The author, Scott Denning, is a climate scientist at Colorado State University.

On May the Fourth, it’s fun to compare the Ice Planet Hoth to the hot desert planet Tatooine. This comparison helps us better appreciate the Star Wars saga and also to understand how our own home planet works.

Hoth is too cold, Tatooine is too hot, but Earth is just right!

Hoth vs. Earth

Why is Hoth so cold? Maybe its sun is too dim or far away. But even if Hoth’s star is about the same brightness as our sun and  at the same distance as Earth is from our sun, that wouldn’t necessarily make it warm. Without sufficient amounts of atmospheric carbon dioxide (CO2), a heat-trapping gas, it could still be an ice world.

In fact, Earth was previously Hoth-like as recently as 700 million years ago: Ice expanded all the way to the equator, and there was no hydrologic cycle because the oceans were frozen over. It was just cold, dry  and windy all the time, everywhere.

During Earth’s Hoth-like phase, our sun was about 5 percent dimmer than it is now. For Earth to become Hoth-like again, CO2 levels would have to be cut in half three times, from the current concentration of 410 parts per million to about 51 parts per million.

As the CO2 level dropped, the polar ice would spread toward the equator from both poles, so more and more sunlight would be reflected to space, further cooling the planet. Something like this must really have happened to Earth 700 million years ago.

Over geologic time, Earth’s CO2 is regulated by the balance between volcanic outgassing, which adds CO2 to the atmosphere, and CO2 removal by chemical weathering of rocks. It takes tens of millions of years for these geologic processes to do much to Earth’s climate.

After Earth’s oceans froze over completely, the hydrologic cycle shut down, and C02 removal from the atmosphere stopped. The planet then managed to escape the icy  period known as “Snowball Earth.” Volcanic gases, especially CO2, just built up in the air until the resulting greenhouse effect warmed the planet enough to melt the ice.

So how did Hoth get so cold and end up with so little CO2? Maybe some kind of collision of Hoth’s continents raised huge basaltic mountains, and the CO2 was all used up as all that steep, fresh rock weathered? This might have resulted in the frozen wilderness in which Luke had to sleep in the carcass of his beast before the imperial walkers invaded.

Tatooine vs. Earth

Tatooine is hot and weird.

We know it’s got two suns, so maybe that’s partly why it’s so hot. It would also be easy to make a hot world by adding lots of CO2 or other greenhouse gases to the atmosphere.

We know it’s a desert planet, but we also know that there’s a bit of water vapor in the atmosphere because, of course, Luke’s family harvests it from the air.

Where does the water vapor come from? Is there a small, distant ocean someplace that supplies the water vapor Luke harvests? If so, it must also rain somewhere on Tatooine, or the ocean would dry up. In that case, how come people don’t live in the rainy part of the planet? Maybe Jabba and his minions have the prime real estate all locked up and regular folk can only afford to live in the desert?

Getting Earth to become like Tatooine would be really hard. You could add a lot of CO2 to make it hot, but that would just evaporate more water out of our oceans and make it rain even harder in the tropics and the polar regions. If you kept raising CO2 high enough you could evaporate all of the oceans, but that would make a very thick water vapor atmosphere with hardly anything else to breathe!

The mass of Earth’s oceans is about 400 times the mass of the atmosphere, so if this happened, the surface pressure would be about 6,000 pounds per square inch! Besides the crushing pressure, it would be so hot that nothing could live here, not even Hutts. No Jabba. No Luke.  No Obi-wan. No Mos Eisley.

This is kind of what happened to Venus. Then ultraviolet radiation dissociated the water vapor in the upper Venusian atmosphere, turning it into molecular oxygen and hydrogen. The extremely light hydrogen escaped Venus’s gravity, and the oxygen was consumed by rusting into the rocks. Pretty much only the CO2 is left, about 1,500 pounds per square inch of it (compared with 0.006 pounds per square inch of CO2 on the present Earth). Yikes!

Tatooine probably never had much water to begin with. As our solar system formed, the early sun blasted the inner planets with radiation and high-speed plasma, which drove off most of the gases (hydrogen, water vapor  and CO2). Earth probably picked up our water from impacts as comets and asteroids rained down on it around 3.8 billion years ago.

Maybe Tatooine formed close to its suns, so it got dried out by all that early solar radiation, but it never got re-wetted by a late, heavy bombardment as Earth did.

So there you have it: Hoth lost most of its CO2, so it froze solid. If it’s still got active volcanoes, there’s “a new hope” that over time, the CO2 will build up, since it no longer has rain to remove CO2 by chemical weathering. Tatooine is pretty much out of luck from now on.

In contrast to Hoth and Tatooine, the mixture of gases in Earth’s atmosphere is just right to support a thriving biosphere and a flourishing human civilization. It would be a good idea to keep it that way!

May the fourth be with you!

Read more: The Star Wars Universe & Planetary Climates, via Climate Central