Why are terrestrial planets smaller

As the mass of a planet increases, so does its gravitational pull on the dust and gas molecules through which it passes. During this time of planet formation, the light pressure and solar wind from the star will tend to strip any atmosphere from the forming planets and asteroids, unless they are massive enough to retain it by gravity.

In particular lighter elements like hydrogen and helium will only be retained in significant proportion in planets over a certain mass. Once planets have such a mass they will grow considerably, collecting gases from the surrounding disk, while their smaller cousins will tend to stay smaller. Points L1, L2 and L3 are unstable, but points L4 and L5 - which form an equilateral triangle with the star and the planet are stable.

Why are the Jovian planets so much larger than the terrestrial planets? Venus and Mercury have no moons, while Earth has only the one the Moon. Mars has two satellites, Phobos and Deimos , but these are more akin to large asteroids than actual moons.

Unlike the gas giants, terrestrial planets also have no planetary ring systems. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core. Io and Europa are also satellites that have internal structures similar to that of terrestrial planets.

Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water. Dwarf planets, like Ceres and Pluto , and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.

Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since , hundreds of potentially terrestrial extrasolar planets have been found — mainly by the Kepler space mission. Examples of extrasolar terrestrial planets include Gliese d , a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese , which is located approximately 15 light years from Earth.

This disc, called the solar nebula , was composed mainly of hydrogen and helium, but also had other elements in smaller proportions. The nebula had a certain amount of angular momentum orbiting the forming Sun. Particles in the spinning disc began to clump together as gravity attracted them to each other.

Over a few million years many of these chunks had merged together and there were about 10 9 objects called planetesimals , with diameters of about m. Over time the planetesimals continued to collide and join together, attracted by gravity. Uranus and Neptune were less efficient at attracting hydrogen and helium gas, so they have much smaller atmospheres in proportion to their cores.

Chemically, each giant planet is dominated by hydrogen and its many compounds. Nearly all the oxygen present is combined chemically with hydrogen to form water H 2 O. Chemists call such a hydrogen-dominated composition reduced. Throughout the outer solar system, we find abundant water mostly in the form of ice and reducing chemistry.

The terrestrial planets are quite different from the giants. In addition to being much smaller, they are composed primarily of rocks and metals.

These, in turn, are made of elements that are less common in the universe as a whole. The most abundant rocks, called silicates, are made of silicon and oxygen, and the most common metal is iron. We can tell from their densities see Table 2 in Overview of Our Planetary System that Mercury has the greatest proportion of metals which are denser and the Moon has the lowest.

Earth , Venus , and Mars all have roughly similar bulk compositions: about one third of their mass consists of iron-nickel or iron-sulfur combinations; two thirds is made of silicates. Because these planets are largely composed of oxygen compounds such as the silicate minerals of their crusts , their chemistry is said to be oxidized.

When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result the sinking of heavier elements due to the pull of gravity. This leads us to conclude that, although the terrestrial planets are solid today, at one time they must have been hot enough to melt.

The heavier metals sink to form a core, while the lightest minerals float to the surface to form a crust. Later, when the planet cools, this layered structure is preserved. In order for a rocky planet to differentiate, it must be heated to the melting point of rocks, which is typically more than K. The three largest moons—Ganymede and Callisto in the jovian system, and Titan in the saturnian system—are composed half of frozen water, and half of rocks and metals.

Most of these moons differentiated during formation, and today they have cores of rock and metal, with upper layers and crusts of very cold and—thus very hard—ice Figure 2. Figure 2: Ganymede. The brownish gray color of the surface indicates a dusty mixture of rocky material and ice.

The bright spots are places where recent impacts have uncovered fresh ice from underneath. Most of the asteroids and comets, as well as the smallest moons, were probably never heated to the melting point.