IN THE JOURNALScience,OliverMüller
(University of Basel, Switzerland) and
colleagues report that satellite galaxies
are moving around the huge elliptical
Centaurus A (NGC 5128) in an orderly
fashion. The finding may challenge our
understandingofgalaxyevolution.
Current cosmological theory predicts
that large galaxies should surround
themselveswithachaoticswarmof
dwarf galaxies. Instead, out of the 16
Centaurus A satellites with known
line-of-sight velocities, 14 orbit in the
same direction along a broad plane that
crosses perpendicular to the galaxy’s
famousdustband.Theresultechoes
similarfindingsforourownMilkyWay
Galaxy,aswellasfortheAndromeda
Galaxy (M31).
Sometheoristshaveproposed
that such order might come from
gravitational interactions within our
Local Group of galaxies. But according
to Müller and his coauthors, the
likelihood of finding just one example
of coordinated motion among dwarf
satellites is smaller than 0.5% in
current cosmological simulations.
“Findingthreesuchsystemsinthe
nearby universe seems extremely
unlikely,” they write.However, as Eline Tolstoy (University
of Groningen, The Netherlands) points
out, “It’s still small number statistics.”
Müller’steamcouldonlymeasure
the velocities of the brightest satellite
galaxies, which could skew the results.
Moreover, previous observations have
shown that Centaurus A has experienced
a major merger in its past, which might
have organised its satellites’ orbits.
More complete observations of
galaxies and their satellite systems
can help settle the issue, but the
measurements are difficult to make.
New data from the European Space
Agency’s Gaia mission should at least
clarify the motions of dwarf galaxies
around the Milky Way.
■GOVERT SCHILLINGOrderly dwarf galaxies challenge
cosmological wisdomCENTAURUS A: ESO; VENUS
: JAXA / ISAS / DARTS / DAMIA BOUIC
New dividing line
between giant
planets and brown
dwarf stars
Scientists have long set the dividing line
between giant planets and brown dwarf
stars at 13 times Jupiter’s mass, the
minimum required to ignite deuterium
fusion in an object’s core. However,
writing in theAstrophysical Journal, Kevin
Schlaufman (Johns Hopkins University)
sets a different upper boundary: between
4 and 10 times Jupiter’s mass. The ne
deinition is based on how the objects
form: Giant planets grow from the bott
up in a process calledcore accretion,
so they should form more easily in
protoplanetary disks enriched with he
elements. Brown dwarfs grow from the
top down, collapsing directly from a cl
of gas, which doesn’t depend so much
on heavy elements. Indeed, Schlaufm
found that, in 146 carefully selected
planetary systems, objects with mass
less than 4 to 10 Jupiters tend to form
around stars rich with heavy elements
while objects with more than 10 Jupit
masses form around all kinds of stars.
These results won’t reclassify any planets
within our own Solar System, but they do
have implications for where we draw t
line between giant exoplanets and bro
dwarfs. Read more athttps://is.gd/
planetdefinition.
■SHANNON HALLJapan’s Venus-bound orbiter Akatsuki survived great odds to reach its objective
and is now wowing scientists with results. Akatsuki has been at Venus since
December 2015, and in its final science orbital configuration since April 4, 2016.
The very elliptical orbit has a period of 10.5 days, travelling from a closest
approach of around 10,000 km to its farthest point 360,000 km from the planet.
Akatsuki carries five cameras to view Venus in different wavelengths, each one
penetrating into the planet’s atmosphere to a different depth. Unfortunately, its
two infrared cameras suffered an electrical fault in December 2016, but three
cameras capturing ultraviolet, visible and long-infrared wavelengths still work.
The ultraviolet imager (UVI) records high-altitude clouds illuminated by sunlight,
as pictured here. Drifting along at elevations of 65 to 75 km, these clouds consist
mostly of sulfuric acid. Amateur image processor Damia Bouic combined UVI
images taken from different distances to create this composite. Read more and
see additional stunning images at https://is.gd/Akatsukiviews.
■ EMILY LAKDAWALLAAkatsuki’s views of Venus
S Centaurus Ahttp://www.skyandtelescope.com.au 9