Sky & Telescope - August 2019 - 36

Life As We Don't Know It

Habitability Gamble
There are many reasons why red dwarf stars might be the best
places to look for habitable planets. First among them is sheer
numerical superiority. These puny stars comprise about 70%
of all stars in the Milky Way. This means that when astronomers swivel their telescopes toward a random patch of the
sky, most of the planets they'll ﬁnd will orbit red dwarfs.
Furthermore, the two most widely used techniques for
detecting exoplanets - the transit and radial velocity methods - work better with small stars. An Earth-size planet
orbiting a small star will eclipse a higher percentage of the
star's visible surface (and thus its starlight) than the same
q NEVER-ENDING STORMS Red dwarf stars are tumultuous. On a typical day, they display gigantic arcing prominences and a wealth of dark
sunspots, but they also erupt with intense flares that, over time, could
strip a nearby planet's atmosphere. One look at this artist's conception
and it's easy to see why astronomers initially thought that any planets
around stars like this one would be sterile.

AUGU S T 2 019 * SK Y & TELESCOPE

Because these stars are much
cooler than most other stars, their
habitable zones are signiƓcantly
closer to them, much like a person
must stand closer to a small
campƓre than to a large one to
feel the same amount of heat.
planet orbiting a large star. Similarly, an Earth-size planet
orbiting a less massive star will cause that star to wobble more
than if it orbited a heftier star. Thus, astronomers discover
more planets around small stars using these methods.
Furthermore, the gas clouds that collapse to make red
dwarf systems appear to form small planets more easily
than they do big ones. NASA's Kepler spacecraft found that
although red dwarfs host far fewer gas giant planets than
Sun-like stars do, they have 3.5 times more small planets in
the Earth-size regime. In fact, 1 in 4 red dwarf star systems is
estimated to have an Earth-size planet in the habitable zone.
For all these reasons, detecting habitable planets is not
only easier but also more likely around red dwarfs than
around other types of stars.
Unfortunately, because these stars are much cooler than
most other stars, their habitable zones are signiﬁcantly closer
to them, much like a person must stand closer to a small
campﬁre than to a large one to feel the same amount of heat.
This close distance presents a number of complications to the
habitability of orbiting planets.
For starters, such proximity creates strong tidal forces,
which can slow down the planet's rotation rate and affect its
atmospheric circulation. This effect, known as tidal locking,
can play out in a number of ways. It can slow down the planet
only a little, or it can slow down the planet a lot, to a point
called synchronous rotation, in which the same side of the
planet always faces the star - meaning it's always daytime
on that side of the planet and nighttime on the other. Who
would want to live on a planet where it's always day or night?
Could life even survive on such a world?
Indeed, the prevailing concern has long been that the
nightside of a synchronously rotating planet could become so
cold that the entire atmosphere would freeze out, condensing
onto the surface to create an icy, airless world. Not the greatest prospect for a fun nightlife.
But all is not lost. Recent research using sophisticated
global climate models has shown that a thick atmosphere
could transport enough heat to the nightside of a synchronously rotating planet to prevent atmospheric freeze-out.
Additionally, synchronous rotation might even create an
advantage for habitability: If a tidally locked planet has an
ocean, then the stronger convection in its atmosphere - a
consequence of longer daytime illumination - could gener-

N ASA / ESA / STSCI / G. BACON

tances from a star where conditions on an Earth-like planet
would permit liquid water to ﬂow on its surface. But true
habitability involves a glorious cocktail of factors, from the
host star's luminosity and magnetic activity to the planet's
rotation rate and the composition of its atmosphere and
surface. When it comes to red dwarfs, the prospects are not
all sunshine and rainbows, but life - perhaps an unfamiliar
form of it - could still thrive.

Sky & Telescope - August 2019

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