NEWS NOTES
12 FEBRUARY 2020 • SKY & TELESCOPE
FOLLOW-UP OBSERVATIONS of an
amateur-dis covered planetary system
show that its planet has Neptune’s mass
and orbits its star in the region where
ice giants are thought to form.
On October 25, 2017, Japanese
amateur astronomer Tadashi Kojima
was monitoring stars for the sudden
brightening that might indicate a nova.
Rescanning a fi eld in Taurus he found
one particular star that had brightened
from magnitude 13.0 to 11.7. By Hal-
loween, the star had brightened further
to magnitude 10.8.
The star was faint in Kojima’s obser-
vations, but calls for follow-up observa-
tions (via the Central Bureau for Astro-
nomical Telegrams and the American
Association of Variable Star Observers)
soon showed that the brightening pat-
tern he had observed was characteristic
of gravitational microlensing.
When one star passes directly behind
another from Earth’s perspective, the
foreground star’s gravity bends the
light from the background star so thatEXOPLANETS
Observations Confi rm Amateur-Discovered Exo-Neptuneit appears to brighten from Earth’s
perspective. If the foreground star has a
planet, the planet’s gravity adds a brief
fl ash to the overall gleaming.
Astronomers have found several
dozen exoplanets using this technique,
but the event that Kojima discovered,
named Kojima-1, is unique. For one,
most microlensing events occur toward
the galactic center, as there are more
stars in that direction. Kojima’s star
is in the sparser region opposite thepArtist’s concept of an exo-Neptune orbiting a
red dwarf starcenter. It’s also relatively close, at 1,
light-years away.
Because the foreground star is in an
uncrowded region and closer to Earth
than the typical microlensing star,
it made for an easy target for follow-
up observations. Multiple observing
campaigns ensued, including one led
by Akihiko Fukui (University of Tokyo).
His team used 13 ground-based tele-
scopes to repeatedly image the star and
obtain its spectrum over 2½ months.
In the November issue of the Astro-
nomical Journal, the team confi rms the
existence of this Neptune-mass planet
in the outskirts of our galaxy. Desig-
nated Kojima-1Lb, the planet has 20
times Earth’s mass and follows an orbit
similar to Earth’s, with an average dis-
tance from its star of 1.1 astronomical
units. Because the star is less massive
and less luminous than the Sun, that
orbit puts the exo-Neptune near its
system’s snow line, beyond which water
vapor and other gases condense into
ice. The discovery hints that Neptune-
mass planets might indeed be common
outside this boundary.
■MONICA YOUNGRefreshing Galactic Wind
Most “normal” or baryonic mass is contained not in stars
but in gas that extends far beyond any galaxy. This gas is
thought to fall onto galaxies over time, providing fuel for
star formation. In turn, stars — or rather, the winds they
produce as they’re born and as they die — are thought
to replenish that reservoir. For the fi rst time, David Rupke
(Rhodes College) and colleagues directly observed evi-
dence of that replenishment using the Keck Observato-
ry’s Cosmic Web Imager, they report in the October 31st
Nature. The KCWI, an integral fi eld spectrograph, took
simultaneous images and spectra of a giant wind fl owing
out of a galaxy into the sparser medium that surrounds
it. The galaxy, named Makani (“wind” in Hawaiian), has
probably driven this tempest via bursts of star formation
ignited during a merger with another galaxy. Makani is
currently forming 100 to 200 solar masses of stars every
year. The wind from these stars fi lls a region 260,000 by
330,000 light-years with hot (10,000 K), ionized gas that’s
enriched with elements heavier than hydrogen and he-
lium. Galactic winds have been observed before but not
so far out from a galactic center.
■ MONICA YOUNGEXO-NEPTUNE: UNIVERSITY OF TOKYO; MAKANI GALAXY: JIM GEACH / DAVID TREE / PETER RICHARDSON / UNIVERSITY OF HERTFORDSHIRE