An image of Venus captured by the Galileo spacecraft
Scientists in the United States say clouds high in the atmosphere of the planet Venus contain chemicals that may suggest the presence of life.
Venus: The facts
Second planet from the Sun
Similar in size and mass to Earth
Thick, poisonous atmosphere of carbon dioxide and sulphuric acid
Greenhouse effect keeps the surface hot enough for molten metal to flow
Space probes have never found any sign of life on Venus, which has an extremely hot surface and an atmosphere that contains a mixture of poisonous chemicals.
But Dirk Schulze-Makuch and Louis Irwin, from the University of Texas, say the Venusian atmosphere is "relatively hospitable" and may be home to large numbers of bacteria.
"From an astrobiology point of view, Venus is not hopeless," the scientists claim after finishing their research, reported in the New Scientist magazine.
However, most astronomers remain sceptical and the general consensus is that life on the Earth's closest neighbour would be impossible.
Using data from the Russian Venera space missions and also the US Pioneer Venus and Magellan probes, the researchers have been studying the high concentration of water droplets in the Venusian clouds.
Nasa's Magellan mission ended in 1994
They noticed oddities in its chemical composition that they say could be explained by the presence of microbes.
The scientists found hydrogen sulphide and sulphur dioxide - two gases which react with each other, and are not seen in the same place unless something is producing them.
They also say that - despite solar radiation and lightning - the atmosphere contains hardly any carbon monoxide, suggesting that something is removing the gas.
The researchers told the New Scientist that "bugs living in the Venusian clouds could be combining sulphur dioxide with carbon monoxide and possibly hydrogen sulphide or carbonyl sulphide in a metabolism similar to that of some early Earth bugs".
Past missions to Venus
Nasa's mission Magellan orbited the planet for 4 years before plunging into its atmosphere in 1994
Nasa's Mariners 2, 5 and 10 also visited Venus
Soviet missions have landed several spacecraft on Venus
Chemical analyses of rocks indicate a composition similar to that of volcanic rocks found on Earth
They also believe the temperatures of Venus was once much cooler and there could have been oceans on the planet.
"Life could have started there and retreated to stable niches once the runaway greenhouse effect began," Mr Schulze-Makuch says.
But most scientists are sceptical.
They say that tiny droplets of water are not enough to support life.