By Drew Lichtenstein | Updated Mar 24, 2022
The gravitational pull of a planet or star grows with its mass. This force, described by Isaac Newton’s Universal Law of Gravitation, determines whether nearby objects stay in orbit or drift away. Newton’s equation is expressed as:
F = G \(\dfrac{M_1 M_2}{r^2}\)
where F is the gravitational force, G is the gravitational constant (6.674×10-11 N·m²/kg²), M1 and M2 are the masses of the two bodies, and r is the distance between their centers. The equation shows that larger masses and closer distances both strengthen gravity.
In our Solar System, the Sun’s immense mass—about 1.989×1030 kg—holds the eight planets, dwarf planets, comets, and asteroids in orbit. Planets themselves keep their moons bound; a more massive planet can support moons that are farther away. For example, Saturn, one of the gas giants, hosts 83 confirmed moons, the largest being Titan.
Newton’s three laws of motion provide additional insight. The first law (inertia) explains why a planet or moon continues in uniform motion unless acted upon. The third law (action‑reaction) accounts for phenomena such as Earth’s tides, which arise from the moon’s gravitational pull on our oceans.
While Newton described how gravity behaves, Einstein’s General Theory of Relativity, published in 1915, explained why. Einstein showed that mass curves spacetime, and objects move along the resulting curvature. This model unifies gravity with the behavior of light and other massless particles, which also follow curved paths around massive bodies.
Understanding the mass‑gravity relationship is essential for astronomy, spacecraft navigation, and predicting celestial motions.
