In Focus
October 02, 2007

How Do Artists Portray Exoplanets They've Never Seen?

	Image: COURTESY OF GEOFF MARCY
	WHY SO BLUE?  A planet recently discovered around the star Gliese 436 was depicted in blue in this artist's rendering because its atmosphere was likely baked into small molecules that scatter blue light like Earth's does. CLICK HERE FOR MORE ON THE LOGIC OF EXOPLANET IMAGES

Stargazers have yet to lay eyes on any of the nearly 240 planets detected outside our solar system. These so-called exoplanets are too faint for current telescopes to distinguish from the stars they orbit*; instead astronomers rely on indirect methods to infer their existence. Yet popular news accounts, supplied by space agency press services, overflow with bold, almost photo-realistic images of distant worlds.

Naturally, people can get confused. When San Francisco artist Lynette Cook painted a particularly striking image of a newly discovered planet passing in front of the star HD 209458 for a 1999 NASA press release, she received e-mail asking what kind of amazing image processing software she had used. "A lot of people didn't understand that it was a rendering," she says.

When astronomers make a sexy new discovery that doesn't lend itself easily to visual inspection, they turn to artists like Cook to bring it to life. A compelling image has the power to transform even the most complex data into an easy-to-grasp mental snapshot, but it runs the risk of fooling people into believing an iffy scientific case is closed.

Well aware of the double-edged sword they wield, artists and astronomers who dream up images of astronomical exotica often spend considerable time deciding how best to illustrate new discoveries. In the case of exoplanets, they are guided by a few key pieces of information and a healthy dose of educated guesswork.

Researchers frequently discover exoplanets by searching for regular variations in the color of a star's light. A planet circling in orbit will tug its star back and forth like a wispy figure skater spinning around a heavier one.

If the system is aligned so that the star wobbles toward and away from us, the light will appear to oscillate between bluer and redder as the wavelength expands or contracts along the line of sight to Earth.

This oscillation, which would not be especially eye-catching plotted as a graph, contains several crucial bits of information. Encoded in the timing of the oscillating light are the shape and distance of the planet's orbit around its star, which in turn determine how warm it is likely to be and, therefore, whether it could have liquid or frozen water, in the case of a rocky, Earth-like planet.

The degree of the color shift indicates the planet's mass and makeup. Astronomers believe that a planet as hefty as the solar gas giants Saturn (95 Earth masses) or Jupiter (318) would almost certainly be gaseous. Smaller planets of up to a few dozen Earth masses might range from those made mostly of rock and ice to larger ones with a solid core enshrouded by a mixture of thick gas.

Orbital distance would also influence a planet's color, says astronomer Geoffrey Marcy of the University of California, Berkeley, whose team has discovered 145 of the known exoplanets.

Jupiter and Saturn have brownish-orangey tints because their atmospheres swirl with carbon-based molecules that reflect light of those colors, Marcy says; a hotter atmosphere would evaporate water clouds and break down those molecules into carbon dioxide and methane, which scatter blue light. The effect might result in "deep blue billiard balls," he says.

Less familiar colors may also be out there. In May astronomers reported that planet HD 149026 b seems to absorb so much light that it should appear nearly black.