Venus is Like an Exoplanet that’s Right Next Door

We’re lucky to have a neighbour like Venus, even though it’s totally inhospitable, wildly different from the other rocky planets, and difficult to study. Its thick atmosphere obscures its surface, and only powerful radar can penetrate it. Its extreme atmospheric pressure and high temperatures are barriers to landers or rovers.

It’s like having a mysterious exoplanet next door.

We’ve been watching Venus with the naked eye for millennia and with telescopes for centuries. In many ways, it’s still shrouded in mystery. We’re in a similar predicament with exoplanets; only it’s their distance that shrouds them.

Venus’ inhospitable nature means we struggle to explore it. We know way more about other worlds in our Solar System—Mars, for example—because they’re more amenable to observation by orbiters. And in Mars’ case, visitation by multiple landers and rovers has revealed a lot about that planet’s history. So even though we’ve sent orbiters—and ill-fated landers—to Venus, our lack of understanding means that, in some ways, it’s more like an exoplanet than another planet in our Solar System.

In the last 30 years, scientists have found thousands of exoplanets. Every one of them is interesting in its own right, but much of our interest in exoplanets concerns their atmospheres and their potential habitability. This is where Venus and exoplanet science intersect.

Learning more about Venus can teach us about exoplanets, and the reverse is also true, according to the authors of a new research article. The article is “Synergies between Venus & Exoplanetary Observations.” It’s published in Space Science Reviews, and the lead author is Dr. Michael Way. Way is a Physical Scientist at NASA’s Goddard Institute for Space Studies. The study examines how General Circulation Models (GCMs) are used to understand exoplanets and how we can leverage them to unravel Venus’ history.

“Here, we examine how our knowledge of present-day Venus can inform terrestrial exoplanetary science and how exoplanetary science can inform our study of Venus,” the authors explain in their introduction. “In a superficial way, the contrasts in knowledge appear stark.” But are they?

This image shows how different spacecraft and the Very Large Array ground-based telescope have observed Venus. Image Credit: O’Rourke et al, 2023.

Even though Venus is challenging to study, scientists have made progress. Evidence suggests the planet was once habitable. Venus may have been warm and wet in the past before runaway greenhouse heating cooked the planet. The planet may even have harboured surface oceans before it became too hot. If Venus was habitable in the past, what processes led to its extremely inhospitable climate today? What can its transformation tell us about exoplanets?

To try to answer that question, researchers turn to one of their main tools: General Circulation Models (GCMs.) They use them to model how atmospheres and oceans influence the climate on Earth, and they’re also useful in the study of other planets. GCMs are mathematical models of how atmospheres circulate, and researchers use them to understand how insolation and rotation rates affect planetary climates.

This is the first video in a series of three videos that explain GCMs. Credit: The UK Met Office

Can exoplanet climate modelling help explain Venus’ climate?

“It may sound preposterous to propose that terrestrial exoplanets, which are far from being explored in-situ, and which present challenges even to detection of their atmospheres, can in any way inform Venus’ evolutionary history,” the authors write. “Yet exoplanetary science has already provided a means to put ancient Venus 4.2 billion years ago within the habitable zone.”

Scientists know a few things about ancient Venus. It received about 40% more solar radiation 4.2 billion years ago compared to present-day Earth. On the face of it, that would preclude any habitability.

But a paper published in 1971 showed that Venus could’ve had temperate conditions despite higher solar radiation if it had 100% cloud cover. With that much cloud, the albedo would’ve been high enough to reflect much of the Sun’s radiation. The cloud cover could’ve lowered the planet’s surface temperature to less than 27 Celsius (80 F.) At that temperature, Venus would’ve been able to maintain surface water and oceans.

That paper didn’t provide any rationale for 100% cloud cover. But as our knowledge of exoplanets grew, discoveries provided a rationale. Among the more than 5,000 exoplanets we know of, a good portion of them are very close to their stars and are either tidally locked or rotate very slowly. A 2014 study used GCMS to show that slowly rotating planets close to their stars could maintain surface temperatures of 26 Celsius (80 F) even when the planet received 2.5 times more solar insolation than Earth does. But only if the cloud cover was thick and high, just like the 1971 paper proposed.

The exoplanets’ slow rotation allowed this type of cloud cover to form. This ties in with Venus, which takes 243 Earth days to complete one rotation, the slowest rotation in the Solar System.

Other studies on slowly rotating exoplanets close to their stars used different GCMs and showed similar results. These results shaped how we think of Venus. Previous thinking showed that Venus may have had a relatively brief habitable period before becoming inhospitable. But if slowly rotating exoplanets with thick cloud cover maintained long-term habitability even when receiving abundant solar insolation, maybe Venus did, too.

We’re all probably wishing that Venus was habitable for a long period of time. It’s in our nature. Thus far, multiple studies based on five different GCMs have largely confirmed what the 2014 study showed. They’ve all shown that slowly rotating planets can produce the type of thick, high cloud cover that keeps an exoplanet’s surface temperate even when close to their stars. But while heartening, this still isn’t conclusive. Scientists need better observations of exoplanets before they can be certain.

There are lots of exoplanets in what exoplanet scientists call the Venus Zone (VZ.) The Venus Zone is an orbital zone around a star where an Earth-similar planet would suffer a runaway Greenhouse Effect like Venus has. Observations of these planets are critical to understanding Venus because of the underlying questions that apply to both exoplanets in the Venus Zone and Venus itself.

This image shows the Venus Zone and the Habitable Zone. The Venus Zone is where planets could experience the Runaway Greenhouse Effect. Image Credit: Chester Harman/NASA/JPL

How long did magma ocean phases last, and how does that affect atmosphere development and evolution? How does it affect the types of clouds that form? Since the exoplanets we find in Venus Zones have different ages, observations can help answer these questions by building a sort of evolutionary timeline for slowly rotating planets close to their stars. This will strengthen our GCMs and their results for both Venus and exoplanets.

Venus is like an exoplanet in our backyard. The authors of this paper point out that we can gather detailed observations of Venus and, eventually, in-situ observations, which will not only shed light on Venus but on Venus Zone exoplanets, too. At the same time, surveys of Venus Zone exoplanets at different ages and different proximities to different types of stars can also help us understand Venus.

This is in line with what the recent Planetary Science and Astrobiology Decadal Survey said. That survey covers a lot of territory, and it showed the links between Venus and exoplanets and how the study of each can inform the other. The authors of the paper made a table of parts of the Decadal Survey that link Venus and Exoplanet science.

The authors created a table from the Decadal Survey that highlights the links between Venus science and exoplanet science. Image Credit: Way et al. 2023.

There’s no strong consensus on Venus and its ancient oceans and potential habitability. Different studies have arrived at different conclusions. Some showed that Venus may have had oceans for a few hundred million years, some for two billion years, and some showed that there were never any oceans.

If it was once habitable, what happened? What can that tell scientists about exoplanets? “If Venus did evolve from an earlier temperate period with surface water reservoirs to its present hothouse state, exactly how did it occur, and what are the key processes involved?” the authors ask. Scientists have struggled for decades to understand how Venus could have evolved from an early habitable period to such an extremely inhospitable state.

Fortunately, we may gain some clarity soon. NASA and other space agencies are planning missions to Venus in the near future. NASA is planning the VERITAS and DAVINCI+ missions, and the ESA is planning the EnVision mission. All three are orbiters, and DAVINCI+ also includes an atmospheric probe. More ambitious surface missions are in the conceptual stage.

This illustration shows DAVINCI+’s atmospheric probe descending through Venus’ atmosphere. The probe will help scientists understand the connections between Venus’ surface and its atmosphere, which could shed light on the planet’s history and potential ancient habitability. Image Credit: NASA/GSFC – NASA/GSFC, http://science.gsfc.nasa.gov/690/photos.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=47317724

With three Venus missions coming in the future, some are calling the 2030s the Venus Decade. And when it comes to exoplanets, that field of study is already in full bloom. The JWST, CHEOPS, TESS, and others are pushing the frontier of exoplanet science. TESS uses the transit method of finding exoplanets, and that method is biased toward planets with shorter orbital periods. It discovered a large number of rocky planets in the Venus Zone. While all planets in the VZ won’t have liquid surface water, some might. In any case, TESS’s VZ planets can act as a guide for more follow-up observations with the JWST and other missions.

This figure from the study shows 208 terrestrial Venus Zone planets. Way et al. 2023.

The JWST has already detected the chemical makeup of one exoplanet named WASP 39-b. Though it’s not a terrestrial planet, it does orbit its star very closely.

“We expected JWST to be a powerful tool to study exoplanet atmospheres, and these observations are among the first real evidence that that is true,” said JHUAPL astrophysicist Erin May, who was involved in the JWST WASP 39-b results. “The precision of these measurements is unmatched by previous telescopes, and we’re really just scratching the surface of what we’ll be able to learn about exoplanets going forward.”

A similar sense of optimism surrounds the upcoming missions to Venus. “No previous mission within the Venus atmosphere has measured the chemistry or environments at the detail that DAVINCI’s probe can do,” said DAVINCI principal investigator Jim Garvin. “DAVINCI will build on what Huygens probe did at Titan and improve on what previous in situ Venus missions have done, but with 21st-century capabilities and sensors.”

Venus and Earth started out quite similar. But somehow, Venus followed a different evolutionary pathway than Earth. While it may seem like its proximity to the Sun is the obvious reason why, this study shows how there could be more to it.

“How can a world that was receiving, 4.2 billion years ago, 1.4 times the incident solar radiation that Earth receives today be inside the habitable zone?” the authors ask in their paper. The answer comes from GCMs. ” … an efficient cloud albedo feedback from a slowly rotating Venus may have kept ancient Venus temperate according to GCM modelling assuming sufficient surface liquid water and a short-lived magma ocean phase,” they explain.

Artist’s concept of the VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) spacecraft. VERITAS is scheduled to launch in 2031. Image Credit: NASA/JPL-Caltech

There’s only one path forward to test how accurate our understanding of Venus and VZ exoplanets is. Along with missions to Venus, we need to find more VZ planets and study them. And as this study makes clear, both efforts are linked.

” … observing a statistically relevant sample of VZ worlds in different evolutionary phases could help us bound the parameter space in ways we may only scarcely comprehend today,” the authors write. It’ll also help us understand Venus, our exoplanet next door.