Hopes that life ever existed on Venus are thinning, after a new study has suggested the planet has always been too dry to support it.

The research, published in Nature Astronomy, examined water signatures in Venus’ atmosphere to learn how wet it was over its history.

“We won’t know for sure whether Venus can or did support life until we send probes at the end of this decade,” said first author Tereza Constantinou, a PhD student in astronomy at the University of Cambridge, UK.

“But given it likely never had oceans, it is hard to imagine Venus ever having supported Earth-like life, which requires liquid water.”

The researchers say their findings also have implications for the search for life on exoplanets: if Venus can’t have supported it, that means similar planets from other stars probably can’t either.

“Even though it’s the closest planet to us, Venus is important for exoplanet science, because it gives us a unique opportunity to explore a planet that evolved very differently to ours, right at the edge of the habitable zone,” says Constantinou.

The idea of life on Venus got a boost 4 years ago, when astronomers found phosphine in the atmosphere – which, on Earth, is only made by microbes.

But for Earth-like life, liquid water is also necessary – and it’s uncertain how much there’s been on Venus.

It was possible Venus once had liquid water, but volcanoes caused a runaway greenhouse effect leading to its current super-hot conditions, with thick clouds made from sulphuric acid.

Another theory holds that Venus has always been inhospitably dry.

“Both of those theories are based on climate models, but we wanted to take a different approach based on observations of Venus’ current atmospheric chemistry,” says Constantinou.

“To keep the Venusian atmosphere stable, any chemicals being removed from the atmosphere should also be getting restored , since the planet’s interior and exterior are in constant chemical communication with one another.”

These chemicals – including water – are restored from volcanoes on the planet’s surface, which means that the speed of chemicals being destroyed in the atmosphere can indicate how much there must have been on the surface in the past. Volcanic gases on Earth are mostly steam from water.

The team used computer modelling to estimate the depletion rates of 3 volcanic gases – water, carbon dioxide, and carbonyl sulphide – in the Venusian atmosphere.

They found that volcanic gases must have been a maximum of 6% water. This means the planet has probably been dry the whole time.

“We would have loved to find that Venus was once a planet much closer to our own, so it’s kind of sad in a way to find out that it wasn’t, but ultimately it’s more useful to focus the search on planets that are mostly likely to be able to support life – at least life as we know it,” says Constantinou.

The researchers say that Venus-like planets, a big target for the James Webb Space Telescope, are “improbable candidates” for liquid-water based life.

“If Venus was habitable in the past, it would mean other planets we have already found might also be habitable,” says Constantinou.

“Instruments like the James Webb Space Telescope are best at studying the atmospheres of planets close to their host star, like Venus. But if Venus was never habitable, then it makes Venus-like planets elsewhere less likely candidates for habitable conditions or life.”