Planet hunters are right on the verge of finding a world beyond the sun where life could plausibly exist — a planet more or less the size of Earth, with balmy temperatures and enough water to sustain biological activity. At this point, it's a numbers game — exoplanets are being discovering by the net full already, so figure a year or two at the outside before a just right world is found.
But determining whether life actually does exist on this warm, wet (and still hypothetical) planet will be another thing entirely. You don't have to see a planet to prove its existence; detecting the gravitational tug it exerts on its parent star is enough. Finding evidence of life, however, requires a direct, visual sighting, and that's a much tougher challenge. NASA once had grand plans for a space telescope called the Terrestrial Planet Finder, or TPF, which would launch as early as 2020. But that work has been put on the far back burner, thanks in part to the agency's ongoing budget woes.
The money hasn't dried up completely, however: funds are still trickling out for research into technologies that could someday make TPF a reality — and one of the ideas under development is breathtaking in both its simplicity and its audacity. What makes an exoplanet so hard to detect is the much brighter light that streams from the star it orbits, which washes out the image any faint bodies nearby. The trick, then, is to block that light — much the way you can use your thumb to block out the glare of the sun. A team at Princeton and the Jet Propulsion Laboratory (JPL) now proposes to work a similar optical trick by flying a giant "starshade" in space, positioning it tens of thousands of miles away from a big orbiting telescope and covering up just enough stellar light to make a planet pop into view.
It's not an entirely new idea: a wildly creative Princeton astrophysicist named Lyman Spitzer conceived of the concept in 1962. (Spitzer also came up with the idea that became the Hubble Space Telescope, and with the notion of producing energy through controlled nuclear fusion, so the man was nothing if not full of nifty inspirations.) "At the time," says Jeremy Kasdin, the Princeton mechanical and aerospace engineer who runs the new project, "the technology didn't exist to actually build it." But in recent years, the starshade has been revived, largely through the efforts of University of Colorado scientist Webster Cash.
In part, the rebirth came about because the original idea NASA had for blocking starlight turned out to be so hellishly complicated. The plan was to send up not one, but four big space telescopes that would fly in perfect formation. By adjusting the telescopes' separation and merging their images in a process called interferometry, astronomers would make a star's light cancel itself out, letting the planets shine through. But after spending $600 million on a much smaller, less ambitious version of the technology, the agency canceled that preliminary mission a year or two ago.
Not that the starshade, or, more formally, the occulter, is simple. You can't just put up a disk-shaped (or thumb-shaped) hunk of material because light from the star will bend around the edges to contaminate the image, just as ocean waves coming in at an angle will bend around a promontory. To eliminate that problem, the edge of the starshade has to be sculpted, and the ideal shape makes the whole thing look something like a giant sunflower about 160-ish ft. (49 m) across in total, with 20-ft. (6 m) petals.
In the first phase of the project, the Princeton-JPL team had to prove they could engineer the petals so that their edges were accurate to tens of microns. "Now we know we can do that," says Kasdin. They're currently in Phase 2: demonstrating that they can get the petals to unfurl from a stowed position — since there's no way to get a 160-ft. object into space without folding it up — and arrange them to within a millimeter of where they're supposed to be. Then, if the mission ever flies, the starshade will have to maintain its alignment with a telescope that's maybe 36,000 miles (58,000 km) away — with a margin of error of no more than about 3 ft.
Any actual attempt at so ambitious an undertaking is still far in the future. "There's no mission," says Kasdin. "NASA is funding our project and others, so that in five or six years they'll know enough to feel more comfortable proposing a mission."
One of the other competing projects Kasdin speaks of is a concept known as a coronagraph, which would put star-blocking technology into the telescope itself (Kasdin's team has funding to work on one of these as well). And NASA is putting money into the original, light-canceling multitelescope idea too. "I give them a lot of credit," says Kasdin. "They're looking at many different pieces of the puzzle."
Still, this one, flower-shaped piece has something going for it that the competition doesn't: the telescope itself would be just a telescope, without any of the fancy add-ons the coronagraph would require; and it would be a lot easier to manage than the four-telescope fleet in the interferometry scheme. In fact, you could even use a starshade with the James Webb Space Telescope. That's a mission that does exist and, despite budget problems of its own, appears to be moving ahead with new momentum. It's not crazy to think that the detecting life on a distant world may a lot closer than everyone imagines — it just may take a telescope with sunglasses to do the job.