Under pressure: what might life look like on another planet?

00:00 09:05

Adam Frank has narrated this article for you to listen to.

Over the past three decades, astronomers have discovered planets orbiting Sun-like stars throughout the universe. This discovery ended 2,500 years of debate about whether worlds existed beyond our solar system, but it came with a shock. The most common kind of planet in the universe is the type of world that doesn’t exist in our small corner of the cosmos: what astronomers call ‘Super-Earths’ and ‘Sub-Neptunes’, planets with much greater masses than ours and which could, in theory, sustain life.

Astronomers concluded a little over a decade ago that every star in the night sky hosts a family of worlds. Importantly for the search for life, one in five of those stars will have a planet in the ‘Goldilocks’ zone (sometimes referred to as the ‘habitable zone’). That’s the band of orbits that are just right for water to pool up on a planet’s surface into puddles, lakes and oceans. It’s a good bet that life needs liquid water to form, so the discovery of Goldilocks worlds (of which there are billions) pushed the search for extraterrestrial life to the front of science’s to-do list.

Planet hunting across interstellar distances is spectacularly difficult. At first, all scientists could learn from their telescopes was the size and mass of newly discovered worlds. That was enough to see that the galaxy’s average planet – the kind that appears most often in most solar systems – looks nothing like the worlds orbiting our Sun.

Our solar system has two kinds of worlds. In the inner solar system, there are ‘terrestrial’ worlds – Mercury, Venus, Earth and Mars. These are basically spherical cakes of layered metal and rock; their cores mostly iron and nickel; and their outer layers made of rock-stuff – silicon, oxygen and magnesium.