An alien world just 70 light-years from Earth is one of the strangest we have found yet.
It clocks in at 20 Jupiter masses, has temperatures that
could quickly melt aluminum, and has a 10,000-year orbit around not one but two
stars. And, oh yeah: It's ravaged by a constant, tempestuous storm of sand.
Astronomers have used the James Webb Space Telescope to
obtain the most high-fidelity observations yet of the planetary-mass object,
revealing roiling clouds of silicate grains circulating in the atmosphere of
the world named VHS 1256 b.
The discovery, published last year on the preprint server
arXiv, has gone through the peer review process and is due to appear in The
Astrophysical Journal Letters.
In addition, the team identified many of the components of
VHS 1256 b's atmosphere. Those include unambiguous detections of methane,
carbon monoxide, and water, with additional evidence of carbon dioxide.
"No other telescope has identified so many features at
once for a single target," says astrophysicist Paul Mollière of the Max
Planck Institute for Astronomy in Germany. "We're seeing many molecules in
a single spectrum from the JWST that detail the planet's dynamic cloud and
weather systems."
VHS 1256 b is somewhat of an enigma. Its mass straddles the
boundary between giant planets and brown dwarfs, "failed stars" that
aren't massive enough to fuse hydrogen but that can fuse the heavier hydrogen
isotope deuterium in their cores, which has lower fusion temperature and
pressure than hydrogen.
It's thought that the two types of objects form pretty
differently. Brown dwarfs generally form like stars, collapsing from a dense
knot of material in a cloud of gas and dust and then sucking in more material
to grow. Deuterium fusion is an intermediary step as the star grows, but some
stars – the brown dwarfs – stop growing at that point and stay as they are.
On the other hand, planets are thought to form from the
bottom up, from the material left over after a star has formed, clumping
together to grow into a planet. That material is thought to be usually pretty
close to the star. The wide orbital separation of VHS 1256 b from its two suns
suggests that it formed by cloud collapse, but that's not diagnostic.
Theoretically, planets can also form from the cloud collapse model; the estimated minimum mass for a cloud collapse object is one Jupiter. The dividing line between a planet and a brown dwarf is, therefore, the deuterium-burning mass limit, which means that VHS 1256 b's precise nature is unknown.
But it's that great distance that allowed such spectacular
observations.
"VHS 1256 b is about four times farther from its stars
than Pluto is from our Sun, which makes it a great target for Webb," says
astronomer Brittany Miles of the University of Arizona, who led the
international research team. "That means the planet's light is not mixed
with light from its stars."
JWST's observation range is the infrared and near-infrared,
the range that includes thermal radiation. And VHS 1256 b is very young, at
just 150 million years old, and still quite hot from the process of forming.
Its atmosphere, where the sand clouds can be found, reaches 830 degrees Celsius
(1,526 degrees Fahrenheit).
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The infrared spectrum of VHS 1256 b. (NASA, ESA, CSA, J.
Olmsted/STScI, B. Miles/UA, S. Hinkley/UOE, B. Biller/UE, A. Skemer/UCSC) |
This heat, along with its low gravity, is what makes its
skies so turbulent. Scientists analyzed the light detected by JWST, studying
the spectrum in minute detail to pick out the features produced by various
elements absorbing specific wavelengths.
This is how they identified the various gasses they found in
the object's atmosphere – and the clouds of sand that are constantly changing,
likely composed of enstatite, forsterite, or quartz.
So detailed was the data that researchers were able to
identify different sizes of grains, too, from finer grains like particles of
smoke to larger grains like sand. These larger grains, the researchers
hypothesized, are too heavy to remain in the upper atmosphere and rain back
down into the interior, as smaller particles rise.
This produces a dramatic variation in the world's brightness
over its 22-hour day, suggesting that silicate clouds might be a common mechanism
for making such variations in brown dwarfs. The team believes the observations
could be easily replicated for other brown dwarfs, which could help us learn
more about these strange objects.
And VHS 1256 b has given us a lot to chew on.
"We've isolated silicates, but better understanding
which grain sizes and shapes match specific types of clouds is going to take a
lot of additional work," says astrophysicist Elisabeth Matthews of the Max
Planck Institute for Astronomy.
"This is not the final word on this planet – it is only
the beginning of a large-scale modeling effort to understand JWST's complex
data."
The research has been published in The Astrophysical JournalLetters.