Planetary mass

Planetary mass is a measure of the amount of matter within a planet-like astronomical object. Understanding mass is vital because it dictates how an object behaves in space. It influences an object's shape, its internal temperature, and its ability to hold onto an atmosphere. In our Solar System, astronomers often use the solar mass as a primary unit. The solar mass is the mass of the Sun. Because the Sun is so large, planets are tiny by comparison. Jupiter is only 0.09% of the solar mass. Earth is even smaller, at approximately 0.0003% of the Sun's mass.

Scientists cannot weigh a planet on a scale like we weigh objects on Earth. Instead, they must infer mass by observing how gravity works. One common method involves looking at natural satellites, or moons. By using Newton's law of universal gravitation, scientists can calculate the mass of a planet and its moons together. This method relies on a generalization of Kepler's third law. This mathematical rule describes how objects orbit one another. This technique allowed for an early measurement of Jupiter's mass in units of solar mass.

Another way to find mass is to observe the gravitational influence on other bodies. A planet's gravity pulls on its neighbors, slightly changing their orbital paths. Space probes also provide excellent data for these calculations. For example, the Voyager probes studied the outer planets. The MESSENGER spacecraft provided data regarding Mercury. Sometimes, scientists use the rotation of an object to find a limit on its mass. The dwarf planet Haumea rotates very quickly on its axis. To avoid being ripped apart by centrifugal forces, Haumea must have a very high density. If the size is known, this density limit helps determine the mass.

Mass also determines the physical structure of a celestial body. When an object forms, it undergoes a process of accretion, where it grows by gathering matter. If a body has enough mass, its own gravity overcomes its compressive strength. This allows the object to achieve a rounded shape, a state called hydrostatic equilibrium. Objects that are not massive enough often remain irregular. For instance, Vesta is the second largest body in the asteroid belt. As seen in images from the Dawn spacecraft, Vesta is not a perfect sphere.

There are specific rules for how we classify these objects based on mass. Since 2006, an object is called a dwarf planet if it orbits the Sun but has not cleared its neighboring region of other objects. This process of "clearing the neighborhood" requires significant mass. The amount of mass needed depends on where the object is located. Mars is massive enough to clear its orbit in its current position. However, it would not be able to do so if it orbited in the distant Oort cloud. Smaller objects, like asteroids, are simply called small Solar System bodies.

Mass also plays a role in what a planet is made of. Small, terrestrial planets like Earth or Mars retain mostly silicates and metals. If a protoplanet grows to more than twice the mass of Earth, its gravity becomes much stronger. This strong gravity allows it to retain hydrogen in its atmosphere. Such objects typically grow into ice giants or gas giants. Earth and Venus are near the maximum size a planet can reach while remaining rocky. If a planet moves closer to its star, the star's radiation might strip its atmosphere away.

Calculating these masses requires very precise mathematical models called ephemerides. The DE405 ephemeris from the Jet Propulsion Laboratory is a widely used model from 1998. It provides a self-consistent set of masses for the whole Solar System. Measuring these values is difficult because it requires knowing the Newtonian constant of gravitation, or G. This constant is very hard to measure with high precision. Because of this, astronomers often use different standard units for comparison. They use the mass of Jupiter for gas giants and the mass of Earth for rocky planets. This makes it easier to compare the different worlds in our universe.