Somewhere Out There

May 01, 1999

Speculators on the night sky, astronomical and other, have long suspected that we were not alone. Putting aside the question of extraterrestrial life for the moment, weren't there any other planets out there, beyond our cozy family of nine? Other ordinary planets circling other ordinary stars?


It always seemed a good bet. Yet no —extrasolar— planet was actually discovered until 1991, when Alexander Wolszczan, now at Penn State and then working on the giant Arecibo radio telescope in Puerto Rico, picked up some quirky signals from a tiny star in the constellation Virgo. What Wolszczan heard, a semi-regular pulse from space, turned out to be three planets orbiting a pulsar, or dead star, some 7,000 trillion miles away. —Alex was so careful to confirm what he had observed that he didn't actually announce it until 1994,— said Larry Ramsey, Wolszczan's Penn State colleague.

The following year, Ramsey went on, two Swiss astronomers detected a planet circling the star known as 51 Pegasi, in the constellation Pegasus. —This was the first planet found around a solartype star.—

Since then, Ramsey said, new planets have been cropping up like daisies. The grand total, as of mid-February, was 18. It is hardly a coincidence, he suggested, that over roughly the same period, —A whole new generation of very large telescopes has come on line.—

Things Are Looking Up

Ramsey, a telescope designer himself, gave the sixth of this year's Penn State Lectures on the Frontiers of Science, —How Huge Telescopes Work in the Search for Other Planets.—

To start things off, he ran down the list: In the northern hemisphere, there are the Keck twins, Subaru, and Gemini North, all on Mauna Kea in Hawaii; the Magnum Mirror, or MMT, near Tuscon, Arizona; and the Hobby-Eberly, in west Texas.

Down south, in Chile, you have the European Very Large Telescope, the Magellan pair, and Gemini South. The smallest of these telescopes boasts a primary mirror that measures 6 meters across. The biggest mirror, at 11 meters, belongs to the Hobby-Eberly, conceived by Ramsey and Penn State colleague Daniel Weedman.

Sheer size is one of the keys to finding new planets. In terms of basic optics, both light-gathering power and resolution—the ability to make out fine detail—
increase with the diameter of the mirror. Both are crucial for picking out distant, smallish objects like planets, which throw off no light of their own.

The real problem, Ramsey said, is blocking out the light from a planet's parent star. As Andrew Fraknoi of the Astronomical Society of the Pacific puts it, seeing a planet in the glare of its —sun— is like trying, from the far end of a long, dark hall, to see a grain of rice suspended an inch or two away from a 100-watt lightbulb. In the realm of visible light, Ramsey estimated, the planet is a million times fainter than the star.

Shift over to infrared light, however, and you've got a different story. In the infrared wavelengths, a star throws less energy, relatively speaking, and a planet reflects more. As a result, Ramsey said, the planet is only a thousand times fainter. —Sensitive instruments can pick it up.— The infrared camera on the Hubble Space Telescope, which has the added advantage of operating above the muddle of Earth's atmosphere, has recently sent back amazing images of dust disks clustered around central stars—important information for the study of how stars form. But Hubble's mirror, at 2.4 meters, is too small to search for new planets, Ramsey said. For now, that job is left to the giants on the ground.

Fine Optics


A couple of recent technological advances have cleared the way. First, Ramsey said, —You have to have active optics.— A piece of glass this big, he explained, changes shape with the slightest change in its environment. It sags under its own weight from simple gravity. Its metal support structure expands and contracts with temperature. A shift as small as 1/100th the thickness of a human hair can skew the clarity of an image from deep space. With active optics, Ramsey said, computer-controlled sensors are mounted all over the mirror's back, and tiny motors can adjust to the slightest change in its shape, moving it precisely to keep it aligned.

The second necessary feature, he continued, is adaptive optics. Incoming light, he explained, is distorted by turbulence in our atmosphere. (—You can see this turbulence by looking across a field with binoculars on a hot summer day.—) The light waves are actually bent; a telescope only magnifies the distortion, making stars appear fuzzy and shimmery. With adaptive optics, a flexible secondary mirror is placed within the telescope to counteract this effect: The incoming light is bounced off the secondary mirror, which has been —deformed— to cancel out the distortion. By the time the light reaches the primary mirror, it's all straightened out again.

—It's very sophisticated stuff, and it's just in its infancy,— Ramsey said, —but it promises to make ground-based imaging as good as that from space.—

Watching the Wobbles

Thus far, though, the most productive approach to finding new planets has not been through direct imaging at all, but through spectroscopy.

—We all know how incoming light, dispersed by a prism, splits into different colors. Spectroscopy is just splitting it into more colors.—

A high-resolution spectrograph, he explained, splits a star's light through a grating, a super-sensitive —prism— which looks like a long, fine-toothed comb. The spectrum produced is then spread out on a charge-coupled device, or CCD, a light-sensitive receptor. What you see are stacked-up bands of light, broken into subtly colored segments by narrow black lines. These spectral lines, caused by the absorption of certain wavelengths as light passes through the atmosphere, are the key to finding planets.

A planet orbiting a star, Ramsey explained, exerts a gravitational pull, which causes the star to wobble. From our perspective here on Earth, the star moves slightly toward us, and then slightly away, as the planet circles around it.

domed telescope

Very slightly, since the planet is relatively tiny, the star huge, and the whole system light years away. In fact, the wobble is too slight (and too slow, at once per orbit) for anyone to actually see it. But this movement, and its speed, can be determined because of the Doppler effect.

The Doppler effect, Ramsey explained, causes the light waves streaming from a star that is moving away from the Earth to lengthen slightly, so that they appear to us more red in color. Those coming from an object that's getting closer to us are shortened, and so they appear more blue.

On a CCD, the Doppler effect causes those black spectral lines to shift—ever so slightly—as the star's light leans toward either the red or the blue end of the spectrum. Those infinitessimal shifts indicate the presence of a wobble, and if they occur periodically—first red, then blue, then red, then blue—that wobble suggests an orbit, which probably means a planet. —The name of the game for detecting planets is measuring those wobbles very, very precisely,— Ramsey said.

None of the new-found extrasolar planets has actually been seen. But careful spectral analysis can yield an estimate of each planet's mass, as well as the size and frequency of its orbit. —So far,— Ramsey said, —the planets discovered have been huge, Jupiter-like in size, and fairly close to their suns.— Not surprising, he added, since the largest objects with the shortest orbital periods are the easiest to detect.

Other Earths?

Are there other, smaller planets out there? Planets more like Earth? The search is in fullswing. Around the globe, Doppler surveys are currently keeping watch on over a hundred stellar candidates; when the Hobby-Eberly Telescope becomes fully operational in a year or two, Ramsey said, it will begin to track several hundred more.

Still more help is on the way. Ramsey's final image was a conceptual drawing for the aptly named Extremely Large Telescope, or ELT, to be built some time early in the coming century. The futuristic dome around the massive instrument, drawn to scale, looked as big as the Roman Colosseum. The primary mirror, Ramsey said, will be 35 meters across.

Lawrence W. Ramsey, Ph.D., is professor of astronomy and astrophysics, 517 Davey Laboratory, University Park, PA 16802; 814-865-0333. He is project scientist for the Hobby-Eberly Telescope, a collaboration between Penn State and the University of Texas at Austin.

Last Updated May 01, 1999