Periodic comets account for some of the more spectacular vistas, visible at specific intervals in the night skies and possible sources for annually observable periodic meteor showers. Comet 12P/Pons-Brooks, currently closing in on its perihelion passage (its closest point to the sun in its orbit), which it will reach on April 21, is one such example of a periodic comet, with an orbital period around the sun of 71 years. While it had been previously observed in 1385 and 1457, the comet was definitively discovered by Jean-Louis Pons in 1812 before being confirmed on its next passage in 1883, with observations by William Robert Brooks.
Comets are predominantly made of icy materials, which are solid at the colder temperatures prevalent in the outer solar system, where they also pass most of their orbital time. As opposed to the planets, comets travel on highly elliptical (oval-shaped) orbits, meaning that their orbit takes them out to the far reaches of the solar system before they make a return to the inner solar system.
While entering the inner solar system, as they are heated up by the sun, they experience outgassing, with volatile compounds being released in their wake.
We know that specific comets on particular trajectories are responsible for particular meteor showers
This can, in turn, result in dust being dislodged and released from the comet as well, with the comet achieving two distinct tails – a gas (or ion) tail and a dust tail. In certain cases, infrequently, comets can experience more rapid disintegrations, resulting in outbursts of material being lost from the comet.
If a comet crosses the path of the Earth, the stream of material released by the comet would be annually crossed by the Earth at a very specific point in its orbit, and ergo at a very specific time of year, giving rise to periodic meteor showers. Thus, most periodic meteor showers see their origin in the passages of such comets as they enter the inner solar system, crossing Earth’s path and thus leaving debris for the Earth to collide with as we, in turn, cross that point in our orbit around the sun.
In this way, we know that specific comets on particular trajectories are responsible for particular meteor showers – the Perseid meteor shower, for example, is attributed to Comet Swift-Tuttle, with an orbital period of 133 years. It is thought, though unconfirmed, that Comet 12P/Pons-Brooks might be responsible for the weak December κ Draconids.
Josef Borg completed a PhD in astronomy at the Institute of Space Sciences and Astronomy, University of Malta, and is currently a researcher at the Faculty of Health Sciences at the University of Malta. He is also Malta’s representative on the European Astrobiology Network Association (EANA) council.
Sound Bites
• The James Webb space telescope (JWST) observes hints of exomoons in a newly forming solar system!: Astronomers using JWST to observe the system had already discovered two planets forming in the protoplanetary disk for this infant star. A third planet was also discovered, but a stream of heated material surrounding the two other, previously discovered, planets might be a glimpse at the formation of moon systems!
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DID YOU KNOW?
• Comet collisions with planets are very rare – but we have seen one! Comet Shoemaker-Levy 9 was likely captured by Jupiter two decades prior to its eventual disintegration and collision with the gas giant planet. It is likely that the comet was, in fact, captured from a solar orbit some time in the early 1970s, where it fell into orbit around Jupiter before eventually breaking up in 1992 and colliding with the gas giant in July 1994. Images from the event are shown in today’s ‘Photo of the Week’ on this page.
• Comets are classified as long or short period comets, depending on their orbital period: Short period comets are those comets that take less than 200 years to complete one orbit around the sun, while long period comets take longer than 200 years. The origin of short period comets is the Kuiper belt or the scattered disc, while longer period comets might even originate from the Oort cloud, in the outermost reaches of the solar system, particularly for very long period comets.
For more trivia, see: www.um.edu.mt/think.