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You are standing on a sandy plain lit only by the harsh, cold light of a blue-white spark flashing overhead in steady metronomic bursts. The pinprick of light almost seems like a distant star, but each flash raises a disturbing tingling sensation within you, suggesting the stroboscopic light source is somehow nearby, and must be the shrunken sun of this desolate world. There is no substantial atmosphere to shield you from the vacuum, and you feel strangely disconnected as you collapse and begin to lose consciousness. As your vision fades to black, you feel saliva bubbling to a boil on your tongue, and you notice pale blue wisps of ionized atoms subliming off the x-ray-sizzled skin of your face.

Encased in a spacesuit, you are floating in the void of interstellar space when you feel an unfamiliar sensation, a gentle, persistent tug at the soles of your feet. You are falling. Looking down, a great circular shadow has blotted out the stars below. Against astronomical odds, you have been gravitationally captured by an unbound world wandering between the stars. Surface details sharpen as you fall ever-faster toward the wanderer’s looming surface, and shortly before impact you glimpse jagged rocky cliffs rising up from a scalloped, cratered crust of a frozen sea. Your shattered remains freeze upon the surface.

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You are bobbing up and down on a wind-thrashed ocean. Warm, brackish water stretches from horizon to horizon beneath a murky overcast sky of pinkish-orange smog. As you kick and thrash to stay afloat your limbs bump against globs, filaments, and mats of grayish-purple jelly—vast colonies of bacteria. The microbial slime clogs the surface and extend down as far as you can see into the depths. You gasp for breath, but there is no oxygen in the thick, heavy air to sustain you, only the putrid stench of rotting biomass. Within moments, you have vanished beneath the heaving waves.

By now you’re probably wondering what each of these grim, speculative scenarios has in common other than ending in your grisly demise. The answer is that all of these planetary environments could be called “Earth-like,” depending on the assumptions used. It may come as a surprise, but despite the potentially historic nature of pronouncing another world quite similar to our own, there are no standard criteria for what exactly “Earth-like” means, and attempts to create them inevitably struggle with major assumptions and uncertainties. (Last week I wrote in more detail about how little we actually know about some of the planets that are described as Earth-like: “The Fun-House Mirror Earths.”)

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I suspect the most useful interpretation of “Earth-like” is akin to the famed subjective definition of obscenity: We’ll know it when we see it.

If your definition of “Earth-like” is simply a rocky planet orbiting a star, then the world in the first scenario meets all those criteria. It is a pulsar planet, like the very first confirmed extrasolar worlds of the 1990s. The world in the second scenario is even more Earth-like—it’s a rocky planet with the added bonus of abundant surface water. Almost all that water just happens to be frozen to ice, because the planet is no longer orbiting, and absorbing energy from, a star. Such free-floating “Earth-like” worlds may be quite common throughout the cosmos, produced when a small planet is slingshotted out of its home system after coming too close to a giant planet like Jupiter. With warm temperatures, liquid-water oceans, a protective atmosphere, and apparently some form of indigenous life, the world in the last scenario is easily the least alien of the three, though it’s still quite unearthly. It is actually our own Earth as it was some three billion years ago before the rise of complex multicellular organisms and the dominance of oxygenic photosynthesis, when life was mostly aquatic, anoxic, and single-celled. In fact, given that the Earth has spent well over three-quarters of its existence in that primitive, anoxic state, this ancient environment could be considered more “Earth-like” than our modern oxygenated world, though of course none of us could live there.

The impetus for using the term “Earth-like” at all stems from its undeniable public-relations appeal: Everyone likes thinking of there being other planets like ours. But this approach also carries risks: As the preceding examples show, if the term’s definition is too broad, it can be misleading or nearly meaningless. If it becomes too narrow, even the Earth itself will sometimes not be deemed “Earth-like,” and the search for familiar planets will seem forever doomed. Either extreme would disappoint and confuse people, potentially reducing interest and support for exoplanet searches, a prospect I’ve written about it before.

However, the greater problem with “Earth-like” is arguably a scientific one. Even limiting ourselves to planets that could support life like us, the emerging diversity of planets makes clear that constraining our search to “Earth-like” locales can hurt more than help. The clearest example of this may be in the conventionally defined boundaries of habitable zones around stars. The inner edge of the habitable zone is defined by where starlight would heat a planet so much that the world steams its water off into space; since life as we know it requires water, a planet losing all its water would become uninhabitable. For the actual Earth, this inner edge is conservatively pegged to lie only a few percent inward of our current orbit. Small, rocky worlds orbiting other Sun-like stars closer than that boundary are thus not typically defined as “potentially habitable.”

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Yet recent studies suggest that planets could remain habitable much closer to a Sun-like star, well inward of the orbit of Venus, if they were unearthly “desert worlds” with far less water than Earth. This is because water vapor exhibits a self-reinforcing greenhouse effect: Higher temperatures push more water vapor into the air, which raises temperatures higher still, pushing even more water vapor skyward. In this controversial new view, Earth’s apparently crucial feature for life—its abundant water—can paradoxically decrease its habitability. Conversely, a desert world with only a minuscule fraction of water could be considered more habitable, since it could still hold on to its water, and thus some conceivably biosphere, much closer to a star. Similar counterintuitive conclusions apply to the habitable zone’s outer edge, which is defined by where all of a planet’s water would freeze: If a small rocky world is insulated beneath an atmosphere with a much stronger greenhouse effect than Earth’s, it could conceivably circle a Sun-like star well past the orbit of Jupiter and still maintain some semblance of habitability.

There are many more physical factors, such as orbital configurations and planetary composition, that could make a planet less like ours yet more suitable for life than they would otherwise be. Further, on any world where life does manage to arise, the co-evolution of that biosphere with its planetary environment can have additional second-order effects on habitability that are not readily predictable, as we see in Earth’s stark transformation from anoxic slime world to the familiar planet we know today.

Consequently, in the broad search for life beyond the solar system, I sometimes suspect the most useful interpretation of “Earth-like” is also the most vague, akin to former U.S. Supreme Court Justice Potter Stewart’s famed subjective definition of obscenity: We’ll know it when we see it.

In my next blog post, I’ll discuss how we could one day see faraway, potentially habitable planets and test exactly how Earth-like they are. 

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Lee Billings is a freelance writer living in New York City. Five Billion Years of Solitude, his book on the search for Earth-like exoplanets, will be published this October by Current/Penguin.

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