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
Sky & Telescope - February 2022 - 52
Sky & Telescope - February 2022 - 53
Sky & Telescope - February 2022 - 54
Sky & Telescope - February 2022 - 55
Sky & Telescope - February 2022 - 56
Sky & Telescope - February 2022 - 57
Sky & Telescope - February 2022 - 58
Sky & Telescope - February 2022 - 59
Sky & Telescope - February 2022 - 60
Sky & Telescope - February 2022 - 61
Sky & Telescope - February 2022 - 62
Sky & Telescope - February 2022 - 63
Sky & Telescope - February 2022 - 64
Sky & Telescope - February 2022 - 65
Sky & Telescope - February 2022 - 66
Sky & Telescope - February 2022 - 67
Sky & Telescope - February 2022 - 68
Sky & Telescope - February 2022 - 69
Sky & Telescope - February 2022 - 70
Sky & Telescope - February 2022 - 71
Sky & Telescope - February 2022 - 72
Sky & Telescope - February 2022 - 73
Sky & Telescope - February 2022 - 74
Sky & Telescope - February 2022 - 75
Sky & Telescope - February 2022 - 76
Sky & Telescope - February 2022 - 77
Sky & Telescope - February 2022 - 78
Sky & Telescope - February 2022 - 79
Sky & Telescope - February 2022 - 80
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
https://www.nxtbook.com/nxtbooks/aas/st_202406
https://www.nxtbook.com/nxtbooks/aas/st_202405
https://www.nxtbook.com/nxtbooks/aas/st_202404_qr
https://www.nxtbook.com/nxtbooks/aas/st_202404
https://www.nxtbook.com/nxtbooks/aas/st_202403
https://www.nxtbook.com/nxtbooks/aas/st_202402
https://www.nxtbook.com/nxtbooks/aas/st_202401_aus
https://www.nxtbook.com/nxtbooks/aas/st_202401
https://www.nxtbook.com/nxtbooks/aas/st_202312_aus
https://www.nxtbook.com/nxtbooks/aas/st_202312
https://www.nxtbook.com/nxtbooks/aas/st_202311
https://www.nxtbook.com/nxtbooks/aas/st_202310
https://www.nxtbook.com/nxtbooks/aas/st_202309
https://www.nxtbook.com/nxtbooks/aas/st_202308
https://www.nxtbook.com/nxtbooks/aas/st_202307
https://www.nxtbook.com/nxtbooks/aas/st_202306
https://www.nxtbook.com/nxtbooks/aas/st_202305
https://www.nxtbook.com/nxtbooks/aas/st_202304
https://www.nxtbook.com/nxtbooks/aas/st_202304_qr
https://www.nxtbook.com/nxtbooks/aas/st_202303
https://www.nxtbook.com/nxtbooks/aas/st_202302
https://www.nxtbook.com/nxtbooks/aas/st_202301
https://www.nxtbook.com/nxtbooks/aas/st_202212
https://www.nxtbook.com/nxtbooks/aas/st_202211
https://www.nxtbook.com/nxtbooks/aas/st_202210
https://www.nxtbook.com/nxtbooks/aas/st_202209
https://www.nxtbook.com/nxtbooks/aas/st_202208
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com