This artist’s concept shows a brown dwarf — an object
larger than a planet but not massive enough to kickstart fusion in its core
like a star. Brown dwarfs are hot when they form and may glow like this one,
but over time they get closer in temperature to gas giant planets like Jupiter.
NOIRLab/NSF/AURA/R. Proctor
An unusual cosmic object is helping scientists better understand the
chemistry hidden deep in Jupiter and Saturn’s atmospheres — and potentially
those of exoplanets.
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’s James Webb Space Telescope sheds light on this
question by focusing on a peculiar object that astronomers discovered by chance
in 2020 and called “The Accident.”
The results were published on Sept. 4 in the journal Nature.
As shown in this graphic, brown dwarfs can be far more
massive than even large gas planets like Jupiter and Saturn. However, they tend
to lack the mass that kickstarts nuclear fusion in the cores of stars, causing
them to shine.
NASA/JPL-Caltech
The Accident is a brown dwarf, a ball of gas that’s 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 and others seen only in ancient ones. Because of those features, it
slipped past typical detection methods before being discovered five years ago by a citizen scientist participating in Backyard Worlds: Planet 9. The program lets people around the globe look for
new discoveries in data from NASA’s now-retired NEOWISE (Near-Earth Object Wide-field Infrared Survey
Explorer), which was managed by NASA’s Jet Propulsion Laboratory in Southern
California.
The brown dwarf nicknamed “The Accident” can be seen
moving in the bottom left corner of this video, which shows data from NASA’s
now-retired NEOWISE (Near-Earth Object Wide-Field Infrared Survey Explorer),
launched in 2009 with the moniker WISE.
NASA/JPL-Caltech/Dan Caselden
The Accident is so faint and odd that researchers needed NASA’s most
powerful space observatory, Webb, to study its atmosphere. Among several surprises, they found evidence of
a molecule they couldn’t initially identify. It turned out to be a simple
silicon molecule called silane (SiH4). Researchers have long expected — but been unable — to find silane not
only in our solar system’s 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.
Scientists are fairly confident
that silicon exists in Jupiter and Saturn’s atmospheres but that it is hidden.
Bound to oxygen, silicon forms oxides such as quartz that can seed clouds 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.
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’s table.
That such molecules haven’t appeared anywhere except in a single, peculiar
brown dwarf suggests something about the chemistry occurring in these
environments.
“Sometimes it’s the extreme objects
that help us understand what’s happening in the average ones,” said Faherty, a
researcher at the American Museum of Natural History in New York City, and lead
author on the new study.
Happy accident
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.
Webb’s 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.
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.
“We weren’t looking to solve a
mystery about Jupiter and Saturn with these observations,” said JPL’s Peter
Eisenhardt, project scientist for the WISE (Wide-field Infrared Survey
Explorer) mission, which was later repurposed as NEOWISE. “A 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’t expecting silane. The universe
continues to surprise us.”
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 extend to all kinds of planets, including ones
outside our solar system that might feature potential signs of
habitability.
“To be clear, we’re not finding
life on brown dwarfs,” said Faherty. “But at a high level, by studying all of
this variety and complexity in planetary atmospheres, we’re 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’re going to get data that is complicated and confusing and
doesn’t fit their models, just like we are. They’ll have to parse all those
complexities if they want to answer those big questions.”
More about
WISE, Webb
A division of Caltech, JPL managed
and operated WISE for NASA’s Science Mission Directorate. The mission was
selected competitively under NASA’s Explorers Program managed by the agency’s
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’s
Planetary Defense Coordination Office.
For more information about WISE, go to: https://www.nasa.gov/mission_pages/WISE/main/index.html
The James Webb Space Telescope is
the world’s premier space science observatory, and an international program led
by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space
Agency).
To learn more about Webb, visit: https://science.nasa.gov/webb
Source: NASA Study: Celestial ‘Accident’ Sheds Light on Jupiter, Saturn Riddle - NASA
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