In the search for life beyond Earth, humans have sent robots to the rocky surface of Mars, deployed spacecraft to investigate the moons of Jupiter and Saturn, and aimed their most powerful telescopes toward distant solar systems.
But now, in an unexpected twist, a group of scientists say they have found possible signs of extraterrestrial life in a place where few had thought to look: high in the thick, toxic clouds of Venus, our closest planetary neighbor.
In that noxious environment, they discovered a gas called phosphine that is associated with life on Earth.
The notion that the Venusian phosphine could have been produced by living organisms may seem absurd, the team members acknowledged. And yet it's one of the most plausible theories they have.
"There are two possibilities for how it got there, and they are equally crazy," said Massachusetts Institute of Technology astrobiologist Sara Seager, a member of the team that reported the discovery Monday in the journal Nature Astronomy. "One scenario is it is some planetary process that we don't know about. The other is there is some life form living in the atmosphere of Venus."
Seager emphasized that she and her colleagues are not claiming to have found evidence of life on Venus. Instead, they are saying they found a robust signal of a gas that doesn't belong in the planet's atmosphere, and that it will take a lot more work to understand how it got there.
"What we need now is for the scientific community to come and tear this work to shreds," said Clara Sousa-Silva, a molecular astrophysicist at MIT who worked on the paper. "As a scientist, I want to know where I went wrong."
Phosphine is a pyramid-shaped molecule with a phosphorus atom on top and three hydrogen atoms at the base. It is hard to make on rocky planets like Earth and Venus because it takes tremendous pressures and temperatures to get the atoms to bond, Sousa-Silva said.
Those conditions exist deep within the interior of the gas giants Jupiter and Saturn, but rocky planets like Earth and Venus simply don't have the thermal environments that would allow phosphine to form spontaneously.
On Earth, the production of phosphine is associated with anaerobic life, which does not need oxygen to survive. It has been detected in marshlands, rice fields, sewage plants, animal feces and the intestinal tracts of fish and human babies, Sousa-Silva said.