Sky & Telescope - March 2022 - 11

EXOPLANETS
MOONS
Did We Find a Planet in
Another Galaxy?
PLANETS ABOUND in the Milky Way.
They probably do in other galaxies, too,
but the usual techniques can't reach
far enough to tell. Using an innovative
method, Rosanne Di Stefano (Center
for Astrophysics, Harvard & Smithsonian)
and colleagues announced a possible
extragalactic exoplanet in Nature
Astronomy on October 25th.
In X-ray binaries, a black hole or
neutron star siphons material off a
companion star. Infalling material
heats up until it emits X-rays. This hot
inner region is small enough that a
planet passing in front of it could block
most or all of the radiation.
Di Stefano and collaborators set
out looking for such a signal, examining
observations of 238 X-ray binaries
in three nearby spiral galaxies: M51,
M101, and M104 (the Whirlpool, Pinwheel,
and Sombrero galaxies).
The hunt struck gold 28 million
light-years away in the Whirlpool
Galaxy. There, the bright M51-ULS-1
system hosts a compact stellar remnant
feeding off of a massive, bright-blue
star, surrounded by an X-ray-emitting
region some 50,000 kilometers
(30,000 miles) across.
During the team's observations, the
X-rays dropped dramatically, staying
dim for three hours before returning to
baseline. The dip was symmetric and
appeared the same across frequencies,
extends 500 km down at most.
" Most of the scientific community
was thinking the Great Red Spot was
shallow, " Parisi says. " We were surprised
that it goes so deep. "
Despite shrinking over the past
few decades (S&T: Apr. 2002, p. 24),
the Great Red Spot is still wider than
Earth's diameter. So the storm itself
is somewhat pancake-shaped - just
a thicker one than scientists had
expected. For context, Jupiter's stripes,
the brown-red belts and whitish zones,
extend much deeper, down to about
Did a Piece of the Moon
Become a Quasi-satellite?
pA box shows the location of the X-ray binary
system in the Whirlpool Galaxy (M51) that
might host an exoplanet, 28 million light-years
from Earth.
as expected from a solid body crossing
the field. Di Stefano's team concludes
the transiting object was probably
smaller than Jupiter and in a wide orbit.
Scott Wolk (also at Center for Astrophysics),
who wasn't involved in the
study, applauds the results but cautions,
" For this to actually have been a planet
would be incredibly lucky. " The chance
of catching a transit in the 2,624 lightcurves
examined so far is on the order
of a million to one.
But those chances depend on how
common planets are in such systems;
this result might not be so unexpected.
" We need more discoveries to make a
real comparison between observations
and theory, " Di Stefano adds.
While we may never know if a planet
caused this particular dip in X-rays, the
observations provide a proof of concept
that Di Stefano and colleagues hope to
use for future discoveries, both in other
galaxies and in the Milky Way.
¢MONICA YOUNG
3,000 km, or about 4% of the way to
the center.
Nevertheless, the unexpected depth
means that the pumpkin-colored vortex
is rooted well beneath the cloud layer as
well as the water and ammonia condensation
layers below that, and certainly
well beyond the reach of sunlight. This
depth will be something scientists need
to consider as they puzzle over the
mechanisms that could drive the storm.
¢MONICA YOUNG
Read more about other Jupiter storms at
https://is.gd/GRSdeep.
THE MOON has orbited Earth for
billions of years, but quasi-satellites
travel in phase with our planet
around the Sun for only a few hundred
years. Now the first detailed
observations of the quasi-satellite
469219 Kamo'oalewa suggest that it
(and perhaps others) might actually
come from the Moon itself.
Discovered in 2016, Kamo'oalewa
is both the closest and smallest
known quasi-satellite and comes
within range of large telescopes every
year. Benjamin Sharkey (University
of Arizona) and colleagues have
observed it over the past five years
using the Large Binocular Telescope
and the Lowell Discovery Telescope,
and reported their results on November
11th in Nature Communications
Earth & Environment.
Data collected at visible and
infrared wavelengths showed
that Kamoʻoalewa reflects more
light at longer wavelengths (1.5 to
2.5 microns), making it more like the
samples collected during the Apollo
missions than other near-Earth
asteroids and meteorite samples.
The similarity with lunar samples
suggests that Kamoʻoalewa might
be a chunk of the Moon, perhaps
blasted away by an asteroid impact.
In addition to its color, the object's
eccentricity and orbital inclination
are also quite small for what
is typical of captured objects.
Kamoʻoalewa's slow speed near the
Earth-Moon system, less than a
quarter of typical near-Earth asteroids,
also supports a Moon origin.
Kamo'oalewa's close approaches
to Earth offer unique possibilities
in the future. The quasi-satellite
comes close enough that the China
National Space Administration has
designated it the target for a complex
mission to collect and return
samples from an asteroid. Plans call
for launch in 2024.
¢JEFF HECHT
sk yand tele scope .o r g * MARCH 2022 11
X-RAY: NASA / CXC / SAO / R. DI STEFANO, ET AL.;
OPTICAL: NASA / ESA / STSCI / GENDLER
https://www.is.gd/GRSdeep http://skyandtelescope.org
Sky & Telescope - March 2022

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