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Seeing dim distant worlds

Q: Astronomers claim to have a photograph of a planet circling a brown dwarf 225 light years away. Must of been a heck of a flash bulb! How did they take the picture? (L.A., Sandia Park, New Mexico)

A: A remarkable feat in more ways than one. We now know that it is a planet. The astronomers* who discovered the planet in April 2004 have watched its motion over the subsequent months.

First image of an extrasolar planet — heck of a flash bulb? [Courtesy of European Southern Observatory]

The planet and the brown dwarf (2M1207) travel together, hence gravity binds them together, and therefore the two objects are in the same system. Furthermore, the planet-like object’s light spectrum displays signs of water, which indicates the body must be comparatively small and light. Finally, it shines infrared light (heat). The data, taken together, indicates it is a planet — which makes this image a first. The first image we have ever captured of a planet not in our solar system.

How was it possible? "With great efforts," say the team. "At the technical limit" of the 320-inch (8.2-m) infrared telescope they used in the arid plains of Chile.

The team had a couple of things going for them — the nature of the planet’s light itself and recent advances in correcting distortions introduced by the atmosphere (a field called adaptive optics). Not to mention some brilliant sleuthing.

Yepun infrared telescope at the Paranal Observatory in Chile. [Courtesy of European Southern Observatory]

The nature of the light. The brown dwarf (which is a failed star with too small a mass for nuclear reactions to occur in its core) shines a dim red light at its brightest. Fortunately, it is only 8 million years old (compared with our 4,600-million-year old Sun). Young brown dwarfs are brighter than older ones of the same mass. Furthermore, brown dwarfs shine little visible light but much infrared (heat) radiation. This helps most of all, as we shall see.

The planet is, of course, dimmer and cooler than the star — about 100 times dimmer. But, like the star, it shines in the near-infrared spectrum (long wavelength: 0.8 to 8 millionth of a meter). The planet has surface temperatures around 1800 degrees Fahrenheit (1000 C) — hot still from inward contraction. It’s about 10 times hotter than Jupiter — a much older planet that is also still producing heat in its interior.

So, both bodies radiate heat in the infrared spectrum. The light wave fronts reach Earth’s atmosphere as plane waves (parallel smooth sheets, each stretching infinitely in all directions). This 2-source light is the reader’s "heck of a flash bulb."

The job of the telescope is to grab as much of the brown dwarf’s and planet’s infrared light as it can and clean it up enough that we can discern both light sources. A cute trick — called "adaptive optics." Otherwise, we’ll never know that the planet exists.

Adaptive optics. The Yepun infrared telescope in the parched Chilean air gathers the 2-source heat radiation with a huge 320-inch (8.2-m) mirror. A secondary mirror reflects this radiation to a detector or scientific instrument. The initially undistorted plane waves that hit Earth’s atmosphere, however, are no longer smooth. Uncorrected, we can’t see both sources. See figure. Adaptive optics to the rescue!

The top image shows a blob without adaptive-optics correction. The bottom one shows the same image after an adaptive-optics correction that resolves the top blob into a double star. [Courtesy of European Southern Observatory]

Traveling the last 11 miles (18 km) through a jumble of moving air molecules (that part of the atmosphere closest to Earth’s surface, called the troposphere) has thrown the infrared radiation wave out of kilter. Phase errors have crept into the waveform (a few millionths of a meter). The telescope must correct the error to no more than 1/50 of a millionth of a meter and do so every hundredth of a second. That’s how fast our turbulent atmosphere changes and, therefore, how fast the atmosphere changes the distortion.

The telescope senses the distortion, measures it, and corrects it with a small deformable mirror. The adaptive optics system measures the distortion by monitoring how our atmosphere disturbs the image of a bright reference star. Knowing the effects on that known starlight, the system can correct the same effects on the infrared light of the planet and brown dwarf. The controlling computer cranks out several hundred million operations for each set of commands sent to the little mirror. VoilB! The corrected waveform reveals both light sources.

"If these images had been obtained without adaptive optics, that object [the planet] would not have been seen," says astronomer Gaël Chauvin.

*Astronomer team: Gaël Chauvin and Christophe Dumas of ESO-Chili, Inseok Song now of Gemini, Anne-Marie Lagrange and Jean-luc Beuzit of LAOG-France, Benjamin Zuckerman UCLA-USA

Further Reading:

European Southern Observatory: Is this speck of light an exoplanet?

Gemini Observatory, National Science Foundation, and the university of Hawaii Adaptive Optics Group: Straightening out bent starlight

European Southern Observatory: What is active and adaptive optics?

Paris Observatory: Notes for 2M1207 by Jean Schneider

(Answered Sep. 23, 2005)