Sky & Telescope - January 2024 - 11

EXOPLANETS
Webb, Hubble Hint at
Ocean Worlds
WITH RECENT OBSERVATIONS,
astronomers have found signatures
hinting at water-rich compositions for
two planets between Earth and Neptune
in size - but other scenarios can't
be ruled out just yet.
By their average density, sub-Neptunes
have compositions unlike rocky
Earth but also unlike fluffy Neptune.
These worlds could be made almost
entirely of different phases of water, or
they could just as easily be dense cores
wrapped in thick blankets of hydrogen.
To distinguish between scenarios,
astronomers can take a spectrum as a
planet passes in front of its star. Molecules
in the upper atmosphere absorb
light, changing the planet's apparent
size, at particular wavelengths.
Nikku Madhusudhan (University
of Cambridge, UK) and colleagues
announce in an upcoming issue of
Astrophysical Journal Letters that
James Webb Space Telescope spectra
reveal methane and carbon dioxide
in the upper atmosphere of K2-18b, a
habitable-zone planet. (Starlight can't
penetrate the atmosphere's depths, so
IN BRIEF
Japan, India Launch X-ray,
Solar Telescopes
In an ambitious mission pairing, Japan
launched a new X-ray observatory and an
innovative lunar lander on September 6th. The
X-Ray Imaging and Spectroscopy Mission
(XRISM) carries a microcalorimeter, jointly developed
by Japan's space agency and NASA.
The instrument precisely measures the energies
of incoming X-rays, shedding light on the
hot gas spiraling around black holes, flying
outward from supernovae, and drifting within
galaxy clusters. XRISM is the fourth mission
to attempt to bring such an instrument into
orbit - and will hopefully be the first one to
report success. The Smart Lander for Investigating
Moon (SLIM) mission accompanied
XRISM on the launchpad. It's now headed for
the Moon, with a landing expected in early
2024. Its primary objective is to demonstrate
the lower atmosphere is still beyond
the reach of direct measurement.) But
the researchers did not see ammonia,
a molecule expected in thick atmospheres.
Ammonia's nondetection permits
the possibility of a global surface
ocean under thin, hydrogen-rich air.
But Raymond Pierrehumbert (University
of Oxford, UK), who was not
involved in the study, isn't convinced.
The absence, he says, might instead
indicate an aspect of ammonia chemistry
or atmospheric mixing that we still
don't understand.
Elsewhere in the galaxy, the atmosphere
of hotter and fluffier GJ 9827d
shows signs of water vapor in observations
taken by the Hubble Space
Telescope. Publishing the data in the
September 10th Astrophysical Journal
Letters, Pierre-Alexis Roy, Björn Benneke
(both at University of Montreal),
and colleagues make the case that GJ
9827d is another candidate water world
- albeit a steamy one.
The detection is uncertain, though,
due to the limited range of wavelengths
Hubble covers. In fact, in 2019, astronomers
thought Hubble had spotted water
vapor in K2-18b's upper atmosphere;
however, Madhusudhan's analysis of
JWST infrared data revealed that the
a pinpoint landing, with an accuracy of 100
meters (300 feet). Meanwhile, on September
2nd, India launched the Aditya-L1 mission,
which carries a suite of seven instruments to
study the Sun's photosphere, chromosphere,
and corona. Three of these instruments will
also carry out in situ studies of solar wind particles
and magnetic fields at the spacecraft's
position at the Earth-Sun L1 Lagrange point.
Aditya will also for the first time resolve the
solar disk at near-UV wavelengths.
¢DAVID DICKINSON
NASA Panel's UAP Report
NASA has announced the results of an independent
study commissioned to determine
how the agency can contribute to understanding
unidentified anomalous phenomena
(UAPs). The panel in charge of this study was
not commissioned to evaluate existing reports
of UAPs, though they did note that there
was no evidence to suggest such phenompThis
artist's concept, based on science data,
shows what exoplanet K2-18b could look like.
signal actually came from methane; the
same might be true for GJ 9827d. Additional
JWST observations are already
planned for both worlds as well as a few
other sub-Neptunes.
" In the end, the main goal is not
to find some methane, or to find some
water vapor, " Benneke says. " What
we really want to do is to distinguish
between those two scenarios . . . [that
is] what Webb can eventually deliver. "
¢ELISE CUTTS
See the Webb and Hubble spectra at
https://is.gd/WaterWorlds.
ena are extraterrestrial in origin. Instead, the
goal was to determine how NASA can help
evaluate future UAP reports. The agency's
Earth-observing satellites lack the resolution
to resolve individual events but can help
monitor background conditions. Commercial
satellites, on the other hand, offer resolution
from several meters to under a meter.
However, they are not looking everywhere
all the time, so any detections would require
some serendipity. The panel also suggests
NASA could help provide a framework for
civilians to report UAPs. Once scientific data
are collected, the panel then advocates either
following up on individual reports or, perhaps
more profitably, searching for unusual events
that deviate from expected conditions. While
NASA is still reviewing the panel's report, it
has already created a new position to oversee
the study of UAPs, appointing Mark McInerney
as director of UAP research.
¢MONICA YOUNG
sk yand tele scope .o r g * JANUARY 2024 11
NASA / CSA / ESA / J. OLMSTEAD (STSCI) / N.
MADHUSUDHAN (CAMBRIDGE UNIVERSITY)
https://www.is.gd/WaterWorlds http://skyandtelescope.org
Sky & Telescope - January 2024

Table of Contents for the Digital Edition of Sky & Telescope - January 2024
