Where do comets come from?
To answer this question, we must reconstruct the orbit of comets before they enter the inner solar system for the first time, thus before the disturbances by the gravitational attraction of the most massive planets are felt. This is a difficult problem, because we discover new comets when they are quite close to us, that is, after their orbit has been affected. So we must assess these disturbances and determine by calculation the comet’s initial orbit. From 1914 the Swedish-Danish astronomer Elis Strömgren (1870-1947) and his colleagues have shown that this initial orbit is generally a very elongated ellipse and therefore that comets do not come from the infinite. Based on this idea, the imaginative Estonian astronomer Ernst Öpik (1893-1985) considered in 1932 the existence of a “reservoir of comets” located at more than 10,000 astronomical units from the Sun. Öpik’s hypothesis was the basis of the final work of the Dutchman Jan Hendrik Oort (1900-1992). Using the initial orbits calculated by different authors, Oort showed in 1950 that their aphelion (the point where they are farthest from the Sun) is in a thick spherical shell of radius between 50,000 and 150,000 astronomical units (an astronomical unit is roughly 150 Million kilometers or 93 million miles, the distance between the Earth and Sun).
Orbites de quelques comètes connues en 1887, avec pour comparaison l’orbite des planètes Ces comètes sont périodiques, à l’exception des comètes C/1729 P1 et C/1853 L1. Il y a sensiblement autant de comètes à mouvement direct (comme celui des planètes) qu’à mouvement rétrograde. D’après Les Comètes / Guillemin, 1887.
Crédit : Observatoire de ParisOccasionally, a gravitational disturbance drives a comet into the solar system. Oort thought that this disturbance was related to the passage of a nearby star. We now think that the density waves that run through our Galaxy disturb the Oort cloud of comets, rather than the gravitational action of the stars. The objects that inhabit this reservoir were probably formed near the orbits of Uranus and Neptune, and were subsequently sent to the outer solar system by the gravitational perturbations of Jupiter and Saturn.
The scientists also identified another reservoir of comets, much closer than the Oort cloud but nevertheless located beyond the orbit of Neptune: the Kuiper belt of icy objects, between 30 and 55 astronomical units from the Sun.
To summarize, there are three types of comets, from two large reservoirs.
Some new comets come to us from a distant reservoir, the Oort cloud; their orbit is a highly elongated ellipse, almost a parabola, and their direction of arrival is arbitrary.
Others come to us from a closer reservoir, the Kuiper belt; they also have almost parabolic orbits, but they are relatively close to the plane of the ecliptic where the planets circulate.
Finally, periodic comets bring together the two previous categories; their trajectory has undergone gravitational perturbations from Jupiter and Saturn, and their orbit is a more or less elongated ellipse. Some of these comets, “captured” by Jupiter and called comets of the “Jupiter family,” gravitate closer to the Sun than the planet does, within five astronomical units from the Sun, with periods of less than 10 years. The 2P/Encke comet is the prototype of this class. Their orbit is close to the plane of the ecliptic because they come mostly from the Kuiper Belt. Having passed many times near the Sun, they have lost much of their reserve of ice as a result of the temperature increase, and are generally not very active and therefore not very bright. Other comets, those of the Halley family, have periods between 20 and 200 years and sometimes high inclinations; they generally come from the Oort cloud. These comets are the best known today; they can be very bright, like Halley’s comet in 1910 or comet C/1995 O1 Hale-Bopp in 1997.
Orbites de quelques comètes à courte période, « capturées » par le champ gravitationnel de Jupiter. Ce sont des comètes dites « de la famille de Jupiter ». Les deux cercles labellisés « orbite de Jupiter » ont pour rayon la distance au périhélie et à l’aphélie (le point le plus éloigné du Soleil). D’après L’Astronomie populaire / Flammarion, 1880.
Source : James Lequeux