Surface gravity

The surface gravity, shown as g, tells us how strong the pull of gravity is on the surface of a planet, moon, or star. It is the speed at which things would fall if they were dropped close to the surface. This pull depends on how big the object is and how fast it spins.

We measure surface gravity in meters per second squared. On Earth, this value is about 9.8 m/s², which is why things fall the way they do here. Scientists sometimes compare other objects to Earth by saying how many times stronger or weaker their gravity is.

Some objects in space have very strong gravity. For example, a white dwarf—a small, dense star left after a normal star burns out—has a surface gravity much stronger than Earth’s. Neutron stars, which are even smaller and denser, have gravity so strong that anything near them would be pulled in very quickly. Their surface gravity can be trillions of times stronger than Earth’s. Because of this extreme gravity, the speed needed to escape a neutron star is close to the speed of light itself.

Relationship of surface gravity to mass and radius

The pull of gravity on a planet or star depends on two things: its mass and its size. A bigger mass means stronger gravity. But if the object is also larger, the gravity might not feel as strong. This is because gravity gets weaker with distance.

For example, think of a planet that has five times the mass of Earth but is also bigger. If it is only a little bigger, its surface gravity would feel stronger than Earth’s. But if it is much bigger, the gravity might feel only a little stronger. Scientists use special math to find out exactly how strong the gravity is on the surface of these faraway worlds.

Gas giants

For big planets made mostly of gas, like Jupiter, Saturn, Uranus, and Neptune, we talk about their surface gravity at a special level in their atmosphere. This level has a pressure of 1 bar. These planets can be up to 100 times the mass of Earth, but their surface gravity is close to 1 g. This is the same as Earth’s gravity. We call this area the gravity plateau.

Non-spherically-symmetric objects

Most real space objects, like stars and planets, are not perfect balls. This is often because they spin, which mixes together the pull of gravity and the push of spinning. Because of this, these objects are usually a bit squashed, meaning their surface gravity is weaker at the middle, or equator, than at the ends, or poles. This idea was used in a science fiction book called Mission of Gravity by Hal Clement.

Scientists can learn about what’s inside an object by measuring its surface gravity. This method was first used in 1915–1916 by Roland Eötvös, who used a special tool called a torsion balance to search for oil near the city of Egbell, now called Gbely in Slovakia. Later, in 1924, the same tool helped find oil fields in Texas.

Sometimes, it’s helpful to imagine simple shapes that don’t exist in real life, like endless flat surfaces or hollow shells, to better understand how real objects behave.

Black holes

Thinking about gravity near a black hole is tricky because black holes are very different from regular objects. A black hole does not have a surface like Earth does, so we cannot measure gravity the same way. Instead, scientists look at a special area called the event horizon.

For a type of black hole called the Schwarzschild black hole, there is a way to describe what gravity would act like if we could measure it. This idea helps scientists study black holes even though the actual gravity at the event horizon would seem infinitely strong.

Different kinds of black holes, like those that spin or have electric charge, have their own special ways to describe this “surface gravity.” Scientists are still working on the best way to describe gravity for black holes that are changing or moving.