1. The Origin of Comets

Empirical data are confronted with different hypotheses on the origin of comets. The hypotheses are classified into three categories: 1) Comets were condensed from the solar nebula and ejected later into the Oort’s cloud. 2) Comets were condensed in situ, more or less recently, on their present trajectories; 3) Reversing the arrow of time in the traditional evolution of comets. Only two hypotheses, both from the first category, are found to be in agreement with all empirical data. The first hypothesis explains the origin of the Oort’s cloud by the perturbations of the giant planets (mainly Uranus and Neptune and possibly Pluto) on a ring of proto-comets, during the final accretion stages of the solar system. The second hypothesis uses the fast mass loss of the solar nebula to expell an outer ring of proto-comets into elliptic trajectories. Although no empirical evidence requests that the Oort’s cloud be older than a few million years, its matter is not likely to be from a different reservoir than solar system stuff, and no satisfactory theory explains its formation more recently than 4,5 billion years ago.