This Is How Earth Would Look to Alien Astronomers

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It has been less than 30 years since the discovery of the first exoplanets, and we’re still in the dark when it comes to the possibility of life on any of them. Our techniques for finding other planets isn’t particularly sensitive, and we’re not even certain what signals we need to prioritize. A new study could shed light on that question by using the only habitable planet known to exist: Earth. 

The study comes from the California Institute of Technology’s Division of Geological and Planetary Sciences (GPS) and the NASA Jet Propulsion Laboratory, led by graduate student Siteng Fan. The goal was to start with a planet we know supports life (Earth), and work backward to extrapolate what an alien looking in our direction might see. In that way, Fan and the team hope to nail down the “look” of a life-supporting exoplanet. To do that, they used 9,740 images of Earth taken by NASA’s Deep Space Climate Observatory. They include data on light reflected from Earth in multiple wavelengths. 

At our current level of technology, we can’t directly observe exoplanets. They’re too small and dim compared with stars, but we can infer their presence via gravity or the way they obscure light from their parent stars. Often, the most we can say about an exoplanet’s habitability is whether it has a chance of harboring liquid water based on proximity to the star. 

The team discovered that the second principal component of Earth’s reflected light curve correlates to the fraction of land in the illuminated hemisphere. Using the original images, they were able to pick out the values matching land and cloud cover and applied them to a contour map (see top). The green highlighted areas are landmasses, and the blue is water — Africa is in the center, and Asia is to the right. North America is peeking in at the left edge. 

If we apply these same light curve values to a distant exoplanet, it could help us determine if there’s cloud cover and liquid water on the surface. Confirming that a planet has a water cycle could be a big step toward proving habitability. Will we actually be able to gather such data? It’s possible! Upcoming instruments like the James Webb Space Telescope and Thirty Meter Telescope could have enough resolution to pick up some reflected light from small, rocky exoplanets. We could be on the verge of finding more Earth-like worlds.

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