Our solar system is home to eight different planets classified into three different types: rocky planets, gas giants and ice giants. The four inner planets, Mercury, Venus, Earth and Mars, are all rocky worlds. Jupiter and Saturn are both gas giants, while the outermost planets, Uranus and Neptune, are ice giants. Whether a planet is a rocky world, a gas giant or an ice giant depends on its composition. What is each planet made of?
Mercury is the innermost planet in the solar system, orbiting the sun at an average distance of 58 million kilometers. Mercury is also the smallest planet in the solar system with a diameter of 3,031 miles (4,878 kilometers). Despite its small size, Mercury is actually the second densest planet in the solar system after Earth, and this is due to the composition of its core. Mercury has a core of iron, a fairly dense material. Interestingly though, relative to Mercury’s size, it has the largest iron core of any inner planet, and it makes up over 40% of the volume inside Mercury. Meanwhile, Mercury’s surface is mostly composed of silicate rock. Scientists estimate that 70% of Mercury is metal and the remaining 30% is rock.
Venus is the closest planet to Earth and the most similar to our world in terms of size and mass. Since Earth and Venus are located so close to each other, they have similar compositions. Venus is believed to have an iron core surrounded by a mantle and crust composed of rock and metal. Unlike Earth, however, Venus’ surface is dominated by volcanic rock. Moreover, the atmosphere of Venus is very different from our world. Venus’ atmosphere is 96% carbon dioxide, causing the planet to experience a runaway greenhouse effect that creates the highest surface temperatures in the solar system.
Like Mercury and Venus, Earth has a core composed mostly of iron, along with small amounts of nickel and other metals. The surface of our world is dominated by silicate rock. Thus, the Earth has many similarities to the other rocky planets in terms of composition, but one very unique aspect of our world is its atmosphere. Unlike all other planets in the solar system, Earth’s atmosphere contains large amounts of oxygen. Oxygen makes up 21% of the atmosphere, while nitrogen makes up 78%. The remaining 1% is made up of various other elements and compounds such as argon, carbon dioxide, and methane.
Mars is the most distant of the rocky planets, and it is perhaps the most studied planet in the solar system after Earth. Like the other three rocky planets, Mars has an iron core ringed by a surface of silicate rock, but there are unique features of Mars, such as frozen carbon dioxide and monoxide at its poles. Mars’ atmosphere is about 90% carbon dioxide, but the atmosphere itself is too thin to induce any significant greenhouse effect, so the surface remains extremely cold.
Jupiter is the largest and most massive planet in the solar system. Despite its large size, its composition is rather simple. Jupiter is primarily made up of just two elements: hydrogen and helium, along with traces of other chemicals such as water, sulfur, methane, and ammonia. About 90% of Jupiter is hydrogen, while the remaining 10% is almost entirely helium. However, although other chemicals only exist in trace amounts, they play an important role. For example, sulfur and organic compounds are thought to cause the various shades of red in Jupiter’s atmosphere. Although hydrogen makes up most of Jupiter’s composition, not everything comes in the same form. Deep in Jupiter’s upper cloud layers, pressures and temperatures become so high that hydrogen becomes a metallic liquid. This layer of liquid metallic hydrogen is able to conduct electricity, which in turn generates Jupiter’s magnetic field, which is the largest of all the planets.
Saturn is very similar to Jupiter in terms of composition. Like Jupiter, Saturn is mostly made up of hydrogen and helium, along with traces of other chemicals. Despite its almost identical composition, however, Saturn is very different from Jupiter, almost as if it were a watered down version of Jupiter. This is probably due to the fact that Saturn is located much further from the sun. It is also probably due to the presence of a layer of ammonia ice crystals in the upper atmosphere of the planet.
Uranus and Neptune
Like Jupiter and Saturn, Uranus and Neptune are mostly made up of hydrogen and helium. However, the two outermost planets are somewhat different in composition. Hydrogen and helium are not as abundant on Uranus and Neptune, and there is a greater presence of other chemicals. Interestingly, the densities of Uranus and Neptune are primarily determined by the existence of various ices in their atmospheres, and so rather than being classified as gas giants, they are classified as ice giants. Both Uranus and Neptune contain higher amounts of methane, ammonia, and water. In fact, the presence of methane is believed to cause Uranus and Neptune to turn blue.