Astronomy - February 2014

101 102 103 104 105

Intensity (relative)

Wavelength (nanometers)

15,000 kelvin star

Sun (5800 K)

3000 K

Visible light

M-type

K-type

G-type

Ha

bita

ble^ zo

ne

0.5 AU

0.2 AU

0.1 AU

0.14 AU

1 AU

0.3 AU

Inner edge of

habitable zone

0.8 AU

Outer edge of

habitable zone

2 AU

Earth’s orbit

Earth 1 AU

AFGKM

30 ASTRONOMY t FEBRUARY 2014

warm enough to have liquid water sloshing
over their surfaces. Such planets will race
around their suns in orbits even smaller
than Mercury’s. But huddling so close
comes at a price. These worlds will quickly
become “tidally locked,” like the Moon is
tidally locked to Earth. Their rotational and
orbital periods will be identical, and one
hemisphere will endlessly roast in the star’s
warmth, while the other will shiver in an
eternal stygian night.
This sounds like a recipe for climatic
Armageddon, as any atmosphere will soon
freeze out on the dark side and pile up in
snowy drifts. If you’re a land-dwelling life-
form, the lack of air will ruin your whole
day, every day.
A second problem dogging the dwarfs is
yet another consequence of their half-
hearted shine. Their surface temperatures
range from 5700° Fahrenheit (3150° Cel-
sius) to 5900° F (3260° C). By comparison,
the Sun’s photosphere is far hotter, close to
10,000° F (5540° C). Because of their lower
surface temperatures, dwarfs radiate less
energy, and their light is skewed toward the
red end of the spectrum. These guys pump
only low-octane fuel and produce few high-
potency blue photons. Consequently, some
researchers argue that dwarfs’ light doesn’t
have the oomph to power photosynthesis.
A further concern is the small size of
the habitable zone (often called the Goldi-
locks zone) that surrounds red dwarfs.
This is the temperate “doughnut” of space

around a star where a planet might have a

temperature suitable for maintaining liq-

uid water. Simple high-school geometry

tells you that the small doughnuts around

dim stars have less volume than the larger

ones around their brighter cousins. This

suggests that planets around dwarfs are

less likely to be located in that happy

region where they might qualify to host

biology.

A final black mark against the dwarfs is

that the space weather in their neighbor-

hoods could be nasty. The stellar surfaces

occasionally erupt in giant f lares, and the

radiation they spew into space could deal a

knock-out blow to life gaining a foothold

on a closely orbiting world.

This short list of demerits has tradition-

ally kept red dwarfs from being first-round

draft picks for SETI research. But new

insights and data suggest that these worka-

day stars may have been the victims of a

bum rap.

Belt of life

Perhaps the most stinging criticism from

the SETI camp has been the tidal locking

problem. A yin-and-yang world of blister-

ing heat and unremitting cold hardly

sounds like a good bet for biology. Even

microbial extremophiles would have dif-

ficulty in such outrageous climes.

But the notion of a planet that’s half

parched rock and half frozen ocean — with

no atmosphere anywhere — is not only

grim, it’s unduly simplistic. Any atmo-

sphere that’s at least moderately dense

could save itself from turning to ice. In par-

ticular, the large temperature difference

between the two hemispheres would imme-

diately produce winds, high-altitude air

streams that would bring heat from the hot

side to the cold. This would prevent the

atmosphere from condensing out and pil-

ing up in worthless, frozen heaps.

Astronomers Martin Heath of the Bio-

sphere Project in London; Laurance Doyle

of the SETI Institute in Mountain View,

California; and Manoj Joshi and Robert

Haberle of the NASA Ames Research Cen-

ter in Moffet Field, California, worked out

these details more than a dozen years ago.

They made computer models of tidally

A star’s color depends solely on its tempera-

ture. A star is a “black body” — an opaque

object at a constant temperature. A black

body emits radiation across a range of ener-

gies, but its peak — which corresponds to

the color it appears — is bluer the hotter the

object is. Red dwarfs appear red because they

are relatively cool. ASTRONOMY: ROEN KELLY

The Research Consortium on Nearby Stars

(RECONS) searched the volume of space within

10 parsecs (32.6 light-years) of Earth to take

a complete census. Since 2000, when RECONS

began its study, the number of known nearby

red dwarf (M-type) stars has increased by 25

percent. If these most abundant neighbors are

able to host life, many astrobiological possibili-

ties exist. ASTRONOMY: ROEN KELLY, AFTER RECONS

A star’s habitable zone is the volume of space where liquid water can exist. Although astronomers are still de-
bating the habitable zone boundaries and discovering factors that affect them, it is safe to say that the larger
a star is, the larger its habitable zone will be. Here, the habitable zone of a G-type star, a Sun-like type, is
compared to those of a smaller K-type star and a tiny red dwarf, or M-type, star. The red dwarf ’s zone extends
from 0.14 to 0.1 astronomical unit (AU). An AU is the distance from Earth to the Sun. ASTRONOMY: ROEN KELLY

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