A team of astronomers has discovered that Venus was never habitable, despite decades of speculation that our nearest planetary neighbor was once much more similar to Earth than it is today.

The University of Cambridge researchers studied the chemical composition of Venus’ atmosphere and concluded that its interior is now too dry for there ever to have been enough water for oceans to exist on its surface. Instead, the planet has likely been a sweltering, inhospitable world throughout its history.

The results, reported in the journal Natural astronomyhave implications for understanding the uniqueness of Earth and for the search for life on planets beyond our solar system. While many exoplanets are Venus-like, the study suggests that astronomers should limit their focus to exoplanets that more closely resemble Earth.

From a distance, Venus and Earth look like siblings: it is almost identical in size and is a rocky planet like Earth. However, when viewed up close, Venus looks more like an evil twin: it is covered in thick clouds of sulfuric acid and its surface has an average temperature of around 500°C.

Despite these extreme conditions, astronomers have been studying for decades whether Venus once had liquid oceans capable of harboring life, or whether a mysterious form of “aerial life” now exists in its dense clouds.

“We won’t know for sure whether Venus can or does support life until we send probes at the end of this decade,” said lead author Tereza Constantinou, a Ph.D. student at the Cambridge Institute of Astronomy. “Given that there have probably never been oceans on Venus, it is difficult to imagine that Venus has ever supported Earth-like life that requires liquid water.”

In searching for life elsewhere in our galaxy, astronomers are focusing on planets that orbit their host stars in the habitable zone, where temperatures are high enough for liquid water to exist on the planet’s surface. Venus represents a strong boundary for where this habitable zone lies around a star.

“Although Venus is the closest planet to us, it is important for exoplanet research because it offers us a unique opportunity to study a planet that has evolved very differently than us, right on the edge of the habitable zone,” Constantinou said.

There are two main theories about how conditions on Venus may have evolved since its formation 4.6 billion years ago. The first is that conditions on Venus’ surface were once temperate enough to support liquid water, but a runaway greenhouse effect caused by widespread volcanic activity caused the planet to become increasingly hotter.

The second theory is that Venus was born hot and liquid water was never able to condense on the surface.

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

“To keep the atmosphere of Venus stable, any chemicals removed from the atmosphere should also be returned to it, since the interior and exterior of the planet are in constant chemical communication with each other.”

The researchers calculated the current rate of destruction of water, carbon dioxide and carbonyl sulfide molecules in Venus’ atmosphere, which must be restored by volcanic gases to keep the atmosphere stable.

By adding gases to the atmosphere, volcanism provides a window into the interior of rocky planets like Venus. As magma rises from the Earth’s mantle to the surface, it releases gases from the deeper parts of the planet.

Because of our planet’s water-rich interior, volcanic eruptions on Earth usually involve steam. But based on the composition of the volcanic gases necessary to maintain Venus’ atmosphere, the researchers found that volcanic gases on Venus contain at most six percent water. These dry eruptions suggest that the interior of Venus, the source of the magma that releases volcanic gases, is also dehydrated.

At the end of this decade, NASA’s DAVINCI mission will be able to test and confirm whether Venus has always been a dry, inhospitable planet through a series of flybys and a probe sent to the surface. The results could help astronomers sharpen their focus in the search for planets that can support life in orbit around other stars in the galaxy.

“If Venus was habitable in the past, that would mean that other planets we have already found could also be habitable,” Constantinou said. “Instruments like the James Webb Space Telescope are best suited to studying the atmospheres of planets near their parent star, such as Venus. But if Venus was never habitable, then Venus-like planets elsewhere are less likely candidates for habitable conditions or life.”

“We would have liked to find that Venus was once a planet much closer to ours. So it’s sad in some ways to find out that this wasn’t the case, but ultimately it makes more sense to focus the search on planets that are most likely to be able to support life – at least that Life as we know it.”