly hazy and shrouds its deeper layers, but recent observations have seen long-lived storms mar the bland visage.
The Sun is so far away that only a trickle of warmth reaches Neptune. Like Uranus, Neptune has methane in its atmosphere, which creates a blue hue and may serve as a sort of blanket for this ultra-frigid planet. Heat left over from its formation may be trapped by the methane and help drive its extreme weather. Its winds have been clocked at 1250 miles (2000 kilometers) per hour -- even faster than Saturn's -- and massive storm systems move within its atmosphere. The active atmosphere made it difficult for scientists to determine how fast the planet was rotating, but the Voyager spacecraft used bursts of radio emissions generated by the magnetic field to clock the planet's 16-hour day. One of Neptune's 13 moons, Triton, is in deep freeze, with a temperature of -200°C (-391°F).
Magnetosphere and Interior
Like Earth, the gas giants have a magnetic field. Earth's magnetic field is familiar to us through its effects: our compasses point to the magnetic poles; it protects our atmosphere from the blast of the solar wind; and particles interact with it to produce the auroras, or the northern and southern lights.
These magnetic fields originate from processes deep in each planet's interior. Earth's is generated from the electric current caused by the flow of molten metallic material within its outer core. The gases inside Jupiter and Saturn are crushed to such incredible pressures that they are forced beyond the common states of liquid, solid, or gas that we find on Earth. One such layer inside Jupiter and Saturn is metallic hydrogen, and the electric current caused by swirling movements in this substance produces magnetic fields. Jupiter's magnetosphere is so large that its tail end ("magnetotail") extends past the orbit of Saturn.
Liquid metallic hydrogen probably makes up most of Jupiter. There may be a dense core at the planet's center, and it may be slightly larger than the whole of Earth. The thick atmosphere merges seamlessly with a liquid hydrogen layer; there is no solid surface on Jupiter, or any of the gas giants.
Uranus's and Neptune's magnetic fields arise not from layers of liquid metallic hydrogen, but from currents flowing through the salty slush deep within the planets.