Could there be life in the skies of Venus?
Despite its average temperature of 460degC, a new paper by researchers in the US claims there could be microbial life in the clouds which cover the whole planet.
"Venus has had plenty of time to evolve life on its own," said Sanjay Limaye of the University of Wisconsin-Madison, who led the study.
He says it's possible Venus was once habitable and had liquid water, perhaps for as long as 2 billion years.
"That's much longer than is believed to have occurred on Mars."
Bacteria on Earth have been found alive as high as 41km. Other living organisms have been found thriving in extreme circumstances, including acidic lakes and hot springs.
NASA probes to Venus have found temperatures in the clouds can be much lower than on the surface. Mysterious dark patches in the clouds absorb light in a similar way to bacteria, previous studies have found.
"Venus shows some episodic dark, sulfuric rich patches, with contrasts up to 30-40 percent in the ultraviolet, and muted in longer wavelengths," says Dr Limaye.
"These patches persist for days, changing their shape and contrasts continuously and appear to be scale dependent."
The particles in the dark patches are the same size as bacteria on Earth, but NASA's probes in the 1960s and '70s couldn't differentiate between organic and non-organic.
"To really know, we need to go there and sample the clouds," says co-author Rakesh Mogul, professor of biological chemistry at California State Polytechnic University.
"Venus could be an exciting new chapter in astrobiology exploration."
The paper was published in journal Astrobiology.