An unusual cosmic object is helping scientists better understand the chemistry hidden deep in Jupiter and Saturn\u2019s atmospheres \u2014 and potentially those of exoplanets.<\/p>\n
Why has silicon, one of the most common elements in the universe, gone largely undetected in the atmospheres of Jupiter, Saturn, and gas planets like them orbiting other stars? A new study using observations from NASA\u2019s James Webb Space Telescope sheds light on this question by focusing on a peculiar object that astronomers discovered by chance in 2020 and called \u201cThe Accident.\u201d<\/p>\n
The results<\/a> were published on Sept. 4 in the journal Nature.<\/p>\n The Accident is a brown dwarf, a ball of gas that\u2019s not quite a planet and not quite a star. Even among its already hard-to-classify peers, The Accident has a perplexing mix of physical features, some of which have been previously seen in only young brown dwarfs<\/a> and others seen only in ancient ones. Because of those features, it slipped past typical detection methods before being discovered five years ago<\/a> by a citizen scientist participating in Backyard Worlds: Planet 9<\/a>. The program lets people around the globe look for new discoveries in data from NASA\u2019s now-retired NEOWISE<\/a> (Near-Earth Object Wide-field Infrared Survey Explorer), which was managed by NASA\u2019s Jet Propulsion Laboratory in Southern California.<\/p>\n The Accident is so faint and odd that researchers needed NASA\u2019s most powerful space observatory, Webb<\/a>, to study its atmosphere. Among several surprises, they found evidence of a molecule they couldn\u2019t initially identify. It turned out to be a simple silicon molecule called silane (SiH4). Researchers have long expected \u2014 but been unable \u2014 to find silane not only in our solar system\u2019s gas giants, but also in the thousands of atmospheres belonging to brown dwarfs and to the gas giants orbiting other stars. The Accident is the first such object where this molecule has been identified.<\/p>\n Scientists are fairly confident that silicon exists in Jupiter and Saturn\u2019s atmospheres but that it is hidden. Bound to oxygen, silicon forms oxides such as quartz that can seed clouds<\/a> on hot gas giants, bearing a resemblance to dust storms on Earth. On cooler gas giants like Jupiter and Saturn, these types of clouds would sink far beneath lighter layers of water vapor and ammonia clouds, until any silicon-containing molecules are deep in the atmosphere, invisible even to the spacecraft that have studied those two planets up close.<\/p>\n Some researchers have also posited that lighter molecules of silicon, like silane, should be found higher up in these atmospheric layers, left behind like traces of flour on a baker\u2019s table. That such molecules haven\u2019t appeared anywhere except in a single, peculiar brown dwarf suggests something about the chemistry occurring in these environments.<\/p>\n \u201cSometimes it\u2019s the extreme objects that help us understand what\u2019s happening in the average ones,\u201d said Faherty, a researcher at the American Museum of Natural History in New York City, and lead author on the new study.<\/p>\n Located about 50 light-years from Earth, The Accident likely formed 10 billion to 12 billion years ago, making it one of the oldest brown dwarfs ever discovered. The universe is about 14 billion years old, and at the time that The Accident developed, the cosmos contained mostly hydrogen and helium, with trace amounts of other elements, including silicon. Over eons, elements like carbon, nitrogen, and oxygen forged in the cores of stars, so planets and stars that formed more recently possess more of those elements.<\/p>\n Webb\u2019s observations of The Accident confirm that silane can form in brown dwarf and planetary atmospheres. The fact that silane seems to be missing in other brown dwarfs and gas giant planets suggests that when oxygen is available, it bonds with silicon at such a high rate and so easily, virtually no silicon is left over to bond with hydrogen and form silane.<\/p>\n So why is silane in The Accident? The study authors surmise it is because far less oxygen was present in the universe when the ancient brown dwarf formed, resulting in less oxygen in its atmosphere to gobble up all the silicon. The available silicon would have bonded with hydrogen instead, resulting in silane.<\/p>\n \u201cWe weren\u2019t looking to solve a mystery about Jupiter and Saturn with these observations,\u201d said JPL\u2019s Peter Eisenhardt, project scientist for the WISE (Wide-field Infrared Survey Explorer) mission, which was later repurposed as NEOWISE. \u201cA brown dwarf is a ball of gas like a star, but without an internal fusion reactor, it gets cooler and cooler, with an atmosphere like that of gas giant planets. We wanted to see why this brown dwarf is so odd, but we weren\u2019t expecting silane. The universe continues to surprise us.\u201d<\/p>\n Brown dwarfs are often easier to study than gas giant exoplanets because the light from a faraway planet is typically drowned out by the star it orbits, while brown dwarfs generally fly solo. And the lessons learned from these objects<\/a> extend to all kinds of planets, including ones outside our solar system that might feature potential signs of habitability.\u00a0<\/p>\n \u201cTo be clear, we\u2019re not finding life on brown dwarfs,\u201d said Faherty. \u201cBut at a high level, by studying all of this variety and complexity in planetary atmospheres, we\u2019re setting up the scientists who are one day going to have to do this kind of chemical analysis for rocky, potentially Earth-like planets. It might not specifically involve silicon, but they\u2019re going to get data that is complicated and confusing and doesn\u2019t fit their models, just like we are. They\u2019ll have to parse all those complexities if they want to answer those big questions.\u201d<\/p>\n A division of Caltech, JPL managed and operated WISE for NASA\u2019s Science Mission Directorate. The mission was selected competitively under NASA\u2019s Explorers Program managed by the agency\u2019s Goddard Space Flight Center in Greenbelt, Maryland. The NEOWISE mission was a project of JPL and the University of Arizona in Tucson, supported by NASA\u2019s Planetary Defense Coordination Office.<\/p>\n For more information about WISE, go to:<\/p>\n https:\/\/www.nasa.gov\/mission_pages\/WISE\/main\/index.html<\/a><\/p>\n The James Webb Space Telescope is the world\u2019s premier space science observatory, and an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).<\/p>\n To learn more about Webb, visit:<\/p>\n https:\/\/science.nasa.gov\/webb<\/a><\/p>\n Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov<\/a><\/p>\n