Sky & Telescope - August 2019 - 37

ate a thick blanket of clouds on its dayside. Those clouds
would reflect 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 planets, 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, determines 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 significantly throughout the year.
Eccentric orbits aren't all bad, though. As a planet in an
eccentric orbit swings through its closest approach to its

Ultraviolet

10

Visible

Infrared

0K
550

6

K
00
50

Relative light intensity

8

4

45
00
K

4000
K

2

3500K

0
0

500

1000
1500
Wavelength (nanometers)

2000

p PEAK 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 nascent life on an orbiting planet. (Color intervals are approximate.)

q SHIFTING ZONES Shown here are confirmed, 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.

wa

Temperature

WAV ELENGTHS: WIK IPEDIA; ZONES: G REGG DINDER M A N / S&T, SOURCE: JUN YA NG
E T A L. / ASTR OPHYSICAL JOUR NAL LE T TERS 2014 A ND CHESTER H A R M A N, JR.

se

us
Planet Size

se

ou

n
Ve

nh

nt
Tidal loc
king

4000K

Mars

Kepler-62f

radius

Kepler-442b
Kepler-186f
Kepler-1229b

Kepler-438b

(Earth radii)
0.5 Earth

Kepler-1410b

Kepler-1512b

1 Earth

3000K

1.5 Earth

LHS 1140b
Kepler-560b
Ross 128b

Gliese 667 Cc
Proxima Cen b
Trappist-1g
Trappist-1e
Trappist-1f

Trappist-1d
200%

175%

nhou

e
re
yg

ce

5000K

g re e

na

Re
Venus

Earth

Mars

mum

Ru

6000K

Early

Maxi

7000K

150%

125%

100%

75%

50%

25%

Starlight on planet relative to sunlight on Earth

sk yandtele scope.com * AUGU S T 2 019

37



http://www.skyandtelescope.com

Sky & Telescope - August 2019

Table of Contents for the Digital Edition of Sky & Telescope - August 2019

Contents
Sky & Telescope - August 2019 - Cover1
Sky & Telescope - August 2019 - Cover2
Sky & Telescope - August 2019 - 1
Sky & Telescope - August 2019 - Contents
Sky & Telescope - August 2019 - 3
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