NEWS NOTES
10 NOVEMBER 2019 • SKY & TELESCOPE
NASA’S LATEST exoplanet hunter, the
Transiting Exoplanet Survey Satellite
(TESS), has found more than two dozen
new worlds that have been confi rmed in
follow-up observations. Scientists shared
their fi ndings late July at the mission’s
fi rst science conference since the space-
craft’s launch in April 2018.
TESS’s goal is to fi nd planets around
bright stars within 200 light-years of
Earth. The spacecraft has scanned all
13 sectors of the Southern Hemisphere
sky during its fi rst year of operations.
So far, TESS scientists have identi-
fi ed 993 planet candidates in 12 of the
13 southern sectors; 271 of these are
smaller than Neptune. A total of 28 of
these exoplanets have been confi rmed as
real via follow-up observations.EXOPLANETS
TESS Finds Hundreds
of Planet Candidates in
Southern SkyCOSMOLOGY
Hubble Constant Tension ContinuesOne of the newest systems, anchored
by a small, M dwarf star called GJ 357,
hosts three exoplanets. TESS found the
innermost world, then astronomers dis-
covered two more planets with ground-
based observations of the star’s wobble
in response to its orbiting exoplanets.
The system lies 31 light-years away.
The team reports the trio in the August
issue of Astronomy & Astrophysics.
The outermost planet, GJ 357d, is at
least six times Earth’s mass. Technically,
it orbits on the outer edge of the star’s
habitable zone, but unless it has a thick
atmosphere to trap heat, its surface
temperature is likely well below zero.
Lisa Kaltenegger (Cornell University)
is particularly excited about GJ 357d
because the planet should also be rela-
tively bright in refl ected light, which
means near-future telescopes could
characterize its atmosphere.
Besides cataloging exoplanets, TESS
is also catching supernovae, asteroids,
and other transient events, and evenpThe outermost planet of the GJ 357 system
orbits within the star’s habitable zone, so if it
has a dense enough atmosphere and a surface,
liquid water might exist there.tMeasurements of the
universe’s current expan-
sion rate differ depending
on whether cosmologists
assess its value based on
measurements of the cos-
mic microwave background
(black), Cepheid variable
stars (blue), or a special
class of red giant stars (red).A NEW STUDY of a special class of red
giant stars fuels a dispute about the rate
at which today’s universe is expanding.
The universe’s expansion rate has
changed throughout time. However,
the crux of an ongoing debate is that
observations of the early universe
predict a current expansion rate that
doesn’t match measurements of galaxy
recession speeds (S&T: June 2019, p. 22).
Observations of the cosmic microwave
background pin today’s expansion rate,also called the Hubble constant (H 0 ),
between 66.9 and 67.9 km s-1 Mpc-1.
However, measurements of galaxies’
recession speeds — based on so-called
standard candles such as Cepheid vari-
able stars — fi nd H 0 to be signifi cantly
higher, between 73 and 76 km s-1 Mpc-1.
Now, Wendy Freedman (Univer-
sity of Chicago) and colleagues have
employed red giant stars — specifi cally,
those that have just made the transi-
tion from burning hydrogen to igniting
helium — to gauge the
distances to galaxies.
Their study, which willYear of publicationHubble Constant Over TimeCepheids
TRGB
CMBH(^0)
[k
m
s
-^1
Mpc-^1
]2000 2004 2008 2012 2016 2020 20246065707580providing evidence of exocomets (S&T:
Sept. 2019, p. 12).
The TESS mission is set to go at least
through 2022. The team will map the
northern sky over the next year before
moving to an extended observing phase.
By the end of the extended mission,
TESS will have surveyed 94% of the sky.
Among other goals, the extra observa-
tions are expected to net long-period
planets and multiple-planet systems.
■ DIANA HANNIKAINENappear in the Astrophysical Journal,
reports a Hubble constant between 67.
and 71.7 km s-1 Mpc-1.
Taken on its own, this result agrees
both with measurements of the cosmic
microwave background and with nearby
standard candles. However, as Freed-
man and colleagues point out in their
paper, the near-far discrepancy remains.
Statistically speaking, if CMB measure-
ments represented the true value of the
Hubble constant, then you’d expect at
least a couple of measurements to fall
below that number; however, the red
giant measurements still give a Hubble
constant that’s higher compared to
studies of the early universe.
Ultimately, current data won’t settle
the debate. Freedman’s team is looking
to the European Space Agency’s Gaia
mission, which will enable astronomers
to estimate the Hubble constant to bet-
ter than 1% precision within the next
few years.
■ MONICA YOUNG- Read more details at https://is.gd/
HubbleConstant.
Habitable zoneGJ 357d GJ 357c GJ 357bGJ537 SYSTE
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