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

"We are now on the eve of the second transit of a pair, after which there will be no other 'til the twenty-first century of our era has dawned upon the Earth, and the June flowers are blooming in 2004."

--U.S. Naval Observatory astronomer James Harkness, just before observing the 1882 transit of Venus 

On June 8, sky watchers will be treated to a celestial event that no one alive today has ever seen. For the first time in 122 years, earthlings will witness the silhouette of Venus moving across the face of the sun. During this miniature version of a solar eclipse, Venus appears as a black dot against the roiling solar surface. Unlike the moon, which looms so large on the sky that it covers the entire sun during a solar eclipse, Venus during a transit masks only one-thousandth the area of the sun's surface and blocks a mere 0.1 percent of its light.

As seen from Earth, a transit of Venus occurs just twice every century or so, in pairs spaced 8 years apart. Each transit lasts about 6 hours. For the June 8 event, observers in Europe and Asia will be best placed. In the eastern United States, about 1.5 hours of the transit, from sunrise until roughly 7:25 a.m. Eastern daylight time, should be visible. Observers should not look directly at the sun, taking the same precautions as they would when viewing a solar eclipse.

To record these rare events in the 1700s and 1800s, Captain James Cook and other explorers traveled the world. Transits provided the only means at that time for accurately measuring the scale of the solar system (see box p. 248). Now, some astronomers are planning to observe the June 8 transit to learn about solar systems far beyond our own.

"Transits are the one way we can now study key properties of extrasolar planets," says Sara S. Seager of the Carnegie Institution of Washington (D.C.).

Sun-watching space observatories such as SOHO (Solar and Heliospheric Observatory) and TRACE (Transition Region and Coronal Explorer), which orbit above Earth's turbulent atmosphere, should have ringside seats for the transit.

OVERCOMING OBSTACLES Characterizing extrasolar planets in distant solar systems hasn't been easy. Against the glare of the star it orbits, each orb is too faint to be seen outright. So far, astronomers have detected extrasolar planets only indirectly, almost exclusively through the gravitational tug they exert on their parent stars.

Scientists have an opportunity to learn much more about a distant planet when it passes between Earth and its star. The duration of the transit and the amount by which starlight is dimmed by the planet's passage provide the only way astronomers can now determine the mass, size, and orbital inclination of these unseen bodies.

Among the more than 125 known extrasolar planets, astronomers have observed transits for just two. These transits have involved giant, Jupiter-mass bodies that closely orbit their parent stars and therefore block more starlight than smaller planets do.

By analyzing the specific wavelengths of starlight absorbed during a transit, astronomers have also made the first discovery of an extrasolar planet's atmosphere. Using the Hubble Space Telescope to monitor the periodic dimming of the star HD209455, already known to have a massive planet closely orbiting it, scientists deduced that the planet has a bloated atmosphere that contains hydrogen, sodium, carbon, and oxygen (SN: 2/14/04, p. 109).

Therein lies the grand opportunity of Venus' June 8 transit: The planet's passage across the sun will provide the only local counterpart to a distant, atmosphere-bearing planet crossing the face of its own parent star. The upcoming transit of Venus will therefore provide a valuable benchmark for interpreting data from extrasolar transits.

The astronomers observing the June transit will have an ace in the hole: They already know the composition of Venus' atmosphere, thanks to spacecraft that have directly measured it. By matching theory with observations, says Seager, "we can test the models for completeness and accuracy," in a way that hasn't before been possible.

Seager calls the June 8 event a "rare chance." In 2012, she notes, Venus' follow-on transit could double the bonanza. After that, the next transit won't occur until 2117.

During the upcoming transit of Venus, says Seager, she would like to tease out the role of two factors that make it tricky to interpret observations of extrasolar transits. One factor is the rotation of a parent star, which broadens the spectrum of the filtered starlight. The other, which can also alter the spectrum, is the tendency of a planetary atmosphere to act as a lens, bending or distorting the starlight passing through it. Observations of these effects during the June 8 transit should help planetary scientists eke out more information from future glimpses of extrasolar transits, Seager notes.

In attempting to treat the transit of Venus as if it were a replica of an extrasolar transit, researchers face an obstacle. As seen from Earth, all the light filtering through the atmosphere of an extrasolar planet appears to emanate from a single source--the unresolved image of the parent star. In contrast, the relatively nearby sun effectively comprises many points. To put the transit of Venus on equal footing with its extrasolar counterparts, astronomers have to devise ways to observe the sunlight as though it were coming from a single point.

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.

According to Kepler's famous third law, the cube of a planet's distance from the sun is proportional to the square of the time the planet takes to complete one orbit about the sun. Because this period can be easily measured, Kepler's third law provides the planets relative distance from the sun but not their absolute distance.

In 1716, Edmond Halley, of Halley's comet fame, noted that the rare transit of Venus could be used to determine the Earth-sun distance--if observers fanned out over the globe to record the event. Venus appears to take a different path across the sun depending on where on Earth an observer is standing. The phenomenon known as parallax underlies that effect and can also explain why a finger held at arm's length seems to shift position depending on whether you peer at it with your left eye or your right.

If observers could measure such a shift in Venus' position, they could use simple geometry to find the distance to the sun. However, the actual shift is so slight that Halley advised that observers instead compare the times of the start and finish of a transit from widely separated locations. Astronomers could then use surveying methods to directly measure the distance between Earth and Venus.

For the next pair of transits after Halley's proposal, in 1761 and 1769, British and French expeditions observed the events from several remote locations. One of the most famous observers was Captain James Cook. While circumnavigating the globe and exploring the southern Pacific ocean, he stopped in Tahiti to view the 1769 transit. Most of the transit observations that year were of poor quality. Nonetheless, the data narrowed the Earth-sun distance to between 150 and 153 million kilometers.

The United States had a leading rote in observing the 1874 and 1882 transits. For the 1874 expeditions alone, Congress appropriated what was then a whopping $177,000.

Since 1882, astronomers have developed even more-accurate ways to determine the Earth-sun distance. The methods include bouncing ground-based radar off the surface of Venus.

Now, the transit is luring astronomers who seek to investigate worlds far beyond the solar system.--R.C.