The Inner Planets of Our Solar System

Our Solar System is an immense and amazing place. Between its eight planets, 176 moons, 5 dwarf planets (possibly hundreds more), 659,212 known asteroids, and 3,296 known comets, it has wonders to sate the most demanding of curiosities. Our Solar System is made up of different regions, which are delineated based on their distance from the Sun, but also the types of planets and bodies that can be found within them.

In the inner Solar System, we find the “Inner Planets” – Mercury, Venus, Earth, and Mars – which are so named because they orbit closest to the Sun. In addition to their proximity, these planets have a number of key differences that set them apart from planets elsewhere in the Solar System.

For starters, the inner planets are rocky and terrestrial, composed mostly of silicates and metals, whereas the outer planets are gas giants. The inner planets are also much more closely spaced than their outer Solar System counterparts. In fact, the radius of the entire region is less than the distance between the orbits of Jupiter and Saturn.

The positions and names of planets and dwarf planets in the solar system. Credit: Planets2008/Wikimedia Commons
The positions and names of planets and dwarf planets in the solar system.
Credit: Planets2008/Wikimedia Commons

This region is also within the “frost line,” which is a little less than 5 AU (about 700 million km) from the Sun. This line represents the boundary in a system where conditions are warm enough that hydrogen compounds such as water, ammonia, and methane are able to take liquid form. Beyond the frost line, these compounds condense into ice grains.Some scientists refer to the frost line as the “Goldilocks Zone” — where conditions for life may be “just right.”

Generally, inner planets are smaller and denser than their counterparts, and have few to no moons or rings circling them. The outer planets, meanwhile, often have dozens of satellites and rings composed of particles of ice and rock.

The terrestrial inner planets are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather. All of them have impact craters and tectonic surface features as well, such as rift valleys and volcanoes.

Mercury:

Of the inner planets, Mercury is the closest to our Sun and the smallest of the terrestrial planets. This small planet looks very much like the Earth’s Moon and is even a similar grayish color, and it even has many deep craters and is covered by a thin layer of tiny particle silicates.

Its magnetic field is only about 1 percent that of Earth’s, and it’s very thin atmosphere means that it is hot during the day (up to 430°C) and freezing at night (as low as -187 °C) because the atmosphere can neither keep heat in or out. It has no moons of its own and is comprised mostly of iron and nickel. Mercury is one of the densest planets in the Solar System.

The inner planets to scale. From left to right: Earth, Mars, Venus, and Mercury. Credit: Wikimedia Commons/Lsmpascal
The inner planets to scale. From left to right: Earth, Mars, Venus, and Mercury. Credit: Wikimedia Commons/Lsmpascal

Venus:

Venus, which is about the same size as Earth, has a thick toxic atmosphere that traps heat, making it the hottest planet in the Solar System. This atmosphere is composed of 96% carbon dioxide, along with nitrogen and a few other gases. Dense clouds within Venus’ atmosphere are composed of sulphuric acid and other corrosive compounds, with very litter water.

Only two spacecraft have ever penetrated Venus’s thick atmosphere, but it’s not just man-made objects that have trouble getting through. There are fewer crater impacts on Venus than other planets because all but the largest meteors don’t make it through the thick air without disintegrating. Much of Venus’ surface is marked with volcanoes and deep canyons — the biggest of which is over 6400 km (4,000 mi) long.

Venus is often called the “morning star” because, with the exception of Earth’s moon, it’s the brightest object we see in the sky. Like Mercury, Venus has no moon of its own.

Earth:

Earth is the third inner planet and the one we know best. Of the four terrestrial planets, Earth is the largest, and the only one that currently has liquid water, which is necessary for life as we know it. Earth’s atmosphere protects the planet from dangerous radiation and helps keep valuable sunlight and warmth in, which is also essential for life to survive.

Inner Solar System. Image credit: NASA
Illustration of the Inner Planets and their orbits around the Sun Image credit: NASA

Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy metal core. Earth’s atmosphere contains water vapor, which helps to moderate daily temperatures. Like Mercury, the Earth has an internal magnetic field. And our Moon, the only one we have, is comprised of a mixture of various rocks and minerals.

Mars:

Mars is the fourth and final inner planet, and also known as the “Red Planet” due to the rust of iron-rich materials that form the planet’s surface. Mars also has some of the most interesting terrain features of any of the terrestrial planets. These include the largest mountain in the Solar System – Olympus Mons – which rises some 21,229 m (69,649 ft) above the surface, and a giant canyon called Valles Marineris. Valles Marineris is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi)!

For comparison, the Grand Canyon in Arizona is about 800 km (500 mi) long and 1.6 km (1 mi) deep. In fact, the extent of Valles Marineris is as long as the United States and it spans about 20 percent (1/5) of the entire distance around Mars. Much of the surface is very old and filled with craters, but there are geologically newer areas of the planet as well.

A top-down image of the orbits of Earth and Mars. Image: NASA
A top-down image of the orbits of Earth and Mars. Credit: NASA

At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.

Mars’ thin atmosphere has led some astronomers to believe that the surface water that once existed there might have actually taken liquid form, but has since evaporated into space. The planet has two small moons called Phobos and Deimos.

Beyond Mars are the four outer planets: Jupiter, Saturn, Uranus, and Neptune.

We have written many interesting articles about the inner planets here at Universe Today. Here’s The Solar System Guide as well as The Inner and Outer Planets in Our Solar System.

For more information, check out this article from NASA on the planets of the Solar System and this article from Solstation about the inner planets.

Astronomy Cast also has episodes on all of the inner planets including this one about Mercury.

The Inner and Outer Planets in Our Solar System

In our Solar System, astronomers often divide the planets into two groups — the inner planets and the outer planets. The inner planets are closer to the Sun and are smaller and rockier. The outer planets are further away, larger and made up mostly of gas.

The inner planets (in order of distance from the sun, closest to furthest) are Mercury, Venus, Earth and Mars. After an asteroid belt comes the outer planets, Jupiter, Saturn, Uranus and Neptune. The interesting thing is, in some other planetary systems discovered, the gas giants are actually quite close to the sun.

This makes predicting how our Solar System formed an interesting exercise for astronomers. Conventional wisdom is that the young Sun blew the gases into the outer fringes of the Solar System and that is why there are such large gas giants there. However, some extrasolar systems have “hot Jupiters” that orbit close to their Sun.

The Inner Planets:

The four inner planets are called terrestrial planets because their surfaces are solid (and, as the name implies, somewhat similar to Earth — although the term can be misleading because each of the four has vastly different environments). They’re made up mostly of heavy metals such as iron and nickel, and have either no moons or few moons. Below are brief descriptions of each of these planets based on this information from NASA.

Mercury: Mercury is the smallest planet in our Solar System and also the closest. It rotates slowly (59 Earth days) relative to the time it takes to rotate around the sun (88 days). The planet has no moons, but has a tenuous atmosphere (exosphere) containing oxygen, sodium, hydrogen, helium and potassium. The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is currently orbiting the planet.

The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute

Venus: Venus was once considered a twin planet to Earth, until astronomers discovered its surface is at a lead-melting temperature of 900 degrees Fahrenheit (480 degrees Celsius). The planet is also a slow rotator, with a 243-day long Venusian day and an orbit around the sun at 225 days. Its atmosphere is thick and contains carbon dioxide and nitrogen. The planet has no rings or moons and is currently being visited by the European Space Agency’s Venus Express spacecraft.

Earth: Earth is the only planet with life as we know it, but astronomers have found some nearly Earth-sized planets outside of our solar system in what could be habitable regions of their respective stars. It contains an atmosphere of nitrogen and oxygen, and has one moon and no rings. Many spacecraft circle our planet to provide telecommunications, weather information and other services.

Mars: Mars is a planet under intense study because it shows signs of liquid water flowing on its surface in the ancient past. Today, however, its atmosphere is a wispy mix of carbon dioxide, nitrogen and argon. It has two tiny moons (Phobos and Deimos) and no rings. A Mars day is slightly longer than 24 Earth hours and it takes the planet about 687 Earth days to circle the Sun. There’s a small fleet of orbiters  and rovers at Mars right now, including the large NASA Curiosity rover that landed in 2012.

The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute

The Outer Planets:

The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas. They all have rings and all of plenty of moons each. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers the solar system was bigger than previously thought. Below are brief descriptions of each of these planets based on this information from NASA.

Jupiter: Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot — a raging storm happening for the past 400 years at least (since we were able to view it through telescopes). NASA’s Juno spacecraft is en route and will visit there in 2016.

Saturn: Saturn is best known for its prominent ring system — seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years). Saturn is currently being visited by the Cassini spacecraft, which will fly closer to the planet’s rings in the coming years.

Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

Uranus: Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. There are no spacecraft slated to visit Uranus right now; the last visitor was Voyager 2 in 1986.

Neptune: Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. The only spacecraft to ever visit it was NASA’s Voyager 2 in 1989.

To learn more about the planets and missions, check out these links:

Solar System Exploration: Planets (NASA)
NASA Photojournal (NASA)
Missions (NASA)
Space Science (European Space Agency)
USGS Astrogeology (U.S. Geological Survey)
The Solar System And Its Planets (European Space Agency)