WIRED | February 23, 2013
Extreme Exoplanet-Hunting Telescope to Go Online This Fall
The search for exoplanets is about reach the next level with the Gemini Planet Imager, a new smart-car-sized telescope instrument that will use “extreme” adaptive optics to directly see distant planets around other stars.
Most large telescopes on Earth use adaptive optics — mirrors that wiggle a thousand times a second — to compensate for distortion from the atmosphere that causes the familiar “twinkling” effect of stars. With the technology, fuzzy globs of starlight are transformed into sharp pinpoints. GPI will take the method a step farther, using a mirror made from advanced silicon microelectromechanical systems (MEMS) instead of glass.
The system uses two quarter-coin-size silicon wafers to erase blurred light. GPI’s computer will send electrical signals to more than 4,000 actuators to warp the super-thin mirror painted on the upper layer. To accommodate that many sensors, a conventional adaptive optics mirror would have to be more than 15 inches across, bigger than a MacBook Pro. That would make GPI far too big to fit on its intended telescope: the 8-meter Gemini South telescope in Chile. MEMS’s compact deformable mirror will provide an image much brighter and sharper than that of any other ground-based telescope.
Most exoplanet searches rely on indirect methods, for instance inferring the presence of a planet by noticing how it tugs gravitationally on its parent star. GPI will actually take direct images of distant exoplanets. A component of the instrument called a coronagraph blocks light from the extremely bright star, but lets light coming from the hot, young planets circling the star to pass through, enabling astronomers to see those planets.
“There’s something satisfying about seeing the little dot,” said astronomer Bruce Macintosh of Lawrence Livermore National Laboratory and principal investigator for GPI.
|A simulation of GPI’s view of exoplanets (right) compared with current capabilities (left). Images: Christian Marois/Herzberg Institute of Astrophysics and Marshall Perrin/Space Telescope Science Institute
Another GPI instrument, a spectrograph, detects infrared heat from planets and reads the signature of compounds like methane, water vapor, or carbon dioxide in their atmospheres. “The most exciting thing is that it can really analyze the planets’ compositions,” said astronomer Michael Liu of the University of Hawaii. “That’s a completely new tool we never had before.”
One limitation is that MEMS can only move about 4 micrometers — not enough to completely correct the distortion. So GPI also has a conventional glass deformable mirror — the woofer to MEMS’s tweeter, as Macintosh puts it — to collect and correct a wider range of light. The machine also needs built-in coolers to keep it at -203 Celsius to prevent internal heat from interfering with its readings.
The “adolescent” stars GPI will study are up to 500 million years old, a tenth the age of our solar system. GPI won’t be able to see Earth-size planets, but it will be able to see larger exoplanets with orbits similar to or wider than our own gas giants Jupiter and Saturn. Spotting arrangements of these planets will reveal details of how planetary systems form.
A nervous energy and excitement is building among the exoplanet-hunting community as it waits for GPI to come online.
“For all we know, GPI could be the next Kepler,” said astronomer Sara Seager of Massachusetts Institute of Technology, referring to the space-based telescope that has so far spotted more than 800 exoplanets. “They’re bound to find many, many fascinating objects, because it’s a new eye on the sky.”
But both Liu and Seager point out that there’s a slim chance GPI won’t find planets in the orbital distances it will search. “You just have to hold your breath because…what if there aren’t planets?” said Seager. “What it finds kind of depends on what’s out there.”
If GPI finds a dearth of planets in the orbital outskirts, it will tell us that the layout of other solar systems are very different from our own, and it will also mean fewer chances to study exoplanet atmospheres. GPI can’t distinguish planets that have snug orbits around their stars.
The instrument’s components are currently being tested at UC Santa Cruz. Macintosh hopes GPI will be online in September or October, ahead of SPHERE, a similar project led by European astronomers. It’s turned the quest to see exoplanets from Earth, considered nearly impossible a decade ago, into a tight sprint to the finish line.