Exo-Venus analogs orbiting other stars can help us learn about how Venus came to resemble the hellscape we know today.
The planet Venus is by all accounts a pretty hellish place. Pressures at its surface are 92 times greater than the air pressure on Earth, its temperatures soar to over 900 degrees fahrenheit during the day (the hottest planetary surface in the solar system), and its atmosphere is a deadly cocktail of sulphuric acid and carbon dioxide. Yet many planetary scientists think that Venus may have once hosted oceans of liquid water and maybe even microbial life before a runaway greenhouse effect turned it to the hellscape we know today.
In this sense, Venus is something of a warning about the dangers of not keeping greenhouse gases in check on Earth, but scientists still don’t really understand the processes that occurred billions of years ago that led to its current state. According to a new paper published on arXiv this week by planetary scientists Giada Arney and Stephen Kane studying Venus-like planets around other stars can help us understand what Venus may have looked like in the past and its evolutionary processes. They argue that this line of research would help polish the “magic mirror” that “reflects the future of Earth back to forward-thinking observers.”
That Venus may once have hosted liquid water and may even have been habitable was a thesis first put forward in the 1970s by James Hansen, a renowned NASA climate scientist. A few years ago, I spoke about Hansen’s theory with Geoffrey Landis, a NASA scientist who has investigated building cloud cities on Venus, and he said the theory still holds up.
"Certainly Venus is a prime example of a greenhouse effect planet—maybe it's the future of the Earth, but the far, far future of the Earth," Landis told me. “We learn about our planet by learning about other planets.”
Read More: Why We Should Build Cloud Cities on Venus
Yet if we want to learn what lessons Venus may hold for Earth, we must first learn about Venus’ own history. According to Arney and Kane, the best way to do that may be by studying exo-Venuses around other stars.
“Beyond the solar system, exo-Venus analogs are likely common types of planets, and we likely have already discovered many of Venus’ sisters orbiting other stars,” wrote Arney and Kane, who do research at NASA and UC Riverside, respectively. “By observing exo-Venus planets of differing ages in different astrophysical contexts, these distant hot terrestrial worlds may likewise allow us to witness processes that occurred on Venus in the past.”
Arney and Kane are hopeful that the next generation of exoplanet hunting telescopes, such as the recently launched Transiting Exoplanet Survey Satellite (TESS), will help find dozens of new Venus analogs in addition to some of the ones already identified.
The problem of course, is that our knowledge of exoplanets is still quite limited. Although astronomers have discovered several thousand exoplanets to date and future telescopes will allow astronomers to ‘see’ these planets’ atmospheric composition, we are still very limited in what we may know about them. In this sense, Arney and Kane argue, Venus can also help us learn about planetary evolution outside our solar system by acting as the “exoplanet laboratory next door.”
“An incomplete understanding of the evolution of Venus and the processes that govern its atmosphere and interior today will hinder our ability to interpret observations of hot terrestrial exoplanets,” they write. “When we work to understand the basic properties of exoplanets, we must have a robust understanding of our local planetary neighborhood to guide us.”
So in order to understand Earth better we need to understand how Venus evolved, and in order to understand how Venus evolved we need to study Venus-like exoplanets. But in order to accurately study Venus-like exoplanets we need to learn more about Venus and the authors concede that “many basic parameters of Venus are still unknown.”
Unfortunately, Japan has launched the only mission to Earth’s twin sister in the last few decades, and NASA doesn’t have any future missions on the books yet. But as Arney, Kane, and a growing number of other planetary scientists have convincingly argued, it’s time to place Venus in the spotlight.
“Venus teaches us that habitability is not a static state that planets remain in throughout their entire lives,” Arney and Kane conclude. “Habitability can be lost, and the runaway greenhouse is the final resting place of once watery worlds. Exoplanets may someday allow us to witness planetary states lost to our solar system’s past, and Venus may show us the possible future of Earth.”