Are we alone? Answer inches closer as tech advances help hunt for alien life

Toward the end of his life, the cosmologist Stephen Hawking was asked about the odds of finding intelligent alien life in the next two decades. "The probability is low," he declared in 2017, and took a lengthy pause before adding: "Probably."

- Ian Sample

This week, other scientists from the University of Cambridge reported tentative evidence for two compounds in the atmosphere of a planet, K2-18b, that sits in the constellation of Leo 124 light years away. On Earth, dimethyl sulphide and dimethyl disulphide are hallmarks of life, emanating only from microscopic organisms.

And while marine phytoplankton may not rank as particularly intelligent, the claim unleashed a wave of excitement: the answer to the question "Are we alone?" has never seemed closer.

"This is the strongest evidence to date for biological activity beyond the solar system," Nikku Madhusudhan, an astrophysicist on the team, told the Guardian before the announcement.

"Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach."

Two more years of observations with the James Webb Space Telescope (JWST) will nail down whether or not the compounds really exist in the planet's atmosphere, Madhusudhan believes. The telescope is uniquely placed to make such measurements.

When a planet wanders across the face of a star, JWST can detect subtle shifts in the light it gathers, caused by compounds in the atmosphere absorbing starlight. Make enough observations and the measurements can confirm which compounds are abundant.

But even if K2-18b's atmosphere contains the two compounds, the question of life will not be solved: scientists cannot rule out that the chemicals form in other ways on other worlds, without calling on a cast of tiny aliens.