Volume 2, Issue 10
Berkeley's Star Planet Hunter
When most of us gaze up at the sky on a clear night, we marvel at the majesty of the constellations. UC Berkeley professor Geoffrey Marcy sees much more than that though. He's inspired by the fact that the thousands of visible suns could each be the center of a planetary system. Marcy is the world's foremost planet hunter. In the last decade, the team of astronomers that he leads has found more than 100 planets outside our solar system. Some of those planets could even have a rocky surface, ponds, or oceans like our Earth. A few of them may even support life.
Professor Geoff Marcy is consulting with NASA on their SIM PlanetQuest mission. Scheduled for launch in 2011, the unmanned spacecraft will help locate Earth-like planets with temperatures suitable for liquid water and, therefore, the possibility of life.
"Our astrophysicist geekiness makes us want to understand the full family of planets including their masses and chemical compositions," Marcy says. "But it's also true that there's an extraordinary question of habitability. The possibility that these planets may harbor some kind of life brings a twinkle to my eye."
The planet hunt picked up steam in 1995 when Marcy and collaborator Paul Butler, now a staff scientist at the Carnegie Institution of Washington, made their first discovery of a planet orbiting a star. Approximately 70 light years away, the planet's star, 70 Virginis, is visible to the naked eye but the bodies orbiting it are far too small and faint to be seen against the star's glare. Since they couldn't observe the planet directly, the scientists inferred its existence using a technique they devised nearly a decade earlier. Since that Eureka moment, Marcy and his colleagues have found planets as far as 150 light years away from Earth by measuring their parent stars' stellar wobble.
This artist's concept shows a newfound Neptune-size planet — one of the smallest extrasolar planets detected to date — circling the star 55 Cancri. In this depiction, the new planet has a rocky composition, like Earth. In reality, astronomers do not know if the planet is rocky or gaseous, like Jupiter. The planet's temperature is at least a scorching 1,500 Celsius (2,700 Fahrenheit). (Image courtesy NASA)
"There are 2,000 to 3,000 stars that are within 150 light years of us and we're surveying most of them," Marcy says. "That's the equivalent of looking in our galactic backyard."
To spot the planets, the scientists use giant telescopes like the WM Keck Observatory in Hawaii and the University of California's Lick Observatory east of San Jose to measure the change in the color, or wavelength, of light coming from a star over days, months, and years. Known as the Doppler shift, that change in wavelength is caused by the star orbiting a common center of mass with a companion planet tugging on it. For example, in our solar system Jupiter's gravitational pull causes the Sun to wobble in a circle at a velocity of 12 meters per second. The Doppler shift data can be used to infer an unseen planet's approximate mass, orbital size, and the time it takes to make one trip around the star.
Until last year, all of the 120-plus extrasolar planets discovered by Marcy or anyone else have been gaseous giants, much larger than even Jupiter and Saturn, the biggest planets in our solar system with respective masses 318 and 95 times that of the Earth. In August 2004 though, Marcy and Butler's team co-discovered a pair of planets comparable to Neptune, which has 17 times the mass of Earth. The composition of that planet remains a mystery, Marcy says. It could be gaseous, rock and ice, or rock and iron like our own Mercury. Still, the discovery opened a world of possibility of what their pioneering technique might reveal, Marcy says.
This illustration compares the size of the newfound Neptune-size planets to the sizes of Earth and Jupiter. The new planets are only about 20 times the mass of Earth -- much smaller than the majority of Jupiter-size extrasolar planets detected so far. Astronomers don't know if the new planets are rocky or gaseous, but a rocky planet (pink) would have a smaller diameter than a gaseous one (blue) of the same mass. (Image courtesy NASA)
"Finding the Neptunes showed us that we can look farther away for planets even lower in mass, possibly five or 10 times the mass of the Earth," he says. "Planets that size would almost certainly be rocky. So in the next two years or so, we're hoping to discover definitively rocky planets that you could stand on."
While searching for Earth-like planets with chemistry "that could allow life to flourish" is exciting, Marcy says, it's only one of his research goals. The other challenge is to detect "Jupiter clones" that are identical in two ways to our neighbor planet.
"We have not yet found a planet that's as far from its star as our Jupiter is from the sun and maintains a circular orbit," he says. "All of the planets in our solar system have a gorgeous circular orbit like the grooves in a phonograph record."
Marcy's team has discovered one Jupiter-sized planet with the right orbit distance, but it circles its star in an elliptical pattern. A true "Jupiter clone," Marcy says, "would serve as a signpost that there could well be Mars, Venus, or Earth clones between the Jupiter clone and the star in an architecture like our planetary system."
The scientists are keeping a close watch on several dozen "top secret" stars they believe could harbor Jupiter clones. A Jupiter analog would take approximately 12 years to orbit its parent star. So when Marcy and his colleagues notice the velocity of a star's wobble speeding up from year to year, it's a good indicator that a planet is coming around the bend. Within the next few years, the 12 year period will be up and Marcy will run statistical tests he developed to ensure that there's less than a one percent chance they're wrong about their suspected Jupiter clones. Then he'll announce the discoveries to his peers.
Once the data are scientifically confirmed, it'll be up to the International Astronomical Union to name the heavenly bodies. Someday, perhaps the scientists who discover the planets will have that right. And if that day comes, Berkeley's number one planet hunter will be ready. School children from around the world regularly send Marcy funny and poignant suggestions that he proudly keeps in a desk drawer, but he has his own naming scheme in mind.
"My idea is to name the extrasolar planets from words in Earth's languages that mean things like cooperation, harmony, and peace," he says.