Sky & Telescope - February 2020 - 12

EXOPLANETS

Observations Conﬁrm Amateur-Discovered Exo-Neptune
FOLLOW-UP OBSERVATIONS of an

amateur-discovered 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 ﬁeld in Taurus he found
one particular star that had brightened
from magnitude 13.0 to 11.7. By Halloween, the star had brightened further
to magnitude 10.8.
The star was faint in Kojima's observations, but calls for follow-up observations (via the Central Bureau for Astronomical Telegrams and the American
Association of Variable Star Observers)
soon showed that the brightening pattern 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 that

p Artist's concept of an exo-Neptune orbiting a
red dwarf star

it appears to brighten from Earth's
perspective. If the foreground star has a
planet, the planet's gravity adds a brief
ﬂash 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 the

Refreshing 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 ﬁrst time, David Rupke
(Rhodes College) and colleagues directly observed evidence of that replenishment using the Keck Observatory's Cosmic Web Imager, they report in the October 31st
Nature. The KCWI, an integral ﬁeld spectrograph, took
simultaneous images and spectra of a giant wind ﬂowing
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 ﬁlls 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 helium. Galactic winds have been observed before but not
so far out from a galactic center.
■ MONICA YOUNG

FEBRUARY 2 02 0 * SK Y & TELESCOPE

center. It's also relatively close, at 1,600
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 followup observations. Multiple observing
campaigns ensued, including one led
by Akihiko Fukui (University of Tokyo).
His team used 13 ground-based telescopes to repeatedly image the star and
obtain its spectrum over 2½ months.
In the November issue of the Astronomical Journal, the team conﬁrms the
existence of this Neptune-mass planet
in the outskirts of our galaxy. Designated Kojima-1Lb, the planet has 20
times Earth's mass and follows an orbit
similar to Earth's, with an average distance 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 Neptunemass planets might indeed be common
outside this boundary.
■ MONICA YOUNG

E XO -NEPT UNE: UNIV ERSIT Y OF TOK YO; M A K A NI G A L A X Y: JIM G E ACH /
DAVID TREE / PE TER RICH A RDSON / UNIV ERSIT Y OF HERTFORDSHIRE

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Sky & Telescope - February 2020

Table of Contents for the Digital Edition of Sky & Telescope - February 2020