Sometimes it’s possible to be too close to a problem. For example, how would a citizen of Whoville living on a speck of dust know what another speck of dust several light-years away is supposed to look like?
The situation is much the same on Earth.
Earth, as seen from Mars in 2004
—Image courtesy NASA/JPL/Cornell/Texas A&M
So far we’ve found more than 300 examples of specks in the distant universe that we have good reason to believe are planets circling other stars—so-called extrasolar planets, or exoplanets.
Almost all of these specks are too far away and/or too small to see directly. But we know they’re there, because we can see the gravitational tugs they make on their host stars, or we see a star dim for a bit as a planet passes between it and us.
With current technology, we can tell that quite a few of the worlds we see outside the solar system look a lot like Jupiter, with relatively similar masses, densities, and compositions, but often much closer to their stars than would be expected.
Not long after we spotted the first “hot Jupiter,” we found the first exoplanet that looks like it might be a rocky world like Earth, raising hopes that there’s at least one distant orb out there that could be habitable for life as we know it.
The problem is, the only habitable world we know of is Earth, and how do we know what Earth looks like from veeeeeery far away?
—Video courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Luckily, a few of the orbiters we’ve sent out to examine the solar system’s other denizens are being asked to shoot a few glances back home and give us a better picture of what we should be looking for when scanning the skies for other Earths.
The European Space Agency’s Venus Express, for example, has been collecting visible and near-infrared pictures of Earth taken from venusian orbit about 26 million miles (42 million kilometers) away for the past two years.
These images are helpful based on the notion that less is more—Earth is merely a featureless point of light to the Venus-bound craft, simulating what other habitable worlds might look like once we [hopefully] find a way to look at them directly.
The MESSENGER mission to Mercury also did an Earth flyby during an early leg of its journey, showing what the planet looks like in different light wavelengths to get a picture of what plants and oceans might look like from afar.
In near-infrared, for example, lush landmasses appear deep maroon because the chlorophyll in plants makes them highly reflective at that wavelength. This means a “red edge” seen in near-infrared images of exoplanets could be a strong indicator of vegetation.
—Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
NASA’s Deep Impact probe, which achieved its primary goal of studying comet debris in 2005, is now on an extended mission to look, in part, for exoplanets that pass in front of, or transit, their stars.
One phase of the mission involved looking back at Earth, and in May 2008 the probe captured stunning video that showed the moon transiting Earth as seen from from 31 million miles (50 million kilometers) away.
—Video by Donald J. Lindler, Sigma Space Corporation and NASA/JPL-Caltech/GSFC/UMD
In addition to showing what the moon looks like to distant observers—darker and redder than what we see from Earth’s surface—the video revealed that watery worlds like Earth would have glints of polarized light that vary in brightness as oceans rotate in and out of view.
All these projects illustrate how good our space probes are getting at being multitaskers. Of course, an even bigger question would be, if we find an exoplanet that we are fairly sure is habitable, what do we do about it?
The most Earthlike exoplanet we know of right now is more than 20 light-years away. In other words, it’s 20 times the distance that light can travel in a vacuum in a year, or 20 times 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That’s a big number.
Still, even though we’re hampered by the fact that we can’t plan a road trip to Earth mark 2, there’s no harm in looking. At lease we’ll have an idea of how likely it is that somewhere out there an eager set of alien earlike organs might be enjoying the dramatic tones of Orson Welles.