A 'hot Jupiter's' dark side is revealed in detail for first time
The planet's night side likely hosts iron clouds, titanium rain, and
winds that dwarf Earth's jetstream.

Date:
February 21, 2022
Source:
Massachusetts Institute of Technology
Summary:
Astronomers have obtained the clearest view yet of the perpetual
dark side of an exoplanet that is 'tidally locked' to its star. The
planet is WASP-121b, a massive gas giant nearly twice the size
of Jupiter.



FULL STORY ==========================================================================
MIT astronomers have obtained the clearest view yet of the perpetual
dark side of an exoplanet that is "tidally locked" to its star. Their observations, combined with measurements of the planet's permanent day
side, provide the first detailed view of an exoplanet's global atmosphere.


========================================================================== "We're now moving beyond taking isolated snapshots of specific regions of exoplanet atmospheres, to study them as the 3D systems they truly are,"
says Thomas Mikal-Evans, who led the study as a postdoc in MIT's Kavli Institute for Astrophysics and Space Research.

The planet at the center of the new study, which appears in Nature
Astronomy, is WASP-121b, a massive gas giant nearly twice the size of
Jupiter. The planet is an ultrahot Jupiter and was discovered in 2015
orbiting a star about 850 light years from Earth. WASP-121b has one
of the shortest orbits detected to date, circling its star in just
30 hours. It is also tidally locked, such that its star-facing "day"
side is permanently roasting, while its "night" side is turned forever
toward space.

"Hot Jupiters are famous for having very bright day sides, but the
night side is a different beast. WASP-121b's night side is about 10
times fainter than its day side," says Tansu Daylan, a TESS postdoc at
MIT who co-authored the study.

Astronomers had previously detected water vapor and studied how the
atmospheric temperature changes with altitude on the planet's day side.

The new study captures a much more detailed picture. The researchers were
able to map the dramatic temperature changes from the day to the night
side, and to see how these temperatures change with altitude. They also
tracked the presence of water through the atmosphere to show, for the
first time, how water circulates between a planet's day and night sides.



========================================================================== While on Earth, water cycles by first evaporating, then condensing into
clouds, then raining out, on WASP-121b, the water cycle is far more
intense: On the day side, the atoms that make up water are ripped apart
at temperatures over 3,000 Kelvin. These atoms are blown around to the
night side, where colder temperatures allow hydrogen and oxygen atoms
to recombine into water molecules, which then blow back to the day side,
where the cycle starts again.

The team calculates that the planet's water cycle is sustained by winds
that whip the atoms around the planet at speeds of up to 5 kilometers
per second, or more than 11,000 miles per hour.

It also appears that water isn't alone in circulating around the
planet. The astronomers found that the night side is cold enough to host
exotic clouds of iron and corundum -- a mineral that makes up rubies
and sapphires. These clouds, like water vapor, may whip around to the
day side, where high temperatures vaporize the metals into gas form. On
the way, exotic rain might be produced, such as liquid gems from the
corundum clouds.

"With this observation, we're really getting a global view of an
exoplanet's meteorology," Mikal-Evans says.

The study's co-authors include collaborators from MIT, Johns Hopkins University, Caltech, and other institutions.



==========================================================================
Day and night The team observed WASP-121b using a spectroscopic camera
aboard NASA's Hubble Space Telescope. The instrument observes the
light from a planet and its star, and breaks that light down into its constituent wavelengths, the intensities of which give astronomers clues
to an atmosphere's temperature and composition.

Through spectroscopic studies, scientists have observed atmospheric
details on the day sides of many exoplanets. But doing the same for the
night side is far trickier, as it requires watching for tiny changes in
the planet's entire spectrum as it circles its star.

For the new study, the team observed WASP-121b throughout two full
orbits - - one in 2018, and the other in 2019. For both observations,
the researchers looked through the light data for a specific line,
or spectral feature, that indicated the presence of water vapor.

"We saw this water feature and mapped how it changed at different parts
of the planet's orbit," Mikal-Evans says. "That encodes information about
what the temperature of the planet's atmosphere is doing as a function of altitude." The changing water feature helped the team map the temperature profile of both the day and night side. They found the day side ranges
from 2,500 Kelvin at its deepest observable layer, to 3,500 Kelvin
in its topmost layers. The night side ranged from 1,800 Kelvin at its
deepest layer, to 1,500 Kelvin in its upper atmosphere. Interestingly, temperature profiles appeared to flip-flop, rising with altitude on
the day side -- a "thermal inversion," in meteorological terms -- and
dropping with altitude on the night side.

The researchers then passed the temperature maps through various models to identify chemicals that are likely to exist in the planet's atmosphere,
given specific altitudes and temperatures. This modeling revealed the
potential for metal clouds, such as iron, corundum, and titanium on the
night side.

From their temperature mapping, the team also observed that the planet's hottest region is shifted to the east of the "substellar" region directly
below the star. They deduced that this shift is due to extreme winds.

"The gas gets heated up at the substellar point but is getting blown
eastward before it can reradiate to space," Mikal-Evans explains.

From the size of the shift, the team estimates that the wind speeds
clock in at around 5 kilometers per second.

"These winds are much faster than our jet stream, and can probably
move clouds across the entire planet in about 20 hours," says Daylan,
who led previous work on the planet using NASA's MIT-led mission, TESS.

The astronomers have reserved time on the James Webb Space Telescope to
observe WASP-121b later this year, and hope to map changes in not just
water vapor but also carbon monoxide, which scientists suspect should
reside in the atmosphere.

"That would be the first time we could measure a carbon-bearing molecule
in this planet's atmosphere," Mikal-Evans says. "The amount of carbon
and oxygen in the atmosphere provides clues on where these kinds of
planet form." This research was supported in part by NASA through a
grant from the Space Telescope Science Institute.

Video: https://youtu.be/XnMXf-uASzo ========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Mikal-Evans, T., Sing, D.K., Barstow, J.K. et al. Diurnal
variations in
the stratosphere of the ultrahot giant exoplanet WASP-121b. Nat
Astron, 2022 DOI: 10.1038/s41550-021-01592-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220221115406.htm

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