Earth's Place in the Solar System

The Search for Water

On Mercury, which is the closest planet to the sun, temperatures can reach 427 degrees Celsius (801 degrees Fahrenheit). On Neptune, the farthest planet from the sun, temperatures can reach -218 degrees Celsius (-360 degrees Fahrenheit). Earth has an average temperature of 14 degrees Celsius (57 degrees Fahrenheit).

Earth’s temperature allows for another difference from any of the other planets. Earth is unique because unlike any other known planet, water is found almost everywhere on Earth, in all three states. Water molecules make up the ice found in glaciers and snow, the liquid water in oceans, lakes, and rivers, and the water vapor gas in Earth’s atmosphere. Water is able to exist in all three states because of Earth’s temperature. Any closer, and the sun would cause temperatures to be too high. Any farther away, and the temperature would be too low.

Scientists have known that solid water (ice) is present on Mars and throughout the solar system. Scientists believe that Mars’ permanent northern ice cap is mostly frozen water, while the southern cap is made up of frozen water and carbon dioxide. However, Curiosity has been searching for liquid water on Mars because it is necessary for almost every known living thing. Without water, there would be no life on Earth.

In September 2015, scientists announced that they had found evidence that liquid water does indeed sometimes flow on Mars. “It took multiple spacecraft over several years to solve this mystery. Now we know there is liquid water on the surface of this cold, desert planet,” said NASA scientist Michael Meyer. “It seems that the more we study Mars, the more we learn how life could be supported and where there are resources to support life in the future.”

Patterns in Planets

The evidence of liquid water comes from dark streaks in gullies and craters on Mars. Scientists believe the streaks are caused by salty water, but they still aren’t sure where that salty water comes from. On Earth, the majority of the planet is covered by oceans filled with salt water.

The surface of Mars, in contrast, is mostly covered by red dust. This red dust gives the planet its nickname of the red planet. These dark streaks only appear in the summer. Mars experiences the same four seasons that Earth does: summer, fall, winter, and spring. This is because Mars has a very similar tilt to Earth, at 25.2 degrees on its axis. However, Mars is farther away from the sun. Because of this, it takes almost twice as long for Mars to complete one orbit around the sun—687 Earth days. As a result, seasons on Mars last longer than seasons on Earth.

All of the planets have seasons. As the planets orbit the sun, different amounts of sunlight reach different parts of the planet at various times throughout the orbit. Every planet is tilted on its axis, but each planet is tilted at a different angle. For example, Jupiter has almost no tilt, at 3 degrees. Because of this, Jupiter experiences very little change between its seasons. However, Jupiter is much farther from the sun than Earth is. As a result, the length of each season is about three Jupiter years, compared to three months on Earth.

In contrast to Jupiter, Venus has an almost 180-degree tilt, which means Venus is spinning upside down. Uranus has a 90-degree tilt, which means it is spinning on its side. And each planet has a different orbit because of its distance from the sun. For example, Mercury has the shortest orbit, taking just 88 Earth days to complete its turn around the sun. Neptune has the longest orbit because it is farthest from the sun. Its orbit takes 164.8 years (60,182 Earth days).

There are also patterns in the length of a planet’s day and night. Like Earth, all planets rotate on their axis. Earth and Mars have about the same length of one day because their rotation speed is very similar. It takes Mars 24 hours and 40 minutes to complete one rotation. This is one day on Mars, which is also called a sol. It is very close to Earth’s rotation of 23.5 hours.

In contrast, the gas giants rotate much faster than any of the terrestrial planets. It takes Jupiter just 10 hours to complete one rotation. Venus has the slowest rotation, taking 5,832 hours to rotate once.

Every planet in the solar system is held in its orbit by the force of the sun’s gravity. The distance a planet is from the sun determines how strong the sun’s gravity will pull on the planet. Planets that are closer to the sun experience a stronger pull of the sun’s gravity than planets that are farther away.

In addition to the planets, the sun is also orbited by non-planets. The asteroid belt between Mars and Jupiter is made up of rocks left over from planets that didn’t form. Comets are small objects made up of mostly ice and dust, and they are usually found farther from the sun. Planet-like masses, called dwarf planets, are also found throughout our solar system. Pluto, Ceres, and Charon are examples of dwarf planets.

Each of the planets has its own gravity that is determined by its mass. This is why some planets are orbited by moons. Earth is orbited by just one moon, while Mars is orbited by two moons. Jupiter has so much mass that it holds in orbit the most moons of any planet. Jupiter is so large that all of the other planets in the solar system could fit inside it. More than 1,000 Earths would fit inside Jupiter.

Planet Mass and Gravity

The gravitational force exerted on an object by a planet or moon is called weight. In science, mass and weight are not the same, even though here on Earth, the distinction between mass and weight is typically ignored by non- scientists. This is because acceleration due to gravity is nearly identical everywhere on Earth (9.8 m/s2) except with highly sensitive instruments.

On other planets, which have noticeably different gravities, an object’s weight would change dramatically. It would more than double on Jupiter, which has the highest gravity, at 25.9 m/s2. That same object’s weight would drop by 85 percent on the moon, which has a gravity of 1.6 m/s2. The object’s mass, on the other hand, would remain the same regardless of where it was.

Mars’ weaker gravity allows for a unique surface feature. Mars has the largest volcano in our solar system, called Olympus Mons. Olympus Mons is 24 kilometers high and measures 550 km across. By comparison, Earth’s largest volcano, Mauna Loa in Hawaii, rises 9 km high and measures 120 km across. Olympus Mons could not exist on Earth. Earth's stronger gravitational pull would cause the massive volcano to collapse under its own weight if it were here.