The Sciences

The Search for Earth-Like Worlds

The gravitational tug of a giant like Jupiter could lead us to Earth II

By Robert IrionOct 24, 2005 5:00 AM


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After a decade of hunting for planets circling other stars, astronomers have turned up hot planets, puffed-up planets, and planets in bizarre, looping orbits. What they have not found is the thing they dearly seek: a planetary system that meets two crucial conditions for supporting life. First, it should have a small, rocky body orbiting in the habitable zone, where conditions are right for liquid water on the surface. Second, it should have a more distant, giant planet to protect that inner world. Both of these goals are almost in sight.

To zero in on Earth-like worlds, astronomer Steven Vogt of the University of California at Santa Cruz and his colleagues are developing an $8 million robotic telescope. The facility is called the Automated Planet Finder, “but it’s really the rocky-planet finder,” Vogt says. “It’s optimized to find rocks in habitable zones.”

The Automated Planet Finder will detect planets by tracking subtle wobbles in the light from nearby stars. Such wobbles are caused by the back-and-forth gravitational tugs of planets circling around them. Vogt’s group, led by Geoffrey Marcy of the University of California at Berkeley, has already detected more than 100 planets this way, but the tugs produced by small, Earth-like worlds are particularly elusive. The new telescope employs several tricks to aid the hunt. A computer program will direct its 95-inch mirror at dozens of targets each night, zipping efficiently from star to star. And the search will concentrate on stars already known to have at least one planet—locations where we know the raw materials exist for building other worlds. “The best place to find a planet is where you’ve already found another planet,” Vogt says.

The final advance is a custom-built spectrograph—a device that analyzes a star’s light—nearly three times as sensitive as the ones used in other planet searches. It will detect stellar motions as slight as three feet per second toward or away from us, about the speed of a person walking. By next spring, the planet finder will start scanning the skies on every clear night from Lick Observatory near San Jose, California. Vogt expects to detect worlds just five times the mass of Earth and possibly some as small as Earth itself—bodies that would almost certainly have solid surfaces.

Meanwhile, Marcy’s team and a competing Swiss team are closing in on “true Jupiters,” bodies whose masses and orbits mirror those of our solar system’s biggest planet. Such planets may create stable regions where Earth-like worlds can form and may shield those worlds from collisions with comets and asteroids. Finding a true Jupiter is mostly a matter of patience. Researchers need to track a suspected planet for at least one full orbit, or about a decade, for objects at Jupiter’s distance from the sun. The American and European astronomers anticipate announcing true Jupiters within the next few years. Other teams hope to observe them directly. The Keck Observatory in Hawaii, the Very Large Telescope in Chile, and the new Large Binocular Telescope in Arizona will soon combine light from multiple mirrors in ways that could isolate a true Jupiter from the glare of its parent star.

Ultimately, the planet hunt will move into space, above Earth’s blurring atmosphere. In about two years, NASA’s Kepler probe will look for the shadows of small planets passing in front of their stars. Sometime after 2014, NASA’s Terrestrial Planet Finder and the European Space Agency’s Darwin mission will attempt to make the first direct images of rocky worlds. However, those space missions will cost billions of dollars, making them prime targets for budget cuts.

For a fraction of the cost, the Automated Planet Finder and other ground-based programs are at least kicking off a sophisticated search. Within just a few years we may have such an interesting planet catalog of Earth-like possibilities that we won’t be able to resist putting the Terrestrial Planet Finder, now in development, out there.

Planets with a layer of diamond?

So far astronomers have found at least 161 planets orbiting other stars. Most of these alien worlds circle middleweight stars that are superficially similar to the sun (but in many cases somewhat cooler and redder). The known planets have a bewildering variety of compositions, masses, and orbits. Nonetheless, the universal rules of physics and chemistry suggest that they will broadly fall into just a few types, the most likely of which (above) are mind-boggling.

Alan Boss, an astrophysicist at the Carnegie Institution of Washington, develops models to explain where and how planets form. The past decade has kept him busy.

We’ve seen 10 years of discovering exoplanets. What has it been like for you?

B: It’s been a major change. I spent the first 10 to 15 years of my career worried primarily about star formation. These days nearly all of my research is going into modeling planet formation. I’ve been so swayed by this that I’m now trying to find some planets myself, even though I have been a theorist all my professional life. I’m trying to learn to become an astronomer at the ripe old age of 54.

What have been the biggest surprises?

B: When we knew only about Jupiter, we thought that was the most massive planet that could be. We know now of planets as much as 10 times as massive. Another surprise is that a number of these big planets are on fairly eccentric orbits, not circular like those of Jupiter and Earth. That means their gravitational force extends over a large range of orbital distances, making it much harder to grow a planet like Earth and to keep it stable. And from the very first detection, we learned that some giant planets migrated far inward from where they formed. Somehow they stayed on parking orbits roughly 100 times closer to their stars than Jupiter is to our sun. No one had predicted that.

Can we now say our solar system is rare?

B: We don’t know. With current technology, we wouldn’t yet have been able to detect ourselves around another star. We’ve basically ruled out 10 percent of the nearby stars as being like our solar system, but the other 90 percent could be just like us, and we wouldn’t know it yet. But we’re right on the verge now.

Are you confident we’ll find rocky planets in orbits similar to Earth’s?

B: It’s inevitable that such worlds exist. The question is the frequency. A lot of people think that 10 percent of nearby stars have Earth-like planets close enough to their stars to have liquid water, but it could be 1 percent or even lower. That still would mean millions of Earths in our galaxy.

What will we know about alien planets a decade from now?

B: We should know how common Earths are. We’ll find out which nearby stars have them. The first Terrestrial Planet Finder mission will take optical images of nearby stars and essentially put a thumb over the star itself to block the light and see the dim planet. It will start giving us our first real pictures from sunlight reflected off the planet’s atmosphere. That will tell us something about the planet’s atmospheric composition and whether it has the molecules associated with life. It’s going to be an exciting decade.

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