The team discovers that younger exoplanets are preferable possibilities while searching for other Earths.

An SwRI-led study suggests that host-star age and radionuclide abundance will help determine both an exoplanet’s history and its current likelihood of being temperate today. For example, the red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-sized planets ever found in a single stellar system with seven rocky siblings including four in the habitable zone. But at around 8 billion years old, these worlds are roughly 2 billion years older than the most optimistic degassing lifetime predicted by this study and unlikely to support a temperate climate today. Credit: NASA/JPL-Caltech

As scientists look for worlds circling nearby stars that may house life, new research headed by the Southwest Research Institute indicates that younger rocky exoplanets are more likely to have temperate, Earth-like climates.

Historically, astronomers have concentrated their efforts on planets located inside a star’s habitable zone, a region of the star’s atmosphere that is neither too hot nor too cold for liquid surface water to exist. Even within this so-called “Goldilocks zone,” though, planets can acquire hostile climates. Sustaining temperate climates also requires a planet to have enough heat to power a global carbon cycle on a large scale. The decay of radioactive isotopes of uranium, thorium, and potassium provides a significant source of this energy. This essential source of heat can power the mantle convection of a rocky exoplanet, the gradual creeping motion of the region between a planet’s core and crust that eventually melts near the surface. Surface volcano degassing is a major supplier of CO2 to the atmosphere, which contributes to the planet’s heat retention. Without mantle degassing, planets like the Earth are unlikely to have moderate, habitable climates.

“We know these radioactive elements are required for climate regulation, but we don’t know how long they can do so because they decay over time,” said Dr. Cayman Unterborn, main author of the research paper published in the Astrophysical Journal Letters. “Additionally, radioactive elements are not equally distributed throughout the Galaxy, and as planets age, they may run out of heat, resulting in the cessation of degassing. Because planets can contain more or fewer of these elements than the Earth, we wanted to understand how this variance might affect the duration of temperate, Earth-like climates on rocky exoplanets.”

Exoplanet research is difficult. Today’s technology is incapable of determining the composition of an exoplanet’s surface, let alone its innards. However, scientists may use spectroscopy to determine the quantity of elements in a star by observing how light interacts with the components in the star’s higher layers. Scientists can deduce the composition of a star’s circling planets using stellar composition as a rough proxy for the planets.

“By estimating the amount of these elements that would enter planets over the course of the Milky Way’s history, we calculated how long planets will have enough volcanism to support a temperate temperature before they run out of power,” Unterborn explained. “We estimate that this critical age is roughly 2 billion years old for Earth-mass planets and 5–6 billion years for higher-mass planets under the most pessimistic scenarios. For the few planets for which we know ages, we discovered that just a few are young enough for us to firmly assert that they can have surface degassing of carbon today, as observed by the James Webb Space Telescope, for example.”

This study used direct and indirect observational data with dynamical models to determine which parameters had the most impact on an exoplanet’s capacity to support a temperate climate. Additional laboratory studies and computational modelling will help quantify the reasonable range of these characteristics, especially in the era of the James Webb Space Telescope, which will allow for greater in-depth assessment of individual targets. It will be feasible to measure the three-dimensional variation of extraterrestrial atmospheres with the Webb telescope. These measurements will increase scientists’ understanding of atmospheric processes and their interactions with the planet’s surface and interior, allowing them to better estimate the age of a rocky exoplanet in habitable zones.

“Exoplanets that do not exhibit active degassing are more likely to be icy, snowball worlds,” Unterborn explained. “While we cannot rule out the possibility that other planets are currently degassing, we can argue that they would require specific conditions to do so, such as tidal heating or plate tectonics. This contains the TRAPPIST-1 star system’s high-profile rocky exoplanets. Regardless, younger planets with temperate climates may be the most straightforward locations to search for extra-Earths.”

Further information: Cayman T. Unterborn et al, Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets Can Support Temperate Climates, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac6596

Journal information: Astrophysical Journal Letters

Source: Southwest Research Institute