Sky & Telescope - February 2022 - 23

zone - the region around a star where local
temperatures would permit the existence of
liquid water. In fact, K2-18b receives an almost
identical amount of energy from its star as we
do from the Sun.
Astronomers discovered K2-18b in 2015
and have since oscillated between classifying
it as a mini-Neptune or a super-Earth. The
planet's status was called into question again
after Nikku Madhusudhan (University of
Cambridge, UK) used Benneke's data to create
different models of K2-18b's structure.
Madhusudhan suggests that hydrogen makes
up no more than 6% of the planet's mass and,
based on Benneke's estimate of the world's size
(2.6 Earth radii), K2-18b's density is partway
between Earth's and Neptune's. To square the
amount of hydrogen with this middling density,
K2-18b likely has a rocky core.
In Neptune, scientists think ices make up
a mantle that lies between the planet's core
and the hydrogen-rich envelope around it. But
Madhusudhan argues that K2-18b's overall density
might indicate something else is going on
inside this exoplanet. He considered three possible
models of K2-18b's interior, one of which
suggests the planet could have an abundance
of liquid water. Unlike the thin veneer of water
Water world
INTERIOR DESIGN This
diagram shows three representative
interiors that could explain
observations of K2-18b. These
examples are by no means exhaustive;
for example, the extreme
case at bottom could be made
a bit less extreme by trading in
some of the iron core for rock.
Gaseous,
hydrogenrich
envelope
Water layer
Rocky
(silicate)
outer core
Iron inner
core
on Earth, K2-18b's ocean may take up most of
the planet's mantle. If so, that would make it
more like a super-Earth or water world - even
though astronomers would classify it as a miniNeptune
based on its radius alone.
Mini-Neptune
Water, Water Everywhere . . .
Water seldom gets mentioned in the exoplanet
arena without piquing interest about whether
life could be exploiting that liquid. " Whenever
we talk about habitability and life, we immediately
think about Earth-like planets, " Madhusudhan
says. " We've shown that even if it's
a mini-Neptune [based on radius alone], the
planet could still be habitable. "
Benneke also claims to have found water
Iron-core world
vapor in K2-18b's atmosphere, a find that
Tsiaras confirmed. (However, it's worth noting
that Bruno Bézard at the Paris Observatory
SUPER-EARTH OR MINI-NEPTUNE?
Astronomers have detected water vapor
in the hydrogen-dominated atmosphere
of K2-18b, shown here in an artist's illustration.
The planet's density suggests
it most likely has a rocky core.
PLANET MAKE-UPS: TERRI DUBÉ; DATA: NIKKU MADHUSUDHAN;
K2-18B: ESA / HUBBLE, M. KORNMESSER

Sky & Telescope - February 2022

Table of Contents for the Digital Edition of Sky & Telescope - February 2022

Contents
Sky & Telescope - February 2022 - Cover1
Sky & Telescope - February 2022 - Cover2
Sky & Telescope - February 2022 - 1
Sky & Telescope - February 2022 - Contents
Sky & Telescope - February 2022 - 3
Sky & Telescope - February 2022 - 4
Sky & Telescope - February 2022 - 5
Sky & Telescope - February 2022 - 6
Sky & Telescope - February 2022 - 7
Sky & Telescope - February 2022 - 8
Sky & Telescope - February 2022 - 9
Sky & Telescope - February 2022 - 10
Sky & Telescope - February 2022 - 11
Sky & Telescope - February 2022 - 12
Sky & Telescope - February 2022 - 13
Sky & Telescope - February 2022 - 14
Sky & Telescope - February 2022 - 15
Sky & Telescope - February 2022 - 16
Sky & Telescope - February 2022 - 17
Sky & Telescope - February 2022 - 18
Sky & Telescope - February 2022 - 19
Sky & Telescope - February 2022 - 20
Sky & Telescope - February 2022 - 21
Sky & Telescope - February 2022 - 22
Sky & Telescope - February 2022 - 23
Sky & Telescope - February 2022 - 24
Sky & Telescope - February 2022 - 25
Sky & Telescope - February 2022 - 26
Sky & Telescope - February 2022 - 27
Sky & Telescope - February 2022 - 28
Sky & Telescope - February 2022 - 29
Sky & Telescope - February 2022 - 30
Sky & Telescope - February 2022 - 31
Sky & Telescope - February 2022 - 32
Sky & Telescope - February 2022 - 33
Sky & Telescope - February 2022 - 34
Sky & Telescope - February 2022 - 35
Sky & Telescope - February 2022 - 36
Sky & Telescope - February 2022 - 37
Sky & Telescope - February 2022 - 38
Sky & Telescope - February 2022 - 39
Sky & Telescope - February 2022 - 40
Sky & Telescope - February 2022 - 41
Sky & Telescope - February 2022 - 42
Sky & Telescope - February 2022 - 43
Sky & Telescope - February 2022 - 44
Sky & Telescope - February 2022 - 45
Sky & Telescope - February 2022 - 46
Sky & Telescope - February 2022 - 47
Sky & Telescope - February 2022 - 48
Sky & Telescope - February 2022 - 49
Sky & Telescope - February 2022 - 50
Sky & Telescope - February 2022 - 51
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Sky & Telescope - February 2022 - 54
Sky & Telescope - February 2022 - 55
Sky & Telescope - February 2022 - 56
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Sky & Telescope - February 2022 - 70
Sky & Telescope - February 2022 - 71
Sky & Telescope - February 2022 - 72
Sky & Telescope - February 2022 - 73
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Sky & Telescope - February 2022 - 76
Sky & Telescope - February 2022 - 77
Sky & Telescope - February 2022 - 78
Sky & Telescope - February 2022 - 79
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Sky & Telescope - February 2022 - 81
Sky & Telescope - February 2022 - 82
Sky & Telescope - February 2022 - 83
Sky & Telescope - February 2022 - 84
Sky & Telescope - February 2022 - Cover3
Sky & Telescope - February 2022 - Cover4
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