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".