Why do larger planets tend to mostly be made up of gases?
In: 2
It’s hard for a planet to hold on to light gases. Random thermal motion can accelerate light gases to escape velocity, especially with extra kicks provided by the solar wind. The Earth, for example, can’t hold on to either hydrogen or helium at its size and temperature: they both escape quickly (on planetary timescales) into space due to those effects (escape velocity dominates for hydrogen on Earth, and the solar wind kicks dominate for helium, but either effect would be enough to get rid of both gases over the age of the Earth).
To do it, a planet needs to be both big and, ideally, cold, although a big enough planet can do without the cold.
When the planets formed early in the history of the solar system, the protoplanets that would eventually end up as the gas (Jupiter, Saturn) and ice (Uranus, Neptune) giants were larger than the protoplanets that would eventually end up as the terrestrial planets (Mercury, Venus, Earth, Mars). As a result, they were able to hold on to light gases. Since those gases then added to the mass of the protoplanet, that in turn let them hold on to even *more* light gases, in a feedback loop that eventually captured most of the free gas that wasn’t part of the Sun.
Jupiter and Saturn were big enough to hold on to hydrogen and helium, so they ended up the biggest. Uranus and Neptune couldn’t hold on to those, but they could hold on to light molecules like methane, oxygen, ammonia, and water, and so they captured those gases from the outer Solar System. Since there is a *lot* more hydrogen and helium than other elements in the Universe, Jupiter and Saturn ended up much bigger than Uranus and Neptune.
Only a small amount of the remainder was left over, and that last bit (or the elements heavy enough to be held on to by each planet) ended up forming the early atmospheres of the terrestrial planets. Water is close to the limit for Earth and Mars, but Earth’s magnetic field protects us well enough to hold on to water at Earth’s current temperatures, while Mars’ lack thereof has caused it to lose most of its water to space. Earth is expected to suffer the same fate as Mars in the future, as the Sun continues to warm as it ages (well before the Sun becomes a red giant).
Latest Answers