Tuesday, January 10, 2023

3818 - ASTEROID 'Oumuamua - how unusual is it?

 

     -   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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