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03/06/2002 08:00:00

Caltech astronomer to search for "hot Jupiters"with off-the-shelf camera lens

In an age when nearly all astronomical work requires really big telescopes, David Charbonneau is something of an anomaly. The Caltech astronomer will soon begin a multiyear survey for extrasolar planets at Palomar Observatory—not with the 200-inch Hale telescope, but with a tiny desktop-sized device he and JPL researcher John Trauger assembled largely from parts bought at a camera shop.

Basing his instrument on a standard 300-millimeter telephoto lens for a 35-millimeter camera, Charbonneau will begin sweeping the skies this spring in hopes of catching a slew of "hot Jupiters" as their fast orbits take them in front of other stars. Admittedly, the charge-coupled device at the camera-end of the lens is a good bit more costly than the lens itself, but the total budget for the project—$100,000—is still a paltry sum when one considers that the next generation of earthbound telescopes will likely cost upwards of $400 million apiece.

Charbonneau, a recent import to the Caltech astronomy staff from the Harvard-Smithsonian Center for Astrophysics, is one of the world's leading authorities on the search for "transiting planets," or planets that should be detectable as they pass into the line of sight between their host star and Earth. In November, Charbonneau and his colleagues made international news when they discovered the first planetary atmosphere outside our own solar system. But that work was done with the Hubble Space Telescope. The yet-to-be-formally-named telescope at Palomar Observatory will certainly be more modest in cost, but every bit as ambitious a program for searching out other worlds.

"Basically, the philosophy of this project is that, if we can buy the stuff we need off the shelf, we'll buy it," the Canadian native said recently in his new campus office.

At the fore-end of the new instrument is a standard 300-millimeter camera lens. Charbonneau settled on a telephoto lens because he reasoned that the optics have been honed to a fine degree of precision over the years. Too, he assumed that the lens would be robust enough for the duration of the three-year project.

The charge-coupled device (CCD), a standard imaging tool in astronomy for the last couple of decades, is a $22,000 item that accounts for the largest part of the instrument budget. The CCD will be mounted in a specially constructed camera housing to fit at the back of the lens, and the entire device will be fitted onto an inexpensive equatorial mount—also available at many stores carrying amateur astronomical equipment.

Meanwhile, the Palomar staff has stored away a 20-inch telescope so Charbonneau will have a small dome for his new instrument, and are also doing other preparations to mechanize the actual observing so that a telescope operator will not have to be on site at all times.

Palomar Observatory engineer Hal Petrie says the mountain crew is currently busy linking the new telescope with an existing weather-monitoring system at the nearby 48-inch dome, where another automated telescope is located. The system monitors the atmospheric conditions to determine whether the dome should be opened.

"The new telescope is a very good use of space," says Petrie. "The potential for results is very exciting."

Charbonneau will be able to photograph a single square of sky, about 5 degrees by 5 degrees. That's a field of view in which about 100 full moons could fit. Or, if one prefers, a field of view in which an entire constellation can be seen at one time.

With special software Charbonneau helped develop during his time at Harvard-Smithsonian and at the National Center for Atmospheric Research, he will compare many pictures of the same patch of sky to see if any of the thousands of stars in each field have slightly changed. If the software turns up a star that has dimmed slightly, the reason could well be that a planet passed in front of the star between exposures.

Repeated measurements will allow Charbonneau to measure the orbital period and physical size of each planet, and further work with the 10-meter telescopes at the Keck Observatory will allow him and his colleagues to get spectrographic data, and thus, the mass and composition of each planet.

"Once you get the mass and size, you have the density," he says. Weather permitting, Charbonneau will be able to get up to 300 images during an ideal night. Assuming that he can have 20 good nights per month, he should have about 6,000 images each month show up in his computer.

The ideal time will be in the fall and winter, when the Milky Way is in view, and an extremely high number of stars can be squeezed into each photograph. This, too, is an anomaly in astrophysical research, particularly to cosmologists, for whom the Milky Way is pretty much a blocked view of the deep sky.

"It's estimated that about one in three stars in our field of view will be like the sun, and that one percent of sunlike stars will have a hot Jupiter, or a gas giant that is so close to the star that its orbit is about four or five days," he says.

"One-tenth of this 1 percent will be inclined in the right direction so that the planet will pass in front of its star, so that maybe one in 3,000 stars will have a planet we can detect," Charbonneau adds. "Or if you want to be conservative, about one in 6,000."

Compared to other research programs in astronomy, the search for hot Jupiters is fairly simple and straightforward to explain to the public, Charbonneau says.

"An amateur could do this, except maybe for the debugging of the software, which requires several people working 10 hours a day.

"But it's easy to understand what's going on, and cheap to build the equipment. That's why everyone thinks it's an ideal project—if it works."

The new Palomar telescope is the final instrument in a network of three. Of the other two, one is located in the Canary Islands and operated by the National Center for Atmospheric Research; the other is near Flagstaff, Arizona, and is operated by Lowell Observatory.

The large span in longitude of the three-instrument network will allow Charbonneau and his colleagues to observe a patch of sky with one telescope while the patch is above the horizon in the night sky, and then pass it off to the next westward telescope as the sun comes up.

Contact: Robert Tindol (626) 395-3631