Sky and Telescope - April 2018 - 11

IN BRIEF

GAMMA-RAY BURSTS

Did Colliding Neutron Stars Choke Their Own Jet?
ing debris. To test these
scenarios, Mooley and
colleagues tracked
detected last year might
GW170817's radio afterpoint toward a new class
glow for more than 100
of sources.
days after the collision.
A quick burst of
If the jet were simgamma rays followed the
ply pointed in another
ﬁrst-ever gravitationaldirection, Mooley's team
wave detection of mergwould have detected
ing neutron stars, known
Artist's concept of failed-jet
its signature as a sudas GW170817. Some
scenario
den increase in radio
astronomers heralded
waves. Instead, they saw
this event as deﬁnitive
a gradual brightening. They interpret
proof that neutron-star collisions cause
the radio glow as a cocoon surrounding
short gamma-ray bursts (GRBs), highthe smashup: If the jet failed to punch
energy ﬂashes less than 2 seconds long.
through the material kicked out in the
A short GRB would have sent a
merger, it would have transferred most
gamma-ray-emitting jet toward Earth.
But in a study to be published in Nature, of its energy to the debris, pushing it
outward into a bubble-like structure.
Kunal Mooley (University of Oxford,
The cocoon model also jibes with
UK) and colleagues report radio-wave
new Chandra X-ray Observatory data
observations that don't match this
reported January 18th in Astrophysical
simple jet scenario. Instead, the data
Journal Letters, which show that X-rays
add to a growing argument that this
from the event continue to brighten.
source is anything but typical of short
If this scenario holds up, GW170817
GRBs (S&T: Feb. 2018, p. 32).
could mark an entirely new class of
GW170817 posed a problem from
GRBs. But the debate rages on: Several
the beginning. This event was 10,000
papers in the pipeline argue that a suctimes dimmer than other short GRBs.
cessful, off-axis jet can better explain
Many astronomers thought that might
the latest data, returning this event to
be because the jet was facing slightly
its status as a typical short GRB.
away from Earth, but others proposed
that the jet had choked on surround■ SHANNON HALL

NR AO / AUI / NSF / D. BERRY

THE NEUTRON-STAR
SMASHUP that LIGO

15,000 Kepler light curves previously
studied by humans. Andrew Vanderburg
(University of Texas, Austin) then ran a
preliminary sample of 670 stars through
the standard pipeline to identify signals
to feed to the algorithm. The neural net
caught two rocky planets, Kepler-90i
and Kepler-80g, both having roughly
14-day orbits. The discovery appears in
the January 18th Astronomical Journal.
Kepler-90i is the eighth planet
discovered in its tightly packed system.
t Kepler-90i is the third planet from the left in
this artist's illustration of its scrunched system.
All eight planets would fit within Earth's orbit.

Meteoroids Explode
from the Inside
Space rocks breaking up in Earth's atmosphere might be their own Trojan horses.
Spurred by an attempt to understand why
the 2013 Chelyabinsk meteoroid fragmented so easily, Marshall Tabetah and
Jay Melosh (both at Purdue University)
simulated the rock's interaction with the
surrounding air as it dove through the
atmosphere. The key was the meteoroid's
porosity, which hadn't been included in
previous simulations. Asteroids are more
like piles of coarse gravel than solid
boulders. As one of these rocks shoots
through the atmosphere, a bow shock
forms around it, with piled-up air in front
and a vacuum in the wake. This setup
creates something of a suction effect,
with the high-pressure air in front shooting
through the rock's fractures to fill the vacuum behind. The air's infiltration breaks
the meteoroid apart from the inside out.
The team's paper appeared December
11th in Meteoritics & Planetary Science.
■ CAMILLE M. CARLISLE

Exoplanet Loops
Over Star's Pole

Kepler-80g is the sixth planet in its system, orbiting its star in resonance with
four of its companions. The resonance
keeps the orbits stable, even though
they're more compressed than Kepler90's. Neither planet offers a pleasant
place to live: Surface temperatures are
710K (820°F) and 420K, respectively.
Eventually, Shallue and Vanderburg
want to expand their project to study
all 150,000 stars in the Kepler ﬁeld, but
they'll proceed with caution. A neural
network is only as good as it was taught
to be; it can't generalize what it has
learned in the same way humans can.

Reporting December 18th in Nature,
Vincent Bourrier (Geneva University
Observatory, Switzerland) and colleagues
have found that the Neptune-mass
exoplanet GJ 436b orbits around its
star's poles rather than its equator. The
default in planet formation is for worlds to
coalesce from a dusty disk of gas around
a newborn star's middle, a consequence
of the conservation of angular momentum. Furthermore, huge planets close to
their parent stars "should" be on circular,
equator-aligned orbits due to tidal interactions. But GJ 436b is wonky: The team's
3D mapping of the planet's trajectory,
coupled with measurements of the star's
rotation, reveal that the gaseous world is
following an elongated orbit that passes
over the star's pole. This setup suggests
that an outer, unseen exoplanet might
have driven GJ 436b inward to its current
location. The migration would also have
exposed the planet's atmosphere to
more starlight and might have triggered
GJ 436b's comet-like atmosphere.

■ MONICA YOUNG

■ CAMILLE M. CARLISLE
sk yandtelescope.com * A PR I L 2 018

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