Venus is one of Earth’s closest neighbors, but astronauts won’t be stepping foot on the second planet from the sun anytime soon. Venus is a true hellscape, sporting an atmosphere thick enough to crush a person, temperatures high enough to melt lead and pervasive clouds of sulfuric acid. But new simulations suggest that wasn't always the case. Venus was downright Earth-like for 2 to 3 billion years and didn’t turn into the violent no-man’s land we know today until 700 million years ago.

Venus was a cloudy mystery to astronomers until 1978, when the Pioneer Venus Project reached the planet and found indications that it was once home to shallow seas. To understand whether the planet could have ever supported liquid water, and possibly life, researchers from NASA’s Goddard Institute for Space Science ran five simulations each representing different levels of water covering the planet. In all scenarios, they found that the planet would have been able to maintain a stable temperate climate for a couple billion years. The research was presented at the European Planetary Science Congress—Division for Planetary Sciences Joint Meeting 2019 in Geneva, Switzerland.

NASA’s Michael Way and Anthony Del Genio calculated three scenarios based off the topography of Venus was see today: one with a 1,017-foot average ocean, one with a shallow 30-foot-deep ocean and one in which the moisture was locked in the soil. The team adjusted their model to account for changing atmospheric conditions and for the sun heating up over time. They found that in all scenarios the planet could maintain an average temperature between 68 and 122 degrees Fahrenheit.

“Venus currently has almost twice the solar radiation that we have at Earth. However, in all the scenarios we have modeled, we have found that Venus could still support surface temperatures amenable for liquid water,” Way says in a press release. “Our hypothesis is that Venus may have had a stable climate for billions of years. It is possible that the near-global resurfacing event is responsible for its transformation from an Earth-like climate to the hellish hot-house we see today.”

Soon after it first formed around 4.2 billion years ago, Venus cooled off quickly and had an atmosphere dominated by carbon dioxide, the researchers hypothesize. If the planet followed similar patterns to the early Earth, much of that carbon dioxide would have been absorbed by silicate rocks and locked into the surface over the course of 3 billion years. Roughly 715 million years ago, the Venusian atmosphere would have been pretty similar to Earth, with a predominance of nitrogen with some trace amounts of carbon dioxide and methane.

Around that time, however, massive amounts of carbon dioxide re-entered the atmosphere, setting off the runaway greenhouse effect that transformed the planet into what it is today. The researchers believe it was likely a volcanic event that released gas trapped in massive amounts of magma but prevented the carbon dioxide from being reabsorbed.

“Something happened on Venus where a huge amount of gas was released into the atmosphere and couldn’t be re-absorbed by the rocks,” Way says. “On Earth we have some examples of large-scale outgassing, for instance the creation of the Siberian Traps 500 million years ago which is linked to a mass extinction, but nothing on this scale. It completely transformed Venus.”

There are still some big questions about whether Venus was habitable. First, researchers need to learn more about how quickly Venus cooled off after its formation. It’s possible that it never cooled down enough for liquid water to form. It’s also unknown whether the event that reshaped the planet was one mega-cataclysm or if it was a series of smaller events over billions of years that gradually turned Venus into what it is today.

If Venus was habitable for billions of years, it opens up the possibility that exoplanets spotted in a so-called “Venus Zone,” or roughly the same distance as Venus is from the sun, could be candidates for supporting life in their solar system. But confirming the hypothesis will take more missions to study the planet.

There are plenty of compelling reasons to go back. A study released last month shows that cyclical dark patches that appear and disappear in the upper reaches of Venus’s thick atmosphere are associated with changes in the planet’s brightness and energy levels. Astronomer Carl Sagan and other notable scientists have hypothesized that the unusual darkening could be caused by microscopic life in the clouds.