Scientists will soon be "listening" to planets orbiting 20,000 stars in expanded attempt to find alien life.

The stars are red dwarfs, which are relatively dim compared to our own sun.

But a statement from the SETI Institute (Search for Extra Terrestrial Intelligence) expressed optimism that many of them host planets:

"Exoplanet data have suggested that somewhere between one sixth and one half of red dwarf stars have planets in their habitable zones, a percentage comparable to, and possibly greater than, for Sun-like stars," the statement says.

By that logic, between 3,000 and 10,000 of these stars can be expected to host planets.

Scientists previously thought it unlikely that planets orbiting a red dwarf could realistically support life.

Red dwarfs have small "habitable zone," meaning a planet would have to fall in just the right orbital "ring" to hit the optimal temperature. Further, the planet would be so close to the star that one side of the planet would become "tidally locked" toward the star, permanently facing it. That means one side of the planet would be extremely hot and the other cold and devoid of direct sunlight, which isn't ideal for life formation.

Scientists have recently reassessed that view, however, after recently learning that heat could transferred to the dark side of these planets, creating life-sustaining conditions across much of the planet.

Red dwarf stars are also billions of years older than our own sun on average, meaning life would have had far more time to form there. Three-fourths of all stars are red dwarfs. Scientists will focus on the red dwarfs nearest to us in our own galaxy.

The new batch of 20,000 stars will be examined three at a time with a group of 42 antennas. The antennas are based at the SETI Institute's Allen Telescope Array in northern California.

"We'll scrutinize targeted systems over several frequency bands between 1 and 10 GHz," said SETI scientist Gerry Harp. "Roughly half of those bands will be at so-called 'magic frequencies' - places on the radio dial that are directly related to basic mathematical constants."

Listening to these frequences will begin in earnest in two years' time. Right now, they have a list of 70,000 red dwarf candidates they must narrow down to 20,000. They'll be using data from NASA's TESS satellite to include any candidate stars known in advance to have exoplanets.

cd/fs (AFP, dpa)