Knowra Interstellar object Interstellar object A natural object originating beyond the Solar System that travels through interstellar space. Some pass through the Solar System on unbound trajectories.
ʻOumuamua : The first confirmed interstellar object detected passing through the Solar System, in 2017. Its unusual brightness changes and unexplained nongravitational acceleration made it the best-known case.
Hyperbolic trajectory : An open orbital path with eccentricity greater than one, not bound to its central body. A sufficiently hyperbolic Solar System orbit signals an object arriving from interstellar space.
Protoplanetary disk : A rotating disk of gas and dust surrounding a young star, where planets and smaller bodies form. Interstellar visitors are likely fragments ejected from such disks during planetary-system formation.
Comet : A small Solar System body rich in ice that can release gas and dust near the Sun. Cometary activity on 2I/Borisov allows comparison with familiar Solar System comets.
Interstellar medium : The gas, dust, and energetic particles occupying space between stars. Visitors travel through this material, which can erode or alter their surfaces over time.
2I/Borisov : The second confirmed interstellar object, discovered in 2019, with a visibly active cometary coma. Its familiar comet-like activity contrasted with ʻOumuamua’s ambiguous appearance.
Orbital eccentricity : A number describing how much an orbit differs from a circle. Eccentricity provides the standard measure for distinguishing unbound visitor paths from closed orbits.
Planetary scattering : Gravitational encounters with planets that alter small bodies’ orbits, sometimes ejecting them from a system. This process can launch asteroids and comets onto permanent interstellar trajectories.
Spectroscopy : The study of matter through its interaction with light across different wavelengths. Spectra reveal the gases and dust released by an interstellar visitor.
Interstellar dust : Microscopic solid particles dispersed through the space between stars. Dust grains may be abundant, but current surveys mainly detect larger, brighter objects.
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