C/2014 S3 (PANSTARRS): The Return Of The Native

Banished from its warm and well-lit birthplace in the inner Solar System, only to wander through the wilderness of the most frigid, dark, and desolate outer limits, a sad little refugee has come home at last. C/2014 S3 (PANSTARRS), a primordial building-block of the rocky inner planets, was likely born in the toasty, bright region around our young Sun at the same time as the Earth itself–but was heartlessly ejected very early in the planet-birthing process. For billions of years, this rejected and ejected little object lived among strangers–the icy comet nuclei inhabiting a thick bubble of frozen debris that is thought to encircle our entire Solar System. This still-hypothetical shell of dancing icy objects is called the Oort Cloud, and it may extend a third of the way from our own Star to the next star–between 5,000 and 100,000 astronomical units (AU). One AU is the average distance between Earth and Sun, which is 93,000,000 miles. In April 2016, a team of astronomers announced the happy ending to this sad story–C/2014 S3 (PANSTARRS) has come in from the cold!

C/2014 S3 (PANSTARRS) is considered to be unique. It appears to have been born in the inner Solar System from the same material that Earth and the other rocky terrestrial planets formed– but was booted out–far, far out! Originally discovered by the University of Hawaii’s Pan-STARRS1 telescope, C/2014 S3 (PANSTARRS) is a weakly active comet that is a bit more than twice as far from our Sun as the Earth. The object’s current long orbital period, of approximately 860 years, indicates that its origin is within the Oort Cloud–and it was nudged out relatively recently into an orbit that carries it closer to our Star.

Additional follow-up observations with the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT)and the Canada France Hawaii Telescope suggest that C/2014 S3 (PANSTARRS) is the very first object to be discovered on a long-period cometary orbit that has the attributes of a pristine, primeval inner Solar System asteroid. Therefore, this small wanderer may provide a treasure trove of precious clues about how our Solar System came to be.

Both comets and asteroids are the relics of a vast population of primordial objects that built-up the major planets of our Solar System, termed planetesimals. The asteroids, that today are found primarily in the Main Asteroid Belt between Mars and Jupiter, are akin to the rocky and metallic bodies that built up the quartet of inner terrestrial planets: Mercury, Venus, Earth, and Mars. Alternatively, the comets–that hail from the more distant Kuiper Belt, Scattered Disc, and Oort Cloud–are the left-over building blocks of the population that constructed the four outer, gaseous giant planets: Jupiter, Saturn, Uranus, and Neptune. The Kuiper Belt and Scattered Disc are much closer to the inner Solar System than the Oort Cloud, and they are the source of short-period comets that come shrieking into the inner Solar System less than every hundred years. Long-period comets are refugees from the very remote Oort Cloud, and invade the domain of the terrestrial planets every one hundred years–or more.

At the current speed of NASA’s Voyager 1 spacecraft, that is zipping towards our Sun’s outermost region of influence, it will not reach the Oort Cloud for about three centuries–and it will require about 30,000 years to reach the other side.

The Oort Cloud

The Dutch astronomer Jan Oort (1900-1992) first proposed the idea of this distant region of space, named in his honor, in order to explain the origins of comets that take thousands of years to circle our Sun. These long-period comets have commonly been observed only once in recorded history. Short-period comets dash into the brilliant light and melting heat of the inner Solar System much more frequently.

There well may be hundreds of billions–possibly even trillions–of dancing icy objects in the remote Oort Cloud. Every so often, something occurs that disturbs one of these icy little worldlets, and sends it streaking inward toward our fiery Star. A duo of recent comets can serve as examples: C/2012 S1 (ISON) and C/2013 A1 Siding Spring. ISON met its catastrophic fate when it passed too close to our Sun. Siding Spring, which made a close brush by the planet Mars, will not return to the balmy inner Solar System for approximately 740,000 years.

The Oort Cloud likely formed considerably closer to our searing-hot, roiling young Sun during the earliest days of our Solar System’s existence. As the planets were born, grew, and then migrated to their current locations, their gravitational tugs–especially that of the behemoth Jupiter–likely scattered and then tossed billions of these icy planetesimals out to their current habitat in the very remote Oort Cloud. Indeed, the Oort Cloud is so far from our Sun that it can be shaken up by the close brush of a passing, neighboring star, nearby nebula, or even by jostling triggered by interactions with the disk of our barred-spiral Milky Way Galaxy. Alas, those jostling events can readily hurl icy cometary nuclei–the leftover relics of primordial icy planetesimals–screaming inward towards the merciless melting flames of our Star.

The inner regions of the Oort Cloud begin at around 2,000 AU from the Sun. The Cloud itself is thought to be composed of an outer Cloud and a torus (doughnut-shaped) inner Cloud. The entire Oort Cloud is spherical in shape.

Within the mysterious deep freeze of this remote bubble, surrounding our entire Solar System, frozen comet nuclei are commonly located tens of millions of kilometers apart. Furthermore, they are only very weakly bound to our Sun’s gravitational grip. Because our Sun’s hold on these frigid little objects is so weak, passing stars and other events can easily mess up the orbits of Oort Cloud comets. There are times when this unwelcome jostling hurls unfortunate Oort Cloud comets out of our Solar System altogether, and into the desolate, dark wilderness of the space between stars. On the other hand, our own Sun may have gravitationally stolen alien comets from the Oort Clouds belonging to other stars that were also being born in the same stellar nursery as our Sun.

The Oort Cloud contains pristine ices that date all the way back in time to the birth of our Solar System. The total mass of the cometary nuclei dwelling within the Oort Cloud has been estimated to be about 40 times that of Earth. This cometary material likely originated at varying distances and temperatures from our baby Sun. These variations explain the great compositional differences observed in comets.

The typical noontime temperature of the Oort Cloud is approximately four degrees Celsius above absolute zero. As the temperature dramatically drops to absolute zero, the kinetic energy of molecules begins to attain a finite value. Absolute zero is not a state of zero energy bereft of movement. Instead, there still exists some small amount of molecular energy–albeit at a minimum.

The Oort Cloud has not yet been discovered. Its existence is still just a theory, even though astronomers have studied a handful of comets thought to have originated from this very remote region of our Solar System.

The Return Of The Native

In a paper published in the April 29, 2016 issue of the journal Science Advances, lead author Dr. Karen Meech of the University of Hawaii Institute for Astronomy and her team propose that C/2014 S3 (PANSTARRS) was born in the inner Solar System, along with the quartet of terrestrial planets. The astronomers’ observations indicate that it is truly a primordial rocky object, and not a more recently formed asteroid that lost its way and wandered outward into the cold murkiness of the outer limits. Alas, according to this study, C/2014 S3 (PANSTARRS) was unceremoniously evicted from the balmy inner Solar System early in its planet-forming era. As such, it is likely a building-block of the rocky planets like our Earth, and it has been preserved in the deep freeze of the Oort Cloud for billions of years.

“We already knew of many asteroids, but they have all been baked by billions of years near the Sun. This one is the first uncooked asteroid we have found: it has been preserved in the best freezer there is,” Dr. Meech noted in an April 29, 2016 University of Hawaii Press Release.

The team of astronomers immediately realized that C/2014 S3 is special; a precious preserved object that does not sport the characteristic flashing, thrashing tail that most long-period comets develop as they travel close to our Sun. The astronomers playfully dubbed it the Manx comet, after a breed of tailless cat. Within only weeks of its discovery, the team obtained spectra of the extremely dim object with ESO’s VLT in Chile. A careful study of the light reflected by V/2014 S3 (PANSTARRS) suggests that it is typical of “S-type” asteroids, which are generally discovered in the inner Main Asteroid Belt. Indeed, it does not resemble typical comets, which form in the frozen outer Solar System and are, as a result, icy instead of rocky. This unique comet’s material has experienced minimal processing, which suggests that it has been preserved in the distant deep freeze far from our Star. The extremely weak comet-like activity associated with C/2014 S3 (PANSTARRS), which is consistent with the vaporization of water ice, is approximately a million times lower than that of active long-period comets at a similar distance from our Star.

The authors of this study propose that the unique object, that is V/2014 S3 (PANSTARRS), is most likely composed of fresh, unpolluted inner Solar System material that has been preserved in the freezer of the Oort Cloud–and is now coming in from the cold, returning to its warm and well-lit place of birth.

There are a number of theoretical scenarios that are able to reproduce the structure that astronomers observe in our Solar System. One important difference between these various models is what they predict about the icy objects that dance around in the mysterious, remote Oort Cloud. The varying scenarios predict significantly different ratios of icy to rocky objects. This first important discovery of a rocky denizen streaking inward from the Oort Cloud is, therefore, a precious test of the varying predictions of the differing scenarios. The authors of the paper estimate that observations of 50-100 of these Manx comets are necessary in order to shed important light on these differing models. These important observations will open up a new window in our scientific quest to understand the mysterious birth of our Solar System, that first formed approximately 4.56 billion years ago. About 15 of these Manx comets are being detected annually–most of them by Pan-STARRS–so this quest is well within reach.

Pan-STARRS is a panoramic survey telescope and rapid response system, designed for wide-field imaging. This innovative technology was developed at the University of Hawaii.

Study co-author Dr. Richard Wainscoat of the University of Hawaii’s Institute for Astronomy commented to the press on April 29, 2016 that “I expect that the discovery of this new class of objects will be one of the lasting legacies of the Pan-STARRS survey.”

The research is published under the title Inner Solar System Material Discovered in the Oort Cloud.

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