SwRI-led team finds younger exoplanets better

image: A study led by the SwRI suggests that the age of the host star and the abundance of radionuclides will help determine both the history of an exoplanet 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 star system with seven rocky siblings including four in the habitable zone. But at about 8 billion years old, these worlds are about 2 billion years older than the most optimistic outgassing lifetime predicted by this study and are unlikely to support a temperate climate today.
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Credit: NASA/JPL-Caltech

SAN ANTONIO – May 3, 2022 – As the scientific community searches for worlds orbiting nearby stars that could potentially harbor life, new research from the Southwest Research Institute suggests that younger rocky exoplanets are more likely to support temperate climates, similar to those of Earth.

In the past, scientists have focused on planets in a star’s habitable zone, where it’s neither too hot nor too cold for liquid surface water to exist. However, even within this so-called “Goldilocks zone”, planets can still develop climates inhospitable to life. Sustaining temperate climates also requires a planet to have enough heat to fuel a planet-wide carbon cycle. A key source of this energy is the decay of radioactive isotopes of uranium, thorium and potassium. This critical heat source can power a rocky exoplanet’s mantle convection, a slow, creeping motion of the region between a planet’s core and crust that eventually melts on the surface. Surface volcanic outgassing is a primary source of CO2 in the atmosphere, which helps keep a planet warm. Without mantle outgassing, the planets are unlikely to support temperate, habitable climates like Earth’s.

“We know that these radioactive elements are needed to regulate the climate, but we don’t know how long these elements can do that because they decay over time,” said lead author Dr Cayman Unterborn. Astrophysical Journal Letters research article. “Furthermore, radioactive elements are not evenly distributed in the Galaxy, and as planets age they may run out of heat and outgassing will cease. Because planets can have more or less of these elements than Earth does, we wanted to understand how this variation might affect how long rocky exoplanets can sustain temperate Earth-like climates.

The study of exoplanets is difficult. Today’s technology cannot measure the composition of an exoplanet’s surface, let alone that of its interior. Scientists can, however, measure the abundance of elements in a star spectroscopically by studying how light interacts with elements in the upper layers of a star. Using this data, scientists can deduce what the planets orbiting a star are made of by using stellar composition as a rough approximation of its planets.

“Using host stars to estimate the amount of these elements that would enter planets throughout the Milky Way’s history, we calculated how long we can expect planets to have enough volcanism to endure a temperate climate before running out of energy,” Unterborn said. . “Under the most pessimistic conditions, we estimate that this critical age is only about 2 billion years old for an Earth-mass planet and reaches 5 to 6 billion years for higher-mass planets in more optimistic conditions. For the few planets for which we have ages, we found that only a few were young enough for us to say with confidence that they may have surface carbon outgassing. todayas we observed it with, say, the James Webb Space Telescope.

This research combined direct and indirect observational data with dynamical models to understand which parameters most affect an exoplanet’s ability to withstand a temperate climate. More laboratory experiments and computer modeling will quantify the reasonable range of these parameters, especially in the era of the James Webb Space Telescope, which will provide more in-depth characterization of individual targets. With the Webb telescope, it will be possible to measure the three-dimensional variation of the atmospheres of exoplanets. These measurements will deepen knowledge of atmospheric processes and their interactions with the planet’s surface and interior, allowing scientists to better estimate whether a rocky exoplanet in the habitable zones is too old to resemble Earth. .

“Exoplanets without active outgassing are more likely to be cold, snowball planets,” Unterborn said. “While we cannot say that other planets are not outgassing today, we can say that they would need special conditions to do so, such as tidal heating or plate tectonics. This includes exoplanets high-level rock formations discovered in the TRAPPIST-1 star system Regardless, younger planets with temperate climates may be the easiest places to search for other Earths.

the Astrophysical Journal Letters The article describing this research is titled “Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets can Support Temperate Climates” and can be viewed at https://doi.org/10.3847/2041-8213 /ac6596.

For more information, visit https://www.swri.org/planetary-science.

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