for comets

The Oort Cloud is a vast, distant reservoir of icy bodies surrounding the Solar System, hypothesized to explain the origins of long-period comets (Oort, 1950). The Oort Cloud is traditionally divided into an inner, flattened region along the ecliptic plane (sometimes called the “Hills cloud” after Hills 1981)

and an outer, more isotropic spherical shell (Dones et al, 2004). Its sparsely populated extent stretches from roughly 2 000 or 3 000 au through to approximately 0.5 parsecs (105 au) from the Sun (Fouchard et al, 2020). The outer fringe serves as the outermost small-body population of the Solar System, marking the region where the Sun’s gravitational influence weakens against external stellar and Galactic perturbations (Dones et al, 2015). Most of our understanding of this distant region comes from analyzing the orbital element distributions of long-period comets. The structure of the Oort Cloud itself is primarily inferred from numerical simulations, which provide the basis for hypotheses about its formation and evolution (Fouchard et al, 2020).
The “Oort peak” refers to a specific, high-density concentration in the distribution of the original orbital energies of long-period comets. This spike indicates that a vast number of these comets possess nearly parabolic, extremely large orbits with semimajor axes usually > 10,000 to 20,000 au. The apparent inner edge of the Oort peak at approximately 10 000 au is attributed to an observational bias, as only comets with small (≲20 au) perihelion distances are typically detectable (e.g. Hills, 1981), and a semimajor axis of ≳10 000 au is needed for the Galactic tide to force a body’s perihelion to such a low value. Duncan et al (1987) developed a model describing the formation of the Oort Cloud through planetary scattering. Their findings suggest that the cloud should have an inner edge at approximately 3 000 au, beyond which it transitions to an isotropic distribution. More recent simulations suggest that, even after 4 Gyr, the Oort Cloud is isotropic only in its outermost regions. Emel’yanenko et al (2013) found that for semimajor axes a < 6 000 au, the cloud retains a strongly flattened structure, while at a > 8 000 au, external perturbations gradually reshape it into a more isotropic distribution. Fouchard et al (2018) further demonstrated that the Oort Cloud becomes fully isotropic beyond 10 000 au, whereas its innermost region, with a < 2 500 au, remains closely aligned with the ecliptic. Once emplaced in the Oort Cloud with a semimajor axis ≳10 000 au, a body’s orbit is affected by tidal forces from the Galactic potential and by passing stars, which can force its perihelion inwards to produce the isotropic comet population (Rickman et al, 2008). The Galactic tidal potential also causes an erosion of the Oort Cloud over Gyr, as bodies are stripped from the Sun to form tidal streams (Portegies Zwart et al, 2021; Forbes et al, 2025), which extend ahead of and behind the Solar System, following similar Galactic orbits. These are subsequently dynamically heated in the potential by encounters with giant molecular clouds (GMCs) and by dynamical structures such as the Galactic bar, dispersing the streams further. GMC encounters do little to Oort clouds themselves, but are surprisingly frequent (Pfalzner et al, 2020). A system’s Oort cloud is also subject to stripping by stellar flybys, with varying likelihood depending on the star’s distance from the Galactic centre. The strength of the eroding tide also varies with the star’s position in the potential as it orbits in the Galaxy. While the tidal force is greatest when a star crosses the midplane of the Galactic disc, it also varies if the star’s Galactocentric location changes with time. This occurs for any star on a non-circular orbit, but even stars on near-circular orbits can radially migrate several kpc in their lifetimes through interactions with the Galactic bar and spiral arms, in a process called “churning” (Sellwood and Binney, 2002;