ASTRONEWS
12 ASTRONOMY • APRIL 2018
FREE FALLING. A French satellite experiment confirmed that the equivalence principle — a key tenet of Albert Einstein’s
general theory of relativity that states all objects fall at the same rate, despite any differences in mass — holds true in space.
As the catalog of known exoplanets grows,
astronomers continue to ask: Could any of
these planets host life? While many habit-
ability studies focus on planets capable of
hosting liquid water, some researchers are
now arguing that “water worlds” may not be
the best place to find extraterrestrial life.
During a five-day workshop in Laramie,
Wyoming, on habitable worlds last
November, some of the world’s foremost
planet hunters met to evaluate which exo-
planets are the best candidates for life. They
concluded that water alone is not the decid-
ing factor in whether life will arise, and
should not be the sole target in the search
for habitable worlds.
That’s because many other factors come
into play when creating an environment
suitable for Earth-like life. The presence of
geological activity and of vital nutrients,
such as phosphorus, are also necessary for
life to thrive on Earth. An exoplanet
sheathed in oceans with no phosphorus-
rich dry land and no geothermal activity to
heat the water and dredge up additional
nutrients is not a good place to look for life.
Furthermore, the type of star a world cir-
cles and the distance between the star and
the world can affect the chemical makeup
of the planet’s atmosphere, helping or hin-
dering the case for habitability. Winds and
storms from a planet’s host star can erode
the atmosphere if the star is too active or if
the planet’s magnetic field is too weak.
While exoplanets rich in water are likely
common, it is planets with the right mix of
water and land, geothermal activity, and
atmospheric chemistry that astronomers now
believe should be the targets in the search for
life. Narrowing down these targets will pre-
vent wasted resources when instruments like
the James Webb Space Telescope come
online, boosting the chances of finding real
signs of life in the coming decades. — A.K.
Rethinking water’s
role in search for life
In the early universe, black holes 100,000 or
more times the mass of the Sun formed fast
and often. But how they formed, especially
so quickly, remains a question. The recent
discovery of a behemoth black hole just
690 million years after the Big Bang may
help astronomers answer it.
J1342+0928 is a quasar — a big, bright
disk of infalling material — around a black
hole already containing 800 million solar
masses that formed when the universe was
just 5 percent its current age. At 13.1 billion
light-years away, it is the new record-holder
for the most distant supermassive black hole.
The quasar was found in data from three
large surveys: the DECam Legacy Survey,
NASA’s Wide-field Infrared Survey
Explorer, and the United Kingdom Infrared
Telescope Deep Sky Survey (UKIDSS) Large
Area Survey. The discovery was published
December 6 in Nature, and details about its
host galaxy appeared in The Astrophysical
Journal Letters.
The number and size of quasars in the
early universe depend on its conditions.
Current models of supermassive black hole
growth predict somewhere between 20 and
100 bright quasars as distant as J1342+
in the entire sky. And J1342+0928 is just the
second (and farthest) quasar in this group
ever found, now doubling the objects avail-
able to study at a unique time in the uni-
verse’s history: the epoch of reionization.
Immediately after the Big Bang, the uni-
verse was smaller, hotter, and denser. As it
expanded, it cooled; after 400,000 years,
it had cooled enough for particles to com-
bine and form neutral hydrogen atoms.
But nothing else was created until gravity
finally caused denser regions of hydrogen
to collapse and coalesce into stars and,
later, galaxies.
These stars and galaxies began shining,
putting out light — photons. Their photons
were capable of knocking electrons off the
neutral hydrogen gas permeating the uni-
verse, ionizing it into the state in which it
exists today. This is called the epoch of reion-
ization, and “it was the universe’s last major
transition and one of the current frontiers of
astrophysics,” said Carnegie astronomer
Eduardo Bañados, leader of the team that
discovered J1342+0928, in a press release.
Astronomers know that J1342+0928 is
shining during this time period in cosmic
history because the majority of the material
surrounding its host galaxy is neutral; it
hasn’t been ionized yet, suggesting that
J1342+0928 is one of the many ionizing
sources at work changing the universe
during this time. That makes this quasar
extremely valuable, and literally a light in
the dark, guiding astronomers to a better
understanding of what the universe was like
less than 700 million years after its birth.
— Alison Klesman
FARTHEST SUPERMASSIVE BLACK HOLE
LIES 13 BILLION LIGHT-YEARS AWAY
10.
kilometers
JUST RIGHT? Ross 128b is a recently discovered
exoplanet with a good chance of harboring liquid
water. But is water enough to sustain Earth-like life
on its surface? M. KORNMESSER/ESO
BABY PICTURE. J1342+0928 is a quasar: a supermassive black hole with a disk of material shining when
the universe was 690 million years old. Its light played a role in transforming the universe into the state in which
it remains today. ROBIN DIENEL, COURTESY OF THE CARNEGIE INSTITUTION FOR SCIENCE
The minimum possible radius of a nonrotating
neutron star with a mass of 1.6 solar masses,
according to gravitational wave measurements.