Exoplanet Earth: what would Earth look like from deep space if inquisitive aliens were scanning for planets?

Natural History, Feb, 2006 by Neil deGrasse Tyson

Whether you prefer to crawl, sprint, swim, or walk from one place to another, you can enjoy close-up views of Earth's inexhaustible supply of things to notice. You might see a vein of pink limestone on the wall of a canyon, a lady-bug eating an aphid on the stem of a rose, a clamshell poking out of the sand. All you have to do is look.

Board a jetliner crossing a continent, though, and those surface details soon disappear. No aphid appetizers. No clams. Reach cruising altitude, around seven miles up, and identifying major roadways becomes a challenge.

Detail continues to vanish as you ascend to space. From the window of the International Space Station, which orbits at about 225 miles up, you might find London, Los Angeles, New York, or Paris in the daytime, because you learned where they are in geography class. But at night their brilliant lights present only the faintest glow. By day, contrary to common wisdom, with the unaided eye you probably won't see the pyramids at Giza, and you certainly won't see the Great Wall of China. Their obscurity is partly the result of having been made from the soil and stone of the surrounding landscape. And although the Great Wall is thousands of miles long, it's only about twenty feet wide--much narrower than the U.S. interstate highways you could barely see from a transcontinental jet.

Indeed, apart from the smoke plumes rising from the oil-field fires in Kuwait at the end of the First Persian Gulf War in 1991, and the green-brown borders between swaths of irrigated and arid land, from Earth orbit the unaided eye cannot see much else that's made by humans. Plenty of natural scenery is visible, though: hurricanes in the Gulf of Mexico, ice floes in the North Atlantic, volcanic eruptions wherever they occur.

From the Moon, a quarter million miles away, New York, Paris, and the rest of Earth's urban glitter don't even show up as a twinkle (unless you build a large telescope before you take a look). But from your lunar vantage you can still watch major weather fronts move across the planet. From Mars at its closest, some 35 million miles away, massive snow-capped mountain chains and the edges of Earth's continents would be visible through a large backyard telescope. Travel out to Neptune, 2.7 billion miles away--just down the block on a cosmic scale--and the Sun itself becomes embarrassingly dim, now occupying a thousandth the area on the daytime sky that it occupies when seen from Earth. And what of Earth itself? It's a speck no brighter than a dim star, all but lost in the glare of the Sun.

A celebrated photograph taken in 1990 from the edge of the solar system by the Voyager 1 spacecraft [see photograph on page 55] shows how under-whelming Earth looks from deep space: a "pale blue dot," as the late American astronomer Carl Sagan called it. And that's generous. Without the help of a picture caption, you might not even find it.

What would happen if some big-brained aliens from the great beyond scanned the skies with their naturally superb visual organs, further aided by alien-state-of-the-art optical accessories? What visible features of planet Earth might they detect?

Blueness would be first and foremost. Water covers more than two-thirds of Earth's surface; the Pacific Ocean alone makes up an entire side of the planet. Any beings with enough equipment and expertise to detect our planet's color would surely infer the presence of water, the third most abundant molecule in the universe.

If the resolution of their equipment were high enough, the aliens would see more than just a pale blue dot. They would see intricate coastlines, too, strongly suggesting that the water is liquid. And smart aliens would surely know that if a planet has liquid water, the planet's temperature and atmospheric pressure fall within a well-determined range.

Earth's distinctive polar ice caps, which grow and shrink from the seasonal temperature variations, could also be seen optically. So could our planet's twenty-four-hour rotation, because recognizable landmasses rotate into view at predictable intervals. The aliens would also see major weather systems come and go; with careful study, they could readily distinguish features related to clouds in the atmosphere from features related to the surface of Earth itself.

Time for a reality check: We live less than a dozen light-years from the nearest known exoplanet--that is, a planet orbiting a star other than the Sun. Most exoplanets lie more than a hundred light-years away. Earth's brightness is less than one-billionth that of the Sun, and our planet's proximity to the Sun would make it extremely hard for anybody to see Earth directly with an optical telescope. So if aliens have found us, they are likely searching in wavelengths other than visible light--or else their engineers are adapting some other strategy altogether.

Maybe they're doing what our own planet hunters typically do: monitor stars to see if they jiggle at regular intervals. A star's periodic jiggle betrays the existence of an orbiting planet that may otherwise be too dim to see directly. The planet and the host star both revolve around their common center of mass. The more massive the planet, the larger the star's orbit must be, and the more measurable the jiggle when you analyze the star's light. Unfortunately for planet-hunting aliens, Earth is puny, and so the Sun barely budges, further challenging alien engineers.