ET's Earth Appears More as Pale, Red Dot

June 26, 2009 -- Scientists looking for Earth-like planets in distant solar systems might find it more productive to focus on pale red dots, rather than blue ones.

So concludes a team of astronomers from Spain and Florida who observed the Aug. 16, 2008, lunar eclipse from a simulated alien perspective and discovered that several biologically relevant molecules, such as oxygen, water, carbon dioxide and methane, show up stronger than expected in longer, redder wavelengths of light.

"The Earth is often referred to as the pale blue dot, but in transmission, the pale blue dot becomes the pale red dot," Enric Palle, with Spain's Instituto de Astrofisica de Canarias, and colleagues wrote in a paper published June 11 in the journal  Nature.

The team used optical and near-infrared spectrographs attached to telescopes at the El Roque de los Muchachos observatory in the Canary Islands to observe the light reflected from the moon during the eclipse.

With the sun positioned behind Earth and the planet's shadow falling on the moon, the light reflecting off the lunar surface back to Earth first passed through the planet's atmosphere. The effect is similar to the geometry observed when an extrasolar planet passes in front of its parent star, Palle told Discovery News.

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When a planet transits a star, part of the starlight passes through the planet's atmosphere where it interacts with the various atoms and molecules. Breaking down the light into its component wavelengths then gives scientists insight into the planet’s composition.

Scientists have discovered more than 350 planets orbiting stars beyond the solar system, including at least 58 that transit their parent stars, relative to the perspective of Earth.

"We have a much better idea about what to do to find planets similar to our own where life may be thriving," said Eduardo Martin, an associate physics professor at the University of Central Florida in Orlando.

The spectra of Earth during the eclipse also revealed the presence of the planet's ionosphere, the layer of ionized gas the sits atop of the atmosphere. Scientists found a telltale sign of ionized calcium atoms, the sixth most abundant element on Earth.

Future investigations could reveal additional ionized elements, such as magnesium, which would appear in shorter wavelengths.

NASA is planning to attempt exoplanet spectroscopy with the James Webb Space Telescope and other future observatories.

"Our ... spectrum suggests that retrieving the major planetary signals might be easier than model calculations suggest," the authors conclude.

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Irene Klotz's Blog: Free Space

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