A rare Venus solar transit sheds light on exoplanet atmospheres

A rare Venus solar transit sheds light on exoplanet atmospheres
by Sophie Jenkins
London, UK (SPX) Dec 06, 2024
In the coming decade, astronomers will begin exploring the atmospheres of Earth- and Venus-sized planets orbiting nearby stars. Despite their physical similarities, Venus and Earth have drastically different atmospheres. This raises the question: could scientists distinguish between these "twin" planets if observed from light-years away?

Researchers from the Instituto de Astrofisica e Ciencias do Espaco (IA) tackled this question by simulating Venus as an exoplanet. Their findings, published in *Atmosphere*, confirm that methods used for studying large, hot exoplanets can effectively apply to much smaller planets, such as Earth or Venus. This research also helps identify markers that distinguish nitrogen-rich atmospheres like Earth's from the carbon dioxide-dominated environment of Venus.

"The techniques currently used to study the atmosphere of exoplanets are effective for giant planets close to their star, thus with a hot atmosphere. However, it is challenging to study the atmosphere of bodies as small as Earth or Venus," said Alexandre Branco, lead author and MSc student at IA and the Faculty of Sciences of the University of Lisbon (Ciencias ULisboa). "The most promising targets are often bathed in a stellar radiation regime much like Venus, so 'ExoVenus' are most likely to be the first small worlds to have their atmosphere characterised. Our work had this aim of looking at Venus as if we were looking at an exoplanet."

Analyzing the 2012 Venus Transit

The study relied on rare data from Venus's solar transit on June 5-6, 2012, the last event of its kind this century. During such transits, the planet's atmosphere imprints its molecular signature onto the light of the star it passes, revealing its composition.

This method becomes increasingly challenging with smaller planets due to weaker signals and more noise. However, upcoming facilities like ESO's Extremely Large Telescope (ELT) and ESA's Ariel mission will bring such planets within reach.

By applying their techniques to Venus transit data, the researchers demonstrated the capability of these methods to work with next-generation instruments. Yet challenges remain, as Venus viewed from afar could resemble Earth, complicating the identification of "true Earth-like" planets.

Pedro Machado, IA researcher and study co-author, explained: "The high temperatures intrinsic to rocky planets with an atmosphere rich in carbon dioxide, and thus subject to intense greenhouse effect, lead to a chemically active environment, with many chemical transitions. This makes this type of atmosphere easy to detect."

Co-author Olivier Demangeon of IA and the Faculty of Sciences of the University of Porto (FCUP) added, "Venus' atmosphere is around 90 times denser than Earth and is also significantly hotter. So much so that, despite being denser, Venus' atmosphere is larger. Larger and denser both imply a strong signature in our observations. We detected some faint signatures of carbon dioxide on the Venus data that are not expected in Earth-like atmospheres. Yet, it is still not the most efficient way to differentiate between the two planets."

Implications for Solar System Studies

The techniques adapted for Venus have also been used to probe other Solar System atmospheres. During the 2012 transit, Pedro Machado's team gathered spectroscopic data using the Dunn Solar Telescope in New Mexico. They successfully detected isotopes of carbon and oxygen in Venus's atmosphere, offering insights into its past climate and evolution.

"These observations help estimate isotopic ratios, which reveal information about temperature and pressure conditions over time," said Branco. This approach aligns with the goals of ESA's upcoming Venus mission, EnVision, which aims to study Venus's atmospheric evolution.

The research underscores the potential of instruments like the ANDES spectrograph for ESO's ELT and ESA's Ariel mission to further characterize exoplanets and Solar System bodies alike. Ariel, set to study the atmospheres of roughly 1,000 exoplanets, will directly benefit from this team's work.

"Adapting sophisticated techniques for nearby planets validates their use in exploring distant worlds," Machado noted. "We are preparing observations that will benefit from this technique to probe the atmospheres of Jupiter and Saturn when a bright star passes behind them as seen from Earth."

Research Report:Transmission Spectroscopy Along the Transit of Venus: A Proxy for Exoplanets Atmospheric Characterization