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Protoplanetary Disks. By R. Hurt (NASA/JPL-Caltech/IPAC)

Protoplanetary Disk

A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gasses or other material may be falling from the inner edge of the disk onto the surface of the star. But this process should not be confused with the accretion process thought to build up the planets themselves.

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Earth Similarity Index. By Krischan

Earth Similarity Index

The Earth Similarity Index (ESI) is a proposed characterization of how similar a planetary-mass object or natural satellite is to Earth. It was designed to be a scale from 0.0 to 1.0, with Earth having a value of 1.0; this is meant to simplify planet comparisons from large databases. It has no quantitative meaning for habitability. The ESI incorporates a planet’s radius, density, escape velocity, and surface temperature into the index.

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Planetary Rings. By ESA/Hubble (M. Kornmesser & L. L. Christensen)

Planetary Ring

A ring system is a disc or ring orbiting an astronomical object that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as a planetary ring system. The most prominent planetary rings in the Solar System are those around Saturn, but the other three giant planets (Jupiter, Uranus and Neptune) also have ring systems.Read More

Planetary Migration. By NASA/ESA/A. Feild, STScI

Planetary Migration

Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite’s orbital parameters, especially its semi-major axis. Planetary migration is the most likely explanation for hot Jupiters, extrasolar planets with jovian masses, but orbits of only a few days.Read More

Asteroids. ESO -


Asteroids are minor planets, the larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets.Read More