This mosaic shows the entire sky imaged by the Wide-field Infrared Survey Explorer (WISE). Infrared light refers to wavelengths that are longer than those visible to the human eye. Many cosmic objects radiate infrared, including gas and dust clouds where stars form, and brown dwarfs. Credits: NASA/JPL-Caltech/UCLA
Brown dwarfs aren’t quite stars and aren’t quite planets, and a new study
suggests there might be more of them lurking in our galaxy than scientists
previously thought.
A new study offers a
tantalizing explanation for how a peculiar cosmic object called WISEA
J153429.75-104303.3 – nicknamed “The Accident” – came to be. The Accident is a
brown dwarf. Though they form like stars, these objects don’t have enough mass
to kickstart nuclear fusion, the process that causes stars to shine. And while
brown dwarfs sometimes defy characterization, astronomers have a good grasp on
their general characteristics.
Or they did, until they found this one.
Can you see the dark
spot moving in the bottom left corner of the screen? It’s a brown dwarf
nicknamed “The Accident,” which was discovered by citizen scientist Dan
Caselden. It had slipped past typical searches because it doesn’t look like any
other known brown dwarfs. Credits: NASA/JPL-Caltech/Dan Caselden
The Accident got its name after being
discovered by sheer luck. It slipped past normal searches because it doesn’t
resemble any of the just over 2,000 brown dwarfs that have been found in our
galaxy so far.
As brown dwarfs age, they cool off, and their
brightness in different wavelengths of light changes. It’s not unlike how some
metals, when heated, go from bright white to deep red as they cool. The
Accident confused scientists because it was faint in some key wavelengths,
suggesting it was very cold (and old), but bright in others, indicating a
higher temperature.
“This object defied all our expectations,” said Davy
Kirkpatrick, an astrophysicist at IPAC at Caltech in Pasadena, California. He and his co-authors posit in
their new study, appearing in the Astrophysical Journal Letters, that The
Accident might be 10 billion to 13 billion years old – at least double the
median age of other known brown dwarfs. That means it would have formed when
our galaxy was much younger and had a different chemical makeup. If that’s the
case, there are likely many more of these ancient brown dwarfs lurking in our
galactic neighborhood.
A Peculiar Profile
The Accident was first spotted by NASA’s Near-Earth
Object Wide-Field Infrared Survey Explorer (NEOWISE),
launched in 2009 under the moniker WISE and
managed by NASA’s Jet Propulsion Laboratory in Southern California. Because
brown dwarfs are relatively cool objects, they radiate mostly infrared light,
or wavelengths longer than what the human eye can see.
Brown dwarfs share
certain characteristics with both stars and planets. Generally, they are less
massive than stars and more massive than planets. A brown dwarf becomes a star
if its core pressure gets high enough to start nuclear fusion, the process that
causes stars to shine. Credits: NASA/JPL-Caltech
To figure out how The Accident could have such seemingly contradictory
properties – some suggesting it is very cold, others indicating it is much
warmer – the scientists needed more information. So they observed it in
additional infrared wavelengths with a ground-based telescope at the W. M. Keck Observatory in Hawaii. But the
brown dwarf appeared so faint in those wavelengths, they couldn’t detect it at
all, apparently confirming their suggestion that it was very cold.
They next set out to determine if the dimness resulted from The Accident
being farther than expected from Earth. But that wasn’t the case, according to
precise distance measurements by NASA’s Hubble and Spitzer Space Telescopes.
Having determined the object’s distance – about 50 light-years from Earth – the
team realized that it is moving fast – about half a million miles per hour
(800,000 kph). That’s much faster than all other brown dwarfs known to be at
this distance from Earth, which means it has probably been careening around the
galaxy for a long time, encountering massive objects that accelerate it with
their gravity.
With a mound of evidence suggesting The Accident is extremely old, the
researchers propose that its strange properties aren’t strange at all and that
they may be a clue to its age.
When the Milky Way formed about 13.6 billion years ago, it was composed
almost entirely of hydrogen and helium. Other elements, like carbon, formed
inside stars; when the most massive stars exploded as supernovae, they
scattered the elements throughout the galaxy.
Methane, composed of hydrogen and carbon, is common in most brown dwarfs
that have a temperature similar to The Accident. But The Accident’s light
profile suggests it contains very little methane. Like all molecules, methane
absorbs specific wavelengths of light, so a methane-rich brown dwarf would be
dim in those wavelengths. The Accident, by contrast, is bright in those
wavelengths, which could indicate low levels of methane.
Thus, the light profile of The Accident could match that of a very old
brown dwarf that formed when the galaxy was still carbon poor; very little
carbon at formation means very little methane in its atmosphere today.
“It’s not a surprise to find a brown dwarf this old, but it is a surprise
to find one in our backyard,” said Federico Marocco, an astrophysicist at IPAC
at Caltech who led the new observations using the Keck and Hubble telescopes.
“We expected that brown dwarfs this old exist, but we also expected them to be
incredibly rare. The chance of finding one so close to the solar system could
be a lucky coincidence, or it tells us that they’re more common than we
thought.”
This artist’s illustration shows a dim, cold brown dwarf in space. Brown dwarfs form like stars, but do not have enough mass to ignite nuclear fusion in their cores – the process that causes stars to burn. As a result they share some physical characteristics with massive planets, like Jupiter. Credits: IPAC/Caltech
A Lucky Accident
To find more ancient brown dwarfs like The Accident – if they’re out there
– researchers might have to change how they search for these objects.
The Accident was discovered by citizen scientist Dan Caselden, who was
using an online program he built to find brown dwarfs in NEOWISE data. The sky
is full of objects that radiate infrared light; by and large, these objects
appear to remain fixed in the sky, due to their great distance from Earth. But
because brown dwarfs are so faint, they are visible only when they’re
relatively close to Earth, and that means scientists can observe them moving
across the sky over months or years. (NEOWISE maps the entire sky about once
every six months.)
Caselden’s program attempted to remove the stationary infrared objects
(like distant stars) from the NEOWISE maps and highlight moving objects that
had similar characteristics to known brown dwarfs. He was looking at one such
brown dwarf candidate when he spotted another, much fainter object moving
quickly across the screen. This would turn out to be WISEA J153429.75-104303.3,
which hadn’t been highlighted because it did not match the program’s profile of
a brown dwarf. Caselden caught it by accident.
“This discovery is telling us that there’s more variety in brown dwarf
compositions than we’ve seen so far,” said Kirkpatrick. “There are likely more
weird ones out there, and we need to think about how to look for them.”
More About the Missions
Launched in 2009, the WISE spacecraft was placed into hibernation in 2011
after completing its primary mission. In September 2013, NASA reactivated the
spacecraft with the primary goal of scanning for near-Earth objects, or NEOs,
and the mission and spacecraft were renamed NEOWISE. JPL, a division of
Caltech, managed and operated WISE for NASA’s Science Mission Directorate
(SMD). The mission was selected competitively under NASA’s Explorers Program
managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland.
NEOWISE is a project of JPL, a division of Caltech, and the University of
Arizona, 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
JPL managed Spitzer mission operations for NASA’s SMD until the spacecraft
was retired in 2020. Science operations were conducted at the Spitzer Science
Center at IPAC at Caltech. Spacecraft operations were based at Lockheed Martin
Space in Littleton, Colorado. The Spitzer data archive is housed at the
Infrared Science Archive at IPAC at Caltech.
For more information about NASA’s Spitzer mission, go to: https://www.nasa.gov/mission_pages/spitzer/main/index.html, https://www.ipac.caltech.edu/project/spitzer
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight
Center in Greenbelt, Maryland, manages the telescope. The Space Telescope
Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science
operations. STScI is operated for NASA by the Association of Universities for
Research in Astronomy in Washington.
For more information about NASA’s Hubble, go to: https://www.nasa.gov/mission_pages/hubble/main/index.html
For more opportunities to participate in NASA Citizen Science Projects, go
to: https://science.nasa.gov/citizenscience
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