Planetesimal

Planetesimals /plænᵻˈtɛsᵻməlz/ are solid objects thought to exist in protoplanetary disks and in debris disks.

A widely accepted theory of planet formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals may form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, but a few of the largest planetesimals may survive such encounters and continue to grow into protoplanets and later planets.

Solar System

The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury.

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

Viktor Safronov

Viktor Sergeevich Safronov (Russian: Виктор Серге́евич Сафронов) (born Velikie Luki, Russia, 11 October 1917 – died Moscow 18 September 1999) was a Soviet astronomer who put forward the low-mass-nebula model of planet formation, a consistent picture of how the planets formed from a disk of gas and dust around the Sun.

Biography and legacy

Safronov graduated from Moscow State University Department of Mechanics and Mathematics in 1941. He defended a dissertation for the Doctor of Physical and Mathematical Sciences in 1968. His scientific interests were planetary cosmogony, astrophysics and geophysics.

His planetesimal hypothesis of planet formation is still widely accepted among astronomers, although alternative theories exist (such as the gravitational fragmentation of the protoplanetary disk directly into planets).

A minor planet 3615 Safronov, discovered by US-American astronomer Edward L. G. Bowell in 1983 is named after him. Safronov's work is discussed at length in the BBC documentary The Planets (TV miniseries)

Chamberlin–Moulton planetesimal hypothesis

The Chamberlin–Moulton planetesimal hypothesis was proposed in 1905 by geologist Thomas Chrowder Chamberlin and astronomer Forest Ray Moulton to describe the formation of the solar system. It was proposed as a replacement for the Laplacian version of the nebular hypothesis that had prevailed since the 19th century.

The theory was based on the idea that a star passed close enough to the sun early in its life to cause tidal bulges to form on its surface, which along with the internal process that leads to solar prominences, caused material to be ejected repeatedly from the sun. Due to the gravitational effects of the passing star, two spiral-like arms would have extended from the sun, and while most of the material would have fallen back, part of it would remain in orbit. This orbiting material would cool and condense into numerous small bodies that they termed planetesimals and a few larger protoplanets. Their theory proposed that as these objects collided over time, the planets and their moons were built up, with comets and asteroids being the leftover debris. The "spiral nebulae" photographed at Lick Observatory were thought to possibly be views of other suns undergoing this process. These nebulae are now known to be galaxies rather than developing solar systems.

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