There may be millions of Earth-like worlds out there, but we won't be sure we've found a second home until we can get a good, hard look at it. That's why a rather unremarkable quarter-sized glass disk has set the exoplanet community abuzz.
"With this new coronagraph we are now looking for planets around nearby stars," said astronomer Jared Males of the University of Arizona in a statement. "We have the capacity to directly detect, or rule out, planets smaller than Jupiter."
Coronagraphs—telescope filters that block starlight—have been around for nearly a century. But the new "vector Apodizing Phase Plate" (vector-APP) coronagraph, developed by astronomers at Leiden University and the University of Arizona, is a bit special. Using advanced liquid crystal technology similar to that found in computer monitors and TV screens, this optic cleverly bends and cancels the light surrounding a star, allowing astronomers to see the faint image of distant worlds.
The device, which saw its first starlight last month, will help us discover new planets in our cosmic neighborhood and begin peering into their atmospheres, hunting for signs of life.
A generation ago, the idea of planets outside our solar system was borderline science fictional. But the cosmic veil was lifted this past decade, as NASA's Kepler space telescope and other exoplanet-hunting missions uncovered thousands of new worlds. Incredibly, astronomers now believe there are more planets than stars in the galaxy. With a billion-plus potential real estate deals out there, the million dollar question is whether there's another Earth waiting for us.
To answer that question, we need more powerful optics. Missions like Kepler detect exoplanets indirectly, by measuring the faint dip in a star's luminosity as a planet transits across its path. This itself is no small feat: Exoplanet researcher Natalie Batalha compares witnessing an exoplanet transit event to detecting the change in light if one person in a fully lit up skyscraper lowers the blinds in one room by an inch. But impressive as this sounds, transit surveys can't spot all planets, and at best, they can only reveal a planet's orbit, size and mass.
To learn whether the air on a distant world contains the telltale signatures of biology, we need telescopes that can pick up the starlight filtered through its atmosphere, or better yet, the light pouring off the planet itself.
So-called "direct detection" is extraordinarily difficult, in most cases impossible, as a star like the Sun is roughly a billion times brighter than the reflected light from an exoplanet in orbit. In a few instances, astronomers have been able to directly image larger-than-Jupiter-sized worlds orbiting at a great distance from their parent star, using giant telescopes outfitted with starlight-blocking coronagraphs. But none of the gaseous supergiants is a very good candidate for life. To measure anything Earth-sized and rocky, we'll need to do a much better job blocking the overwhelming glare of the stars.
Vector-APP differs from earlier coronagraphs in that it doesn't suppress starlight directly. Rather, it manipulates the phase of lightwaves as they enter a telescope, causing them to cancel each other out in the region surrounding a star. Using several layers of liquid crystals thinner than a human hair, the device is able to cancel nearly all starlight across a wide range of wavelengths, including the infrared region, where planets tend to glow brightly but are normally outshone by their stars.
In May, astronomers pilot-tested their new optic on the Magellan Clay Telescope at the Las Campanas Observatory in Chile. This massive telescope produces sharp images of distant stars, which the new coronagraph was able to split up, carving out dark regions where much fainter planets can be detected. With this successful proof-of-concept, astronomers hope to start using their new tool to discover new exoplanets and study their composition in unprecedented detail.
"Once we have directly detected exoplanets, we can immediately start analyzing their light to infer the properties of their atmospheres," Frans Snik, an astronomer at Leiden University who helped develop vector-APP, told me in an email.
The main technique for characterizing atmospheres is spectroscopy—measuring the emission of light from an astronomical body across a broad range of wavelengths. Since different molecules will absorb and reflect different wavelengths of light, spectroscopy offers astronomers a way to "fingerprint" planetary atmospheres and hunt for distinct compounds, including oxygen, methane, and water. According to Snik, vector-APP is not only well-suited for spectroscopy, it can also measure the polarization of visible light bouncing off a planet, which helps reveal how cloudy the skies are.
The team has already installed a version of vector-APP on several of the world's largest ground-based telescopes, and they're hoping to get the next generation of extremely large telescopes outfitted as well. These future ground-based scopes will work in concert with the Transit Exoplanet Survey Satellite (TESS), which will begin searching our neighboring star systems for planets upon launching into orbit in 2017.
"Transit missions like TESS will indirectly detect many rocky planets around nearby stars, perhaps even a few habitable Earth-like planets," Snik told me. "They will thus point the way for us to put these planets into direct view, and scrutinize them for signs of life."
Every time we build a new piece of technology, we see the universe differently. If the first telescopes able to spot the shadows of planets on stars filled a barren galaxy with worlds, I think it's fair to say that when we set our new eyes to the sky, we're going to discover wonders.