Outbound Comets: Where Are You Going, Where Have You Been?
The comets of our own Solar System come shrieking into the brilliant light and melting heat of the inner regions, as they flee from their birthplace in a frigid, dark domain far, far away. In this mysterious region of cold, perpetual twilight, our own Solar System’s sparkling, icy comet nuclei linger as relics of an ancient era when planets were first forming from myriad colliding and merging frozen chunks of primordial material called planetesimals–the building blocks of major planets. But sometimes a comet that wanders inward towards our Sun is the icy offspring of a distant star beyond our own. In January 2020, astronomers at the National Astronomical Observatory (NAOJ) in Japan, announced that they have analyzed the paths of a duo of frozen vagabonds on their way out of our Solar System and determined that they most likely were born in the family of another star. These findings improve astronomers’ understanding of the outer limits of our Sun’s own family–and beyond.
Not all of the comets that we see in our dark night sky travel closed orbits around our Star. Some soar through our Solar System at breathtaking speeds before rushing out into the space between stars–never to return. Even though it is easy for astronomers to calculate where these comets are going, determining where they originated is much more difficult.
Frozen Vagabonds
Most comets are small Solar System objects that travel elongated orbits that carry them close to our Star for part of their orbit–and then into the remote outer limits of our Solar System for the remainder. Comets are frequently classified according to the length of their orbital periods. The longer the period, the more elongated the orbit.
The two classes of Solar System comets are short period and long period.
Short Period Comets: Short period comets are usually defined as those having orbital periods of less than 200 years. These comets normally orbit (more or less) in the ecliptic plane in the same direction as the planets. Their orbits usually carry these frigid wanderers out into the realm of the quartet of giant gaseous outer planets–Jupiter, Saturn, Uranus, and Neptune–at aphelion (when they are farthest from our Sun). For example, the aphelion of the famous Halley’s Comet is a little beyond the orbit of the outermost planet Neptune. Those comets that have an aphelia close to one of the orbits of a major planet are referred to as its “family”. These “families” are believed to have formed when the planet gravitationally pulled what were originally long-period comets into shorter orbits.
At the shorter orbital period extreme, Encke’s Comet sports an orbital period that does not even reach the orbit of the innermost giant planet, the banded-behemoth Jupiter, and is thus known as an Encke-type comet. Short-period comets that sport orbital periods of less than 20 years and have low inclinations to the ecliptic are termed traditional Jupiter-family comets (JFCs). The comets that are similar to Halley’s Comet, that sport orbital periods of between 20 and 200 years and show inclinations extending from zero to over 90 degrees, are termed Halley-type comets (HTCs).
Recently discovered comets, that orbit within the Main Asteroid Belt between Mars and Jupiter, have been designated a distinct class. These comets orbit in more circular orbits within the asteroid belt.
Because their elliptical orbits often carry them close to the quartet of giant gaseous planets, comets experience additional gravitational perturbations. Short-period comets tend to have their aphelia coincide with one of the giant planet’s semi-major axis, with the JFCs populating the largest group. Comets traveling from the remote Oort cloud–that forms a sphere around our entire Solar System reaching halfway to the nearest star beyond our own–have orbits that are powerfully influenced by the gravity of giant planets as a result of close encounters. The enormous planet Jupiter is, of course, the source of the most powerful perturbations. This is because Jupiter is more than twice as massive as all of the other planets in our Solar System combined. These perturbations can deflect long-period comets into shorter orbital periods.
As a result of their observed orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc. This disc is populated by icy objects in the trans-Neptunian region. In contrast, the origin of long-period comets is thought to be in the remote Oort cloud (named for the Dutch astronomer Jan Oort (1900-1992), who hypothesized its existence). It is believed that an enormous population of icy comet-like objects swarm, within these remote regions, in roughly circular orbits around our Sun. Every so often, the gravitational perturbations 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 these icy bodies howling into an elliptical orbit that carries it inward towards the melting heat of our Sun–and a visible comet is born. In contrast to the predictable return of periodic comets, whose orbits have been well-established in earlier observations, the appearance of new comets by this mechanism cannot be predicted. When hurled into the orbit of our Star, being perpetually pulled towards its glaring roiling fires, tons of matter are torn from the comets. This dangerous journey, of course, greatly shortens their “lifespan”.
Long Period Comets
Long-period comets sport periods that range from 200 years to thousands of years. These frozen objects also display highly eccentric orbits. An eccentricity that exceeds 1 when near perihelion (when a comet is closest to our Sun) does not necessarily indicate that a comet will escape from our Solar System.
By definition long-period comets are gravitationally bound to our Star. Comets that are evicted from our Sun’s family usually have been perturbed as the result of a path that has carried them too close to the major planets. As a result, they are no longer considered to have “periods”. The orbits of long-period comets carry them far beyond the realm of the quartet of giant planets at aphelia, and the plane of their orbits need not be situated close to the ecliptic. For example, Comet West–a long-period comet–can have an aphelion distance of almost 70,000 astronomical units (AU), with an orbital period calculated to be approximately 6 million years. One AU is equal to the average distance between Earth and Sun, which is about 93,000,000 miles.
As of 2019, only two comets have been detected with an eccentricity significantly greater than 1: 1I/’Oumuamua and 2I/Borisov. This indicates that the two comets originated beyond our Solar System, and are the vagabond children of another star. While Oumuamua displayed no optical signs of cometary activity during its voyage through the inner Solar System in October 2017, alterations to its trajectory–which suggests outgassing–indicate that it is likely a comet. In contrast, the interstellar comet, 2IBorisov, has been observed to display the tattletale coma feature that is characteristic of comets.
In addition to the comets born in our own Solar System, exocomets circling other stars, have also been detected. Indeed, exocomets are believed to be common throughout our entire Milky Way Galaxy. The first exocomet system to be discovered circles a main-sequence (hydrogen-burning) star named Beta Pictoris. Beta Pictoris is very young by star standards, being “only” around 20 million years old. Eleven such exocomet systems have been detected, as of 2013, by astronomers using the absorption spectrum which is caused by the large clouds of gas emitted by comets when traveling close to their star. For a decade, the Kepler Space Telescope hunted for planets and other bodies beyond our Solar System. The first transiting exocomets were discovered in February 2018 by a team of professional astronomers and citizen scientists studying light curves recorded by Kepler. After Kepler‘s mission ended in October 2018, a new telescope named TESS took over its mission. Since TESS was launched, astronomers have used it to discover the transits of exocomets around Beta Pictoris using a light curve obtained from TESS.
If there is a large population of comets flying around in the space between stars, they would be traveling at velocities of the same order as the relative velocities of stars close to our Sun–that is, a few tens of kilometers per second. If these icy vagabond children of another star wandered into our Solar System, they would possess positive specific orbital energy and would be observed to have hyperbolic trajectories. A rough calculation demonstrates that there might be four hyperbolic comets per century within Jupiter’s orbit–plus or minus one and possibly two orders of magnitude.
Where Are You Going, And Where Have You Been?
Two possible scenarios have been proposed to explain the existence of mysterious outbound comets. According to the first model, a comet is born in a stable orbit very far from out Sun. Alas, gravitational perturbations with a passing object tear the comet from its original orbit. The comet then migrates into the warm and well-lit inner Solar System where it can be observed before it is unceremoniously evicted into interstellar space. In contrast, the second model proposes that a comet is born somewhere very far away, perhaps within a different planetary system altogether. As the frigid wanderer zips through the space between stars, by pure chance it happens to enter our own Solar System before continuing on its journey.
Dr. Arika Higuchi and Dr. Elichiro Kokubo at NAOJ calculated the types of trajectories which would typically be expected in each of the two models. The team then compared their calculations to observations of the duo of strange outbound objects, ‘Oumuamua and 2I Borisov. The astronomers discovered that the interstellar origin scenario provided the better match for the paths of both unusual comets.
The astronomers also demonstrated that it is possible for gas-giant-sized bodies, wandering close to our Solar System, to destabilize long-period comets. According to this scenario, the perturbed comets are then flung onto paths similar to those of the two unusual comets. Survey observations have not revealed any gas-giant-size bodies which can be linked to the mysterious duo of outbound comets. However, further study, both observational and theoretical, of small interstellar objects is needed to better understand the origins of these weird travellers.
