#11: Geoff Marcy Leads the Hunt to Find Planets Like Our Own

By Michale LemonickDec 16, 2010 6:00 AM


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The universe is looking a lot less lonely these days, and Geoff Marcy can take a lot of credit for that. An astronomer at the University of California, Berkeley, he is leading the search for exoplanets: worlds that orbit other stars. His research has uncovered many oddities, such as hot Jupiters (gas balls that bake in thousand-degree heat) and backward-orbiting objects, but he also found the first multiplanet system that is roughly analogous to the solar system. Last fall he estimated that, judging from his observations, our galaxy may contain tens of billions of planets roughly the size and mass of Earth. And now, as coinvestigator on NASA’s Kepler space telescope, he is close to finding some of them. In February, Kepler will release its first major set of observations; early word is that the data will include tentative identification of several hundred planets that are just slightly larger than our own.

Where does exoplanet science stand now? It’s been an explosion. More than 450 exoplanets have been recognized. We can’t keep track of them. Fifteen years ago there was doubt whether we would find a single planet around a sunlike star, and here we are overwhelmed. Just vetting them and cataloging their masses and orbits has become a challenge.

Which advances from last year were most important?Studies of planetary orbits have brought incredibly exciting results. You would expect planets to orbit in the same direction as the star spins: If the star rotates counterclockwise, the planets should orbit it counterclockwise in a plane aligned with the star’s equator. In our solar system and in the first dozen exoplanets that we measured, that’s what happens. Then we started finding some that were misaligned—planets with tilted orbits or planets going around their star in the opposite direction from its spin, in what we call a retrograde orbit.

What is so important about backward planets? Back in 1995, when we found hot Jupiters orbiting close to their stars, everybody said they form far away from the star and then lose energy and migrate closer. But that would never explain why they end up in these retrograde and misaligned orbits. Somehow their orbits get jerked out of that plane. It’s likely that violent gravitational interactions between planets slingshot one of them close to the star, and then the orbit slowly circularized in some cockamamy orientation. This is mind-blowing. It shows our old idea that these planets migrated closer to their star over time was wrong. We’ve all taught our students about migration, and it’s at best only partially right.

How can you estimate the number of Earth-size planets in the galaxy? The goal of this work that I did with Berkeley astronomer Andrew Howard was to measure the fraction of stars that have small planets in close orbits. We surveyed 166 stars for four years. When it was all said and done, the rate was about 12 percent. You can look up into the night sky, and about 12 percent of those stars have a super-Earth—a planet with 3 to 10 times the mass of Earth—orbiting within the distance separating Mercury and the sun. As the size of the planets we looked for decreased, the number that we found increased: We found more planets with 3 times the mass of the Earth than planets with 10 times Earth’s mass, more planets 10 times as massive than 100 times, and so on. This was a lifelong dream of mine, to have the distribution of planets down to three Earth masses, the smallest we could detect. Extrapolation of that trend suggests about one in four stars hosts an Earth-size planet, which we define as one with a mass between one-half and two times the mass of Earth. We’re edging closer, but so far no one has announced the discovery of a truly Earth-size object.

I notice you carefully said “so far.” That’s right. [Laughs.] Results from Kepler will be coming out in February, including the data from the 400 stars that have been held back until now. You can just bet what’s in there. The implications of those planets are so profound that we’ve got to work harder before we make them public. I can’t reveal too much yet, but you get the idea.

There have been some grumblings about the withholding of that data. What’s your take? Aristotle wondered whether Earth was unique, and his question is still with us. We were just asking for another six months to answer it carefully. Only after great deliberation at nasa did we decide that it was in the best interest of science to look really hard at those 400 stars that have interesting candidate planets. For my colleagues who are impatient, I sympathize. But Aristotle’s been waiting 2,400 years—I bet he’s willing to wait another six months.

What does that extra time allow you to accomplish? Kepler looks for recurring decreases in a star’s brightness, indicating that a planet is repeatedly passing in front of the star and blocking some of its light. The littlest planets are very difficult to confirm. We’re working our butts off. There are about 30 people at nasa Ames working 18-hour days to get the photometry [star brightness measurements] right. To find an Earth, you’d better have photometry that’s unassailable at the one-hundredth of 1 percent level. No one has ever measured the brightness of stars to that precision minute after minute for a year. And once Kepler identifies candidate small planets, we have to figure out a way to rule out bogus ones. That’s hard.

Aside from Kepler’s big reveal, what else do we have to look forward to this year? One really exciting thing on the horizon is a new camera called the Gemini Planet Imager, which will be attached to the Gemini South telescope in Chile. It’s supposed to be finished in mid-2011. They’re building a super adaptive optics camera with a coronagraph that blocks a star’s light in order to image its planet. Right now there are three stars that I’m aware of for which there are imaged planets. [Most are studied indirectly, which limits what we can learn about them.] But this thing was designed specifically to find planets, and when it’s done, they’re going to find them in droves. The Europeans are building a competitive instrument for the Very Large Telescope. They’re both in the Southern Hemisphere, so they’ll be seeing the same goddamn stars. It’s a race. The first camera to be completed and go on the telescope is going to find all the low-hanging fruit.

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