Search results
Results From The WOW.Com Content Network
Pebble accretion may accelerate the formation of planets by a factor of 1000 compared to the accretion of planetesimals, allowing giant planets to form before the dissipation of the gas disk. [28] [29] However, core growth via pebble accretion appears incompatible with the final masses and compositions of Uranus and Neptune. [30]
A widely accepted theory of planet formation, the planetesimal hypothesis of Viktor Safronov, states that planets form from cosmic dust grains that collide and stick to form ever-larger bodies. Once a body reaches around a kilometer in size, its constituent grains can attract each other directly through mutual gravity , enormously aiding ...
Pebble accretion is the accumulation of particles, ranging from centimeters up to meters in diameter, into planetesimals in a protoplanetary disk that is enhanced by aerodynamic drag from the gas present in the disk. This drag reduces the relative velocity of pebbles as they pass by larger bodies, preventing some from escaping the body's gravity.
After small planetesimals—about 1 km in diameter—have formed by one way or another, runaway accretion begins. [20] It is called runaway because the mass growth rate is proportional to R 4 ~M 4/3, where R and M are the radius and mass of the growing body, respectively. [65] The specific (divided by mass) growth accelerates as the mass increases.
This model received favorable support for about 3 decades, but passed out of favor by the late '30s and was discarded in the '40s due to the realization it was incompatible with the angular momentum of Jupiter. A part of the hypothesis, planetesimal accretion, was retained. [4]
The protoplanetary disk was too diffuse and the time scales too long [44] for them to form via planetesimal accretion before the gas disk dissipated, and numerical models indicate that later accretion would be halted once Pluto-sized planetesimals formed. [45]
A portion of the theory stating that smaller objects — planetesimals — gradually collided to build the planets by accretion is still well-regarded. From his theories and other geological evidence he concluded that Earth was much older than assumed by Lord Kelvin (ca 100 million years) at the time. His speculations about the source of energy ...
[46] [47] If most of the growth of planetesimals and embryos into terrestrial planets was due to pebble accretion, a small Mars could be the result this process having been less efficient with increasing distances from the Sun. [48] [49] Convergent migration of planetary embryos in the gas disk toward 1 AU would result in the formation of ...