Sky & Telescope - November 2021 - 10

NEWS NOTES
EXOPLANETS
Kepler Finds Possible Outcast Earths
An artist's impression shows a free-floating
Earth-mass planet.
microlensing event; Earth-mass planets
magnify background stars for only a
couple of hours, making them difficult
to differentiate from stellar flares.
A team lead by McDonald pored
ASTRONOMERS HAVE UNCOVERED
four candidate Earth-mass rogue
planets by searching for microlensing
events observed with NASA's Kepler
satellite. Rogue planets drift aimlessly
through space after ejection from their
stellar system during the early stages of
planet formation. Iain McDonald (now
at Open University, UK) and colleagues
announced the new planet candidates
in the August Monthly Notices of the
Royal Astronomical Society.
When a star or planet passes in front
of a distant star, it acts like a magnifying
lens to temporarily brighten the
background star, an effect known as
microlensing. Rogue planets are best
spotted via microlensing because they're
too faint to detect directly. However,
the smaller the " lens, " the shorter the
IN BRIEF
Witnessing Gravitational Instability
Astronomers have reported a gravitationally unstable disk around the young star
Elias 2-27 in the June 20th Astrophysical Journal. Gravitational instability is one path to
planet formation: When disks become massive enough, they may collapse directly into
planets or form spiral arms that trap material for future planet formation. A team led by
Teresa Paneque-Carreño (now at Leiden University, The Netherlands) used the Atacama
Large Millimeter/submillimeter Array to spot a hallmark wiggle, a signature of gravitational
collapse, in the spiral arms of the disk around Elias 2-27. The wiggle is a disturbance
in the disk's rotation on scales coinciding with instability-induced spiral arms. PanequeCarreño's
team saw the wiggle by observing the motions of carbon monoxide in the
disk, which traces the harder-to-observe hydrogen gas. Furthermore, a companion study
concluded that Elias 2-27's disk has 17% the mass of its star, creating conditions ripe for
gravitational instabilities to occur. Together, the spiral, wiggle, and mass all indicate the
disk is gravitationally unstable, providing convincing evidence of this planet-formation
mechanism in action.
¢LAUREN SGRO
10 NOVEMBER 2021 * SKY & TELESCOPE
through data from the rejuvenated
Kepler mission, dubbed K2, to recover
previously detected microlensing
events and find new ones, too. Four
of the new events lasted only a couple
hours at most, implying rogue-planet
masses comparable to Earth's. Previously,
astronomers had detected only a
handful of such short-lived microlensing
episodes, making the new events a
valuable addition.
But Przemek Mróz (Caltech), a fellow
rogue-planet hunter, cautions that these
planet candidates might not be true
drifters. " Figuring out whether these
objects are indeed free-floating or not is
trickier, " he says. Some of these planets
might be orbiting far from their host
star, he explains, so their microlensing
signature would be nearly identical to
that of free-floating planets.
If outcast Earths are truly typical in
our galaxy, future facilities such as the
Nancy Grace Roman Space Telescope
should easily detect their signals.
¢LAUREN SGRO
SOLAR SYSTEM
Amateur Tracks Down
New Moon of Jupiter
AN AMATEUR ASTRONOMER has
pinpointed a new moon of Jupiter,
first spotted in 2003 but whose orbit
was never determined. If approved
by the International Astronomical
Union's Minor Planet Center, the
find would bring the tally of known
Jovian satellites to 80.
Kai Ly, the amateur who last year
recovered four lost Jovian moons
(S&T: May 2021, p. 12), examined
images that David Jewitt, Scott
Sheppard (both then at University
of Hawaii), and their colleagues had
recorded almost two decades ago
with the 3.6-meter Canada-FranceHawaii
Telescope (CFHT). In 2003,
that group announced the discovery
of 23 tiny Jovian moons, and the
images they used remained available
online to the public.
Recently, Ly found another potential
moon in images from the past
survey, one so faint that it's probably
just 1 km across. That led to its
recovery in additional survey images
obtained throughout the spring
of 2003. " From there on, the orbit
and ephemeris quality were decent
enough for me to search observations
beyond 2003, " Ly explains.
Ultimately, Ly tracked the moon -
provisionally designated S/2003 J 24
- through 76 observations taken
over 15.26 years (5,575 days). That's
the equivalent of nearly eight orbits,
says David Tholen (University of
Hawai'i), more than enough to show
it's bound to Jupiter and to consider
the trajectory well-secured for
decades. Ly reported the orbit they
determined to the Minor Planet
Mailing List on June 30th.
The moon is a typical member
of the Carme group, which includes
22 other small retrograde satellites
with periods of around two years.
They are probably all fragments of
Carme, the first of the group to be
discovered and by far the largest.
¢JEFF HECHT
CHRISTINE PULLIAM / CFA

Sky & Telescope - November 2021

Table of Contents for the Digital Edition of Sky & Telescope - November 2021

Contents
Sky & Telescope - November 2021 - Cover1
Sky & Telescope - November 2021 - Cover2
Sky & Telescope - November 2021 - 1
Sky & Telescope - November 2021 - Contents
Sky & Telescope - November 2021 - 3
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