We are well aware that hydrogen and helium are the most common elements in the Universe.   Despite their abundant nature, their abundance is not so great on the inner terrestrial planets.  This is due to the heat energy from the Sun.  The reason being, invoking thermal physics, that temperature is the measure of the kinetic energy of molecules and atoms of the measured system.  The higher the temperature, the greater the velocity of the molecules and atoms and this will give the needed velocity to exceed a planet’s gravitational force (however, this is dependent on the mass of the planet).  This is the case with planets that are close to a star.  Planets that are farther away from the star (such as our Jovian planets) will have lower temperatures and so they maintain the ability to trap in the gases.   

            The issue of temperature’s role in the planet formation process is addressed by the behavior of matter found on various planets.  For instance, compounds of iron and silicon are solids up to the temperature of 1000K.  Elements such as hydrogen and helium are gases except in environments of extreme low temperatures and high pressures.  Compounds such as CO₂, CH₄, NH₃ and H₂O will solidify at temperatures ranging from 200K to 300K (the solids formed from the compounds are known as ices).  So the composition of a planet will depend on the matter present at the time of formation and distance from heat sources (such as a star).

            At the initial state, the solar nebula is roughly less than 50 K (which is less than the condensation temperature of most compounds excluding hydrogen and helium).  Within the solar nebula, ice coated dust grain particles are scattered and soon the gravitational attraction begin to attract all the particles toward the center of the solar nebula.  As the matter is condensed, the pressure and density of the center begins to increase.  This concentration of matter is referred to as the protostar (or protosun).  The gravitational attraction will increase the internal temperature of the protostar and the protostar will possess an overall angular momentum.  The angular momentum is significant because it maintains enough matter for planetary formation.  In a sense, the solar nebula is contracted to a flattened disk with rotation.  This description is supported by astrophysicists since planetary orbits within our own solar system are in similar planes.  Temperature-wise, the center of the solar nebula (the protostar) is roughly 2000 K while the outermost regions of the solar nebula remain at around 50 K or less.  As mentioned before, the composition of planets themselves are dependent on their location.  Near the protostar, ices will be vaporized by the high temperature and a rocky solid portion will remain.  However, the ice covered dust grains are able to sustain further away from the protostar due to a decreased absorption in thermal energy.