As our discoveries of exoplanets continue to accumulate (and we’ve spotted it Over 11,000 possible exoplanets Yet) we need to know if a planet the size of Earth is more likely to look like Earth, or if it is more likely to look like Venus. “We don’t know which of these results are expected or likely,” says Paul Byrne, a planetary scientist at North Carolina State University. And to find out, we need to understand Venus better.
Most scientists agree that any habitable exoplanet needs water.
With surface temperatures of 471 degrees Celsius and surface pressures 89 times worse than Earth’s, it would seem impossible for water to exist on Venus. But Venus and Earth are roughly the same size, same ages, and our best guess is that they’re made of similar materials and born with very similar starting conditions. Venus is 30% closer to the sun than Earth, which is significant, but not significantly so. However, after 4.5 billion years, the performance of these two planets was completely different.
In fact, there is mounting evidence that Venus may have been home to water long ago. The Pioneer Venus missions launched in 1978 made some tantalizing measurements of the ratio of deuterium and hydrogen in the atmosphere, indicating that Venus has lost a ton of water over time. But we have never had a suitable mission that could study the history of water on Venus, or search for ancient water-flow features on the surface, or understand whether it possessed the kind of geological and climatic conditions necessary for water and for habitable conditions. .
“There may have been two habitable worlds side by side for an unknown amount of time in our solar system,” says Giada Arne, deputy principal investigator at DAVINCI+. Although Venus is uninhabitable today, the fact that it may have been habitable at one point means that it wasn’t always destined for such a hellish fate if conditions erupted a little better.
And that’s good news for how we rate distant exoplanets. “Looking beyond the solar system, this may also indicate that habitable planets are more common than we previously expected,” Arney says.
There are two leading theories of what happened to Venus — and each has implications for what we might expect on other exoplanets. The first, consistent with our current and limited observations, is that Venus started out as a hot mess from the start and never turned back. See, the closer a planet is to its orbit around its host star, the more likely it is to slowly rotate (or even gradually lock up where one side is permanently facing the star, like the Moon around the Earth)
Slow engines like Venus generally have a hard time maintaining a cool, comfortable global climate—and for a while it was assumed that this is likely what drove Venus to become so hot and unbearable. The sun’s rays bombarded the planet with heat, and the vapor-rich atmosphere never condensed into liquid water at the surface. Meanwhile, the carbon dioxide, water, and sulfur dioxide in the air act as greenhouse gases that only trap all that heat. And it stayed that way for 4 billion years, more or less.
Then there is a new theory developed by Michael Way and others at NASA’s Goddard Institute for Space Studies. This model shows that if you make some small adjustments in the climates of these planets, they can develop hemisphere-length cloud shapes that are constantly facing the host star, Reflects a lot of stellar heat. As a result, a planet like Venus remains temperate and atmospheric vapor condenses into liquid oceans at the surface. Wai’s work shows that once you reach this point, the planet can self-regulate its temperature as long as Earth-like processes such as plate tectonics (which help remove carbon dioxide from the atmosphere) can mitigate the buildup of greenhouse gases.
It’s a complex hypothesis full of caveats. And if Venus is evidence that slow rotors can develop more habitable conditions, that’s also evidence that these conditions are fragile and potentially transient. The people who buy into the Way model think that what most likely happened on Venus is that a massive amount of volcanic activity The planet was inundated with carbon and around 96% of the carbon dioxide in the atmosphere, exceeding all that plate tectonics can provide.
However, it’s a hypothesis worth testing by DAVINCI+ and VERITAS, because as Arne points out, many of the potentially habitable exoplanets we’ve discovered are slow rotors orbiting low-mass stars. Because these stars are faint, the planets usually have to orbit close to them to receive enough heat to allow liquid water to form. If they form hemisphere-length clouds, they may be able to maintain habitable climates. The only way we can currently verify whether this hypothesis makes sense is to first see if it occurred on Venus.
But before we can apply Way’s model to other exoplanets, we need to determine if it explains Venus. DAVINCI+ will descend on Venus and directly probe the chemistry and composition of the atmosphere, as well as photograph the surface on its way down. It should be able to collect the kind of data that helps tell us whether Venus was really wet early on in its life, and also explain more of the climate history and whether a long cloud in the hemisphere might have already formed.
The VERITAS orbiter will interrogate the planet’s geology, taking high-resolution images through radar observations that may be able to detect evidence of terrain or terrain created by past water or tectonic flows. Perhaps the most exciting target is the tessera: regions of deeply distorted upland that are believed to be the oldest geological features on the planet. If VERITAS spotted evidence of ancient oceans — or at least the kind of geological activity that would have kept the planet much milder long ago — it will support the idea that other slow-rotating exoplanets could achieve the same conditions.
“Thinking about them coming together makes it kind of a huge, complementary mission,” says Lauren Jozwiak, a planetary scientist at the Johns Hopkins Laboratory of Applied Physics working on the VERITAS mission. “This idea that you want to do both geologic mapping and atmospheric exploration was at the heart of how you want to explore Venus,” Joswiak says.
In the end, if Venus has always been uninhabitable, the reason is probably its proximity to the Sun. So any exoplanet of a similar size that is relatively close to its star would likely be like Venus. It would be better for us to focus more investigations on exoplanets far from their stars.
On the other hand, if Venus has gone through a period of cooling before turning into a permanent furnace, that means we should take the “Venus-zone” exoplanets seriously, as they may still be habitable. It also suggests that factors such as plate tectonics and volcanoes play a critical role in mediating habitable conditions, and we need to find ways to investigate these things on distant worlds as well.
The more we thought about what DAVINCI+ and VERITAS could achieve, the more it seemed as if we were actually understating how excited we were. These next missions, Joswiak says, “will completely change the way we think about both Venus and planet formation in general.” “It’s an exciting time to see if Venus is Earth’s one-time and future.”