Life within the Goldilocks zone

Astronomers are in dispute about the existence of a planet that could host life on its surface.

An American team recently announced the discovery of planet Gliese 581g orbiting the red dwarf star Gliese 581, about 20 light years away. Reported to be a rocky planet with a mass of three to four times that of earth and an orbit of 37 days, Gliese 581g is in the habitable zone of space, which is the distance from a star at which an Earth-like planet can maintain both liquid water and Earth-like life on its surface.

This zone is often called the “Goldilocks zone” because its temperatures are neither too hot nor too cold to support life.

Only two weeks later a Swiss team said they had failed to detect the planet, although they had confirmed other previously announced Gliese 581 planets.

Nearly 500 exoplanets (those outside our solar system) have been discovered, but most are inhospitable giant planets thought to resemble Jupiter. Yet the American researchers suggested that the number of hospitable planets in “the immediate solar neighbourhood” could be “of the order of a few tens of per cent” of the total, meaning that the Milky Way could be teeming with potentially hospitable planets.

If Gliese 581g turns out not to exist after all, there are one or two other planets that have already been identified that could support life. The planet 55 Cancri f orbits within the habitable zone of the yellow dwarf star in the 55 Cancri binary star system. This Jupiter-like giant gas planet probably could not itself support life, but its satellite moons could.

Another planet, GJ 1214b, lies just outside the habitable zone but seems to be an ocean planet, covered by a deep liquid ocean of water, similar to some of the Jovian moons of our own solar system — but much warmer.