Planetary atmospheres

key in search for extraterrestrial life

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 11 June, 2009

Astronomers using the William Herschel Telescope and Nordic Optical Telescope have confirmed an effective way to search the atmospheres of planets for signs of life.

Astronomers at the Instituto de Astrofisica de Canarias studied the chemical composition of the Earth’s atmosphere by looking at sunlight that has passed through it, revealing vital information about the chemical makeup of the atmosphere, and therefore details of the planet itself.

Studying sunlight as it passes through a planet's atmosphere could yield vital clues to the planet's habitability. Image: Gabriel Perez Diaz, SMM, Instituto de Astrofisica de Canarias (IAC).

This kind of measurement is known as a transmission spectrum, and although astronomers cannot use the exact same method to look at the Earth’s atmosphere, they were able to gain a spectrum of our planet by observing light reflected from the Moon towards Earth during a lunar eclipse. This is the first time a transmission spectrum has been measured for the Earth, and reassuringly, the spectrum contained strong signs of life, as well as the signature of the Earth’s ionosphere.

“Now we know what the transmission spectrum of an inhabited planet looks like, we have a much better idea of how to find and recognize Earth like planets outside our Solar System where life may be thriving,” says Enric Palle, lead author of the paper discussing the results, which appears in today’s edition of the journal Nature. “The information in this spectrum shows us that this is a very effective way to gather information about the biological processes that may be taking place on a planet.”

The Moon during a lunar eclipse. The red light illuminating the Moon's surface during the eclipse has passed through the Earth's atmosphere, carrying the information of all the major Earth atmospheric components. Image: Daniel Lopez, Instituto de Astrofisica de Canarias (IAC).

Several hundred exoplanets have been discovered in the last twenty years, and ambitious ground- and space-based missions, such as the recently launched Kelper spacecraft, are set to find the first true Earth-like planets. Once these planets have been detected, techniques like transmission spectra will prove invaluable to their further exploration.

“Many discoveries of Earth-size planets are expected in the next decades and some will orbit in the habitable zone of their parent stars,” says Pilar Montañes-Rodriguez. The habitable zone is the “Goldilocks” region in space where conditions are favourable for life as we know it on Earth. “Obtaining their atmospheric properties will be highly challenging; the greatest reward will happen when one of those planets shows a spectrum like that of our Earth.”

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