Venus Could Have Blossomed Like Earth

If conditions had been just a little different an eon ago, there might be plentiful life on Venus and none on Earth. This idea isn't so far-fetched, according to a hypothesis by Rice University scientist Adrian Lenardic and collaborators, who published their thoughts on life-sustaining planets, the planets' histories and the possibility of finding more in their latest article.

If we were to wind the clock back to the early days of our Solar System, some 4.5 billion years ago, we would have seen a young, G-class star with four rocky worlds interior to our asteroid belt. Like many of the star systems the Kepler spacecraft has discovered, this type of configuration is relatively common; there are billions upon billions of chances in our galaxy alone that began just like ours did. But the young worlds in our newborn Solar System were very different from how they are today, and so was the Sun, for that matter.

Venus’ atmosphere was very thin at the beginning, comparable to the thickness of Earth’s atmosphere today. Earth, on the other hand, was very different, with lots of methane, ammonia, water vapor, hydrogen and virtually no oxygen at all. And the Sun was very faint compared to what it is now: less than 80% as luminous as it is today. With all that in mind, perhaps — if we rewound the Solar System to the very beginning and started it again — the ingredients for life would come together on Venus far more easily than on Earth, and perhaps early Venus was teeming with life, while things on Earth were barely getting started.

Lenardic and his colleagues suggest that what we think of as the 'Goldilocks zone' in extra-solar systems may actually be much broader — and more variable — than we’ve thought up to this point. The Goldilocks zone has long been defined as the band of space around a star that is not too warm, not too cold, and with the right conditions for maintaining surface water and a breathable atmosphere. But that description, which to date scientists have only been able to calibrate using observations from our own solar system, may be too limiting.

In expanding the notion of habitable zones, the researchers determined that life on Earth itself isn't necessarily a given based on the Goldilocks concept. A nudge this way or that in the conditions that existed early in the planet's formation may have made it inhospitable. By extension, a similarly small variation could have changed the fortunes of Venus, Earth's closest neighbor, preventing it from becoming a burning desert with an atmosphere poisonous to terrestrials. "If we find a planet in another solar system, sitting where Venus is that actually has signs of life, we'll know that what we see in our Solar System is not universal," Lenardic says.

The paper also questions the idea that plate tectonics is a critical reason Earth harbors life. "There's debate about this, but the Earth in its earliest lifetimes, let's say 2-3 billion years ago, would have looked for all intents and purposes like an alien planet," according to Lenardic. "We know the atmosphere was completely different, with no oxygen. There's a debate that plate tectonics might not have been operative."

"Yet there's no argument there was life then, even in this different a setting. The Earth itself could have transitioned between planetary states as it evolved. So we have to ask ourselves as we look at other planets, should we rule out an early Earth-like situation even if there's no sign of oxygen and potentially a tectonic mode distinctly different from the one that operates on our planet at present?" he says. "Habitability is an evolutionary variable. Understanding how life and a planet co-evolve is something we need to think about."

"There are things that are on the horizon that, when I was a student, it was crazy to even think about," Lenardic says. "Our paper is in many ways about imagining, within the laws of physics, chemistry and biology, how things could be over a range of planets, not just the ones we currently have access to. Given that we will have access to more observations, it seems to me we should not limit our imagination as it leads to alternate hypothesis."