Neil_Boekend wrote: keithl wrote:
DHT wrote:Yes, the Oort cloud is real. It is the region where the gravitational pressure of the sun (inward) and the radiative pressure of the sun (outward) roughly equalize.
Both gravity and radiation pressure from the Sun are inverse-square - they balance at all >1 AU distances. ...
Also: the radiation pressure depends mostly on your surface area while the gravity depends on your mass. Whether they balance out depends on your surface/mass ratio. A solar sail will have winning radiation pressure (on all sane distances) while a uranium sphere will have winning gravity.
That is true, but irrelevant to the Oort cloud. Of course, the balance depends on the sun-facing area to mass ratio - at ALL distances, not just the Oort cloud. It also depends on velocity; centrifugal acceleration compensates for gravity
. What is "out there" is Really Cold hydrogen and water molecules, and tiny amounts of heavier elements, mostly ejected by light pressure unless the particles are big enough for the mass-to-surface area to approximately balance. Albedo and shape is important - reflected light increases light pressure somewhat (for a perfect sphere, perhaps 45%).
Light pressure for a black particle is power flux divided by c, and the power flux is inverse square. At earth radius, the power flux is 1367 W/m², the light pressure is 4.56e-6 N/m², and the solar gravitation is 5.93 mm/s². That supports a black particle with a mass to area ratio of 0.77 g/m², or a reflective spherical particle with a mass to area ratio of 1.12 g/m². And that is true all the way out to the borders of other star systems, it is not sensitive to distance, the first point I hoped to make in my posting.
The numbers I gave apply today, 4.6 billion years after the sun formed, but solar luminosity ramped up from zero over time, so the "balance" occured for smaller particles in the past, and larger stuff fell inwards. A competent stellar astronomer can tell you what that means for the evolution of the Kuiper belt and (hypothetical) Oort cloud.
The (hypothetical) Oort cloud contains few solar sails, uranium spheres, uranium needles, unicorns, or Russell's Teapots. And it remains hypothetical because if it does exist, it is so cold and diffuse and distant that it is virtually impossible to observe. The much easier-to-observe Kuiper belt is still mostly unexplored - there may be dwarf planets larger than Pluto and Eris out there. But then, the vast majority of the matter in the universe is unobservable with current technology.