Sky & Telescope - April 2024 - 8

NEWS NOTES
EXOPLANETS
New Doubt Cast on
Exomoon Candidates
OUR SOLAR SYSTEM is awash in
moons, so it seems reasonable to infer
that moons are common in other stellar
systems, too. However, astronomers
have only found evidence hinting at a
few exomoons so far (S&T: Sept. 2020,
p. 34) - and a recent study casts doubt
on two of those detections.
Exoplanets transiting in front of
their host stars block some of their
light. By measuring dips in stellar light,
astronomers can build a picture of the
objects causing the partial eclipses.
Moons could be found this way, too, but
because their dips might occur before,
during, or after their planet's, they
require additional statistical vetting.
In 2018, Alex Teachey and David
Kipping (both at Columbia University)
announced the discovery of a exomoon
candidate orbiting the super-Jupiter
Kepler-1625b. Based on data from the
Kepler telescope, this discovery later
received support from a Hubble observation.
Then, two years ago, a team
including Teachey and Kipping identified
another candidate orbiting a different
giant world, Kepler-1708b.
Now, a study published December
7th in Nature Astronomy casts doubt
on those exomoon claims. René Heller
(Max Planck Institute for Solar System
Research, Germany) and Michael HipSOLAR
SYSTEM
Buried Polygons on Mars Hint at Stark Early Transition
POLYGONS BURIED beneath the
surface of Mars indicate an abrupt
environmental transition in the planet's
early history.
Ground-penetrating radar on
China's Zhurong rover has revealed
large, wedge-like structures well below
ground level on the Red Planet. While
previous missions have observed polygonal
mud cracks on the Martian surface,
created by repeated wet-dry cycles on
early Mars (S&T: Dec. 2023, p. 8), these
8
APRIL 2024 * SKY & TELESCOPE
larger polygons indicate a different kind
of cycle. Lei Zhang, Chao Li, and Jinhai
Zhang (all at Chinese Academy of Sciences)
and colleagues report the results
November 23rd in Nature Astronomy.
Analysis of Zhurong's radar data has
revealed these structures are more than
35 meters (115 feet) below the surface.
The cracks between polygons are themselves
quite wide (27 meters), almost
half again the polygons' average width
of 67 meters. The difference in the
Buried polygon wedge
pThis artist's illustration shows exoplanet
Kepler-1625b transiting its star with an alleged
moon in tow. A new study suggests the moon
signal may not be real.
pke (Sonneberg Observatory, Germany)
reanalyzed previous observations with
the help of a new and improved computer
code called Pandora, developed to
validate exomoon transits.
Heller and Hippke suggest the
original team might have overestimated
putative exomoon signals in part due
to stellar limb darkening, which can
alter the shape of transiting objects.
The effect makes it harder to separate a
planet's signal from other variations.
Heller and Hippke also find that
the detrending method previously used
to remove extraneous signals, such as
telescope vibrations and stellar activity,
might have inadvertently injected
moons into the data.
" These moons are extremely challenging
to detect, since their signature
is very subtle, " says Johanna Vos
(Trinity College Dublin), adding that
independent studies such as this one
are important for the field. She praises
the new study for developing " advanced
tools for detecting and validating possible
signatures of exomoons. "
In addition, for each candidate, the
researchers tested 128 models including
only a planetary transit as well as 128
models including both a planet and its
moon. Comparing these models to each
other, they conclude that Kepler data
can only detect massive moons on wide
orbits - specifically orbits beyond 30%
of the planet's Hill radius, within which
the planet's gravity dominates over its
star's. The solar system's giant planets
have no massive moons so far away.
¢ KIT GILCHRIST
pThis diagram shows wedges from polygonal
structures buried 35 meters beneath the Martian
surface. (The surface image comes from
the Tianwen 1 lander's navigation camera.)
crack-to-polygon ratio rules out their
formation by wet-dry cycles, Zhang
says, and instead indicates the presence
EXOMOON: DAN DURDA; POLYGONS DIAGRAM: ZHANG ET AL. /
NATURE ASTRONOMY 2023 / CC BY 4.0 DEED
Sky & Telescope - April 2024

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