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<b>Astral Arcs</b><br>Star trails over the Canada-France-Hawaii Telescope, one of Mauna Kea's thirteen observatories. <br><i>Photo by Richard J. Wainscoat / Photo Resource Hawaii</i>
Vol. 13, no. 3
June/July 2010


View from the Top (Page 2)



Other Worlds: A Profusion of Planets


NGC 6791, a star cluster in the Milky Way's
Lyra Constellation. Photo: NASA, ESA and
L. Bedin (STScI).
To begin, let’s go back to the planets. Astronomers now know of 350 other planetary systems but have only actually seen one. How do they know about the other 349? The answer to that question gets to just what a modern telescope is. Forget the idea of a person putting an eye up to a long tube and gazing at the heavens. The telescopes on Mauna Kea have outgrown humans. Step inside one of the fantastically complicated and huge W.M. Keck telescopes, and you’re confronted by an aperture that stands ten meters tall and holds thirty-six mirrors.


This high-tech honeycomb is surrounded by a maze of secondary devices, all used to refine data. There’s the adaptive optics (AO) system, for example, a crucial device that adjusts for atmospheric turbulence about 2,000 times a second. AO ensures that the planets in the picture of HR 8799 actually are planets and not just specks of dust that the wind blew across the telescope’s field of view.


Then there’s the photometer, an über-precise device used to count the number of photons coming from a star. The photometer allows astronomers to detect dips in the light curve of a star. And those dips? They’re caused by planets. When an orbiting planet passes in front of its star, it eclipses the star’s light and blocks its photons a little—a very little. Find a dip and you’ve found a planet. Astronomers call this method of detection “the transit technique.”


The first “exoplanets”—planets around other stars—were found almost simultaneously in the mid-1990s by two groups: a Swiss astronomer working in Chile and two California astronomers working on Mauna Kea. Those detections were made in a slightly different way: by taking spectra readings that showed the red and blue shifts of stars’ orbits (a red shift occurs when an object is moving away from us, a blue shift when it’s moving toward us). The readings revealed periodic wobbles in the stars’ orbits, wobbles that could only have been caused by … voilà! … orbiting planets. This one was dubbed—no surprise—“the wobble technique.”


“It was an absolute breakthrough in our science,” says Rolf Kudritzki. Kudritzki, head of the University of Hawai‘i’s Institute for Astronomy, is a German who loves wearing vivid aloha shirts and discussing everything under, over and around the sun. “It was the detection of a new world. And every day now you read about another star with planets.”