Outbound Comets: Where Are You Going, Where Have You Been?

Our own solar system’s comets shriek through the dazzling illumination and molten heat of the inner regions as they migrate far, far away from their origins in a deep, dark realm. In this enigmatic area of ice, eternal dusk, the shiny, icy comet nuclei of our own Solar System lingered as remnants of an ancient age when planets first evolved from myriads of colliding and frozen pieces of primordial material called planetesimals — the building blocks of modern planets. But a comet that wanders inward towards our Sun is sometimes the icy offspring of a distant star beyond our own.

Astronomers at Japan ‘s National Astronomical Observatory (NAOJ) in January 2020 revealed that they studied the movements of a pair of frozen vagabonds on their way out of our solar system and concluded that they were most definitely raised in another star’s nest. These findings improve the understanding by astronomers of the outer limits of our own family’s Sun — and beyond.

Almost all of the comets we display in our deep night sky fly around our Sun in closed orbits. Some climb up at breathtaking speeds through our Solar System before rushing out into the space between stars — never to return. While it is easy for astronomers to calculate where those comets are going, it is much more difficult to determine where they originated.

Frozen Vagabonds

Most comets are tiny Solar System bodies that ride elongated paths that take them for part of their path near to our Star — and then, for the remainder, into the distant outer reaches of our Solar System. The comets are also identified by the duration of their orbital cycles. The longer the period the longer the orbit has elongated.

The two classes of Solar System comets are short period and long period.

Short Period Comets: Comets with short periods are generally defined as those with orbital periods of less than 200 years. Those comets normally orbit (more or less) in the same direction as the planets in the ecliptic plane. Their orbits usually carry these frigid wanderers out into the quartet realm of giant outer gaseous planets — Jupiter, Saturn, Uranus, and Neptune — at the aphelion (when they ‘re farthest from our Sun). The aphelion of the famous Halley’s Comet, for example, is somewhat beyond the orbit of the outermost planet Neptune. Those comets which have an aphelia close to one of the major planet’s orbits are called their “family.” These “families” are thought to have formed when the planet gravitationally pulled into shorter orbits what were originally comets of a long period.

Encke’s Comet boasts an orbital period at the shorter orbital drastic duration that does not quite enter the orbit of the innermost major planet, the banded-behemoth Jupiter, and is therefore known as an Encke-type comet. Classic Jupiter-family comets (JFCs) are considered short-term comets that display orbital cycles of less than 20 years and have small inclinations to the ecliptic. The comets close to Halley’s Comet are named Halley-type comets (HTCs), which have orbital cycles between 20 and 200 years and inclinations varying from zero to over 90 degrees.

Comets recently discovered that orbit between Mars and Jupiter within the Main Asteroid Belt has been designated as a distinct class. These comets orbit within the asteroid belt in more circular orbits.

Because their elliptical orbits often carry them close to the giant gaseous planet quartet, comets experience further gravitational disturbance. Short-term comets tend to have their aphelia coincide with one of the semi-major axes of the giant planet, with the largest group of JFCs populating. Comets traveling from the remote Oort cloud — that forms a sphere around our entire Solar System that reaches halfway beyond our own to the nearest star — have orbits powerfully influenced by the gravity of giant planets as a result of close encounters. Of example, the massive planet Jupiter is the cause of the most strong perturbations. This is because Jupiter is more than twice as large as all the other planets together in our Solar System. These disturbances can deflect comets of long periods into shorter orbital periods.

Because of their observed orbital characteristics, it is thought that short-period comets originate from centaurs and the Kuiper belt / scattered disk. In the trans-Neptunian zone, this disk is inhabited by ice objects. In comparison, it is assumed that the nucleus of long-term comets is in the distant Oort cloud (named after the Dutch astronomer Jan Oort (1900-1992), who speculated its existence). The massive number of frozen comet-like bodies is thought to cluster in nearly spherical orbits around our Earth, inside these distant territories.

Every so often, the gravitational disturbances caused by the outer giant planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may hurl one of those icy bodies hurling into an elliptical orbit that carries it inwards towards our Sun’s melting heat — and a visible comet is born. As the expected return of regular comets, whose orbits were well-established in earlier observations, this method can’t forecast the arrival of new comets. Once thrust toward our Star’s orbit, dragged endlessly into its flashing roiling flames, loads of matter are ripped from the comets. This dangerous journey, of course, greatly shortens their “lifespan”.

Long Period Comets

Long-term comets boasting cycles varying from 200 years to thousands of years. These frozen bodies exhibit extremely irregular orbits, too. An eccentricity that approaches 1 as it is past perihelion (where a comet is nearest to our Sun) does not automatically mean whether a comet would leave our Solar System.

Long-period comets are by nature gravitationally linked to our Star. Comets evicted from the families of our Sun is typically disrupted by a course that took them so close to the main planets. As a result, “periods” are no longer considered to have They are carried far beyond the realm of the quartet of giant planets at aphelia by the orbits of long-period comets, and the plane of their orbits need not be near the ecliptic. Comet West, for example,—a long-period comet — can have an aphelion distance of nearly 70,000 astronomical units ( AU), with an orbital period estimated to be about 6 million years. One AU is equal to the Earth-Sun average distance which is about 93,000,000 miles.

As of 2019, only two comets with an eccentricity significantly greater than 1 were detected: 1I/’Oumuamua and 2I / Borisov. This suggests that the two comets formed outside of our Solar System, and are another star’s vagabond babies. Although Oumuamua shows no visible indicators of cometary movement on its October 2017 trip through the internal solar system, adjustments to its trajectory — which indicates exhaust — indicate that it is likely to be a comet. In comparison, it was found that the interstellar comet, 2IBorisov, exhibits the tattletale coma attribute typical of comets.

In addition to the comets born in our own Solar System, exocomets were also detected that circled other stars. Indeed, exocomets are thought to be common throughout the Milky Way Galaxy as a whole. A principal sequence (hydrogen-burning) star named Beta Pictoris is the first exocomet system to be discovered. By star standards Beta Pictoris is very young, being “only” around 20 million years old. As of 2013, astronomers have observed eleven such exocomet structures utilizing the absorption range that is induced by the large amounts of gas released by comets while passing past their star.

For a decade the Kepler Space Telescope has hunted beyond our Solar System for planets and other bodies. In February 2018 a team of qualified astronomers and citizen scientists investigating light curves observed by Kepler discovered the first transiting exocomets. Since Kepler ‘s career concluded in October 2018, the work was carried over by a new telescope called TESS. Since the launch of TESS, it has been used by astronomers to discover exocomet transits around Beta Pictoris using a light curve obtained from TESS.

If there were a large population of comets flying around in the space between stars, they would travel at speeds of the same order as the relative velocities of stars near our Sun — that is, a few tens of kilometers per second. If these other star’s icy vagabond children wandered into our Solar System, they would possess positive orbital specific energy and hyperbolic trajectories would be observed. A rough calculation shows that within Jupiter’s orbit there could be four hyperbolic comets per century-plus or minus one, and possibly two orders of magnitude.

Where Are You Going, And Where Have You Been?

Two plausible explanations for understanding the presence of enigmatic outbound comets have been suggested. A comet is born, in a stable orbit very far from the Sun, according to the first model. Unfortunately, tidal interactions from a moving body are pulling the comet from its initial path. The comet then migrates to the warm and well-lit inner Solar System where it can be observed before being unceremoniously expelled to interstellar space. The second model, by contrast, proposes that a comet be born very far away somewhere, perhaps entirely within a different planetary system. As the frigid wanderer zips through the space between stars, it happens by pure chance to enter our own Solar System before proceeding on its journey.

At NAOJ, Dr. Arika Higuchi and Dr. Elichiro Kokubo estimated the types of trajectories that will usually be anticipated in either model. The team then compared their calculations with observations from the duo of odd outbound objects, ‘Oumuamua and 2I Borisov. The astronomers found the stronger fit for the orbits of both peculiar comets was given by the interstellar origin scenario.

The astronomers also revealed that gas-giant objects would destabilize long-period comets, roaming near to our solar system. The perturbed comets are then flung on paths similar to those of the two unusual comets according to this scenario. Survey observations have not revealed any gas-giant-size bodies which can be linked to the mysterious duo of outbound comets. Nevertheless, further analysis of tiny interstellar bodies, both empirical and theoretical, is required to help grasp the nature of these mysterious travelers.

NASA Reveals Alien Composition of 2I/Borisov, 1st Interstellar Comet