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.