Atmosphere

A planet has a layer of air around it.

This layer is called an atmosphere. A planet's pull holds the air in place. This pull is called gravity. Some planets have very thick air. Jupiter has a huge amount of air. Other planets like Mars have very thin air. This happens because of heat and gravity. The air can even change over time. It is amazing to think about the air above us!

An atmosphere is a layer of gases around a planet. Gravity holds these gases in place.

Planets get their air in a few ways. They can pull in dust and gas. They can also let gases out from inside. This is called outgassing.

Different planets have different kinds of air. Large planets like Jupiter have huge atmospheres. They are made mostly of hydrogen and helium. These planets have strong gravity and are cold. This helps them keep their gas. Small rocky planets have different air. Earth has mostly nitrogen and oxygen. Venus has a very thick atmosphere. It is 80 times thicker than Earth's. Mars has a very thin atmosphere. It is only 0.6% as thick as Earth's.

Space can also take air away. The solar wind can strip it off. A magnetosphere, or a magnetic shield, helps stop this. This shield protects Earth's air. We can even find air on planets far away. Scientists study these using transit spectroscopy. This is a way to look at light. It helps us see what gases are there.

An atmosphere is a layer of gases that wraps around an astronomical object. Gravity is what holds these gases in place so they do not float away.

A planet gets most of its atmosphere during its very early years. This can happen by pulling in matter or through outgassing. Outgassing is when gases are released from inside the planet. The way an atmosphere works depends on its balance. This balance is called hydrostatic equilibrium. It is a tug-of-war between air pressure and gravity. Pressure comes from moving gas molecules. Gravity pulls those molecules toward the surface.

Planets form in a spinning disk of gas and dust around a star. Small pieces of dust collide and grow into larger objects called planetesimals. These grow into protoplanets near the center of the disk. Further out, planetary embryos form from gathering volatiles. These embryos can grow to ten times the mass of Earth. They then pull in huge amounts of gas from the disk to become gas giants. The first atmosphere forms if gravity is strong enough to keep the gas. Later, heat from early collisions can cause outgassing to create a second atmosphere.

Different planets have very different kinds of air. The gas giants like Jupiter and Saturn have huge atmospheres. They are mostly made of hydrogen and helium. These planets have high gravity and low temperatures. This helps them keep their light gases. On the other hand, rocky planets like Earth have denser air. Earth's air is 78.08% nitrogen and 20.95% oxygen. Venus has a very thick atmosphere made mostly of carbon dioxide. Its pressure is 80 times higher than Earth's. Mars has a very thin atmosphere. Its pressure is only 0.6% of Earth's air pressure.

Space can be a hard place for an atmosphere to stay. The solar wind works to strip away a planet's outer gases. This happens more easily if a planet is close to the Sun. A magnetosphere can help slow this process down. Earth has a magnetosphere that acts like a shield. It fends off incoming plasma at a distance of 10 Earth radii. Other moons also have air. Saturn's moon Titan has an atmosphere made mainly of nitrogen. The dwarf planet Pluto has nitrogen and methane too. These gases freeze when Pluto is far from the Sun.

Scientists can even find air on planets orbiting other stars. These are called exoplanets. One way to study them is through transit spectroscopy. This method looks at light as a planet passes in front of its star. In 2002, scientists used this to find sodium on a planet named HD 209458b. This planet is a gas giant with a very hot atmosphere. Other distant planets have shown signs of hydrogen, oxygen, or carbon. Some planets are so close to their stars they have magma oceans. Their atmospheres might be made of vaporized rock like silicon oxide.

An atmosphere is a layer of gases that surrounds an astronomical object. This layer is held in place by the object's gravity.

Without gravity, the gases would simply drift away into space. The atmosphere is vital because it can change the surface of a planet through chemical or photochemical interactions. For example, the Sun's light can interact with the gases to change their makeup. A planet's ability to keep its atmosphere depends on two main factors. High gravity and low temperatures help a planet retain its gases for a long time.

Most atmospheres begin forming during a planet's earliest stages. This process often starts within a rotating disk of gas and dust around a star. In this disk, dust particles collide and stick together to form planetesimals. Near the star, these bodies grow into protoplanets made of rocky, refractory materials. Further out, planetary embryos form by accumulating volatiles, which are substances like water or gases. These embryos can grow to ten times the mass of Earth. Eventually, they pull in massive amounts of gas from the surrounding disk to become gas giants.

There are two main types of atmospheres: primary and secondary. A primary atmosphere is created when a planet's gravity is strong enough to hold onto gases during its formation. For terrestrial planets, a secondary atmosphere often forms later. This happens through outgassing, where volatiles are released from inside the planet due to intense heat. This heat can come from the heavy bombardment of early collisions. The final composition depends on the chemistry of the original stellar nebula and the temperature of the system.

Atmospheres exist in a state called hydrostatic equilibrium. This is a balance between two opposing forces. First, there is the air pressure created by the constant motion of gas molecules. Second, there is the restraining force of gravity that pulls those molecules down. Because there is less gas mass above you at higher altitudes, the pressure decreases as you go up. This creates a pressure-gradient force. The temperature of an atmosphere is also managed by an energy budget. This budget balances the energy received from the Sun against the heat the planet radiates back into space.

Different planets in our Solar System have very different atmospheric compositions. The gas giants, such as Jupiter and Saturn, have massive atmospheres made mostly of hydrogen and helium. Their high gravity and low temperatures allow them to keep these light gases. In contrast, terrestrial planets like Earth have denser atmospheres. Earth's dry air is 78.08% nitrogen, 20.95% oxygen, and 0.93% argon.

Venus has a very thick atmosphere that is 96.5% carbon dioxide. This creates a pressure 80 times higher than Earth's. Mars has a very thin atmosphere with a surface pressure only 0.6% of Earth's. It has lost about 80% to 85% of its original water to space.

Space environments can be harsh on these gas layers. The solar wind constantly works to strip away a planet's outer atmosphere. This stripping happens more quickly if a planet is close to the Sun. However, a magnetosphere can act as a shield to slow this process. Earth's magnetosphere fends off incoming plasma at a distance of about 10 Earth radii. Some moons also have significant atmospheres. Saturn's moon Titan has an atmosphere made mainly of nitrogen. The dwarf planet Pluto has nitrogen and methane, though these gases freeze when it moves farther from the Sun.

Astronomers can even study atmospheres on exoplanets, which are planets orbiting other stars. One method used is transit spectroscopy. This involves watching a planet pass in front of its host star. By comparing the light at different wavelengths, scientists can detect specific gases. In 2002, researchers detected sodium in the atmosphere of HD 209458b. This is a hot gas giant with temperatures over 1,000 K. Other planets have shown signs of hydrogen, oxygen, or carbon. Some "lava planets" may have atmospheres made of vaporized rock, including silicon oxide, sodium, and potassium.