In Search of Distant Planets

by Marshall Perrin

The search for distant worlds has been a staple of science fiction stories since the dawn of the genre. Who hasn’t read or watched as bold explorers travel the void “to explore new worlds and seek out new life,” to quote the famous Captain Kirk? Central to this vision has been the idea that the way to discover new worlds around distant stars is by someday traveling there in person. Yet today, scientists around the world are already discovering worlds around other stars, worlds by the dozen, which are gradually yielding their secrets to us. I am a graduate student in astrophysics at UC Berkeley, home of one of the most successful planet searches presently underway. It’s a tremendously exciting time to be starting a career in this field, as questions first asked millennia ago are finally being answered. The tools for finding these new worlds are telescopes and computers, not spaceships, and the discoveries being made today are changing the way we understand our own place in this universe.

Wobble, Wobble Little Star

We can see the planets in our own solar system by reflected sunlight, but only at night when the sun’s glare does not obscure them. If we try looking directly for planets around other stars, we find the planets’ reflected light is millions of times fainter than their parent stars. Lost in the glare, we must find some other way to detect them.

In the early 1990s, several groups of astronomers came to the same realization. As a planet orbits a star, its gravitational pull tugs on the star and causes it to “wobble” back and forth slightly. Astronomers hoped that painstaking measurements, using some of the world’s largest telescopes, might detect planets not by reflected light, but by the wobbling of the stars they orbit. In theory, careful measurements could reveal the masses, distances from the star, and orbital periods of planets we cannot see directly.

Planets Unexpected

Only in the past decade have astronomical detectors reached the precision necessary for making the necessary observations and computers reached the speeds and power needed for analyzing the data. Two major teams, one led by Michel Mayor and Didier Queloz in Switzerland, and one led by Geoff Marcy here in California, set out to attempt these observations.

The easiest planets to detect by this method are massive ones, like Jupiter and Saturn in our own solar system, which exert a strong gravitational pull on their star. These two planets move in slow and stately orbits, taking decades to swing once around the sun. Everyone expected the radial velocity observations to take similarly long periods of time to record slow wobbles of distant stars. It thus stunned the world in 1995 when Mayor and Queloz reported a planet much larger than Jupiter which orbited not once every four decades, but every four days. In the years since, Marcy’s team here at Berkeley has led the way in discovering dozens more such “close-in giant planets,” as they came to be called. Perhaps even more surprisingly, planets which orbited further and more slowly were eventually discovered, but they were found to have very elliptical orbits. Planets four or five times larger than Jupiter were found in orbits more like that of a comet than the nearly circular orbits of our own solar system. Some of these planets are real monsters, too, weighing in at an astonishing ten or fifteen times the weight of Jupiter!

Understanding These New Worlds

These discoveries posed a serious challenge for those seeking to understand planetary systems--no one ever expected worlds like these! Planets are thought to form from a disk of gas and dust swirling around an infant star. Astronomers believed that giant planets could only form far from a star, like the giant planets in our own solar system. Closer in, higher temperatures prevented most ices and gases in the disk from condensing, resulting in small, rocky planets like Earth and Mars. Planet formation should proceed in an orderly fashion, resulting in the tidy, circular orbits of our own solar system  or so we thought.

The picture that has emerged from the study of extrasolar planets is a far more complex one. Baby planets exert gravitational forces on the “circumstellar disks” of gas and dust around a young star, perhaps creating spiral density waves like those familiar in spiral galaxies. These waves can push planets around, in effect forcing them into highly eccentric orbits. Tidal drag forces from the disk can make giant planets which formed at great distances from a star migrate inwards until they skim barely above the sun’s surface in four day orbits. In some cases, planets may even migrate so close as to be swallowed whole by their parent star! Planets can interact with each other, too: Close encounters and indeed direct collisions between massive worlds can change their orbits drastically, creating eccentric orbits or sometimes tossing one out of the system entirely. The processes at work in infant solar systems are many, and our current understanding of them is only at the roughest of levels. There’s truth to the old saying that every answer we obtain raises two new questions, but tremendous progress is being made all the time by scientists and students working around the globe.

Moving Forward

This revolution in planetary astronomy continues to be driven forward by technology. Today, we can detect only the largest planets; the wobbles induced by smaller ones are just too hard to see. But techniques are rapidly improving. Astronomy today is one of the most computerized of all the sciences; long gone are the days of the noble observer with his eye pressed to the back of a gleaming glass and chrome telescope peering at the heavens. Today that astronomer is much more likely to be found sitting in front of a computer, controlling electronic instruments with far greater sensitivity than the human eye (and much greater patience during long winter nights, too!). An increasing number of observations are carried out with space based instruments as well, although general purpose telescopes like Hubble aren’t that well suited to the task of planet hunting. Rather, a number of dedicated missions are planned for coming decades, with names like the Space Interferometry Mission, Terrestrial Planet Finder, and ultimately perhaps the Terrestrial Planet Imager.

Some of these missions will try new methods for detecting planets instead of the tried-and-true radial velocity wobble, in the hopes of seeing different types of planet. For instance, one proposed mission, called Kepler, would monitor a hundred thousand stars continuously for five years, looking for rare eclipses of stars by Earth sized planets. Meanwhile, more advanced ground based telescopes will surely continue to find planets by tracking the wobbles of stars. In addition, now that we know for sure that planets are out there, some astronomers are starting to think harder about that problem of directly imaging a planet in reflected light amidst the glare of a star a million times brighter. This is where I myself am working, as part of a team using specialized techniques called adaptive optics and coronography to try to image the companions to distant stars. It’s a challenging proposition, but the potential payoff is immense.

There’s No Place Like Home?

This past August, Dr. Marcy’s team announced a long awaited discovery: a solar system that looks a lot like our own, at last. With five dozen planetary systems found so far that look nothing like ours, astronomers were starting to wonder if our solar system was one of a kind. But Marcy and his team discovered evidence for two planets orbiting a star in the Big Dipper named 47 Ursa Majoris, two worlds moving in nice circular orbits and sized similarly to Jupiter and Saturn. But each answer begets more questions: Are there also small rocky worlds we cannot yet detect in that system? How common are Earth like planets? Ultimately, do any harbor life, even intelligent life? The chances are greater than ever that these questions will be answered, at least in part, within our lifetimes. Maybe they’ll even be answered by some of you!