Why are the compositions of the planets different from each other, and why are they different from stars?

• Planets of the solar system

• How do we know about planetary compositions?
  • measurement of densities. How? Masses and radii.
  • absorption in atmospheres of planets (for those that have atmospheres)

• What do we know about planetary compositions?
  • When we measure mean planetary densities, we find that planets do not all have the same density. The inner four planets have densities of about 5gm/cm³ , while the outer planets have densities of about 1gm/cm³ . Pluto doesn't quite fit with the other; it has a density of about 2gm/cm³ . To get a feel for what these numbers mean, note that water has a density of 1gm/cm³ . The units, gm/cm³ , can be understood by noting that the density in gm/cm³ just gives the mass of a small cube of the material which is 1cm on each side.

  • We see that planets are split into 3 classes: the inner planets (terrestrial) which are more dense, the outer planets (Jovian) which are less dense, and Pluto (and other Kuiper belt objects), which is intermediate.

  • Since the densities of the planets are different, we infer that the compositions of planets differ. The inner planets are made of rocky material, which has higher density; these are often known as terrestrial (earth-like) planets. The outer planets are made mostly of low density gases; these are often known as Jovian (Jupiter-like) planets. Pluto may be made of some combination of rocks and ice, and may represent one of the largest of a third category of objects, called Kuiper belt objects, that we discussed a bit in the first unit of the class.

• Why are the compositions of planets different from each other? Why are the compositions different from those of stars? What determines the composition of planets?
  • The reason for different composition of planets has to do with how the solar system formed. In the early solar system, there was a disk of material rotating around the Sun, from which the planets eventually formed. We've used this model to understand:
    • how the planets get the transverse velocity needed to keep them orbiting around Sun
    • why all the planets orbit in the same direction (and in roughly circular orbits)
    • why all the planets orbit in the same plane

  • To form planets required some kind of initial clump in the protoplanetary disk. These clumps were probably formed from collisions of solid particles. Different elements become solid at different temperatures: the most common elements, hydrogen and helium, never became solid, but hydrogen compounds (like water) can solidfy at colder temperatures, like those in the outer solar system. In the inner solar system, it was too hot for these compounds to solify; only rocks and metals can solidify at these temperatures. Hence, only small planetessimals formed in the inner solar system.

  • In the atmospheres of planets, atoms are held to planets by gravity. However, if an atom can move fast enough, it can escape the gravitational pull of the planet, in the same way that we can launch spacecraft which can escape the gravitational pull of the Earth by shooting them off fast enough.

  • In fact, atoms do move around. The amount they move is related to their temperature. In a hotter environment, atoms move faster. However, the speed that atoms move also depend on the type of element; at the same temperature, heavier atoms move slower than lighter atoms.

  • These properties can be used to generally understand why the compositions of the inner planets differ from those of the outer planets.
    • The outer planets are sufficiently massive and sufficiently cool that no elements can escape their gravitational attraction, so their composition is similar now as to what it was when they originally formed. This is also true of the Sun. This primordial composition is the same as that of most of the Universe: about 90% hydrogen, 9% helium, and only 1% for everything else combined. The normal matter in the Universe is mostly hydrogen.

    • The inner planets are less massive and hotter than the outer planets, so many elements can escape their gravity. On Earth, hydrogen and helium escape so they are not an important constituent of our atmosphere. Instead, our atmosphere is mostly nitrogen and oxygen.

    • On the smallest objects, e.g., Mercury and the moons of most planets, gravity isn't sufficiently strong to retain any atoms - so these objects don't have any atmospheres at all.

• Although these principles are the most important in determining the composition of planetary atmospheres, other effects can also be important, such as the presence of volcanism, life, or manmade pollution.