NASA's Spitzer Telescope casts new light on search for alien life

n the search for alien life, the big question has always been, where to look? After all, the universe is a big place.

In recent years, some experts have argued that we ought to concentrate our search on stars cooler than our sun, where it would be easier to spot nearby planets that might be suitable habitat. But new results from NASA's Spitzer Space Telescope raise questions about whether the chemistry on such planets would let life blossom.

A research team at Johns Hopkins University in Baltimore used the big infrared telescope to search for hydrogen cyanide in the dust and gas swirling around 61 young stars. Hydrogen cyanide is a component of a compound basic to DNA, which is found in every living creature on Earth.

After breaking down the light of these stars with a spectrograph, the researchers found hydrogen cyanide in 30% of the yellow, sun-like stars. They found none around cooler, smaller stars, such as M-dwarfs and brown dwarfs.

"Around cooler stars, there might not be enough hydrogen cyanide" to kick-start the complex chemical reactions necessary to form life, said Ilaria Pascucci of Johns Hopkins, lead author of the research, which is appearing this week in the Astrophysical Journal.

Planet hunters in the U.S. and Europe have found more than 340 planets around stars in the last decade or so, but none is in the so-called zone of habitability, where life could be expected to evolve.

Most are gas giants orbiting so close to the stars that life-forms, at least the kind we know, could not gain a foothold.

In recent years, a number of scientists trying to find extraterrestrial life have suggested concentrating on relatively cool, dim stars.

This is because a key method of finding planets is to observe the dimming of a star's light caused by a planet crossing in front of it, a process known as a transit. The zone of habitability around cool stars would be close to the star, making it easier to spot transits.

In their research, the team focused on young stars from 1 million to 3 million years old, which are surrounded by what are known as "planet-forming disks" of dust and gas. These disks supply the raw material from which the planets are made.

The fact that the disks around cooler stars contain no hydrogen cyanide isn't the first evidence that those stars might not be the right place to look for planets where life can grow. M-dwarfs are prone to extreme magnetic outbursts that could interfere with developing life-forms.

"Although scientists have long been aware that the tumultuous nature of many cool stars might present a significant challenge for the development of life," said a statement by Douglas Hudgins, the Spitzer program scientist at NASA headquarters in Washington, "this result begs an even more fundamental question: Do cool stars even contain the necessary ingredients for the formation of life?"

Compounding the challenge of finding life elsewhere in the universe is the fact that we don't know for sure how life got started on our own planet.

The possibility that the recipe for life was delivered in a neat package from space is only one theory. Other theories suggest that under the right conditions, the planet itself could cook up its own stew of living organisms.

Before excluding cooler stars from the search for life, Pascucci said, she would like to do more studies of planetary disks to find out how they vary.