Jeremy Bailey of the Australian Center for Astrobiology at Macquarie University said looking at the way light bounces off droplets in a planet's atmosphere would be a sensitive indicator of liquid water in its clouds.

Bailey presented his case for using polarization, the same property of light that produces rainbows, to look for liquid water in the latest issue of the journal Astrobiology.

"A rainbow is caused by light that is reflected in a water droplet that is scattered at a particular angle," he said.

"That angle depends on the refractive index of the liquid that is scattering the light, so you can get rainbows at different angles depending on the types of liquids that might exist in clouds of other planets.

"That's how we determined that the clouds of Venus were droplets of concentrated sulfuric acid."