The
farthest reaches of our solar system remain the most mysterious areas around
the sun. Solving the mysteries of the outer solar system could shed light on
how the whole thing emerged — as well as how life on Earth was born.
Why the rainbow of colors in the Kuiper belt?
For
instance, the Kuiper belt past Neptune is currently the suspected home
of comets that only take a few decades or at most centuries to complete
their solar orbits — so-called "short-period comets." Surprisingly, Kuiper
belt objects "show a wide range of colors — neutral or even slightly blue
all the way to very red," said University of Hawaii astrophysicist David Jewitt.
The
color of an object helps reveal details about its surface composition. It
remains a mystery why Kuiper belt objects show a much wider range of color — and
thus surface composition — than other planetoids, such as the asteroids.
Some
researchers had suggested volcanic activity could have led to all these
colors — "absurd in the context of 100-kilometer-sized (60-mile) bodies,"
Jewitt said, as volcanism needs something bigger.
Jewitt
and his colleagues had suggested that cosmic rays could have made Kuiper belt
objects redder, while impacts with rocks could have dug up more pristine matter
that made them less red. Nowadays Jewitt thinks there must be another
explanation for this rainbow, but it remains unknown.
What is ultra-red matter?
There
appears to be a material dubbed "ultra-red matter" that exists only
on about half of all Kuiper belt objects and their immediate progeny, known as
centaurs — icy planetoids orbiting between Jupiter and Neptune that very recently
escaped from the Kuiper belt.
This
ultra-red matter does not exist in the inner solar system, "not even on
the comets which come from the Kuiper belt. This suggests that the ultra-red
matter is somehow unstable at the higher temperatures close to the sun," Jewitt
explained.
The
red colors suggest this substance might contain organic molecules. Comets and
other planetoids are often thought to have helped bring
organic molecules to Earth.
"In
the Kuiper belt objects, organics might have been 'cooked' by cosmic ray
radiation, giving them dark red surfaces, but there is no proof," Jewitt
said. Ideally spacecraft could go out there and find out, he added.
Has the Kuiper belt shrunk?
Theoretical
calculations suggest the Kuiper belt was once hundreds or maybe even thousands
of times more populated than it is now. "How was 99 percent or 99.9
percent of the mass lost, and when?" Jewitt asked.
One
conjecture suggests when Saturn and Jupiter shifted their orbits roughly 4
billion years ago, their gravitational pulls slung Kuiper belt objects out of
the solar system. Another says the Kuiper belt objects pulverized themselves to
dust, which then was swept away by the sun's radiation. Yet another possibility
"is that we are missing something crucial and the conclusion that the belt
is heavily depleted is wrong," Jewitt said. "All these possibilities
are comparably hard to swallow, but would each be amazing, if true."
Secrets in the Oort cloud?
A distant
reservoir of trillions of comets known as the Oort cloud theoretically lies up
to 100,000 astronomical units from the sun — an
astronomical unit or AU being about 93 million miles (150 million kilometers).
This means the Oort cloud is a fifth of the way to the nearest star, so
far away that objects within it have never been seen directly, only
inferred — but it must exist, given all the comets seen over the years.
The Oort
cloud is the conjectured source of comets that require centuries or millennia
to complete their long journeys around the sun. Since these "long-period
comets" come from all directions, the Oort cloud is often thought to be
spherical. However, while comets such as Halley's do not come from the Kuiper
belt, their orbits also do not jibe with a spherical Oort cloud, Jewitt
explained. This suggests there may be an "inner Oort cloud" shaped
kind of like a doughnut.
Astrophysicists
think the Oort cloud is a remnant of the protoplanetary disk that formed around
the sun roughly 4.6 billion years ago. Learning more about the Oort cloud could
shed light on how our solar system — and Earth — were born, Jewitt said.
Are
there more dwarf planets?
So far,
three dwarf planets are recognized — Ceres, Pluto and Eris. The Kuiper belt,
which lies about 50 AU from the sun, could hold some 200 more. Beyond that
there could be scores of dwarf-planet-sized bodies
beyond roughly 100 AU from the sun "that nobody had seen before due to
their faintness and slow motion," said astronomer Chad Trujillo at Gemini
Observatory in Hawaii. "Even a body as big as Mars could be missed in our
current surveys if it were moved beyond a couple hundred AU."
Trujillo noted projects such as
Pan-STARRS (Panoramic Survey Telescope And Rapid Response System) and the LSST
(Large Synoptic Survey Telescope) "should fill this gap in our knowledge
in the coming decade."
Where do the dwarf planets come from?
There
are theories that the dwarf planets of the outer solar system may have dwelt in
the inner solar system billions of years ago, based on their current orbital
trajectories. If so, "why are there so many ices on their surfaces?" Trujillo asked. Bodies in the
inner solar system are generally expected to lose their ice due to sunlight.
Trujillo and his colleagues
suspect the ice now seen on these dwarf planets is relatively new, with such
replacement ice coming perhaps from within these worlds, erupting out during
"cryovolcanism." Of course, further research is needed to see if such
ice renewal would be enough to cover the dwarf planet after they voyaged from
the inner to the outer solar system, he added.
Do cosmic rays come from a bubble around the solar system?
When
the supersonic wind of charged particles that flows from our sun collides with
the thin gas found
between the stars, the solar wind essentially blows a bubble in this interstellar
medium — a ball known as the heliosphere.
Scientists
have thought unusually weak cosmic rays — energetic particles that zip from
space at Earth — come from the heliosphere. Specifically, these rays are thought to come
from the "termination
shock" — a shock wave of compressed, hot particles that results when the
solar wind abruptly brakes against interstellar gas. (The termination shock
appears to be about 75 to 85 AU from the sun.)
However,
Voyager 1 saw no sign these anomalous cosmic rays were produced at the
termination shock. "Perhaps it crossed the shock at the wrong time or
place," said
MIT astrophysicist John Richardson, or perhaps the standard view on how these
anomalous cosmic rays are generated is wrong. Voyager 2 crossed the termination
shock in 2007 about 10 billion miles away from where Voyager 1 crossed it in
2004, and its data, which is still being analyzed, "may help us understand
where these particle are produced," he explained.
"Cosmic
rays have been reported to affect Earth's weather so understanding their source
is important," Richardson added. Moreover, high-energy particles from shock waves triggered
by huge eruptions from the sun known as coronal mass ejections can damage
spacecraft and astronauts, and better understanding the termination shock could
help understand these other, potentially dangerous particles.
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