Are we alone? The question hangs over each discovery of an Earth-sized planet as we speculate on its habitability. But how different and varied could these worlds really be? Perhaps the best way to get a flavor of this potential diversity is to build a few planets.
This is the idea behind Earth-Like: a website and twitter bot that lets you build your own Earth-like world. Earth-Like begins with a planet that resembles our Earth today, with oceans flowing over the surface and an atmosphere that maintains the global average temperature at a comfortable 15°C (59°F) on our orbit within the habitable zone. By making changes to the fraction of exposed land, the volcanic rate and position within the habitable zone, you can change the conditions on our planet into wildly different environments from desert to snowball.
The concept for Earth-Like began during a workshop on planet diversity held at the Earth-Life Sciences Institute (ELSI) in Tokyo. The discussions highlighted that the potential for variation between rocky worlds is vast. A planet rich in carbon could have a mantle of diamond. A stagnant surface rather than mobile continental plates could throttle volcanism. The gravity on a large rocky planet might flatten the topology to allow shallow seas to cover all the land.
At the moment, observations can only tell us the physical size (either radius or mass) and the orbit of the majority of extrasolar planets. As we do not know what the surface of these worlds is like, we dub new discoveries Earth-like or potentially habitable if their size and the amount of radiation they receive from the star is similar to Earth. But this fails to convey how incredibly alien these worlds could be.
Earth-Like was spearheaded by undergraduate student, Kana Ishimaru, at the University of Tokyo (now a graduate student at the University of Arizona), working with myself, Julien Foriel (now a researcher at Harvard University) and Nicholas Guttenberg at ELSI. We wanted to build a model that would give a feel of the diversity of potentially habitable worlds and which could be run easily on a web browser. The aim was to show how even tiny changes to a planet can create drastically different environments.
All the planets built in Earth-Like orbit within the habitable zone (although there is an ‘advanced toolkit’ that lets you wander beyond these boundaries). Yet the small changes you can make with Earth-Like can render many of these worlds too cold or warm for life. The options for exposed land, volcanism and habitable zone orbit would likely differ even between planetary systems that formed identically to our own.
For example, water is thought to have arrived on Earth during impacts with icy meteorites. Varying these chance encounters would deliver larger or smaller seas. Similarly, a re-run of the chaotic collisions that formed the Earth could place the planet on a wider or closer orbit around the sun, while the volcanic action on Earth has varied over our history. In short, even the most Earth-like of Earth-sized planets will differ from our own in these properties. The different planets produced by Earth-Like therefore hint at the wild diversity that might truly exist across the galaxy.
The Earth-Like project was recently accepted as a paper in the International Journal of Astrobiology. This paper lifts the lid on the code to take a look at how the climate is calculated.
Earth-Like simulates your new world by modeling the carbon-silicate cycle. This cycle on Earth can adjust the level of carbon dioxide in the atmosphere over about 100,000 years. As carbon dioxide is a greenhouse gas that traps heat, changes in its abundance raise or lower the surface temperature of the planet.
The cycle begins with carbon dioxide dissolving in rainwater and reacting with the silicate rocks on our planet’s surface. Minerals containing carbon form and washing into the ocean, before becoming solid sediment. Eventually, the carbon is return to the atmosphere via volcanos, completing the carbon-silicate cycle.
If the surface temperature on Earth increases, the chemical reaction between the rainwater and rock speeds up. This pulls more carbon dioxide from the atmosphere and reduces the greenhouse blanket, allowing the planet to cool. Conversely, if the surface temperature drops, then this reaction slows and allows carbon dioxide to build back up in the atmosphere.
With Earth-Like you can interfere with this cycle, forcing the planet to shuffle carbon between the atmosphere, ocean and sediment until it finds a new balance point where the rate of carbon leaving each location equals its rate of arrival. For example, if you increase the volcanic rate, your planet will push carbon into the atmosphere and increase the amount of greenhouse warming. This will heat the planet and the rate of carbon moving to the ocean and sediment will also rise. Eventually, a new balance point will be reached, but one where there is more carbon dioxide in the air than the Earth today, creating a warmer planet.
The model used by Earth-Like is very simple compared to the global climate models that consider planets very different from the Earth. But given the diversity of worlds it can produce, can you imagine exactly how different the planets we are discovering might truly be?
The nature of the rocky worlds we have discovered will begin to be explored by the next generation of telescopes. Instruments such as James Webb Space Telescope or the new Extremely Large Telescopes (ELTs) will be able to detect molecules in a planet’s atmosphere, providing the first hint at what might be on their surface. If Earth-Like is anything to go by, we need to be ready for something wild.