By Laurence Tognetti, MSc
March 27, 2025
What can Venus-like exoplanets, also known as exoVenuses, teach us about our own solar system and potentially finding life beyond Earth, and how can the planned Habitable Worlds Observatory (HWO) provide these insights? This is what a recent study presented at the 56th Lunar and Planetary Science Conference (LPSC) hopes to address as a team of scientists discussed the difficulties of studying exoVenuses and how HWO can help alleviate these challenges by directly imaging them. This study has the potential to help astronomers develop advanced methods for better identifying and understanding potentially life-harboring exoplanets throughout the cosmos.
Here, Universe Today discusses this incredible research with Dr. Stephen Kane, who is a Professor of Planetary Astrophysics in the Department of Earth and Planetary Sciences at the University of California, Riverside, regarding the motivation behind the study, significant takeaways, the importance of studying exoVenuses, how HWO will help us better understand exoVenuses, and which exoVenuses Dr. Kane thinks HWO should target. Therefore, what was the motivation behind this study?
“We are entering an exciting era when we will be able to spectroscopically characterize the atmospheres of terrestrial planets,” Dr. Kane tells Universe Today. “However, a major challenge for such characterization is trying to distinguish between a Venus analog and an Earth analog. Transmission spectra, particularly at the wavelengths of JWST, can look very similar for Venus and Earth since both exhibit strong carbon dioxide absorption within that regime. This is a critically important issue to resolve since Venus and Earth lie at opposite ends of the habitability spectrum, and distinguishing between them will have a profound effect on the search for life in the universe. The primary motivation is thus: how can we more effectively identify planets that, like Venus, are in a post runaway greenhouse state?”
For the study, the researchers evaluated the ongoing study of exoVenuses and what makes them so difficult to observe and study with indirect methods, as mentioned above. The team emphasized the importance of identifying terrestrial (rocky) exoplanets using observational methods while comparing them to our solar system because “we will functionally never obtain in situ data for exoplanet surface environments” because of the vast distances required to travel there and return to Earth with samples.
For context, the closest star to our solar system is Alpha Centauri, which is 4.37 light-years from Earth. That means a spacecraft traveling at the speed of light would take 4.37 years to get there then another 4.37 years to return to Earth. However, it is estimated that the Voyager 1 and 2 spacecraft would take more than 80,000 years to reach Alpha Centauri (though neither are headed in its direction).
Additionally, the researchers discussed the importance of the planned Habitable Worlds Observatory (HWO) and its ability to directly image terrestrial exoplanets, which could open a new era in exoplanet discoveries and characterization. So, what are the most significant takeaways from the study?
“Terrestrial planets are extremely common, and studying their atmospheres will teach us how common Venus-like planets really are,” Dr. Kane tells Universe Today. “Direct imaging (via Habitable Worlds Observatory (HWO) and Large Interferometer For Exoplanets (LIFE)) will provide reflectance and emission spectra that extend spectral coverage into the UV [ultraviolet], covering crucial features such as a SO2 [sulfur dioxide] and O3 0ozone] absorption. In the mid-term, the coming Venus missions (DAVINCI (atmospheric probe), VERITAS (surface and interior), and EnVision (surface geology)) will provide critical geology-atmosphere-chemistry models that are essential for understanding our sister planet and correctly interpreting exoplanet data.”
As noted, exoVenuses are Venus-like exoplanets, meaning they potentially possess atmospheric and surface characteristics like Venus, which is one of the most unique and mysterious planets in our solar system. This is because despite it orbiting so close to Earth and slightly closer to the Sun, its surface temperatures are hotter than the planet Mercury due to its runaway greenhouse effect.
Size comparison of Venus and Earth, illustrating why astronomers are so interested in finding exoVenuses. (Credit: NASA)
For example, while Mercury’s surface temperatures range from 430°C (800°F) on its dayside to -180°C (-290°F) on its nightside, Venus has an average surface temperature of 464°C (867°F) across its entire surface. Additionally, the surface pressure on Venus is approximately 92 times greater than Earth’s, or equivalent to the pressures experienced at approximately 3,000 feet underwater. Therefore, with these hellish conditions, why is it so important to study exoVenuses?
“Our perception of Venus has been transforming dramatically over the past few years,” Dr. Kane tells Universe Today. “Ideas about abundant life at the surface of Venus persisted up until the mid-60s when then US Mariner and Soviet Venera programs confirmed that the surface of Venus was a hellish landscape with high temperatures and pressures that could not support life. At that point, conversations about the relevance of Venus to planet habitability more or less stopped and the attention turned to Mars. However, we now understand that Venus has EVERYTHING to do with habitability. Earth and Venus likely had very similar starting conditions, and recent work how shown that Venus may have been habitable, with surface liquid water oceans, as recently as a billion years ago.”
As noted by the study and Dr. Kane, the goal of the planned HWO will be to observe exoplanets using the direct imaging method. This is when astronomers use specialized instruments to block out the immense light from the parent star, revealing the orbiting exoplanets that would have otherwise been blocked by the glare. This is often accomplished with coronagraphs, adaptive optics, and infrared wavelengths that block the parent star’s light, compensate for atmospheric distortions, and decrease the brightness, respectively.
While HWO was recommended by the National Academies’ 2020 Decadal Survey on Astronomy and Astrophysics, it is not currently slated to launch until sometime in the 2040s. Once launched, its primary goal will be to directly image at least 25 potentially habitable exoplanets orbiting Sun-like stars. While the Hubble Space Telescope was used to directly image Fomalhaut b, no space-based telescope currently exists whose primary goal is to directly image exoplanets. Therefore, how will HWO directly imaging exoVenuses help scientists better understand them?
Dr. Kane tells Universe Today, “Direct imaging that extends into optical and UV passbands will be essential in breaking the model degeneracies, helping to positively identify planets that are in a post runaway greenhouse state. Additionally, direct image may provide information regarding the planetary rotation rates, which is a first-order effect in evaluating planetary habitability.”
Of the almost 5,900 confirmed exoplanets, more than 300 exoVenus candidates have been identified to exist within the Venus zone (VZ), which is the orbital region around a star where exoplanets would experience a runaway greenhouse effect. One of the most recently discovered exoVenuses is Gliese 12 b, which was discovered in 2024 whose planetary radius and mass of Earth is 0.958 and 3.87, respectively. It is located approximately 40 light-years from Earth and orbits within the inner boundary of its star’s habitable zone with an orbital period of 12.8 days around an M-type star, which is smaller and cooler than our Sun.
“Gliese 12 b is an interesting case for sure,” Dr. Kane tells Universe Today. “It may in fact be a ‘super-Venus’, a coin I termed in my 2013 paper about Kepler-69c. I’m particularly interested in TOI-1266 c, LHS 1140 c, and L 98-59 d and the prospects for characterizing their atmospheres. I’m also interested in the TRAPPIST-1 planets, insofar as they have been able to retain their atmospheres.”
What insights about exoVenuses will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!