So you’ve got your plot sorted out, and maybe some idea of the mass, radius and gravity of your fictional planet. The orbit puts it in the ‘sweet spot’ goldilocks zone where liquid water can be present on the surface. What sort of factors go into whether that planet, presumably an Earth-like rocky world, will have an atmosphere that can support terrestrial life?
The gravity of the planet is one key variable, along with surface temperature, and the strength of the planet’s magnetosphere, which can protect against atmospheric stripping due to solar wind.
The surface temperature of a planet will determine how much kinetic energy, and so velocity, the gas particles will have. If that temperature, and velocity, is high enough it will exceed the planet’s escape velocity and the molecules will fly off into space like tiny spaceship explorers. Earth has lost most of its very light gases like hydrogen and helium in this way, whereas the gas giants have enough gravity to retain them. We kept our water, and we’ve got a lot of it! If Earth was sitting where Venus is things would be different, the additional temperature would give those lighter gases like water vapour enough energy to escape, and also prevent any being trapped on the planet’s surface itself (whereas some is ‘sequestered’ on Earth as water and ice at our lower surface temperature). But beyond the early, settling down period where the lighter gases are lost, any world larger than Earth, orbiting in that goldilocks zone, will not continue to lose a significant proportion of its atmosphere through thermal processes.
Here’s a cool pictorial on thermal escape (source: Wikipedia).
Beyond that thermal stripping process, is where the magnetosphere comes into its own, deflecting the solar wind – one of the main non-thermal processes leading to atmospheric loss. The very thickness of a planet’s atmosphere (retained due to its gravity, and as a function of surface temperature), will also protect a planet from the solar wind, even in the absence of a magnetosphere. It’s thought that Venus’ thick atmosphere, ionized by solar radiation and the solar wind, produces magnetic moments that act out to 1.2-1.5 planetary radii away from the planet to deflect the solar wind, much like a magnetosphere (but an order of magnitude closer to the planet). In fact, it’s thought the dominant non-thermal atmospheric loss process on Venus is actually from a type of naturally induced electrical acceleration. On Venus, the stripping of the lighter electrons from the atmosphere causes an excess of positive charges, accelerating ions like H+ out of its atmosphere.
Our explorers need a breathable atmosphere, but they also need an atmospheric pressure like our own Earth’s.
My fictional planet of Cru, in the Tau Ceti Diversion, has comparable surface temperatures to Earth, but a higher surface gravity. The higher surface gravity, and its lower density, allowed me to assume a lighter atmospheric composition, and allow an atmospheric pressure, or weight of atmosphere, close to surface much like Earth’s. That atmospheric composition is crucial to having a reasonable atmospheric pressure – its not just the gravity of the planet. Venus, even though it has slighter lower gravity than Earth, has a crushing atmospheric pressure of 90 times Earth’s due to its heavier atmosphere of CO2.
The Tau Ceti Diversion is due to be launched on September 1st 2016! Read more about what happens in the story here!