Sky and Telescope - December 2016 - 11

S&T: LEAH TISCIONE, SOURCE: ESO / G. ANGLADA-ESCUDÉ

Radial velocity
(meters per second)

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This plot shows how the motion
of Proxima Centauri towards
and away from Earth (y-axis)
changed with time over the ﬁrst
months of 2016. At its max,
Proxima Centauri approaches or
recedes from Earth at about 1.4
meters per second - normal
human walking pace. This regular pattern repeats every 11.2
days and results in tiny Doppler
shifts in the star's light.

90

Days since January 1, 2016

block out the star's light to see the world,
which is only 0.035 arcsecond (about
1/100,000°) away from it. They may also be
able to tease out reﬂected light from any
atmosphere, revealing its composition.
In addition, there's the upcoming
ESPRESSO spectrograph at the ESO's
Very Large Telescope facility in Chile,
which will begin operation in 2017. With
the new spectrograph, "demonstrating
the existence of Proxima Cen b should be
like shooting ﬁsh in a barrel," says exoplanet researcher Debra Fischer (Yale).

A Habitable World?
Whether this planet is habitable remains
a wide-open question. Technically it lies
at the right distance from its star that,
given the right composition and atmosphere, liquid water would be stable on
its surface.
But, the discovery team stresses,
astronomers only know three things
about this world: its mass, orbital period,
and distance from the star. It's not even
clear whether Proxima Cen b is rocky -
the 1.3 Earth masses is a lower limit; the
team can't give an explicit, upper cutoﬀ
for how hefty it might be. Still, coauthor
Ansgar Reiners (Georg-August-Universität Göttingen, Germany) says previous
work has shown that massive planets generally don't form around stars this small.
His gut instinct is that the world probably
doesn't weigh more than a few Earths.
That could still make it a mini-Neptune.
If the planet does have a solid surface,
whether it has water will depend on how
it formed and what happened to it soon

after - did it coalesce far from the star,
where ices reign, and then migrate in
carrying that water with it? Did it form
where it is now from dry rocks and never
pick up any water? We don't know.
It's also unclear just how much the M
dwarf's active personality would harm the
planet. Proxima Cen b receives roughly
100 times more energy in X-rays from its
star as Earth does from the Sun. What that
means for habitability is anyone's guess. A

lot will depend on what the star did in its
early days, which astronomers don't know.
They suspect the star is roughly 5 billion
years old, on par with the Sun.
Lastly, the planet is probably tidally
locked in synchronous rotation, such that
the same face always points at Proxima.
That's not a death knell for habitability,
but it does change the kind of atmospheric circulation required.
■ CAMILLE M. CARLISLE

TECH I Could Spacecraft Make It to Proxima Centauri?
Last April, billionaire Yuri Milner
announced a $100 million initiative to
test the feasibility of sending tiny spacecraft on a 20-year journey to the Alpha
Centauri system at 20% the speed of light
(S&T: Aug. 2016, p. 12). At one-ﬁfth lightspeed, even the micron-size dust grains
and gas atoms between stars pose a real
threat. But how much of a threat?
In a paper posted August 18th to the
online preprint server arXiv.org, Thiem
Hoang (University of Toronto) and others
explored what might happen to a tiny,
gram-scale probe like that proposed by
Breakthrough Starshot: a mini spaceship
made of a computer chip that contains
sensors (composed primarily of silica)
and a reﬂective lightsail to accelerate the
craft (composed of graphite).
Because of the probe's high speed, it
essentially would see any lone atom or dust
mote as a relativistic projectile. The few
heavy gas atoms en route would be particularly dangerous to silica-based materials,

but overall dust is the main threat. While
tiny and rare, every dust grain that gets
in the way will both evaporate and melt
material on the spacecraft, generating an
impact crater and potentially damaging
sensors or other key components.
The team calculated that the impact of
a dust grain any bigger than 15 microns,
or roughly the width of a human hair,
would destroy the probe. Fortunately, cosmic dust grains of this size are rare: the
authors peg the chance of running into
one of them at one in 1050 (in other words,
practically never).
To protect a spacecraft, Hoang's
team thus suggests making it long and
slim, like a needle. This will reduce the
surface area the spacecraft presents to
the onslaught of interstellar debris. Also
helpful would be a protective graphite
coating a few millimeters deep. Read
more about the analysis at https://is.gd/
centaurishields.
■ MONICA YOUNG

Sk yandTelescope.com December 2016

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http://www.arXiv.org https://www.is.gd/centaurishields https://www.is.gd/centaurishields http://www.SkyandTelescope.com
Sky and Telescope - December 2016

Table of Contents for the Digital Edition of Sky and Telescope - December 2016
