What’s the Big Deal About Planet Size?

Bigger planets have more gravity. Most people know the most obvious effect of this: on a planet with higher gravity, you’d feel heavier and it would be harder to move. On a planet with lower gravity, you’d feel lighter, and have an easier time running and jumping.

But a planet’s size, and the strength of its gravity, determines far more than just how much muscle strength you need to move around. A planet’s size is a major factor in the question of whether it can protect life on it’s surface from deadly cosmic radiation, and in the question of how long humans can survive on its surface.

Planet Size and Atmospheric Thickness

Bigger planets tend to have thicker atmospheres. This has important implications for life, as a certain level of atmospheric thickness is necessary to protect life on a planet’s surface from deadly cosmic rays, and provide it with fuel for respiration. 

Mars, for example, is too small to hold down an atmosphere, and that’s one of three major reasons why it’s not very hospitable to life today. The second reason, which we’ll cover shortly, is also related to Mars’ small size. The third is simply that Mars is unlucky: it was probably subjected to bombardments from the asteroid belt, which may have helped to blow away its atmosphere.

Bigger planets tend to have thicker atmospheres for two reasons:

  1. Planets with higher gravity can attract more gas from the Giant Molecular Cloud during their formation, and can hold these gases down better in the face of solar winds. 
  2. Bigger planets retain the heat of their formation – the geothermal heat that keeps their cores molten – for longer. This allows them to have active geology, such as volcanic activity, which pumps geologic gases into the atmosphere at a steady rate in planets with a healthy geology.

Earth’s healthy geology makes sure that carbon dioxide and other gases are steadily released into the atmosphere, playing a major role in keeping our plants alive and our atmosphere breathable.

CarbonCycles

When a planet’s core cools and its geological processes die, any life living on its surface is in big trouble. They can expect their atmospheres to begin to thin out and the amount of mutation-causing cosmic radiation reaching the ground to increase gradually over the course of hundreds of millions of years. 

Planets the size of Earth and larger may retain their core heat for over tens of billions of years – in other words, Earth’s geology will still be healthy and active when Earth is swallowed by the Sun going red giant about 5 billion years from now. But smaller planets like Mars cool off quickly, leaving them with no ability to replenish their atmosphere or generate protective magnetic fields.

There may be a way to create planets smaller than Earth with thicker atmospheres, however. Planets orbiting at a distance from their parent stars – perhaps because the star is hotter and brighter than our Sun, allowing them to get sufficient light and heat from far away – might attract and keep thicker atmospheres, thanks to their greater distance from their stars’ solar winds.

Atmospheric thickness isn’t the only reason why a planet’s size is important. It’s also important because of…

Gravity and Human Health

Planets with gravity significantly higher than Earth’s may shorten the lifespan of humans living on them by putting extra gravitational stress on our circulatory systems. As upright-walking beings, our hearts may experience a lot of stress, and our brains may experience decreased oxygen, as a result of the added difficulty of pumping blood up to our heads in high gravity. 

Life native to these worlds may be more inclined to have flat, low-to-the-ground body plans like komodo dragons – animals that manage to have four limbs and decent mobility, but which place almost no vertical demands on their circulatory systems. 

Humans born and raised on these worlds may also be unusually short and stout, with unusually strong muscles, circulatory systems, and bones thanks to being shaped by the demands of high gravity during their growing years.

Planets with much lower gravity than Earth’s, on the other hand, may have different effects on human residents. 

Some science fiction authors have postulated that humans may live significantly longer in lower gravity due to decreased stress on our circulatory systems. But recent studies show that this may actually cause complications too: without the force of gravity pulling liquid down toward our feet, for example, astronauts living in zero gravity for extended periods of time show potentially dangerous stiffening of the arteries and increased pressure of their cerebrospinal fluids on their brain and eyes.

Life native to lower-gravity worlds with thick atmospheres may have an easier time flying than life forms on Earth, which may make flying or gliding more common traits. Flying and gliding animals may be bigger than those on Earth. Life forms may also have an easier time growing tall, resulting in fragile body plans that wouldn’t survive on Earth, but which may tower over human visitors.

Humans born and raised on lower gravity worlds may also be taller than those born and raised on Earth, with weaker muscles, hearts, and bones. Beware: these humans may not be able to survive even a brief trip to Earth, if their bodies did not develop sufficient cardiac musculature and bone structure during their growing years.

Scientists think that even humans who spent too long living on a low gravity world might lose too much muscle and bone mass to be able to return safely to Earth. Some science fiction stories, such as the brilliant “Luna Falls” by Ian McDonald, speak of the dilemma faced by people living on lower-gravity worlds over the fact that if they stay on these worlds for too long, they can never go back to Earth.

The Level of Geologic and Geomagnetic Activity

Bigger planets retain heat longer, just like bigger cups of coffee cool more slowly than smaller ones. This core heat is necessary to maintain a molten metal core, which in turn is necessary to produce a magnetic field that also protects planets from deadly stellar radiation.

We’ll discuss stellar radiation in more detail in future chapters. Right now, just know this: there’s radiation falling from the sky, and if you don’t have an atmosphere that’s rich with oxygen and ozone or another “sunblock chemical” to absorb UV rays AND a magnetic field to block higher-energy radiation, you’re going to have a bad time.

Now, you might want your alien life forms or human explorers to have a bad time. In which case, putting them on a radiation-bombarded surface without adequate atmospheric or magnetic protection is a great way to do that. But if you want the surface to be safe for life as we know it to live on, you’re going to want a planet that’s large enough to host a healthy magnetosphere.

Some scientists think that the “lower limit” for planet habitability, because of the need to maintain core heat and hold down an atmosphere, might be about 2.6% the mass of Earth. The “upper limit,” on the other hand – after which the atmosphere becomes too thick and begins to resemble a gas giant – is about 10 times the mass of Earth.Habitability

Thanks to Harvard SEAS lab for creating this graphic showing the relative size of habitable worlds – and those that are too small to be habitable without help from the tidal forces of a nearby gas giant.

If you know that you want a specific type of gravity (as high as possible, or as low as possible), you can tweak your atmospheric conditions by tweaking your star type and your planet’s orbit. 

Planets orbiting hotter, brighter stars can safely orbit further from their parent star, enabling them to hang onto thicker atmospheres despite the solar winds and gravity of their parent star. Planets orbiting cooler, dimmer stars, on the other hand, will have to orbit closer to their parent star, and may lose more atmosphere to its solar winds.

It’s thought that the upper size limit for “super Earths” before they turn into gas giants may be 10 times Earth mass for planets orbiting close to their parent stars, for example. In a close orbit in the inner habitable zone of a cool star, solar winds would blow away enough atmosphere to prevent a planet that large from turning into a gas giant.

But at the outer edge of the habitable zone as a hot star, that limit may be only 5 times Earth’s mass. Without the solar winds and competing gravity of a star, these planets may accumulate thicker atmospheres and move into gas giant classification more easily.

On the other hand, if you know you want a thick atmosphere on a world with lower gravity than Earth, you probably want that world to be orbiting a hot, bright star that can give it all the heat and light it needs from a safe distance. 

If you want your world to have a thin atmosphere despite having high gravity, it makes more sense to put it in a closer orbit around a cooler, dimmer star which can strip more atmosphere away without overheating its planet.

My YouTube talks have been delayed due to technical issues (apparently my microphone is so sensitive it picks up my hard drive spinning as background noise), but you can subscribe to my newsletter here to be updated on new blog posts, and the availability of the e-book where I’ll be compiling everything I know about building alien planets.

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