Shades of Venus: our neighbor in the solar system holds a lantern on faraway planets

Science News, April 17, 2004 by Ron Cowen

One way to achieve this effect is to use an Earth-orbiting satellite, such as ACRIM, that monitors the total amount of radiation that the sun emits at wavelengths ranging from the near-ultraviolet to the near-infrared. Astronomers plan to analyze the decrease in this solar irradiance recorded by ACRIM during the Venusian transit.

Glenn Schneider of the University of Arizona in Tucson has come up with another strategy. Instead of looking at the sun, he proposes to look at the moon. When light from the sun reflects off the moon, it gets integrated into a single signal akin to the point-like appearance of a distant star.

Even if Schneider wanted to directly image the Venusian transit, he couldn't do so from Arizona, where the transit will be finished before the sun rises. But the moon will be visible just over the horizon at the tail end of the transit, so Schneider and his colleague Paul S. Smith expect to have about a half-hour window in which to view the transit's reflection from the Steward Observatory in Tucson.

TOWARD THE FUTURE By whatever means the June 8 transit is observed, the resulting data could become even more important after the scheduled launch of NASA's Kepler satellite observatory in 2007.

Kepler will for the first time enable astronomers to search for extrasolar planets the size of Earth or even smaller, says Kepler lead scientist William Borucki of NASA's Ames Research Center in Moffett Field, Calif. During its 4-year mission, Kepler will scan 100,000 stars for signs of transiting planets.

By using Kepler data on the orbit of a transiting planet as well as the known properties of its star, astronomers will attempt to determine whether any of the orbs lie in its solar system's habitable zone, where liquid water could exist on the planet's surface. If Kepler fails to find any Earthlike planets, says Borucki, "then such planets must be rare in our galaxy, and we might be the only extant life."

Even if Kepler were to find a multitude of planets the size of Earth, it isn't equipped to record their spectra and thereby indicate 4whether the planets have atmospheres. Indeed, "such studies are currently well beyond the limits of precision of even the largest observatories," says David Charbonneau of the California Institute of Technology in Pasadena.

It's still unclear when scientists will have the tools to characterize extrasolar planets' atmospheres with enough precision to recognize signs of life.

Or, as the U.S. Naval Observatory astronomer James Harkness put it 122 years ago: "What will be the state of science when the next transit season arrives, God only knows.... As for ourselves, we have to do with the present."

Transitory History

Including the little-known fact that Captain Cook explored Venus

Sky watchers since the ancient Greeks have attempted to measure the size of the solar system. But after the work of Johannes Kepler early in the 1600s, the challenge focused on determining just one number--the distance between Earth and the sun. This number, known as the astronomical unit, could then be used to calculate the distance between other planets and the sun.