Scientists said on Monday (Sep 14) they have detected in the harshly acidic clouds of Venus a gas called phosphine that indicates microbes may inhabit Earth's inhospitable neighbour, a tantalizing sign of potential life beyond Earth.

The researchers did not discover actual life forms, but noted that on Earth phosphine is produced by bacteria thriving in oxygen-starved environments.

The international scientific team first spotted the phosphine using the James Clerk Maxwell Telescope in Hawaii and confirmed it using the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in Chile.

"I was just stunned. I mean I did this as an interesting experiment. I'd never really thought about detecting it. I thought we'd get some interesting information, some limits on life and Venus. But yeah, it was stunning to actually see the absorption by the phosphine molecule turn up in our spectrum," said astronomer Professor Jane Greaves of Cardiff University in Wales, lead author of the research published in the journal Nature Astronomy. The existence of extraterrestrial life long has been one of the paramount questions of science.

Scientists have used probes and telescopes to seek "biosignatures" - indirect signs of life - on other planets and moons in our solar system and beyond.

"The key to what we've found is the presence of phosphine gas in the clouds of Venus. This is very unexpected because phosphine is a phosphorous atom and three hydrogen atoms and there's really very little hydrogen available in the atmosphere so we think something is...a process that's making it and one of the possibilities is it's small, floating organisms and the reason we think that is because there are small bacteria on Earth that do actually make phosphine," said Greaves.

Phosphine - a phosphorus atom with three hydrogen atoms attached - is highly toxic to people.

Phosphine was seen at 20 parts-per-billion in the Venusian atmosphere, a trace concentration.

Greaves said the researchers examined potential non-biological sources such as volcanism, meteorites, lightning and various types of chemical reactions, but none appeared viable. The research continues to either confirm the presence of life or find an alternative explanation.

Venus is Earth's closest planetary neighbor. Similar in structure but slightly smaller than Earth, it is the second planet from the sun. Earth is the third. Venus is wrapped in a thick, toxic atmosphere that traps in heat.

Surface temperatures reach a scorching 880 degrees Fahrenheit (471 degrees Celsius), hot enough to melt lead.

"We're talking, if they're there, I think probably single-celled organisms but they'd probably be pretty different to something on Earth. They might perhaps need to surround themselves by some kind of thick shell, like sulphur, to defend themselves against the actual sulphuric acid. So they might not be something we'd recognise very easily. But there is some evidence that there are particles that are about a micron, a millionth of a metre in size, floating around in Venus' atmosphere and that is about the size of microbes. So nobody knows exactly what those particles are but it might tie in with this work that other people have done previously," said Greaves.

Some scientists have suspected that the Venusian high clouds, with mild temperatures around 86 degrees Fahrenheit (30 degrees Celsius), could harbor aerial microbes away that could endure extreme acidity. These clouds are around 90% sulphuric acid. Earth microbes could not survive that acidity.

On Earth, microorganisms in "anaerobic" environments - ecosystems that do not rely on oxygen - produce phosphine.

These include sewage plants, swamps, rice fields, marshlands, lake sediments and the excrements and intestinal tracts of many animals.

Phosphine also arises non-biologically in certain industrial settings. To produce phosphine, Earth bacteria take up phosphate from minerals or biological material and add hydrogen.