There are 10 billion trillion planets in the Universe. With all those worlds out there, it might seem like a sure thing that life - even the intelligent kind - has appeared elsewhere.
Unless…
Unless the conditions on Earth are so special it's crazy improbable that they’ll appear anywhere else (if you want to get technical, a less than 1 in 10 billion trillion chance per planet). If such low odds also seem crazy let me introduce you to the freaky problem of our blue marble world.
Earth has often been called a “blue marble”. It’s not just the swirly white clouds that give our world a marbled appearance. The continents complete the metaphor. All that land sticking up above sea level gives Earth its distinctive look: 29% land; 71% water.
How weird is this?
But now that we know the Universe is awash in planets, just how distinctive is our world in this regard? Is an almost equal mix of land and sea common or will most planets be either entirely desert or entirely ocean?
Over the last few decades, astrophysicists have asked exactly this question. Turns out that planets with almost equal amounts of open ocean and dry land should be rare, maybe even very very rare.
Every planet will have some degree of bumps on its surface – highlands and lowlands. The more water the planet has, the more the lowlands get filled in. But if a planet has a low water inventory it will be a Desert World with nothing more than a bunch of disconnected lakes.
But with enough water, sea level will be above the highest bit of land on the planet. In that case what you have is an Ocean World (astronomers prefer that term to “Waterworld”, sorry Kevin Costner). No land anywhere.
Getting the kind of mix Earth has, however, seems tricky. Even Mt. Everest would be submerged if our planet had just several times more water than it does now.
So somehow, we got exactly the right amount of water to form globe-spanning oceans while also retaining enough highlands to form large-scale continents.
How do planets get their water? Comets and asteroids “deliver” H2O that’s locked up inside their interiors. The delivery happens when they collide with young planets early in a solar system’s history. Studies show the amount of water delivered during planet formation should be random. That means there are far more ways to end up with Ocean World planets or desiccated Desert World planets than the remarkable fine-tuned balance we have on Earth.
Continents and oceans: we have them both. But it didn’t have to be this way and on most planets it probably isn’t. That’s important because there’s lots of reasons to think that the evolution of complex life, including our kind, needed dry land to really get going.
So how special is Earth’s mix of land and oceans? 1 in a million? 1 in a billion? 1 in ten billion trillion? Is it enough to make life like ours incredibly rare? Is it enough to mean we are alone?
The good news is the telescopes we’re building will be able to answer this question over the coming decades. That’s why the field of astrobiology is so exciting right now.
Before you go, there’s one more thing to consider. Even asking this question of water, land and planets shows the real power of science as a force in human culture. Usually, we take stuff for granted. Things just are what they are. We don’t go to the beach, stare at the ocean and think “wow that’s really weird.” But science gives us new eyes. It lets us see that even the map of the world, with its mix of blue and brown, is rich with mystery.
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PS. Got a thought about this? Leave a comment on the website version of the post or email me at [email protected]
PSS. I was not able to have this post proof-read so please excuse any typos.
— Adam Frank 🚀
