Sky & Telescope - April 2019 - 38

Famous Stars, Part II

Planet hunters aren't hopeful
of ﬁnding life-sustaining
conditions on any worlds
around Proxima, but they
have brighter prospects for
Alpha Centauri A and B, which
are more like our Sun - as
long as observers can ﬁnd
planets there, that is.

Looking for Planets

p STAY BETWEEN THE LINES Despite being so close together, both

Alpha Cen A and B have regions of stability (inside the dotted lines)
Learning more about the interactions of Alpha Centauri's
where planets could safely orbit. Green bands mark the habitable zones.
stars could reveal how probable planets are in the system
and how their habitability might have evolved over time.
In short, planet hunters aren't hopeful of ﬁnding lifeAstronomers have already found one planet orbiting Proxima,
using HARPS. Found in 2016, Proxima Centauri b has at least sustaining conditions on any worlds around Proxima. But
they have brighter prospects for Alpha Centauri A and B,
1.3 Earth masses, is probably rocky, and sits very close to
which are more like our Sun - as long as observers can ﬁ nd
Proxima, around 10% of Mercury's distance from the Sun. It
planets there, that is.
completes an orbit every 11 days (S&T: Dec. 2016, p. 10).
Double- and triple-star systems have certain
Proxima Cen b is within Proxima's temperparticularities when it comes to forming and
ate zone, where in theory liquid surface water
%
keeping planets. When stars form, most of the
could exist. (The team avoids the more optiFraction
available material ends up in the stars themmistic term "habitable zone.") But red dwarfs
selves. The leftovers form protoplanetary disks
such as Proxima frequently release waves of
of known
energetic particles that could sterilize the
exoplanets that where planets could grow. But when two stars
form in close quarters, they might disrupt each
surface of a planet and erode its atmosphere,
orbit stars in
other's planet-forming disks.
a process similar to what we think might have
multiple systems
Planets are possible, though; there are sevhappened on Mars (S&T: July 2018, p. 14).
eral safe zones in binary systems. Each star can
Proxima Cen b is so close to its star that it's
harbor its own planet population, depending on how close
probably tidally locked, with the same side always facing the
the stars are to each other. There could also be circumbinary
incoming radiation. That means that while the star-side of
planets orbiting both stars. In fact, about 20% of all known
the planet gets toasted, the farside remains cold, unless some
exoplanets are in binary systems. One example is the case of
convection mechanism (perhaps large-scale mixing in the
HD 196885 AB, a binary similar to Alpha Centauri AB with a
atmosphere) redistributes part of the heat.
planet orbiting the primary star.
"In a planet around an M dwarf that somehow had
Recent analyses have determined that the stability zones
retained a thick atmosphere or maybe an ocean, life could
around Alpha Centauri A and B extend to 2 or 3 a.u. from
survive," Ayres says. But he quickly points out that this life
each star, roughly the distance between the Sun and the
would have to evolve to withstand the high levels of radiamain asteroid belt. These stability regions also happen to
tion, too.

SHIRTTAIL RELATION Proxima Cen lies some 400 times farther from Alpha Cen AB than the stars
A and B lie from each other. The distances here are to scale, but the star sizes are not.

Proxima

A PR I L 2 019 * SK Y & TELESCOPE

STA BILIT Y REGIONS: LE A H TISCIONE / S&T, SOURCE: T. W. A . M ÜLLER A ND N.
HAGHIGHIPOUR / ASTR OPHYSICAL JOUR NAL 2014, H T TP://ASTRO.T WA M.INFO/HZ /;
SYSTEM DIAG R A M: LE A H TISCIONE / S&T

http://astro.twam.info/hz/

Sky & Telescope - April 2019

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