This artist's concept shows a brown dwarf with bands of clouds, thought
to resemble those seen at Neptune and the other outer planets.
Credit: NASA/JPL-Caltech. › Full image and caption
Credit: NASA/JPL-Caltech. › Full image and caption
Dim objects called brown dwarfs, less massive than the Sun but more
massive than Jupiter, have powerful winds and clouds -- specifically,
hot patchy clouds made of iron droplets and silicate dust. Scientists
recently realized these giant clouds can move and thicken or thin
surprisingly rapidly, in less than an Earth day, but did not understand
why.
Now, researchers have a new model for explaining
how clouds move and change shape in brown dwarfs, using insights from
NASA's Spitzer Space Telescope. Giant waves cause large-scale movement
of particles in brown dwarfs' atmospheres, changing the thickness of the
silicate clouds, researchers report in the journal Science. The study
also suggests these clouds are organized in bands confined to different
latitudes, traveling with different speeds in different bands.
"This
is the first time we have seen atmospheric bands and waves in brown
dwarfs," said lead author Daniel Apai, associate professor of astronomy
and planetary sciences at the University of Arizona in Tucson.
Just
as in Earth's ocean, different types of waves can form in planetary
atmospheres. For example, in Earth's atmosphere, very long waves mix
cold air from the polar regions to mid-latitudes, which often lead
clouds to form or dissipate.
The distribution and
motions of the clouds on brown dwarfs in this study are more similar to
those seen on Jupiter, Saturn, Uranus and Neptune. Neptune has cloud
structures that follow banded paths too, but its clouds are made of ice.
Observations of Neptune from NASA's Kepler spacecraft, operating in its K2 mission, were important in this comparison between the planet and brown dwarfs.
"The
atmospheric winds of brown dwarfs seem to be more like Jupiter's
familiar regular pattern of belts and zones than the chaotic atmospheric
boiling seen on the Sun and many other stars," said study co-author
Mark Marley at NASA's Ames Research Center in California's Silicon
Valley.
Brown dwarfs can be thought of as failed stars
because they are too small to fuse chemical elements in their cores.
They can also be thought of as "super planets" because they are more
massive than Jupiter, yet have roughly the same diameter. Like gas giant
planets, brown dwarfs are mostly made of hydrogen and helium, but they
are often found apart from any planetary systems. In a 2014 study using Spitzer, scientists found that brown dwarfs commonly have atmospheric storms.
Due to their similarity to giant exoplanets, brown dwarfs are windows into planetary systems beyond our own. It is easier to study brown dwarfs than planets because they often do not have a bright host star that obscures them.
"It is likely the banded structure and
large atmospheric waves we found in brown dwarfs will also be common in
giant exoplanets," Apai said.
Using Spitzer, scientists
monitored brightness changes in six brown dwarfs over more than a year,
observing each of them rotate 32 times. As a brown dwarf rotates, its
clouds move in and out of the hemisphere seen by the telescope, causing
changes in the brightness of the brown dwarf. Scientists then analyzed
these brightness variations to explore how silicate clouds are
distributed in the brown dwarfs.
Researchers had been
expecting these brown dwarfs to have elliptical storms resembling
Jupiter's Great Red Spot, caused by high-pressure zones. The Great Red
Spot has been present in Jupiter for hundreds of years and changes very
slowly: Such "spots" could not explain the rapid changes in brightness
that scientists saw while observing these brown dwarfs. The brightness
levels of the brown dwarfs varied markedly just over the course of an
Earth day.
To make sense of the ups and downs of
brightness, scientists had to rethink their assumptions about what was
going on in the brown dwarf atmospheres. The best model to explain the
variations involves large waves, propagating through the atmosphere with
different periods. These waves would make the cloud structures rotate
with different speeds in different bands.
University of
Arizona researcher Theodora Karalidi used a supercomputer and a new
computer algorithm to create maps of how clouds travel on these brown
dwarfs.
"When the peaks of the two waves are offset,
over the course of the day there are two points of maximum brightness,"
Karalidi said. "When the waves are in sync, you get one large peak,
making the brown dwarf twice as bright as with a single wave."
The
results explain the puzzling behavior and brightness changes that
researchers previously saw. The next step is to try to better understand
what causes the waves that drive cloud behavior.
JPL manages
the Spitzer Space Telescope mission for NASA's Science Mission
Directorate, Washington. Science operations are conducted at the Spitzer
Science Center at Caltech in Pasadena, California. Spacecraft
operations are based at Lockheed Martin Space Systems Company,
Littleton, Colorado. Data are archived at the Infrared Science Archive
housed at the Infrared Processing and Analysis Center at Caltech.
Caltech manages JPL for NASA. For more information about Spitzer, visit: http://spitzer.caltech.edu - https://www.nasa.gov/spitzer
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Source: JPL-Caltech/News
