The University of Arizona
September 16, 1998


Astronomers have seen hints that other planets circle stars other than our own. Such stars have a tantalizing wobble that suggests something is nearby. But, until now, the brightness of the stars themselves have prevented any conclusive look at whether or not they have their own solar system.

Astronomers at the Center for Astronomical Adaptive Optics (CAAO) at The University of Arizona in Tucson have recently demonstrated a new technique for imaging planets circling nearby stars. The observations, which are reported in the current (Sept. 17) issue of the science journal, Nature, were made at the Multiple Mirror Telescope (MMT) atop Mount Hopkins, Ariz., by a team led by graduate student Phil Hinz. (The MMT is now being converted to a single 6.5-meter mirror telescope.)

Although astronomers have found several planets orbiting nearby stars by measuring the wobble of the star due to a planet orbiting it, none of these planets have been seen directly, even with the Hubble Space Telescope. Hinz says it is very difficult to get a direct look at these planets since their parent stars are roughly millions of times brighter. By current methods the planet is simply washed out in the glare of the star. It is akin to using the naked eye to see planets in our own solar system during the daytime.

The trick for getting past all of that glare is a technique called nulling interferometry, which can be used to cancel out the light from the bright star while leaving the image of the planet intact. The experiment at the MMT is the first time nulling interferometry has been demonstrated on a telescope. Light from two of the MMT's mirrors are combined in such a way that the bright starlight from one mirror cancels out the light from the other, while the light from very dim objects close by to these stars is reinforced.

To demonstrate this the astronomers took images of Betelgeuse, a nearby supergiant star in the familiar constellation of Orion. With the light from the Betelgeuse canceled they were able to see clearly an image of a faint dust cloud that surrounds the star. While astronomers have known of this dust cloud previously, nulling interferometry provided the first direct images of it, free of contamination from star light.

These experiments are the first step in a succession of advances needed to image extrasolar planets and determine if they contain any signs of life. The Large Binocular Telescope, currently under construction on Mt. Graham, Ariz., will be used to search for planets the size of Jupiter and larger, while NASA's planned Terrestrial Planet Finder, a space-based nulling interferometer, is planned for detection and spectral analysis of planets as small as the Earth around nearby stars.

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