Scientific American Space & Physics 2020 08-09.pdf

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The Sun,
Neutrinos detected emanating
from our star’s core confirm
decades-old predictions about
what fuels its fusion
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The Scientific Question Machine
The title of this issue is a bit misleading. A fully explanatory and complete title would have gone something like:
“Scientists Confirm Long-Standing Theory of Sun’s Power, but As with All Science, Many Questions Remain and
New Ones Are Revealed.” Exhaustive, yes. Catchy? No. Though when it comes to attention-grabbing-if-slightly-truncat-
ed headlines, this one still holds water. As reporter Davide Castelvecchi reports, astrophysicists have long hypothe-
sized that a small amount of the sun’s energy is generated by a particular reaction involving carbon and nitrogen in the
star’s core, and can be detected by neutrino emissions (see “Neutrinos Reveal Final Secret of Sun’s Nuclear Fusion”).
It’s always extremely satisfying when a scientific explanation is finally confirmed by direct evidence. In this case, the way
that evidence was collected is fascinating, as are some of the further questions relevant to this research: What are the
precise composition and temperature of the sun? What was our star like before the rest of the solar system formed?
In science, it often goes that as soon as you’ve answered one question, you inadvertently have asked a dozen more.
Call that frustrating or intriguing as you will.
Conflicting evidence about the weight of the cosmos is fueling a growing debate among physicists over the forma-
tion of the universe (see “How Heavy Is the Universe? Conflicting Answers Hint at New Physics”). And the surprise
detection of radio bursts from within our own galaxy may help us resolve a larger cosmological phenomenon (see
“‘Magnetic Star’ Radio Waves Could Solve the Mystery of Fast Radio Bursts”). But, you guessed it, this discovery is
inciting a host of new questions waiting to be answered. I, for one, am intrigued.
Andrea Gawrylewski
Senior Editor, Collections
On the Cover
Our sun, as seen by
NASA’s Solar and
Heliospheric Observatory
August-September 2020
Volume 3
No. 4
Did Galileo
Truly Say,
“And Yet It Moves”?
A Modern
Detective Story
An astrophysicist traces
genealogy and art history
to discover the origin
of the famous motto
The Quantum App
Store Is Coming
Quantum computing
is still the province of
specialized program-
mers—but that is likely
to change very quickly
The World
Doesn’t Need
a New Gigantic
Particle Collider
It would cost many
billions of dollars, the
potential rewards are
unclear—and the money
could be better spent
researching threats such
as climate change and
emerging viruses
Missing Memories
of the Universe
With observatories shut
down because of the
pandemic, the photons
that reveal the secrets
of the cosmos can’t be
recorded or decoded
Planet Nine
Could Be a Mirage
Mysterious patterns in
orbits of small bodies
in the outer solar system
could arise from the
gravity of a massive
disk of icy debris rather
than an undiscovered
giant world
Pi in the Sky:
General Relativity
Passes the
Ratio’s Test
Using gravitational
waves to approximate pi,
physicists see
no problem with
Einstein’s theory
Astronomers May
Have Found the
Closest Black Hole
to Earth
At just 1,000 light-years
away, an object in a
nearby star system could
be our nearest known
black hole—but not
everyone is convinced
Direct Proof
of Dark Matter
May Lurk
at Low-Energy
Mysterious effects
in a new generation
of dark matter
detectors could herald
a revolutionary discovery
Get Earliest Ever
Glimpse of Ancient
Giant Galaxy
The disk of gas and
stars resembles our
own Milky Way but
somehow formed when
the universe was only
about 10 percent
of its current age
A Hydrogen
Iceberg from a Failed
Star Might Have
Passed through
Our Solar System
The interstellar visitor
‘Oumuamua, discovered
in 2017, may represent
an entirely new type
of astrophysical object,
two astronomers say
The First
Footprints on Mars
Could Belong to
This Geologist
astronaut Jessica
Watkins is at the forefront
of a new crop of space
explorers destined for
the moon and maybe
one day Mars
Neutrinos Reveal Final Secret
of Sun’s Nuclear Fusion
The detection of particles produced in the sun’s
core supports long-held theory about how our star
is powered
How Heavy Is the Universe? Conflicting
Answers Hint at New Physics
The discrepancy could be a statistical fluke—
or a sign that physicists will need to revise the
standard model of cosmology
“Magnetic Star” Radio Waves Could
Solve the Mystery of Fast Radio Bursts
The surprise detection of a radio burst from
a neutron star in our galaxy might reveal the origin
of a bigger cosmological phenomenon
Physicists Criticize Stephen Wolfram’s
“Theory of Everything”
The iconoclastic researcher and entrepreneur
wants more attention for his big ideas. But so far
researchers are less than receptive
Planet Nine Could
Be a Mirage
Mysterious patterns in orbits of
small bodies in the outer solar
system could arise from the gravity
of a massive disk of icy debris rather
than an undiscovered giant world
Some four years ago, when Ann-
Marie Madigan first encountered
the idea that there might be an
undetected massive planet lurking
beyond Pluto’s orbit, she felt excited
but skeptical. The evidence for such
a world was then—and remains—cir-
cumstantial: strange patterns in the
orbits of small objects at the outskirts
of the known solar system. Propo-
nents of “Planet Nine” (Pluto no
longer counts in the solar system’s
planetary tally) say such patterns
could be produced by that world’s
hefty gravitational influence. But
Madigan, an astrophysicist now at
the University of Colorado Boulder,
wondered whether some other, more
prosaic explanation could suffice. At
Artist’s illustration of a small icy object at the
outskirts of our solar system. In sufficient
numbers, such objects could explain mysterious
orbital patterns otherwise attributed to an
undiscovered world far from the sun.
the time, she was studying how stars
can jostle one another into different
orbits as they whirl around super-
massive black holes. And she saw no
reason why her work could not also
apply to tinier things orbiting our sun.
Today, from those modest begin-
nings, Madigan and a few of her
collaborators have developed a
totally different theory to explain the
strangeness in the outer solar
system: the “collective gravity” of a
diffuse, sprawling (and so far largely
hypothetical) disk of icy debris far be-
yond Pluto could alter the orbits of
the far distant objects we readily see
in a way that resembles the effect of
a large planet. Such a disk would be
composed of millions of small bodies,
most of them left over from the solar
system’s formation long ago.
“What we’re doing is taking the
gravitational forces between all
these small bodies into account,”
Madigan says. “Including those
gravitational forces turns out to be
really important.” Provided the
putative disk possessed sufficient
mass—several times that of Earth—
over a billion years or so, the tiny
gravitational interactions between
and from its constituent members
could sculpt the trans-Plutonian
outer solar system in ways otherwise
explained by Planet Nine, she
maintains. The effect would be a bit
like the proverbial butterfly flapping
its wings to eventually set a distant
storm in motion.
Madigan and her graduate student
Alexander Zderic have now advanced
their theory in two new studies posted
on the preprint server In
one, submitted to the
they show how collective
gravity can produce the same kinds
of tilted and clumped orbits seen in
about a dozen objects at a distance
250 times that between Earth and
the sun—an observation others had
attributed to a possible Planet Nine.
In the other paper, under review at
Astrophysical Journal Letters,
argue that given enough time,
collective gravity can also explain how
certain objects in far-out orbits can
shift as they twirl around the sun,
which had been taken as evidence for
an unseen planet as well.
From this work by Madigan and her
team, an alternative picture of the
solar system’s plausible history is
beginning to emerge. In the early
days, Jupiter, Saturn, Uranus and
Neptune coalesced in compact,
orderly orbits somewhat closer to our
star, which migrated outward later be-
cause of gravitational interactions.
Back then those worlds were sur-
rounded by a swarm of leftover
chunks of debris that never found
their way to planethood—icy bodies
that the giant planets eventually
kicked outward. Most were left
ostracized in what Madigan calls a
“primordial scattered disk” beyond the
territory of present-day Pluto. And
she suggests that there may be much
more mass in the disk than other
researchers have usually considered.
The icy bodies were propelled into
that ring with orbits far from circular,
making up an unstable system—
much like a wobbly, precariously
spinning top. This system exerted
gravitational effects while it gradually
settled into a more stable configura-
tion, with some orbits sharing similar
planes and orientations. That configu-
ration would, of course, essentially
mirror what one would expect from
the hidden gravitational hand of an
undiscovered large outer planet.
“The fact that collective gravity can
give you all the key observational
features means that you don’t need
anything new. I think Occam’s razor
would lead you to believe that it’s the
simpler solution” than Planet Nine,
Madigan says.
California Institute of Technology
astrophysicists Mike Brown and
Konstantin Batygin have been two
of the foremost proponents of the
Planet Nine hypothesis since they
released a sensational study on the
subject in early 2016, and they have
also been honing their arguments
for the world’s existence. To match
the latest observations, the research-
ers argue that Planet Nine’s mass
must be between five and 10 times
that of Earth and that its distance
from the sun must be between 400
and 800 times that of our planet—
slightly smaller and closer than what
they first proposed.
Batygin says he is intrigued by
Madigan’s idea of a remote ring of
debris. But he thinks that is not what
the solar system looks like. “If there
were such a ring parked far away
[from our sun], you run into the
problem of its stability in the early
solar system, since the solar system
formed in a cluster of stars,” he says.
“Perturbations from passing stars will
mess up this ring. They’re going to
destroy its coherence and disperse it.”
Madigan resolves this problem in
her new simulations with careful
tweaks to timing: if the scattered
disk assembled after the young solar
system left its stellar nursery and the
giant planets formed, it could endure
for eons. Such tweaks are not trivial:
accurately modeling the collective
gravity of a debris disk requires
tracking the motions and interactions
of thousands of particles or more
floating and whipping around in
computer models for the equivalent
of hundreds of millions of years.
That task is far more arduous than
modeling the effects of a single
planet, Madigan says—which is, in
part, why she and her team so often
seem to be one step behind the
pro–Planet Nine contingent.
To date, Madigan’s idea has not
gotten much attention in the scientif-
ic community compared with Planet
Nine. But as telescopic searches
for the planet continue to come up
empty, that situation may be about to
change. “We’re in a minority, but
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