Hopes of finding evidence of life on far off alien worlds by studying their atmospheric chemistry have been dashed by a new study that concludes it's almost impossible.

The research, reported in the Proceedings of the National Academy of Sciences, found atmospheric spectral readings from distant exoplanets will never be good enough to be useful in the search for life.

The findings support an underlying view among astronomers that it was always going to be difficult to take the spectrum of an Earth like exoplanet, according to the study's lead author Hanno Rein of the University of Toronto.

Photos: Top Exoplanets for Alien Life

"I was a bit pessimistic when I calculated these numbers for the first time, they were not what I was expecting," said Rein. "We're not going to get any useful spectra at all."

Astronomers determine the chemical composition of a gas, by looking for specific signatures in light from a star shining through the atmosphere of a planet passing in front of it. The strongest indicators of life on another planet would be chemical signatures for molecules of methane and oxygen in that planet's atmosphere.

"We think these two molecules are likely to be produced by life on Earth and also on other planets," said Rein. "There are few geological mechanisms which produce molecules of methane and oxygen in large quantities. To be really sure, we want both molecules together at the same time, then we can be more certain that there's life on that planet."

Too far away

However, most of the exoplanets being found by astronomers are too far away to provide useful spectra.

"The spectral resolution is so low, that we can hardly say anything about them," said Rein. "It's a fundamental physical limit, even if we had perfect technology that measures every single photon we receive from an exoplanet, there's just not enough photons reaching us to say whether there's life on the planet or not."

The problem would be even worse if the exoplanet was orbited by a moon.

"From a long way away an exoplanet and moon system would appear so close together that they couldn't be separately resolved, and would look like a single atmosphere to us," said Rein. "If one had oxygen and the other had methane, it would suggest an oxygen-methane atmosphere, providing a false positive for signs of life when none exists."

Astronomers have successfully collected spectra from distant exoplanets, but these were hot Jupiters, gas giants very close to their host stars, nothing that looks similar to the Earth.

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"Earth is much smaller than a gas giant, so the combination of finding an Earth-sized planet, in the habitable zone, and then taking its spectra, is just very hard," said Rein.

On the positive side, Rein believes looking for the right objects would allow good atmospheric spectra to be taken.

"Planets that NASA's Kepler spacecraft detects are hundreds and thousands of light years away, but if we search for planets closer to our solar system, we might be able to take spectra with a good enough resolution," said Rein.

Another option involves looking for planets around small red dwarf stars.

"These stars would be fainter and easier to take a spectrum because the light from such a star is not as bright," said Rein.

This article originally appeared on ABC Science Online.