Building An Eclipse Machine
Scientists have surveyed, photographed, and even landed spacecraft on the planets in the Solar System. Now, their drive to understand more about the nature of planets is taking them further afield. More than 100 extrasolar planets—planets that orbit stars other than the Sun—have been detected in recent years, but not one has yet been observed directly. That's because the light from a star is typically a billion times brighter than any planet that orbits it, rendering the planet invisible to scientists on Earth.
To overcome this obstacle, astronomers have turned to coronagraphy, an imaging technique invented in the 1920's by the French astronomer Bernard Lyot. A coronagraph is a device that attaches to a telescope. It consists of a series of mirrors that aim light from a star at a small opaque disk. The disk blocks out the brightest light emitted by the star, thereby permitting dimmer objects near the star to be seen. Think of it as an artificial eclipse inside the telescope. Lyot built the first coronagraph to make pictures of the corona, the pale halo of gas that emanates from the Sun. Lyot's coronagraph was basically an opaque disc; it blocked out the Sun, and the luminous "atmosphere" of gas became visible.
The Lyot Project kicks off
Rebecca Oppenheimer, an astrophysicist at the American Museum of Natural History, has built a coronagraph that he hopes will take the first-ever photograph of an extrasolar planet. The Lyot Project Coronagraph, as it is known, was assembled on the sixth floor of the Museum's Rose Center for Earth and Space in a dust-proof "clean room" designed specially for the construction of delicate stargazing instruments. This one, however, looks less like a sophisticated astronomy device than a playground for pint-size robots. Ten mirrors of different sizes sit in aluminum mounts bolted to a large steel table. The contraption weighs nearly a ton and floats on a cushion of air that protects it from vibrations.
In March 2004, the coronagraph was flown to Hawaii and installed in the Maui Space Surveillance System, an observatory at the summit of Maui's Mount Haleakala. Stationed 3,000 meters (10,000 feet) above sea level, the observatory sits above most of the moisture, cloud cover, and atmospheric turbulence that can disrupt a clear view of the night sky. Indeed, before the telescope even feeds an image to the coronagraph, it will refine the image to compensate for atmospheric turbulence, which would otherwise scatter and blur incoming starlight. The technique is called "adaptive optics," and the Maui Space Surveillance System is equipped with the world's most precise adaptive optics system. It employs 941 tiny motors, or actuators, glued to the back of a mirror 23 centimeters in diameter and 2 millimeters thick. Each actuator can push or pull the mirror to shift its surface by a few thousandths of a millimeter. The telescope feeds the adjusted starlight to the coronagraph, which has its own adaptive optics system consisting of several little motors that track the incoming starlight. A computer analyzes the incoming light and decides whether to further adjust any of the motors. "When you're trying to see something that could be a billion times fainter than the star, a slight misalignment could completely impair your ability to see," Oppenheimer says.
Little room for error
Creating the Lyot Project Coronagraph was a design challenge, requiring a team of at least ten scientists from the American Museum of Natural History, the University of California at Berkeley, the University of Hawaii, Caltech, and the Space Telescope Science Institute. The concept is straightforward: take an image, subtract the star, then make a new image that will be analyzed for visible signs of an extrasolar planet. Of course, Oppenheimer says, "in practice it's more complicated." The Maui telescope feeds starlight to the coronagraph as a very thin beam of light; the beam then bounces off ten small mirrors before passing through a filter, which blocks the brightest light and saves the rest for analysis. For the device to function properly, each mirror must be aligned to within half a micron (one two-thousandth of a millimeter) of perfection. "We're talking about something much more refined than aligning your eyeglasses," Oppenheimer says with a laugh. The work can be "very mundane," he admits, "but it's fun. In the end you have something that works. Instead of simply reducing data on a computer or looking at equations, as theorists do, you're actually building an instrument. You take it out to the telescope, and you get to see two years of your own work actually do what you wanted it to do. It's just fantastic."
Since it was installed on Haleakala, the coronagraph has surveyed about 100 nearby stars, looking for anything faint—disks of debris, or possibly even planets—that might be orbiting them. "To me the most exciting thing about the Lyot Project is that we're exploring a region of space that has never really been explored before," says Oppenheimer. The team took pictures of stars relatively near Earth, including some identified by NASA's Spitzer Space Telescope as having material in orbit around them. The initial survey in Maui has not only studied several planet-forming debris disks around nearby stars, but has also led t o many insights and improvements in the techniques of coronagraphy and extrasolar planet imaging. In the next year and a half, the Lyot Project will move to the Palomar Observatory in Southern California, where a more capable adaptive optics system is under development. There, a longer-term, deeper survey of nearby stars will be conducted with vastly improved sensitivity.
If and when an object is found orbiting a nearby star, scientists will study the light it produces or reflects to determine whether it is a planet or some other celestial object. Closely analyzed, the spectrum of light may reveal telltale signs of certain chemical molecules, which could indicate that the possible planet has an atmosphere. Scientists will also look for signs of distinct minerals to analyze what the planet is made of. "This information tells us about the physics of the object itself, whether it might host life, and what sort of life it might host," Oppenheimer explains. "For example, what would a planet five times larger than Earth look like? Would it be a rocky place with very short-legged animals, because of the intense gravity? And understanding the physics of these things will settle the debate about what a planet is—a subject of considerable debate among astronomers today."
Most everything that scientists know about planets and what to expect from them—what they look like, how and where they form—is based on the study of the planets that orbit the Sun. Oppenheimer's hope is that, in their hunt for extrasolar planets, astronomers will find something wholly unexpected—"something so different from predictions that new theories will have to be formed in order to explain them. The bottom line is that we don't know what we'll find."