10 AUSTRALIAN SKY & TELESCOPE May | June 2017
ILLUSTRATION: NASA GODDARD SPACE FLIGHT CENTER / S. WIESSINGER; PHOTO: RYAN THOMPSONTHE CLOSEST STARto the Sun likely
desiccated its exoplanet and destroyed
that world’s chances for habitability,
astronomersreportedatarecent
American Astronomical Society meeting.
Withaminimummassof1.3Earths,
ProximaCentauribmightbeeither
aterrestrialoraNeptune-likeworld.
Itorbitsitssmall,redMdwarfinthe
star’s putative habitable zone.
Butbeinginthissweetspotmight
haveinsteadspelledProximaCentaurib’s
biological doom. YoungMstars are
significantly brighter and more active
than older ones, shooting out potentially
atmosphere-stripping eruptions and
gads of X-ray and ultraviolet radiation —
roughly100timesasmuchinX-rayand
10to20timesasmuchinUVasdwarfs
asoldastheSundo,saidEdwardGuinan
(Villanova University) on January 4.
Allthesefactorsputtogethermean
that,inProximaCentauri’searliest
days,itshabitablezonewasfartherout
than it is now. If the planet formed
where it currently resides (in the
modern habitable zone), then the world
“underwentalivinghellinitsearly
to 400 million years,” Guinan said.ProximaCentauriblikelytobeadesertworld
A RECENT EUROPEAN STUDY has
confirmed that a silent cosmic rain of
micrometeorites is falling on us.
Matthew Genge (Imperial College
London) and colleagues discovered the
small particles in sediment collected
from the gutters of urban EuropeanVictoria Meadows (University of
Washington), who presented in the
same session, has come to the same
conclusion. Her team’s simulations of
different types of atmospheres and their
evolution over the planet’s 5-billion-
year history suggest that, if the planet
once had surface water, then the
incoming radiation likely would have
evaporated most or all of it. The result
would be a substantial atmosphere rich
in either oxygen or carbon dioxide.
Nevertheless, Proxima Centauri b
might be habitable if it started out with
a protective, hydrogen-rich envelope, orif it formed farther from the star — and
thus farther from the deadly radiation —
and then migrated to its current, close-in
position. Forming farther out would also
enhance its chances for retaining water,
because ices are more prevalent in the
outer reaches of planet-forming disks.
Guinan’s team looked at two other
potentially habitable exoplanets —
Kapteyn b (13 light-years away) and
Wolf 1061 c (14 light-years). Both have
masses of at least 4 to 5 Earths.
Given their stars’ ages and their
X-ray and ultraviolet outputs, the team
determined that Wolf 1061 c also
offers poor chances for habitability. But
Kapteyn b looks favourable. It currently
receives 8 times more X-ray energy
from its star than Earth does from
the Sun, and less than twice Earth’s
ultraviolet level. Kapteyn b is also 3½
times farther from its M dwarf than
Proxima Cen b is from its host. So the
former might potentially have been far
enough out to weather the star’s violent
youth. Notably, Kapteyn’s star is about
11 billion years old and probably stolen
from a dwarf galaxy.
■ CAMILLE M. CARLISLErooftops by Project Stardust. The team
sifted through 300 kilograms of material
from a total collection area covering
some 30,000 m^2. The scientists used
magnets to pick out pieces with high
iron content, then washed and sorted
those candidates by size and shape. The
final suspects were examined under
a binocular microscope for signs of
atmospheric entry. Only 500 particles
passed stringent scrutiny. Forty-eight
were chosen for detailed study.
The micrometeorites collected
are tiny, most 300 to 400
microns in size. The largest
of them are just under a half
millimetre across, barely visibleArtist’s concept of a
young, flaring M dwarf.Potential micrometeoriteto the eye. Their compositions are
consistent with meteoritic origin.
The existence of ‘rain-gutter
micrometeorites’ has been a matter
of minor controversy in meteorite-
collecting circles for several decades.
The study, which appears in the
February Geology, demonstrates that
they do in fact occur.
Scientists estimate that 100 billion
micrometeorites fall to Earth each year,
about one per square metre.
■ DAVID DICKINSONLearn more about the micrometeorite
study and how to go on the hunt at
https://is.gd/gutteritesMicrometeorites found on city rooftops
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