MSGID: 1:317/3 645091f5   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Webb finds water vapor, but from a rocky planet or its star?    
      
     Date:   
         May 1, 2023   
     Source:   
         NASA/Goddard Space Flight Center   
     Summary:   
         The most common stars in the universe are red dwarf stars, which   
         means that rocky exoplanets are most likely to be found orbiting   
         such a star.   
      
         Red dwarf stars are cool, so a planet has to hug it in a tight orbit   
         to stay warm enough to potentially host liquid water (meaning   
         it lies in the habitable zone). Such stars are also active,   
         particularly when they are young, releasing ultraviolet and X-ray   
         radiation that could destroy planetary atmospheres. As a result, one   
         important open question in astronomy is whether a rocky planet could   
         maintain, or reestablish, an atmosphere in such a harsh environment.   
      
      
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   FULL STORY   
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   The most common stars in the universe are red dwarf stars, which means   
   that rocky exoplanets are most likely to be found orbiting such a   
   star. Red dwarf stars are cool, so a planet has to hug it in a tight   
   orbit to stay warm enough to potentially host liquid water (meaning it   
   lies in the habitable zone). Such stars are also active, particularly   
   when they are young, releasing ultraviolet and X-ray radiation that could   
   destroy planetary atmospheres. As a result, one important open question   
   in astronomy is whether a rocky planet could maintain, or reestablish,   
   an atmosphere in such a harsh environment.   
      
   To help answer that question, astronomers used NASA's James Webb Space   
   Telescope to study a rocky exoplanet known as GJ 486 b. It is too close   
   to its star to be within the habitable zone, with a surface temperature   
   of about 800 degrees Fahrenheit (430 degrees Celsius). And yet, their   
   observations using Webb's Near-Infrared Spectrograph (NIRSpec) show   
   hints of water vapor. If the water vapor is associated with the planet,   
   that would indicate that it has an atmosphere despite its scorching   
   temperature and close proximity to its star.   
      
   Water vapor has been seen on gaseous exoplanets before, but to date no   
   atmosphere has been definitely detected around a rocky exoplanet. However,   
   the team cautions that the water vapor could be on the star itself --   
   specifically, in cool starspots -- and not from the planet at all.   
      
   "We see a signal, and it's almost certainly due to water. But we can't   
   tell yet if that water is part of the planet's atmosphere, meaning the   
   planet has an atmosphere, or if we're just seeing a water signature coming   
   from the star," said Sarah Moran of the University of Arizona in Tucson,   
   lead author of the study.   
      
   "Water vapor in an atmosphere on a hot rocky planet would represent   
   a major breakthrough for exoplanet science. But we must be careful and   
   make sure that the star is not the culprit," added Kevin Stevenson of the   
   Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland,   
   principal investigator on the program.   
      
   GJ 486 b is about 30% larger than Earth and three times as massive,   
   which means it is a rocky world with stronger gravity than Earth. It   
   orbits a red dwarf star in just under 1.5 Earth days. It is expected to   
   be tidally locked, with a permanent day side and a permanent night side.   
      
   GJ 486 b transits its star, crossing in front of the star from our point   
   of view. If it has an atmosphere, then when it transits starlight would   
   filter through those gasses, imprinting fingerprints in the light that   
   allow astronomers to decode its composition through a technique called   
   transmission spectroscopy.   
      
   The team observed two transits, each lasting about an hour. They then   
   used three different methods to analyze the resulting data. The results   
   from all three are consistent in that they show a mostly flat spectrum   
   with an intriguing rise at the shortest infrared wavelengths. The team   
   ran computer models considering a number of different molecules, and   
   concluded that the most likely source of the signal was water vapor.   
      
   While the water vapor could potentially indicate the presence of an   
   atmosphere on GJ 486 b, an equally plausible explanation is water vapor   
   from the star.   
      
   Surprisingly, even in our own Sun, water vapor can sometimes exist in   
   sunspots because these spots are very cool compared to the surrounding   
   surface of the star. GJ 486 b's host star is much cooler than the Sun,   
   so even more water vapor would concentrate within its starspots. As a   
   result, it could create a signal that mimics a planetary atmosphere.   
      
   "We didn't observe evidence of the planet crossing any starspots during   
   the transits. But that doesn't mean that there aren't spots elsewhere   
   on the star.   
      
   And that's exactly the physical scenario that would imprint this   
   water signal into the data and could wind up looking like a planetary   
   atmosphere," explained Ryan MacDonald of the University of Michigan in   
   Ann Arbor, one of the study's co-authors.   
      
   A water vapor atmosphere would be expected to gradually erode due to   
   stellar heating and irradiation. As a result, if an atmosphere is present,   
   it would likely have to be constantly replenished by volcanoes ejecting   
   steam from the planet's interior. If the water is indeed in the planet's   
   atmosphere, additional observations are needed to narrow down how much   
   water is present.   
      
   Future Webb observations may shed more light on this system. An upcoming   
   Webb program will use the Mid-Infrared Instrument (MIRI) to observe   
   the planet's day side. If the planet has no atmosphere, or only a thin   
   atmosphere, then the hottest part of the day side is expected to be   
   directly under the star.   
      
   However, if the hottest point is shifted, that would indicate an   
   atmosphere that can circulate heat.   
      
   Ultimately, observations at shorter infrared wavelengths by another Webb   
   instrument, the Near-Infrared Imager and Slitless Spectrograph (NIRISS),   
   will be needed to differentiate between the planetary atmosphere and   
   starspot scenarios.   
      
   "It's joining multiple instruments together that will really pin down   
   whether or not this planet has an atmosphere," said Stevenson.   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Stars # Extrasolar_Planets # Astronomy # Mars # Pluto #   
                   Eris_(Xena) # Jupiter # Kuiper_Belt   
       * RELATED_TERMS   
             o Open_cluster o Red_supergiant_star o Extrasolar_planet o   
             Star_cluster o Chandra_X-ray_Observatory o Globular_cluster   
             o Astronomy o Planet   
      
   ==========================================================================   
   Story Source: Materials provided by   
   NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style   
   and length.   
      
      
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   Journal Reference:   
      1. Moran, S.E. and Stevenson, K.B., Sing, D.K., MacDonald, R.J.,   
      et al. High   
         Tide or Rip-Tide on the Cosmic Shoreline? A Water-Rich   
         Atmosphere or Stellar Contamination on GJ 486b from JWST   
         Observations. Astrophysical Journal Letters, accepted April 2023   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230501143017.htm   
      
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