The Nearest Earth

By Seth Shostak, Senior Astronomer and Director of the Center for SETI Research

It's one thing to miss your exit on the Santa Monica Freeway, but something else to blithely sail past an entire star system.

If you're a fan of 1960s television, you might recall that's exactly what happened to Professor John Robinson and his family in the vaguely imbecilic series, "Lost in Space." These pioneering colonists, shot into the vacuum voids of the cosmos aboard the unambitiously named Jupiter 2, intended to settle on a planet circling the double-star system known as Alpha Centauri. Unfortunately, a saboteur inadvertently forced them off-course, and they eventually alighted on an unknown world around an unnamed star. Danger ensued.

Fifty years ago, the idea that Alpha Centauri - which is the nearest star system to the Sun - might host a livable planet was, like the TV show itself, dubious fiction. But now a team of creative NASA scientists is designing a telescope to see if this world might actually exist. The team reckons they have a better-than-ever chance to find it.

Unlike when the Space Family Robinson decided to quit Earth, today we know a lot about worlds orbiting other suns. In particular, it's now clear that most stars have planetary consorts. The tally of confirmed and suspected exoplanets exceeds 5,000 - impressive bounty from two decades of astronomical effort.

But with so much real estate already on the books, you might wonder why anyone would brighten at the thought of finding one more world. Well, there's now some serious talk (albeit only talk) of launching rockets to the stars, and Alpha Centauri is 30 percent closer than Barnard's star, the next one out. So if humans ever do send emissaries - either instruments or passengers - beyond our own cosmic doorstep, a habitable planet at Alpha Centauri will be a convenient truck stop. And even if we never cross the final frontier, such a planet would be close enough to be intensively studied with king-size telescopes now on the drawing boards.

It's noteworthy that the NASA team won't resort to the usual methods for hunting down this putative planet. They're not going to measure the wobble of the host star or look for a periodic dimming of its light. Instead, their scheme is as straightforward as a desert highway - they hope to take its picture.

Straightforward, but not easy. Stars are bright, and planets are dim. Indeed, an Earth-size planet with an Earth-like orbit in the Alpha Centauri system would be roughly one ten-billionth as luminous as the stars. To reprise the heavily worn metaphor exoplanet hunters routinely trot out at dinner parties, finding such a planet is akin to seeing a firefly next to the brilliant beam of a searchlight. Even Hubble can't do it.

But a special-purpose space telescope with a mirror only 18 inches in diameter can, and it's being designed and built. It's called ACEsat (Alpha Centauri Exoplanet satellite), and is planned for launch before 2020. Its most endearing feature is an ability to blot out most of the starlight from any object it's looking at. This is done by inserting a device into the telescope known as a coronagraph - essentially an opaque, mechanical thumb that blocks the majority of the star's glare, but not the light from any planets.

ACEsat has more up its metallic sleeve. Borrowing an approach used by backyard astronomers to make sharper pictures of Jupiter and Saturn, the telescope will take stacks of photos of Alpha Centauri. "Stacks" is the technical term for tens of thousands. And just as combining piles of thermometer readings permits climate researchers to tease out changes in global temperature of only a fraction of a degree, the ACEsat telescope will be able to pluck the faint shine of an Earth analog from a myriad of noisy photos. An Earth-size world won't show up in any single shot, but when they're averaged, it will.

And if it does, what will it look like? Surprisingly, no more than a small dot.

That doesn't sound like much. If you make wedding photos for your sister, and all the relatives appear no larger than a dot, she probably won't be ordering prints. But in astronomy, that small image can sometimes be enough to make a major discovery. By passing the light through a prism and analyzing the resulting rainbow of colors, researchers can determine what's in the atmosphere of the planet. If there's oxygen or methane in the air, that might be a major tipoff for life.

In addition, by watching how the brightness of the image changes with time, it may be possible to determine the presence of oceans and continents.

There are a trillion planets in the Milky Way. Only one of them can be the nearest cousin of Earth. Such a world, once discovered, might become the most tempting object in the sky. Perhaps even a lure for future space colonists.

But what if ACEsat spends two years staring at Alpha Centauri, and comes up empty? Eduardo Bendek, one of the NASA scientists designing the telescope, is realistic. "Of course it's possible that we won't find a planet," he says. "But even if that happens, this project will still be valuable. That's because we're pioneering new technologies and new strategies that can be used for future planet-hunting telescopes."

Indeed, efforts to image planets is destined to become a major activity in the coming decades. As for Alpha Centauri - well, if explorers were discouraged by the thought of failure, that globe in your den would still be largely blank.