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3818 - ASTEROID 'Oumuamua - how unusual is it? Five years after spotting
asteroid Oumuamua, the first known object from beyond our solar system passing
through, scientists are still figuring out what this strange object says about
planetary systems.
--------- 3818 - ASTEROID 'Oumuamua - how unusual is it?
- Marauding ice giant planets like Neptune
could be flinging many trillions of small bodies into interstellar space, some
of which visit our solar system, as 'Oumuamua notably did in 2017. The population of such rogue objects moving
between the stars could be in the hundreds of trillions of trillions, that's a
digit followed by some 26 zeroes.
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- 'Oumuamua was discovered on October 19,
2017, having arrived from interstellar space, where it is headed once more
after swinging through our solar system. The existence of small bodies visiting
from interstellar space wasn't a surprise. In fact, interstellar interlopers
such as 'Oumuamua and Borisov, the only two discovered so far, had been
predicted long before.
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- We know that when the solar system was
forming, several dozen Earth masses' worth of small, icy bodies would have been
ejected into the interstellar medium. So
if you take our solar system as a representative example, then you would expect
to have quite a bit of stuff drifting through interstellar space.
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- The mechanism that ejects these myriad
small bodies is the result of planetary migration, in particular the rampage of
giant planets. In 2005 astronomers proposed the "Nice model," so
named because the astronomers who developed it worked at the Observatoire de la
Côte d'Azur in Nice, France.
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- The “Nice model” depicts how interactions
within a rich disk of asteroids and comets prompted Saturn, Uranus and Neptune
to migrate outward and Jupiter to migrate inward slightly over hundreds of
millions of years.
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- The Nice model has since fallen out of
favor somewhat, to be replaced by similar alternatives such as the "Grand
Tack" model, which describes how Jupiter initially moved inward, only for
Saturn's gravity to stop it and pull it back.
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- Any model that has any sort of movement of
giant planets as they are forming amidst a large sea of planetesimals is going
to produce interstellar objects.
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- The term "throw line" as a
description of where such ejections can take place. The 'throw line' is just a
riff on the term 'snow line, referencing the distance from a star where water
is more stable as ice than as vapor.
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- The throw line, in turn, is located where a
giant planet is able to slingshot a small body with enough acceleration to
achieve escape velocity from the gravitational pull from its star. The farther
out the planet is, the easier this becomes because the star's gravity decreases
with radial distance.
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- In our solar system the throw line is at
about 372 million miles from the sun, which is about the same distance as the
snow line. All four of the gas giants
in our neighborhood, Jupiter, Saturn, Uranus and Neptune, are beyond the throw
line, and all could have ejected bodies into interstellar space.
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- As the most distant planet that orbits in a
region where the escape velocity is low there are plenty of icy bodies to throw
around. Neptune would have acted as
the solar system's bouncer as the planet migrated outward, ejecting many of the
small bodies that got in its way.
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- If 'Oumuamua is typical, then that's
suggesting that the average star has a Neptune-like planet, just like our solar
system. Planet-forming disks of dust
around young stars appear to have ring-shaped gaps in them that may have been
cleared out by the gravity of Neptune-like worlds.
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- Many gas giant exoplanets discovered so far
are so-called "hot Jupiters" and "hot Neptunes," which have
migrated inward and now orbit very close to their stars. These worlds cannot
eject small bodies into interstellar space because the escape velocity that
close to their star is too great.
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- These systems with hot giant planets are very
unlike our own solar system, whose innermost worlds are small, rocky and
comparatively far from the sun.
However, the predicted abundance of interstellar objects implies that
the architecture of our outer solar system, at least, may be fairly regular.
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- This ejection mechanism would also explain
conventional interstellar comets such as Borisov. 'Oumuamua was anything but conventional. Its
shape was most likely that of a flattened, disk-like sliver, rather than that
of a long shard as was initially suggested. We have seen a somewhat
similarly-shaped body in the form of Arrokoth, the Kuiper Belt object that
NASA's New Horizons spacecraft flew past on New Year's Day 2019.
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- However, most comets are not shaped like
'Oumuamua or Arrokoth. In addition, 'Oumuamua didn't have a comet's signature
coma, the "atmosphere" around the comet's main body. Its acceleration changed as though it were
being pushed by outgassing that was typical of a comet, even though astronomers
couldn't detect any outgassing.
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- 'Oumuamua colud be a chunk of solid
hydrogen ice. The only location where such an object could form would be in the
cold core of a dense molecular cloud of gas. Such clouds, once they are
gravitationally destabilized, become the birthplaces of stars, but are they
cold enough to form a chunk of solid hydrogen like 'Oumuamua?
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- If the hydrogen-ice theory were true, then
all of 'Oumuamua's properties would be explained. The theory suggests that
'Oumuamua would have formed inside a molecular cloud as a much larger object
that became whittled down over time.
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- The problem with this theory is that it's
very hard to get the environment cold enough so that molecular hydrogen freezes
out quickly. Molecular hydrogen freezes
at about 14 kelvin. That's 14 degrees
above absolute zero, or minus 434 degrees Fahrenheit (minus 259 degrees
Celsius).
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- The cores of molecular clouds can reach
similar temperatures, but the conditions would have to be just right for the
hydrogen to condense quickly into a solid, and it’s not clear how regularly
those conditions occur. However, if they are common, then "'Oumuamua would
have been something that was assembled before star and planet formation in its
cloud took place.
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- One piece of supporting evidence for this
lies in 'Oumuamua's path through space before it arrived at our solar system.
Astronomers have traced it back and found that, 45 million years ago, 'Oumuamua
would have been in the same spot where a giant molecular cloud would have been
about to form the stars of the Carina moving group.
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- If 'Oumuamua were a hydrogen iceberg, or
even if it was just some freak of nature ejected from a planetary system like
Borisov was, then surely space should be filled with more of these visitors
from far-off stars. Do astronomers find it surprising that besides 'Oumuamua
and Borisov, we have yet to discover any other interstellar interlopers?
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- The Vera C. Rubin Observatory in Chile will
begin observing by the middle of this decade. With its 8.4-meter, wide-field
survey telescope, it will embark on the Legacy Survey of Space and Time (LSST)
and, if predictions hold true, it is expected to discover at least one
interstellar interloper every year.
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- If objects like 'Oumuamua are discovered in
short order by Rubin–LSST, then that's pointing to a large population of
Neptune-like planets. But if it finds no
such objects, then the degree to which 'Oumuamua was unusual will become more
and more pronounced.
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January 10, 2022 ASTEROID
'Oumuamua - how unusual isit? 3818
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--------------------- --- Tuesday, January 10, 2023 ---------------------------
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