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I admit to ignorance on this, but how can we tell (given the limitations of our telescopes)that the planets have an earthlike composition? Or is this largely a deduction? I can understand that we can gather information about stars merely by studying the light... But can we actually "see" other planets light years away?

I believe and it's my opinion, that one day, Earth based telescopes will actually be able to see some surface detail of planets orbiting other stars.

There is also a possibility that there could be planets out there that may possibly emit a small amount of natural light or the real possibility that a planet that may have life on it, may have some artificial light that is being produced and emitted, like Earth. That light could possibly be detected and analyzed with spectroscopes and other instruments to read the light. We are not quite there yet, but I am confident.

New telescopes are being developed all the time. Like the interferometers and other Earth orbiting optical telescopes.

Thought this may be of some interest.

Novel Telescope Array Achieves Milestone By Joshua Roth

#IMAGE_1# September 26, 2001 | Last week, astronomers achieved an esoteric but significant milestone in the quest to see more and more detail on cosmic objects. Scientists with the Center for High Angular Resolution Astronomy (CHARA) combined starlight from two telescopes atop historic Mount Wilson in California, and created a stable "image" with those beams. This has been done many times before on Mount Wilson and elsewhere. This time, however, the two telescopes were separated by 330 meters, the longest baseline used to date at optical wavelengths, says CHARA director and Georgia State University professor Harold A. McAlister. They are part of the six-telescope CHARA Array, slated to begin routine operations next year. A basic tenet of optics is that a telescope's angular resolution — the size of the smallest details it can show — is inversely proportional to the diameter of its light-gathering lens or mirror. Optical interferometers like the CHARA Array are designed to yield the angular resolution of a single mirror or lens hundreds of meters across. They do this by pointing two or more telescopes at a single target, then mixing the light beams thus gathered. The beams interfere with one another, creating fringes that can be analyzed to produce an image of the target (or at least to measure some of its properties). Several optical interferometers operate worldwide, but the CHARA Array's large mirrors and separations (and Mount Wilson's famed atmospheric stability) make it "uniquely suited" to measuring the fundamental properties — masses, diameters, and surface temperatures — of numerous stars, says McAlister.

While the CHARA array can't take "pictures" of extrasolar planets, it will be able to detect the wobbles that such planets induce on stars within binary systems. It also will measure the cyclical swelling and shrinking of pulsating variable stars and resolve disks and jets associated with young stellar objects, says McAlister. The array will work at visual and near-infrared wavelengths and will obtain spectra as well as images of individual stars in many previously unresolved binaries.

The key is the nature of the reflected light. Just in the last year an earth satellite was used to scan the reflection spectrum of earth to be used as a reference for the time when somewhat detailed reflection spectra start coming in from other planets. That time could be in about 8 years when NASA gets its planet-finder telescope launched into outer space.

They'll be looking for oceans, water clouds, continents, who knows, maybe even prairies and forests.