Although Venus and Earth are so-called sister planets, they are as different as heaven and hell. Earth is a natural paradise where life has survived beneath its azure sky despite multiple mass extinctions. On the other hand, Venus is a scorching hot planet with clouds of sulfuric acid and atmospheric pressure strong enough to crush a human.
But the sister thing won't go away because both worlds have roughly the same mass and radius and are adjacent rocky planets in the inner solar system. Why are they so different? What do the differences tell us about our search for life?
The international astronomical community recognizes that understanding planetary habitability is a critical part of space science and astrobiology. Without a better understanding of terrestrial planets and their atmospheres, habitable or not, we won't really know what we're seeing when we study a distant exoplanet. If we find an exoplanet that shows signs of life, we will never visit it, never study it up close, and never be able to study its atmosphere.
Artist's impression of the exoplanet Ross 128 b orbiting its red dwarf star. Potentially habitable rocky worlds like this lie beyond our physical reach. Photo credit: ESO/M. grain knife. Public domain
This shifts scientific focus to the terrestrial planets in our own solar system. Not because they appear to be habitable, but because a complete model of terrestrial planets cannot be complete without also including those that are literal hellholes, like Sister Venus.
A recent research perspective in Nature Astronomy examines how the two planets diverged and what may have caused the divergence. The title is “Venus as an anchor point for the habitability of the planet”. The lead author is Stephen Kane from the Department of Earth and Planetary Sciences at the University of California, Riverside. His co-author is Paul Byrne from the Department of Earth, Environmental, and Planetary Sciences at Washington University in St. Louis.
“A major focus of planetary science and astrobiology is understanding the habitability of the planet, including the myriad factors that control the evolution and sustainability of temperate surface environments such as Earth's,” write Kane and Byrne. “The few significant terrestrial planetary atmospheres in the solar system serve as a critical resource for studying these habitability factors, from which models can be constructed for application to extrasolar planets.”
From their perspective, the twins of our solar system offer our best opportunity to study how similar planets can have such different atmospheres. The more we understand this, the better we can understand how rocky worlds evolve over time and how different conditions promote or limit habitability.
This illustration from the study shows some of the key, fundamental differences between Earth and Venus. Image source: Kane and Byrne, 2024.
Earth is an exception. Due to its temperate climate and surface water, it has been habitable for billions of years, although with some climatic episodes that severely limited life. But if we look at Mars, it appears to have been habitable for a while and then lost its atmosphere and surface water. The situation on Mars must be more common than on Earth.
Artist's impression of Snowball Earth 650 million years ago during the Marino Ice Age. Earth has had periods of extreme climate, but it is still doing well. Image source: University of St Andrews.
It is a daunting challenge to understand an exoplanet when we know nothing about its history. We only see it in one epoch of its climate and atmospheric history. But the discovery of thousands of exoplanets is helping. “The discovery of thousands of exoplanets and the confirmation that terrestrial planets are among the most common types provide a statistical framework for the study of planetary properties and their evolution in general,” the authors write.
A narrow range of properties allows biochemistry to emerge, and these properties may not last. We need to identify these properties and their parameters and develop a better understanding of habitability. From this perspective, Venus is a treasure trove of information.
But Venus is a challenge. We can only see through the dense clouds with radar, and no one has tried to land a spacecraft there since the USSR in the 1980s. Most of these attempts failed, and those that survived did not last long. Without better data, we cannot understand the history of Venus. The simple answer is that it is closer to the sun. But it's too simple to be helpful.
“Venus' evolutionary path to its current runaway greenhouse state is controversial, as it has traditionally been attributed to its closer proximity to the Sun,” explain Kane and Byrne.
We don't know why Venus has a greenhouse effect. Volcanoes may have played a role. They emit carbon dioxide, and without oceans and tectonic plates, the planet cannot remove carbon from its atmosphere. Image source: NASA/JPL-Caltech/Peter Rubin
But when scientists look closer at Venus and Earth, they discover many fundamental differences between them that go beyond their distances from the sun. They have different rotation speeds, different inclinations, and different magnetic fields, just to name a few. This means we cannot measure the exact effects of increased solar radiation on the planet.
This is the main idea of the authors. The differences between Earth and Venus make Venus an important factor in understanding the habitability of rocky exoplanets. “Venus thus offers us a crucial anchor point in the discourse about the planet’s habitability, as its evolutionary history represents an alternative path to the Earth-based narrative – even if the origins of both worlds are probably similar,” they write.
The authors point out that surface water is the basic requirement for life. The bigger question, however, is what factors determine how long surface water can persist. “By taking this measure, studies of the planet’s habitability can then focus on the conditions that allow liquid surface water to be maintained over geologic timescales,” they write.
This figure from research illustrates some of the factors that can affect the planet's surface water and habitability. Image source: Kane and Byrne 2024, National Academies Press, Ron Pettengill.
Earth and Venus lie at opposite ends of the spectrum of rocky planet habitability. This is an important lesson we can learn from our own solar system. For this reason, “understanding the path to a Venus scenario is as important as the path to habitability that characterizes Earth,” the authors write.
The pair of researchers compiled a list of some of the factors that determine habitability on Earth and Venus.
Most of these factors are self-explanatory. CHNOPS are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, the life-sustaining elements. Redox is the potential of an element or molecule to be reduced or oxidized and made available as chemical energy for life. The fact that there is a question mark next to it for the redox environment of Venus is a major stumbling block. Image source: Kane and Byrne, 2024.
There is so much we don't know about Venus. How big is its core? Was there ever water? Some research shows that there was a lot of oxygen in the planet's atmosphere when it lost its water and became fully habitable. If we saw the same amount of oxygen on a distant exoplanet, we could interpret that as a sign of life. Big mistake. “Venus thus acts as a cautionary tale for interpreting seemingly oxygen-rich atmospheres,” the authors write.
Kane and Byrne's research perspective is a call to action. It reflects what recent Decadal Surveys have said. “Recent decadal studies in astronomy and astrophysics, as well as planetary science and astrobiology, both highlight the need for a better understanding of planetary habitability as an essential goal in the context of astrobiology,” they write. For the authors, Venus can anchor the effort.
But for it to serve as an anchor, scientists need answers to many questions. They need to study the atmosphere more thoroughly at all altitudes. You must examine its interior and determine the type and size of its core. Crucially, they must bring a spacecraft to its surface and examine its geology up close. In short, we must do on Venus what we did on Mars.
This is a challenge given Venus' hostile environment. However, missions are being prepared to explore Venus in more detail. VERITAS, DAVINCI and EnVision are all Venus missions planned for the 2030s. These missions will give scientists the answers we need.
As we learn more about Venus, we must also learn more about Exo-Venus. “A parallel approach to studying the intrinsic properties of Venus is the statistical analysis of the vast (and still rapidly growing) inventory of terrestrial exoplanets,” the authors write.
This number from research represents the Venus zone and habitable zone as a function of the stellar effective temperature and solar flux received by the planet. The Venus zone is shaded red and the habitable zone is blue. The images on the left show main sequence stars with different effective temperatures. The images of Venus show the location of Kepler candidates that lie within the Venus zone, scaled by the size of the planet. The planets of the solar system Venus, Earth and Mars are also shown. Image source: Habitable Zone Gallery/Chester Harman; Planets: NASA/JPL. Kane and Byrne, 2024.
We live in the age of exoplanet discovery. We have discovered over 5,000 confirmed exoplanets and the number continues to grow. We launch spaceships to examine the most interesting of them in more detail. But at some point things will change. How many of them do we need to catalog? Is 10,000 enough? 20,000? 100,000?
Everything is new at the moment and the excitement to find more exoplanets, especially rocky ones in habitable zones, is understandable. But at some point we will reach a certain threshold where returns will decrease. To understand it, it would be more useful to focus our efforts on studying Venus and its very different evolution.
Just like Kane and Byrne suggest.
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