Phantom life in space – how to figure out if we’re being tricked

The search for life on other planets is as vigorous now as it ever has been, and a new study aims to cut the fact from the fiction.

Scientists are increasingly looking to the atmospheres of potentially inhabitable planets to measure their levels of oxygen. But could that send them down the wrong path?

Here on Earth, the presence of oxygen in our atmosphere is known as a "biosignature" – a sign of life on our planet. This is because our oxygen particles are created almost exclusively by living things – plants and algae, converting the Sun’s rays into energy.

It is assumed that if oxygen can be identified in the atmosphere of planets outside our Solar System, there’s a good chance life exists there, too.

New research from the University of Washington aims to shine a light on the fool’s gold of biosignatures – that is, a variety of situations in which oxygen may not be a sign of life.

To identify oxygen, scientists use large telescopes to analyse how light moves through a planet’s atmosphere when it passes in front of its host star.

Astronomer Edward Schwieterman and colleagues say there are factors other than life that could cause the presence of oxygen particles.

"The potential discovery of life beyond our Solar System is of such a huge magnitude and consequence, we really need to be sure we’ve got it right," Schwieterman explains. "That when we interpret the light from these exoplanets we know exactly what we’re looking for, and what could fool us."

For example, a star’s ultraviolet light can split carbon dioxide particles, allowing detached oxygen atoms to form O₂, the type of oxygen found in our atmosphere.

The paper details the key giveaway of this process – the presence of carbon monoxide.

“If we saw carbon dioxide and carbon monoxide together in the atmosphere of a rocky planet, we would know to be very suspicious,” Schwieterman says.

Similarly, starlight can break down water in a planet’s atmosphere, leaving large amounts of oxygen that have detached from their hydrogen counterparts.

Using these identifiers, the authors of the paper are keen to illustrate what they call "biosignature impostors" – the factors that could lead scientists down the wrong track in the search for life outside the Solar System.

“With these strategies in hand, we can more quickly move on to more promising targets that may have true oxygen biosignatures,” he says.

This research was published in The Astrophysical Journal Letters.