MSGID: 1:317/3 627201a8   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Younger exoplanets are better candidates when looking for other Earths   
    Some exoplanets found thus far may be too old to support temperate,   
   Earth-like climates    
      
     Date:   
         May 3, 2022   
     Source:   
         Southwest Research Institute   
     Summary:   
         As the scientific community searches for worlds orbiting nearby   
         stars that could potentially harbor life, new research suggests   
         that younger rocky exoplanets are more likely to support temperate,   
         Earth-like climates.   
      
      
      
   FULL STORY   
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   As the scientific community searches for worlds orbiting nearby stars   
   that could potentially harbor life, new Southwest Research Institute-led   
   research suggests that younger rocky exoplanets are more likely to   
   support temperate, Earth-like climates.   
      
      
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   In the past, scientists have focused on planets situated within a star's   
   habitable zone, where it is 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 have sufficient heat to power   
   a planetary-scale carbon cycle. A key source of this energy is the decay   
   of the 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 at the surface. Surface volcanic degassing is   
   a primary source of CO2 to the atmosphere, which helps keep a planet   
   warm. Without mantle degassing, planets are unlikely to support temperate,   
   habitable climates like the Earth's.   
      
   "We know these radioactive elements are necessary to regulate climate,   
   but we don't know how long these elements can do this, because they decay   
   over time," said Dr. Cayman Unterborn, lead author of an Astrophysical   
   Journal Letters paper about the research. "Also, radioactive elements   
   aren't distributed evenly throughout the Galaxy, and as planets age,   
   they can run out of heat and degassing will cease. Because planets   
   can have more or less of these elements than the Earth, we wanted to   
   understand how this variation might affect just how long rocky exoplanets   
   can support temperate, Earth-like climates."  Studying exoplanets is   
   challenging. Today's technology cannot measure the composition of an   
   exoplanet's surface, much less that of its interior.   
      
   Scientists can, however, measure the abundance of elements in a star   
   spectroscopically by studying how light interacts with the elements   
   in a star's upper layers. Using these data, scientists can infer what   
   a star's orbiting planets are made of using stellar composition as a   
   rough proxy for its planets.   
      
   "Using host stars to estimate the amount of these elements that would go   
   into planets throughout the history of the Milky Way, we calculated how   
   long we can expect planets to have enough volcanism to support a temperate   
   climate before running out of power," Unterborn said. "Under the most   
   pessimistic conditions we estimate that this critical age is only around   
   2 billion years old for an Earth-mass planet and reaching 5-6 billion   
   years for higher-mass planets under more optimistic conditions. For   
   the few planets we do have ages for, we found only a few were young   
   enough for us to confidently say they can have surface degassing of   
   carbon today, when we'd observe 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 support a temperate climate. More laboratory   
   experiments and computational modeling will quantify the reasonable   
   range of these parameters, particularly 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 exoplanet atmospheres.   
      
   These measurements will deepen the knowledge of atmospheric processes   
   and their interactions with the planet's surface and interior, which   
   will allow scientists to better estimate whether a rocky exoplanet in   
   habitable zones is too old to be Earth-like.   
      
   "Exoplanets without active degassing are more likely to be cold, snowball   
   planets," Unterborn said. "While we can't say the other planets aren't   
   degassing today, we can say that they would require special conditions to   
   do so, such as having tidal heating or undergoing plate tectonics. This   
   includes the high-profile rocky exoplanets discovered in the TRAPPIST-1   
   star system.   
      
   Regardless, younger planets with temperate climates may be the simplest   
   places to look for other Earths."   
      
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   Story Source: Materials provided by Southwest_Research_Institute. Note:   
   Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. Cayman T. Unterborn, Bradford J. Foley, Steven J. Desch, Patrick A.   
      
         Young, Gregory Vance, Lee Chiffelle, Stephen R. Kane. Mantle   
         Degassing Lifetimes through Galactic Time and the Maximum Age   
         Stagnant-lid Rocky Exoplanets Can Support Temperate Climates. The   
         Astrophysical Journal Letters, 2022; 930 (1): L6 DOI:   
         10.3847/2041-8213/ac6596   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2022/05/220503100208.htm   
      
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