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Astronomers using the Hubble Space Telescope have gone looking for water vapor in the atmospheres of three planets orbiting stars similar to the Sun – and have come up nearly dry. The planets spectroscopically surveyed have only one-tenth to one one-thousandth the amount of water predicted by standard planet-formation theories. The planets are not habitable because they are gaseous and are as big as Jupiter. They lie so much closer to their host star than Jupiter is to our Sun, so their atmospheres are seething between 1,500 and 4,000 degrees Fahrenheit. Nevertheless, this result suggests that some percentage of Earth-size exoplanets may be more deficient in water than predicted. And, water is a necessary prerequisite for life as we know it. The search for water-bearing terrestrial worlds may be more challenging than thought for future space telescopes. And, scientists may have to revisit their theories of planet formation.

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      Last Updated Dec 19, 2024 Editor Andrea Gianopoulos Location NASA Goddard Space Flight Center Related Terms
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      Hubble observations of NGC 346 from the mid 2000s revealed many stars about 20 to 30 million years old that seemed to still have planet-forming disks around them. This went against the conventional belief that such disks would dissipate after 2 or 3 million years.
      “The Hubble findings were controversial, going against not only empirical evidence in our galaxy but also against the current models,” said De Marchi. “This was intriguing, but without a way to obtain spectra of those stars, we could not really establish whether we were witnessing genuine accretion and the presence of disks, or just some artificial effects.”
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      Image B: Protoplanetary Disks in NGC 346 Spectra (NIRSpec)
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      On the right is the comparison of the top and bottom lines. This comparison shows a large peak in the cold molecular hydrogen coming from the star but not its nebular environment. Also, atomic hydrogen shows a larger peak from the star. This indicates the presence of a protoplanetary disk immediately surrounding the star. The data was taken with the microshutter array on the James Webb Space Telescope’s NIRSpec (Near-Infrared Spectrometer) instrument. Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI) A New Way of Thinking
      This finding refutes previous theoretical predictions that when there are very few heavier elements in the gas around the disk, the star would very quickly blow away the disk. So the disk’s life would be very short, even less than a million years. But if a disk doesn’t stay around the star long enough for the dust grains to stick together and pebbles to form and become the core of a planet, how can planets form?
      The researchers explained that there could be two distinct mechanisms, or even a combination, for planet-forming disks to persist in environments scarce in heavier elements.
      First, to be able to blow away the disk, the star applies radiation pressure. For this pressure to be effective, elements heavier than hydrogen and helium would have to reside in the gas. But the massive star cluster NGC 346 only has about ten percent of the heavier elements that are present in the chemical composition of our Sun. Perhaps it simply takes longer for a star in this cluster to disperse its disk.
      The second possibility is that, for a Sun-like star to form when there are few heavier elements, it would have to start from a larger cloud of gas. A bigger gas cloud will produce a bigger disk. So there is more mass in the disk and therefore it would take longer to blow the disk away, even if the radiation pressure were working in the same way.
      “With more matter around the stars, the accretion lasts for a longer time,” said Sabbi. “The disks take ten times longer to disappear. This has implications for how you form a planet, and the type of system architecture that you can have in these different environments. This is so exciting.”
      The science team’s paper appears in the Dec. 16 issue of The Astrophysical Journal.
      Image C: NGC 346: Hubble and Webb Observations
      Image Before/After The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
      The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt manages the telescope and mission operations. Lockheed Martin Space, based in Denver also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
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      View/Download all image products at all resolutions for this article from the Space Telescope Science Institute.
      View/Download the science paper from the The Astrophysical Journal.
      Media Contacts
      Laura Betz – laura.e.betz@nasa.gov
      NASA’s Goddard Space Flight Center, Greenbelt, Md.
      Ann Jenkins – jenkins@stsci.edu, Christine Pulliam – cpulliam@stsci.edu
      Space Telescope Science Institute, Baltimore, Md.
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      Last Updated Dec 15, 2024 Editor Marty McCoy Contact Laura Betz laura.e.betz@nasa.gov Related Terms
      Astrophysics Galaxies Galaxies, Stars, & Black Holes Goddard Space Flight Center James Webb Space Telescope (JWST) Science & Research Stars The Universe View the full article
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