Sky & Telescope - April 2025 - 10

NEWS NOTES
EXOPLANETS
Webb Telescope Reveals
" Featherweight Giant "
Planet
EVERY NEW YOUNG transiting planet
is a valuable target for planet-formation
scenarios - which is why, for several
hours in February 2023, the James
Webb Space Telescope turned its attention
to what was thought to be an
infant hot Jupiter called HIP 67522b.
But the new observations, published
in December's Astronomical Journal,
show 17-million-year-old HIP 67522b
looks nothing like the giant of our solar
system. If the planet had a Jupiter-like
mass, its gravity would have hugged its
envelope of gases closer. Instead, Webb's
observations indicate the atmosphere
extends farther out than expected. The
puffy atmosphere came as a surprise to
the team, led by Pa Chia Thao (University
of North Carolina at Chapel Hill),
who suggest the planet must have a
much smaller mass than anticipated.
KUIPER BELT
Distant Makemake Might
Have a Hot Spot
DESPITE ITS DISTANCE from the Sun,
the far-flung Kuiper Belt object 136472
Makemake is warmer than expected,
leading to some surprising conclusions.
Averaging 45 astronomical units
from the Sun and with a brightly reflective
methane-ice cover, Makemake
should have a mean surface temperature
around 40 kelvin. Some 15 years
ago, observers indeed found such cold
temperatures across most of the body,
which spans 1,430 km (900 miles) -
but something on (or near) it registered
as significantly warmer.
Now observations from the James
Webb Space Telescope (JWST) have
revealed that some portion of Makemake
is even warmer than previously
realized, prompting new explanations.
The study, led by Csaba Kiss (Konkoly
Observatory, Hungary) and published
November 20th in Astrophysical Jour10
APRIL 2025 * SKY & TELESCOPE
pDistant Makemake and its small, unnamed
moon are depicted in an artist's concept.
nal Letters, combines JWST measurements
with those from the Spitzer and
Herschel space observatories and the
ground-based Atacama Large Millimeter/submillimeter
Array to view Makemake's
emission over a broad range
of wavelengths. According to Kiss's
team, the best-fit scenario involves a
small patch, as little as 20 km across,
of something very warm by Kuiper Belt
standards: 150K (-195°F). The source,
they suggest, could be a " hot spot, " a
cryovolcanic flare-up piping heat from
An artist's illustration
of a sub-Neptune planet.
Further analysis of the atmosphere's
thickness by these researchers implies
that HIP 67522b equals roughly 14
Earths. This makes it one of the leastmassive
giant planets ever discovered
- and more similar to sub-Neptunes
than Jupiters.
Although sub-Neptunes are absent
from our solar system, they're one of
the most common types of planets in
the galaxy. Astronomers think they
might lose their initial gaseous envelopes
through processes that include
star-induced boil-off. HIP 67522b's
low current mass likely dooms it to
this fate. It's probably already losing
its atmosphere at a prodigious rate,
and most of it will be gone in the next
billion years. All that will be left is
the shrunken core.
" Measuring the atmospheric properties
of young planets provides a unique
opportunity to understand the formation
and evolutionary histories of these
planets, " says Munazza Alam (Space
Telescope Science Institute), who was
not involved in the study. " Placing constraints
on atmospheric escape early on
in a planet's lifetime can help us better
understand the processes at play that
sculpt their atmospheres. "
¢BENJAMIN CASSESE
the warmer interior.
Another set of JWST observations
provides circumstantial evidence for
this possibility, revealing that the
methane ice on Makemake's surface
has less heavy hydrogen, or deuterium,
than expected, which could result from
recent chemical reactions with water.
Alternatively, Kiss and his colleagues
suggest that a dark ring could match the
observations. Rings aren't unusual, even
around small solar system bodies, but
the particles circling Makemake would
have to be both unusually small and
have an unusual carbon-rich composition
for the explanation to work - and
the ring's orientation would have to be
just so to have avoided detection when
Makemake occulted a star in 2011.
Kiss and his colleagues point out that
the two explanations aren't mutually
exclusive. Perhaps the purported cryvolcano
erupts tiny carbon-rich particles
along with its cryolava, particles that
then encircle Makemake as a dark ring.
¢EMILY LAKDAWALLA
EXOPLANET ILLO: NASA / ESA / CSA / DANI PLAYER (STSCI);
MAKEMAKE AND ITS MOON: NASA / ESA / ALEX PARKER (SWRI)
Sky & Telescope - April 2025

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