2019-08-01_Sky_and_Telescope

skyandtelescope.com • AUGUST 2019 37

ate a thick blanket of clouds on its dayside. Those clouds

would refl ect the star’s light back to space, lowering surface

temperatures. This process might allow these planets to orbit

their stars at much closer distances than they otherwise

would, essentially buffering them against so-called runaway

greenhouse states.

Besides the tidal interactions between a planet and its

parent star, there may also be interactions between plan-

ets, as worlds around M-dwarfs often form closely packed

together. In such cases, the gravitational pull of a planet’s

neighbors will change its rotation rate and might even

transform the shape of the planet’s orbit, making it more (or

less) elongated. Since the orbit’s shape, or eccentricity, deter-

mines how much starlight reaches the planet throughout its

year, each interaction will change the total amount of light

the planet receives. If the planet’s spin axis is also tilted,

then the interaction will also change how that starlight is

distributed across different parts of the globe. There could

be bizarre planets where climate conditions change signifi -

cantly throughout the year.

Eccentric orbits aren’t all bad, though. As a planet in an

eccentric orbit swings through its closest approach to its

Tidal (^) lockin
g (^) radius
E
ar
ly
(^) M
ar
R s
ec
en
t (^) V
en
us
Ru
na
w
ay
(^) g
re
en
ho
us
e
M
ax
im
um
(^) g
re
en
h
ou
se
Earth
Kepler-1512b
Kepler-560b
Ross 128b
Venus
Mars
Kepler-62f
Kepler-438b
Trappist-1d
Proxima Cen b
Gliese 667 Cc
Kepler-1229b
Kepler-186f
Trappist-1g
Trappist-1e Trappist-1f
LHS 1140b
Kepler-442b
Kepler-1410b
7000K
6000K
5000K
4000K
3000K
200% 175% 150% 125% 100%
Starlight on planet relative to sunlight on Earth
Te
m
pe
ra
tu
re
75% 50% 25%
Planet Size
(Earth radii)
0.5Earth
(^1) Earth
1.5Earth
(^4000) K
Wavelength (nanometers)
(^3500) K
Ultraviolet Visible Infrared
0
0 500 1000 1500 2000
2
4
6
8
10
Re
la
tiv
e^
lig
ht
in
te
ns
ity (^55)
(^00)
K
(^50)
(^00)
K
(^450)
(^0) K
pPEAK WAVELENGTHS Cooler stars emit most of their radiation at
longer wavelengths. While the Sun emits mostly visible and ultraviolet
light, red dwarfs emit more infrared — a difference that could affect na-
scent life on an orbiting planet. (Color intervals are approximate.)
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qSHIFTING ZONES Shown here are confi rmed, potentially rocky exoplanets that fall in their stars’ habitable zones, as compiled by Chester

Harman (NASA Goddard Institute for Space Studies). The tidal locking radius was calculated by Jun Yang (Peking University) and colleagues.