Not Earth-Like?

Why does the website always say "Earth-like"? I created a planet with an average surface temperature of -120℃; that's not Earth-like!

Actually, your planet is extremely Earth-like! It has the same mass and physical size as the Earth, with the same composition in the atmosphere, oceans and rocks. The geological processes are also the same, with the surface temperature being controlled by the Earth's carbon-silicate cycle. While you have simulated an average surface temperature very different from that on Earth today, it is what conditions might be like on the Earth with changes only to the volcanism, amount of sunlight and land fraction; all properties that have varied over the Earth's history.

For most exoplanets, we only know the physical size or minimum mass. These worlds are likely to be far more different from our own planet and not remotely "Earth-like". However, even if we found an Earth-like planet with all the same geological processes as our own world, you have proved the environment might still be very different from our home today!

Venus & Mars?

What values should I use to simulate Venus and Mars?

Sadly, you can't simulate Venus or Mars with "Earth-like"! While both our neighbouring worlds are rocky planets, they are not Earth-like planets. Neither Venus nor Mars have oceans or active vocanoes and so they cannot support a carbon-silicate cycle that would allow the surface temperarture to be controlled by altering the levels of carbon dioxide in the atmosphere.

You can start to approach conditions on Venus by moving your planet very close to the inner edge of the habitable zone. Carbon is rapidly removed from the air, but cannot compensate for the increased heat from the Sun. Beyond the edge of the habitable zone, water would start evaporating rapidly. Like carbon dioxide, water in the atmosphere is a greenhouse gas that traps heat. As more water enters the planet's atmosphere, the temperature will rise even further, accelerating the water evapouration until the oceans are dry. This is the runaway greenhouse effect. The surface temperature on Venus is a staggering 460℃ (870℉), in part due to undergoing a runaway greenhouse. However, this does not completely explain why conditions on Venus are so different from the Earth and much of Venus's past remains a mystery.

Classical habitable zone

The video mentions the 'classical' habitable zone. What other habitable zones are there?

In its broadest definition, the 'habitable zone' is the region around a star where any planet can support liquid water on the surface. Such a region depends on both the brightness of the star and on the planet itself. For example, a more massive planet than the Earth might be able to hold onto hydrogen in its atmosphere, which is light enough to escape Earth's gravity. Like carbon dioxide, hydrogen is a greenhouse gas and very effective at trapping heat. A massive planet with a hydrogen atmosphere might therefore be able to support seas and oceans on an orbit where it received less radiation than the Earth. The habitable zone for such a world would be further out than the habitable zone for the Earth.

However, we do not yet know the atmospheric composition or the surface environment for most exoplanets. Therefore, when an exoplanet is said to be orbiting within the habitable zone, this actually means the Earth's habitable zone. This is referred to in the video as the 'classical habitable zone' to emphasise the fact the planet would only be able to support water if it was Earth-like. For the classical habitable zone, inner and outer edges are defined by the extent our carbon-silicate cycle can keep a planet like the Earth temperate enough for water.

Planet images

Are the planet surface visualisations meaningful?

Yes... and... no. The continents and ocean coverage, land colours and ice reflects the choices you made for your new world and its resulting average surface temperature (or those randomly selected by the twitter bot @EarthLikeWorld when it tweets alone). Even the same properties will not produce the same world twice! But the neural network that creates the image is trained on our own planet; the only planet with a carbon cycle whose surface we have seen. This means the landscape will always resemble our home world, whereas exoplanets with similar properties around other stars will have far more diversity.

But the resulting land map should mean you can make a reasonable guess at the planet's properties based on the image alone: Give it a try! If you tweet @EarthLikeWorld but do not specify any properties, the bot will generate you a random planet. The amount of ice indicates if the planet is warmer or colder than the Earth, the land fraction is the ratio between sea and land and the greens, yellows and greys of the land indicate the level of volcanism (yellows and greys are more volcanic lands). From that, you can make an estimate at where in the habitable zone the planet must be orbiting. Once you've taken a guess, you can click on the link in the tweet to take you to the EarthLike.world page displaying the properties that were chosen.

EarthLike spinning globe

Hit us with your best guess. (Then click here.)