In elementary school science classes, most of us learned that we had eight (or nine, depending on how old you are) planets. Those planets were divided into the rocky planets and the gas giants. In later years, dwarf planets were added to the mix. However, planets are more than just hunks of rock and gas hurtling through space.

The inner planets

The four planets closest to the sun, Mercury, Venus, Earth, and Mars, are classified as rocky planets. They have solid surfaces and molten centers. Mercury, which is a third the Earth’s distance from the sun, is a planet of extremes. The sunward side roasts at 800 degrees Fahrenheit while the shadowed side rests at a frigid -290 degrees.

Despite these incredible differences in temperature, probes have found the building blocks for life in the polar caps on this tiny planet. Solar radiation makes it impossible for Mercury to hold onto an atmosphere, which means the chances of life developing are highly unlikely. Venus is an oddball in a few respects. First, it, like Mercury, has no moons.

For Mercury, it isn’t much of a surprise given how close it is to the Sun and how small of a planet it is. The size and distance from the sun make the lack of natural satellites a more unusual case for Venus. Stranger than the lack of moons, the planet’s slow rotation makes it almost a perfect sphere. Astronomers believe Venus collided with another large body during its formative years, slowing its rotation and knocking it on its head, explaining why it rotates in retrograde.

Finally, if we ever managed to send a probe to the planet’s surface, it would have to survive the sulphuric acid in the atmosphere and the surface temperatures, which are hot enough to melt lead. On the near side of the asteroid belt sits Mars. Interesting facts about the Red Planet could fill several articles. Most notably, it’s the planet most suited to future human colonies and, at one point, it likely looked a lot like Earth.

The gas giants

Beyond Mars sits the asteroid belt, and beyond that lies Jupiter, the largest gas giant. For a while, scientists speculated the Jupiter might have been a failed brown dwarf, but as they studied more brown dwarfs, they ruled out that possibility. Jupiter’s size does make it an impressive force in the solar system. It has a habit of grabbing passing comets and sucking them in, along with wayward asteroids.

Saturn, the second-largest planet in the solar system, is unusual in its own right. Despite its size, Saturn would float if you found a bathtub big enough. Like all gas giants, Saturn lacks a solid surface, and its atmosphere gradually gives way to a liquid center as the pressure increases. Saturn’s north pole features an unusual cloud formation that takes the shape of a hexagon. Scientists aren’t sure why the clouds have taken on an angular shape, but most of the theories have to do with sudden changes in pressure around the northern pole.

Out past Saturn lie Uranus and Neptune. Like their larger siblings, these planets have no solid surface, transitioning from a thick gaseous atmosphere to a liquid mantle before finally reaching their solid rocky cores. Despite their differences in size and composition, Uranus and Venus have something in common. Venus rotates on its head and Uranus rotates on its side, revolving around the Sun with its northern pole pointed almost toward it.

Unlike Venus, the hottest planet in the solar system, Uranus is the coldest, with no internal heat source. Finally, although Uranus is four times the size of Earth, its gravitational pull is 90% that of Earth. Its neighbor, Neptune, is a much wilder member of the solar system family. The winds in Neptune’s atmosphere reach supersonic speeds. Neptune and Uranus both have tiny ring systems. Though Uranus’ is more substantial than Neptune’s, neither holds a candle to Saturn’s.

The little guys

Scattered among the “true” planets in the solar system are numerous dwarf planets that didn’t quite make the cut. These smaller planets often met their categorical fate due to their size or their shape, but for some, like Pluto, the title is more personal. Initially considered a planet in full, Pluto was demoted to the status of dwarf planet in 2006. Pluto lost its planetary status in part because of its inability to clear its orbital path, which overlaps with Neptune’s. Its orbit is also at a 17-degree incline from the rest of the planets and is far more elliptical than any other planet’s.

Despite being the farthest former-planet from the sun, Pluto is still warmer than Uranus by about 10 degrees. It is the largest dwarf planet in our solar system, though it is smaller than most moons. Despite its size and its frigid surface temperature, when Pluto’s orbit takes it closer to the Sun, some of its surface ice vaporizes and forms a very thin atmosphere.

Far out beyond Pluto lies the second largest dwarf planet, Eris. When it was discovered in 2005, astronomers believed it to be larger than Pluto and considered making it the tenth planet in the solar system. Eris’s apparent brightness and the interference of its moon, Dysnomia, led astronomers to believe it to be larger than it truly was. Sometimes, Eris is closer to the sun than Pluto.

Much closer to home, however, is the dwarf planet Ceres. It is the most massive object in the asteroid belt and was the first object in its domain to be observed. Since its discovery in 1803, Ceres has captivated astronomers with the possibility of a subsurface ocean. Geysers of water vapor have been seen on the dwarf planet’s surface. These geysers are thought to contribute to a very thin atmosphere around the spherical planetoid, an exciting prospect to astronomers.