Sky & Telescope - November 2019 - 10

NEWS NOTES
GJ 357d

TESS Finds Hundreds
of Planet Candidates in
Southern Sky
NASA'S LATEST exoplanet hunter, the

Transiting Exoplanet Survey Satellite
(TESS), has found more than two dozen
new worlds that have been conﬁrmed in
follow-up observations. Scientists shared
their ﬁndings late July at the mission's
ﬁrst science conference since the spacecraft's launch in April 2018.
TESS's goal is to ﬁnd planets around
bright stars within 200 light-years of
Earth. The spacecraft has scanned all
13 sectors of the Southern Hemisphere
sky during its ﬁrst year of operations.
So far, TESS scientists have identiﬁed 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 conﬁrmed as
real via follow-up observations.

One of the newest systems, anchored
by a small, M dwarf star called GJ 357,
hosts three exoplanets. TESS found the
innermost world, then astronomers discovered two more planets with groundbased 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 relatively bright in reﬂected light, which
means near-future telescopes could
characterize its atmosphere.
Besides cataloging exoplanets, TESS
is also catching supernovae, asteroids,
and other transient events, and even

COSMOLOGY

Hubble Constant Tension Continues
also called the Hubble constant (H0),
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 variable stars - ﬁnd H0 to be signiﬁcantly
higher, between 73 and 76 km s-1 Mpc-1.
Now, Wendy Freedman (University of Chicago) and colleagues have
employed red giant stars - speciﬁcally,
those that have just made the transition from burning hydrogen to igniting
helium - to gauge the
distances to galaxies.
Hubble Constant Over Time
Their study, which will

A NEW STUDY of a special class of red

H0 [km s-1 Mpc-1]

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,

80
75

Cepheids
TRGB

CMB

65
60

2000

2004

2008

2012

2016

2020

Year of publication

N OV E M B E R 2 019 * SK Y & TELESCOPE

2024

t Measurements of the
universe's current expansion rate differ depending
on whether cosmologists
assess its value based on
measurements of the cosmic microwave background
(black), Cepheid variable
stars (blue), or a special
class of red giant stars (red).

GJ 357b

Habitable zone

p The 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.

providing 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 observations are expected to net long-period
planets and multiple-planet systems.
■ DIANA HANNIKAINEN

appear in the Astrophysical Journal,
reports a Hubble constant between 67.9
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 Freedman and colleagues point out in their
paper, the near-far discrepancy remains.
Statistically speaking, if CMB measurements 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 better than 1% precision within the next
few years.
■ MONICA YOUNG

* Read more details at https://is.gd/
HubbleConstant.

GJ 537 SYSTEM: N ASA GSFC / CHRIS SMITH;
HUBBLE CONSTA N T: W. FREEDM A N E T A L. / A R XIV.ORG

EXOPLANETS

GJ 357c

http://www.ARXIV.ORG https://is.gd/HubbleConstant https://is.gd/HubbleConstant

Sky & Telescope - November 2019

Table of Contents for the Digital Edition of Sky & Telescope - November 2019