Comet lander Philae may be sitting on an object teeming with alien microbial life, according to two leading astronomers.
Distinct features of the comet 67P/Churyumov-Gerasimenko, such as its organic-rich black crust, are best explained by the presence of living organisms beneath an icy surface, they claim.
Rosetta, the European space craft orbiting the comet, is also said to have picked up strange "clusters" of organic material that look suspiciously like viral particles.
But neither Rosetta nor its lander are equipped to search for direct evidence of life after a proposal to include this in the mission was allegedly laughed out of court.
Astronomer and astrobiologist Professor Chandra Wickramasinghe, who was involved in the mission planning 15 years ago, said: "I wanted to include a very inexpensive life-detection experiment. At the time it was thought this was a bizarre proposition."
He and colleague Dr Max Wallis, from the University of Cardiff, believe 67P and other comets like it could provide homes for living microbes similar to the "extremophiles" that inhabit the most inhospitable regions of the Earth.
Comets may have helped to sow the seeds of life on Earth and possibly other planets such as Mars early in the life of the solar system, they argue.
The astronomers present their case for life on 67P at the Royal Astronomical Society's National Astronomy Meeting in Llandudno, Wales.
Philae made history last November after detaching from its Rosetta mothership and bouncing down on to the surface of the comet, coming to rest close to a cliff or crater wall.
After being forced into hibernation by the lack of sunlight reaching its solar panels, the probe has delighted scientists by "waking up" as the comet races towards the sun.
The comet, described as looking like a "rubber duck", has two lobes joined by a thinner neck and measures around four kilometres across. Currently it is about 176.7 million miles from Earth and travelling at more than 73,000 mph.
Prof Wickramasinghe and Dr Wallis have carried out computer simulations that suggest microbes could inhabit watery regions of the comet.
Organisms containing anti-freeze salts could be active at temperatures as low as minus 40C, their research shows.
The comet has a black hydrocarbon crust overlaying ice, smooth icy "seas", and flat-bottomed craters containing "lakes" of re-frozen water overlain with organic debris.
Prof Wickramasinghe said: "What we're saying is that data coming from the comet seems to unequivocally, in my opinion, point to micro-organisms being involved in the formation of the icy structures, the preponderance of aromatic hydrocarbons, and the very dark surface.
"These are not easily explained in terms of pre-biotic chemistry.
"The dark material is being constantly replenished as it is boiled off by heat from the sun. Something must be doing that at a fairly prolific rate."
He said several cracks in the ice had been shown to be "spewing out material" that is falling on to the surface.
"I think the microbiotic activity just under the surface results in gas which builds up to the point where the overlaying layers of ice can't withstand the stresses," said the professor.
Biological mechanisms were the likely explanation for the large quantities of organic gases that had been observed around comets, along with water, he maintained.
Philae had confirmed the presence of "ring and linear chain" organic molecules on the surface of 67P that were more complex than simple hydrocarbons such as methane, said Prof Wickramasinghe.
However it was impossible to say if these represented amino acids, the building blocks of proteins.
One tantalising find was the discovery of organic "particle clusters" by Rosetta in the gases surrounding the comet, which resembled viral particles collected from the Earth's upper atmosphere.
"They might be viral particles," said Prof Wickramasinghe.
As the comet reaches its closest point to the sun - a distance of 195 million kilometres (121 million miles) - its family of micro-organisms is likely to become more active, say the scientists.
Prof Wickramasinghe, director of the Buckingham Centre for Astrobiology, believes it is time for a complete shift of thinking about the possibility of alien life.
He said: "The current estimate for the number of extra-solar planets in the galaxy is 140 billion plus. Planets that can harbour life are really quite abundant in the galaxy, and the next neighbouring system to us is only spitting distance away. I think it's inevitable that life is going to be a cosmic phenomenon.
"Five hundred years ago it was a struggle to have people accept that the Earth was not the centre of the universe. After that revolution our thinking has remained Earth-centred in relation to life and biology. It's deeply ingrained in our scientific culture and it will take a lot of evidence to kick it over."
He pointed out that when proof of organic molecules in space emerged in the 1970s "the rebuttals were fierce" from the scientific establishment.
Future missions to 67P and other comets should include life-seeking instruments, he said. But space agencies appeared reluctant to engage in a serious quest for life that risked challenging "a long established paradigm".
Dr Wallis said: "Rosetta has already shown that the comet is not to be seen as a deep-frozen inactive body, but supports geological processes and could be more hospitable to micro-life than our Arctic and Antarctic regions."