The Earth As An Exoplanet

What would Earth look like as an exoplanet? How would Earth appear to our space telescopes looking for worlds like ours?

Published on 23rd Aug, 2018

Our quest for both habitable and inhabited worlds beyond Earth is key to our understanding of the potential for life in the Universe. Our ongoing search seeks to answer very profound questions: Are we alone? How unique is our Earth? Should the hunt for life beyond Earth uncover a multitude of habitable worlds and few (if any) inhabited ones, humanity would begin to understand just how lonely and fragile our situation is. On the other hand, if our hunt yields a true diversity of inhabited worlds, then we would learn something fundamental about the commonality of life in the cosmos. But how will we recognize a distant habitable world, and how would we know if this environment hosts some form of life? A key opportunity for understanding the remote characterization of habitability and life comes from studying our own planet — Earth will always be our best example of a habitable and inhabited world. Thus, by studying our planet within the context of exoplanet exploration, we can develop ideas, approaches, and tools suitable for remotely detecting the signs of habitability and life. While habitable exoplanets are unlikely to look exactly like Earth, these worlds will probably share some important characteristics with our own, including the presence of oceans, clouds, surface irregularities, and, potentially, life. Of course, Earth is not a static place. Life emerged on our planet into an environment completely unlike the Earth we understand today. The early evolution of our planet was an intimate coupling between life and geophysical processes and produced a world seemingly alien to modern Earth. Ranging from ice-covered “Snowball Earth” scenarios to the hazy, oxygen-free atmosphere of the Archean Period, each evolutionary stage of our planet offers a unique opportunity to understand habitable, life-bearing worlds distinct from the present Earth. So as we embark on our search using spacecraft like the Kepler/K2 Space Telescope, the Transiting Exoplanet Survey Satellite and later, the James Webb Space Telescope, let’s ask: What would Earth look like from far away using these observatories? Let’s restrict the answers to looking at only stars like our Sun, although it is entirely possible to have a habitable planet like Earth around other types of stars, such as red dwarfs. From Kepler/K2, since it is designed to look for tiny dips in brightness as the planet passes between the space telescope and the star, Earth would appear as a small decrease in brightness. The dip would be very specific and smaller than if a planet like say, Jupiter, passed in front, and this dip would reappear about every 360 days or so. Kepler would then tell us, looking at its one spot in the constellation Cygnus, that a possible rocky world about the size of Earth passes in front of its star every 360 days or so, just like our planet does. The Transiting Exoplanet Survey Satellite, or TESS, would see something similar. The main difference here is that TESS would look at almost the entire sky for two years and when it is done, astronomers hope to gather thousands of candidate Earth-like planets around some of the nearest stars to us. Neither TESS, nor Kepler/K2 can tell us much else about what Earth would look like from far away. To get more information, we need more instruments, specifically spectrographs that can analyze the light passing from the host star through the atmosphere of the exoplanet as it passes in front. The James Webb Space Telescope will do exactly that. It will have onboard an instrument called NIRSpec, the Near Infrared Spectrometer, which will analyze the light that has passed through any atmosphere and tell us what’s in it. JWST would see a far-away Earth that not only had an atmosphere, but also contained elements that could harbor life. What JWST sees though, does not necessarily have to be the elements in the atmosphere of the Earth of today. When life formed on Earth, it was a very different place, one that was quite inhospitable for us now, but was obviously quite conducive to starting life from scratch. There were varying amounts of water, methane, carbon dioxide and ozone in the Earth’s atmosphere throughout Earth’s four and a half billion year history. Astronomers have built many models that tell us what to expect when we try to locate Earth from very far away and from different periods in Earth’s history. We want to know how many worlds like ours are out there. Are we the only Earth? Or, are planets like ours common throughout our galaxy? With these models and the observations of our newest and best orbiting space observatories, we are on the threshold of getting the answers to these profound questions.