Sky and Telescope - August 2017 - 13

GALAXIES
SOLAR SYSTEM

Two-Lobed Asteroid
Flies Close by Earth
Asteroid 2014 JO25 passed 1.8 million km
(1.1 million miles) from Earth on April 19,
2017. This was 4.6 times the Earth-Moon
distance, the closest an asteroid 600
meters (2,000 ft) wide or larger has come
since 2004. Prior to this, observations from
the NEOWISE spacecraft had suggested
the asteroid's albedo was 25% - about
twice the Moon's average - and thus
that it had a diameter of 650 m. But radar
images from April's pass (shown below)
reveal that, while the object is indeed about
600 m across its short axis, the longest
part of 2014 JO25 spans a full kilometer. It
turns out that the asteroid is twin-lobed,
much like the contact-binary nucleus of
Comet 67P/Churyumov-Gerasimenko (S&T:
May 2017, p. 14). Planetary scientists have
found about 50 contact-binary near-Earth
asteroids with radar, and about 15% of all
NEAs bigger than 150 m probably have this
shape, notes Shantanu Naidu (Jet Propulsion Laboratory).

■ DAVID DICKINSON

Magnetic Bridge Found Between Magellanic Clouds

Magellanic Clouds over Australia

A MAGNETIC FIELD appears to span
the space between the Large and Small
Magellanic Clouds, the two largest
dwarf galaxies being consumed by the
Milky Way.
The ﬁeld is part of an intergalactic
structure called the Magellanic Bridge,
a 75,000-light-year-long ﬁlament of gas
and dust that stretches between the two
dwarf galaxies. The bridge is probably
a remnant of a close ﬂyby, comprised
of gas torn out of both galaxies as they
passed by each other roughly 200 million years ago.
To detect the ﬁeld's presence, Jane
Kaczmarek (University of Sydney,
Australia) and colleagues observed 167

known radio sources in the same area
of the sky as the Magellanic Clouds but
located far beyond the galaxies themselves. Some of these radio sources lie
directly behind the bridge along our line
of sight, while some of them are off to
either side of it.
When emission from radio sources
passes through a magnetized medium
(such as a large gas ﬁlament) on its
way to our telescopes, the direction of
the light waves' undulation changes.
The change in this polarization from
what it was originally tells us about the
strength of the magnetic ﬁeld inside
the intervening medium that caused
the change.
From the observations, the magnetic
ﬁeld in the bridge is 0.3 microgauss,
a millionth as strong as the ﬁeld we
experience on our planet's surface, the
astronomers report in the May Monthly
Notices of the Royal Astronomical Society.
The result conﬁrms the existence of a
"pan-Magellanic ﬁeld" - a magnetic
ﬁeld that spans the entire bridge.
Due to the particular structure of
the ﬁeld observed in the bridge, the
authors think it might be a combination
of both galaxies' magnetic ﬁelds being
dragged along with the gas into the tidal
structure when it formed.
■ SUMMER ASH

SOL A R SYSTEM: N ASA / JPL- CA LTECH / GSSR;
M AG ELL A NIC CLOUDS: MIK E SA LWAY

Asteroid 2014 JO25

eter of 1.4 Earths, its average density is
an incredible 9.1 to 15.9 g/cm3 - two to
three times higher than our world's.
LHS 1140 might prove to be special
compared to other red dwarfs with
exoplanets, Dittmann argues. Red
dwarfs are often maligned as ﬂare stars,
but this one is quiet. "We don't see any
ﬂares in our years of monitoring it," he
says. That's perhaps unsurprising, the
team reports in the April 20th Nature.
The star's rotation period of 130 days,
combined with its lack of hydrogenalpha emission, suggests the star is at
least 5 billion years old.

IN BRIEF
In Situ Formation for
Warm Neptune?
Astronomers think the "warm Neptune"
HAT-P-26b formed close to its star, instead
of migrating inward from farther out.
Theorists have long suspected that giant
planets could only form in a planetary system's outer regions, where conditions are
cold enough for molecules such as carbon
dioxide and water to freeze into icy clumps
and act as planetary building blocks.
The many gas giants found near their
host stars have thus been a conundrum.
Hannah Wakeford (NASA Goddard) and
colleagues used the Hubble and Spitzer

space telescopes to measure the spectrum of light passing through HAT-P-26b's
atmosphere as the exoplanet transited in
front of its star. As the team reports in the
May 12th Science, the spectrum shows a
clear signature of water. Water serves as
a representative of all the elements in the
atmosphere heavier than helium. The low
water abundance suggests the planet's
composition is mostly hydrogen and helium - more like Jupiter's than Neptune's.
In fact the gas is so untainted by heavier
elements that the world must have come
together relatively late during the system's
formation, meaning it probably formed
where it is now, the team concludes.
■ MONICA YOUNG

■ DAVID DICKINSON
s k y a n d t e l e s c o p e .c o m

* AUGUST 2017

13
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Sky and Telescope - August 2017

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