NIRISS instrument on Webb maps an ultra-hot Jupiter's atmosphere

Date:
May 31, 2023
Source:
University of Montreal
Summary:
There's an intriguing exoplanet out there -- 400 light-years
out there - - that is so tantalizing that astronomers have been
studying it since its discovery in 2009. One orbit for WASP-18 b
around its star that is slightly larger than our Sun takes just 23
hours. There is nothing like it in our Solar System. A new study
about this exoplanet, an ultra-hot gas giant 10 times more massive
than Jupiter.


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FULL STORY ========================================================================== There's an intriguing exoplanet out there -- 400 light-years out there
-- that is so tantalising that astronomers have been studying it since
its discovery in 2009. One orbit for WASP-18 b around its star that
is slightly larger than our Sun takes just 23 hours. There is nothing
like it in our Solar System. A new study led by Universite' de Montre'al
Ph.D. student Louis-Philippe Coulombe about this exoplanet, an ultra-hot
gas giant 10 times more massive than Jupiter, based on new data from
the Canadian NIRISS instrument on the James Webb Space Telescope
(JWST) holds many surprises! Mapping an exoplanet An international
team of astronomers have identified water vapor in the atmosphere of
the exoplanet WASP-18 b and made a temperature map of the planet as it
slipped behind, and reappeared from, its star. This event is known as
a secondary eclipse. Scientists can read the combined light from the
star and planet, then refine the measurements from just the star as the
planet moves behind it.

The same side, known as the dayside, of WASP-18 b always faces its star,
just as the same side of the Moon always faces Earth. This is called
tidal locking.

The temperature, or brightness, map of the exoplanet shows a huge change
in temperature -- up to 1,000 degrees -- from the hottest point facing the
star to the terminator, where day and night sides of the tidally-locked
planet meet in permanent twilight.

''JWST is giving us the sensitivity to make much more detailed maps of
hot giant planets like WASP-18 b than ever before. This is the first time
a planet has been mapped with JWST, and it's really exciting to see that
some of what our models predicted, such as a sharp drop in temperature
away from the point on the planet directly facing the star, is actually
seen in the data!'' said Megan Mansfield, a Sagan Fellow at the University
of Arizona, and one of the authors of the paper describing the results.

The team mapped temperature gradients across the day side of the
planet. Given how much cooler the planet is at the terminator, there is
likely something hindering winds from efficiently redistributing heat
to the night side. But what is affecting the winds is still a mystery.

''The brightness map of WASP-18 b shows a lack of east-west winds that is
best matched by models with atmospheric drag. One possible explanation
is that this planet has a strong magnetic field, which would be an
exciting discovery!'' said co-author Ryan Challener, of the University
of Michigan.

One interpretation of the eclipse map is that magnetic effects force the
winds to blow from the planet's equator up over the North pole and down
over the South pole, instead of East-West, as we would otherwise expect.

Researchers recorded temperature changes at different elevations of the
gas giant planet's layers of atmosphere. They saw temperatures increase
with elevation, varying by hundreds of degrees.

Signs of water vapor The spectrum of the planet's atmosphere clearly
shows multiple small but precisely measured water features, present
despite the extreme temperatures of almost 2,700 degrees Celsius. It is
so hot that it would tear most water molecules apart, so still seeing its presence speaks to Webb's extraordinary sensitivity to detect remaining
water. The amounts recorded in WASP-18 b's atmosphere indicate water
vapor is present at various elevations ''It was a great feeling to look
at WASP-18 b's JWST spectrum for the first time and see the subtle but precisely measured signature of water,'' said Louis-Philippe Coulombe,
a Ph.D. student at the Universite' de Montre'al, member of the Trottier Institute for Research on Exoplanets (iREx) and lead author of the WASP-18
b paper. ''Using this kind of measurements, we will be able to detect
such molecules for a wide range of planets in the years to come!'',
added Bjo"rn Benneke, UdeM Professor, iREx member and co-author of this
paper. Benneke is Coulombe's Ph.D. advisor as well and has been leading worldwide efforts to study WASP-18 b since 2016.

The work of the NIRISS instrument and early career scientists The team
of astronomers observed WASP-18 b for about six hours using one of Webb's instruments, the Near-Infrared Imager and Slitless Spectrograph (NIRISS), contributed by the Canadian Space Agency and several partners including
the Universite' de Montre'al and iREx.

''Because the water features in this spectrum are so subtle, they were difficult to identify in previous observations. That made it really
exciting to finally see water features with these JWST observations,''
said Anjali Piette, a postdoctoral fellow at the Carnegie Institution
for Science and one of the authors of the new research.

The WASP-18 b observations were collected as part of the Transiting
Exoplanet Community Early Release Science Program led by Natalie Batalha,
an astronomer at the University of California, Santa Cruz, who helped coordinate the new research and the more than one hundred researchers
in the team. Much of this ground-breaking work is being done by early
career scientists like Coulombe, Challener, Piette, and Mansfield.

Proximity, both to its star and to us, helped make WASP-18 b
such an intriguing target for these scientists, as did its large
mass. WASP-18 b is one of the most massive worlds whose atmospheres we
can investigate. Astronomers are striving to understand how such planets
form and come to be where they are in their systems. This, too, has some
early answers from Webb.

''By analysing WASP-18 b's spectrum, we not only learn about the various molecules that can be found in its atmosphere but also about the way
it formed.

We find from our observations that WASP-18 b's composition is very
similar to that of its star, meaning it most likely formed from the
leftover gas that was present just after the star was born,'' Coulombe
said. ''Those results are very valuable to get a clear picture of how
strange planets like WASP-18 b, which have no counterpart in our Solar
System, come to exist.''
* RELATED_TOPICS
o Space_&_Time
# Extrasolar_Planets # Astronomy # Stars # Jupiter #
Kuiper_Belt # Pluto # Eris_(Xena) # Solar_System
* RELATED_TERMS
o Jupiter o Extrasolar_planet o Light-year o Planet o
Blue_supergiant_star o Definition_of_planet o Red_giant o Saturn

========================================================================== Story Source: Materials provided by University_of_Montreal. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. LP., Benneke, B., Challener, R. et al. A broadband thermal emission
spectrum of the ultra-hot Jupiter WASP-18b. Nature, 2023 DOI:
10.1038/ s41586-023-06230-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230531150101.htm

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