All they need are photons.

Photons are the eternal messengers of the universe. Carriers of electromagnetic energy, they travel from one point to another at the speed of light, conveying an absolutely dependable array of data, even if their journey lasts billions of years and trillions of billions of miles.

In normal life, all one needs to examine the contents of a photon is a good pair of eyes. Everything we can see - from trees and grass to sky and stars, or the faces of loved ones or enemies - is delivered via these vital manifestations of matter that act as both particles and waves.

Seeing a bit farther and wider requires devices of enhancement. Scientists need powerful telescopes and radio receivers to capture and analyze wavelengths, frequencies and energy levels of photons in all parts of the spectrum: infrared, visible, ultraviolet, radio waves, X-rays and gamma rays.

Armed with the best instruments, they can assemble data about stars and black holes located across the Milky Way galaxy or even across the universe.

They also can study the planets orbiting around those objects, gathering data about mass, orbital speed, luminosity, temperature and chemical composition, to name a few characteristics. All scientists need is equipment of sufficient sensitivity.

At a recent gathering at the Space Telescope Science Institute on the Johns Hopkins University campus in Baltimore, astronomers presented some of the information they are attempting to tease from the cosmic messengers.

Jaymie Matthews, with the University of British Columbia, described how the Canadian MOST satellite - a briefcase-sized spacecraft tagged the "Humble Space Telescope" by its creators - can detect exquisitely subtle shifts in stellar brightness that signify the transits of extrasolar planets.

Imagine, Matthews said, looking at the Empire State Building at night, aglow from all its interior lights shining through the windows.

Then imagine being able to measure the change in the whole building's brightness when someone lowers one window shade by an inch. MOST -- for Microvariability and Oscillations of STars - possesses such sensitivity, he said. It is able to detect a shift in brightness of about 1 part per million.

Fine - an instrument can detect the minute dimming of a star, but how does the MOST team know the dimming is due to a planetary transit, and not just a sunspot or the vagaries of interstellar dust?

"Observing stars is usually like reading a paragraph without all the letters," Matthews said. He meant ground-based instruments and even the Hubble Space Telescope must interrupt their photon-collecting often because of Earth's orbital rotation.

MOST is different in this respect. It follows a polar orbit, which permits it to fix on a single star continuously for up to two months, long enough to rule out stellar surface or interstellar phenomena.

One of its targets is HD209458, a sun-like star about 150 light-years away in the constellation Pegasus. HD209458 has a Jupiter-sized planet orbiting at the blinding speed of every 3.52 days, meaning it is extremely close to the solar surface.

Interesting, but not amazing. Astronomers have been detecting exoplanets for 10 years, and the current count is approaching 150. The "hot Jupiter" -- as close-orbiting gas giants are called - orbiting HD209458 was discovered several years ago.

What is amazing is MOST's data show HD209458 has a small tidal bulge locked to the motion of HD209458 b, the close-orbiting planet. As b circles its parent, it gravitationally grabs a hunk of the star's midsection and yanks it along.

The MOST team knows this because all the photons collected by their little satellite in two months of continuous observation told them so.

Timothy M. Brown, with the National Center for Atmospheric Research in Boulder, Colo., also has been watching HD209458 but for a different purpose. He specializes in transit spectroscopy -- the study of the atmospheres of exoplanets that pass in front of their stars.

This is not theoretical. A recent paper in the British journal Nature described how astronomers already have detected sodium and hydrogen in the atmosphere of HD209458 b. They did this by analyzing the light emitted by the parent star as it passed through the planet's atmosphere during a transit.

Brown said observations of HD209458 b with existing instruments, including the Hubble, show temperatures of 1,800 degrees Fahrenheit (1,000 degrees Celsius) in the planet's atmosphere, hot enough to melt silver.

Eventually, he said, with the improved capabilities of spacecraft such as NASA's Kepler -- scheduled for launch in late 2007 or 2008 -- astronomers hope to be able not only to detect Earth-sized exoplanets, but also to analyze the composition of their air.

Imagine: global warming, on a globe not named Earth, in an alien solar system light-years away, detected remotely, courtesy of nature's indefatigable data carriers -- photons.

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