What if Earth were much bigger?

Earth is a big ball of rock. Its gravity holds us on the ground.

What if Earth were twice as big? How would that change things?

Earth's gravity keeps your feet on the ground, holds the air around the planet and sets how tall its mountains can grow. Gravity comes from mass, and a bigger planet has more of it.

NASA reports that planets bigger than Earth but smaller than Neptune, called super-Earths, are common among the planets found around other stars. So what if Earth itself were much bigger? This is a thought experiment: explore a stronger pull, thicker air and lower mountains, and notice what is calculated and what is an idea.

NASA describes super-Earths as planets more massive than Earth but lighter than Neptune and Uranus, between twice Earth's size and up to ten times its mass, common in our galaxy and unlike anything in our solar system.

This thought experiment scales Earth up to twice its radius while keeping its average density, then follows three consequences: surface gravity and escape velocity (straightforward calculations), the atmosphere a planet can hold, and the height mountains can reach (both reasoned ideas, not predictions).

On Earth, gravity pulls us down just right. We can walk, run and jump.

Earth's highest mountain is Mount Everest. It is almost 9 kilometers high!

NASA's Earth fact sheet gives Earth an average radius of 6,371 kilometers and a surface gravity of about 9.8 meters per second squared. Something must move at about 11.2 kilometers per second to escape Earth's gravity completely.

Earth's highest peak, Mount Everest, rises 8,848 meters. On Mars, where gravity is much weaker, the giant volcano Olympus Mons stands more than 40 kilometers tall from base to summit, NASA says.

NASA gives Earth a mass of 5.97 × 10²⁴ kg, a mean radius of 6,371 km, a mean surface gravity of 9.82 m/s² and an escape velocity of 11.186 km/s. OpenStax derives g = GM/R² from Newton's law of gravitation, and escape velocity from setting total energy to zero, v_esc = √(2GM/R).

For comparison, Mars has 0.38 of Earth's surface gravity, and NASA notes that its Olympus Mons, the largest volcano in the solar system, rises more than 40 km from base to summit, while Everest rises 8,848 m.

Imagine Earth twice as wide, made of the same rock. It would pull twice as hard!

You would feel twice as heavy. Jumping would be much harder.

Imagine Earth twice as wide, made of the same rock. It would hold eight times as much rock, because a ball twice as wide holds eight times as much. But you would also stand twice as far from its center. Putting those together, a simple calculation shows its pull at the surface would be twice as strong: about 19.6 meters per second squared.

You would weigh twice as much, and every step and jump would be harder. The speed needed to escape would double too, to about 22.4 kilometers per second. That might help the planet hold on to more air.

With density fixed, M = (4/3)πρR³, so g = GM/R² = (4/3)πGρR grows in proportion to R. Doubling R gives eight times the mass, twice the surface gravity (≈ 19.6 m/s²) and, since v_esc = √(2GM/R) ∝ R, twice the escape velocity (≈ 22.4 km/s).

This is a deliberate simplification: the calculation keeps the density fixed, which a real planet need not do, and NASA notes that at three to ten Earth masses, planets might be water worlds, snowball planets or gas-rich mini-Neptunes. The calculation shows the trend, not a particular planet.

Gravity comes from all the stuff in a planet. More stuff means more pull.

A bigger planet has lots more stuff. So it pulls harder.

Every bit of a planet pulls on you, so more mass means more pull. But the pull also gets weaker the farther you are from the planet's center. On a bigger planet you stand farther out, which weakens the pull a little.

For a planet made of the same rock, the extra mass wins. Twice as wide means eight times the mass but only twice the distance, and gravity weakens with the square of distance, four times. Eight divided by four gives twice the pull.

Newton's law gives g = GM/R², so surface gravity depends on the competition between mass and radius. For constant density M ∝ R³, giving g ∝ R³/R² = R. Escape velocity, √(2GM/R), likewise scales as √(R³/R) = R.

The University of Alaska Fairbanks Geophysical Institute links escape velocity to atmospheres: the more massive the body, the greater its escape velocity, which is why giant planets keep dense atmospheres while small ones like the Moon cannot hold gas. Indiana University's A Moment of Science links gravity to mountains: the weight of a mountain presses on its base, and gravity sets an upper limit on how high it can grow.

Scientists have found real planets bigger than Earth around other stars. They are called super-Earths.

Some might be rocky. Some might be covered in water.

NASA says super-Earths are common among the planets found so far, but there are none in our solar system, so scientists still know very little about what they are like. Some might be rocky. Others might be covered in deep oceans, or wrapped in thick gas like a small Neptune.

NASA lists some examples: TOI 270 b, found by the TESS telescope in 2019, is likely a rocky super-Earth about 25 percent larger than Earth. Scientists are still working out what life, if any, could be like on such worlds.

NASA notes that super-Earths are common among known exoplanets but that their true nature remains uncertain: it is not yet known at what mass they might lose a rocky surface, and between three and ten Earth masses they might be water worlds, snowball planets or Neptune-like worlds of dense gas.

TESS's TOI 270 b, found in 2019 around a cool dwarf star about 73 light-years away, is likely a rocky super-Earth about 25% larger than Earth with an estimated mass about 1.9 times Earth's, based on statistical studies of similar-sized planets. How surface conditions there compare with Earth's remains an open question.