Arrange the events in the order that they’re likely to occur leading up to the formation of a solar system.
Answers
Any model of Solar System formation must explain the following facts:
1. All the orbits of the planets are prograde (i.e. if seen from above the North pole of the Sun they all revolve in a counter-clockwise direction).
2. All the planets (except Pluto) have orbital planes that are inclined by less than 6 degrees with respect to each other (i.e. all in the same plane).
3. Terrestrial planets are dense, rocky and small, while jovian planets are gaseous and large.
I. Contraction of insterstellar cloud
Solar system formed about 4.6 billion year ago, when gravity pulled together low-density cloud of interstellar gas and dust (called a nebula)(movie).
The Orion Nebula, an interstellar cloud in which star systems and possibly planets are forming.
Initially the cloud was about several light years across. A small overdensity in the cloud caused the contraction to begin and the overdensity to grow, thus producing a faster contraction --> run away or collapse process
Initially, most of the motions of the cloud particles were random, yet the nebula had a net rotation. As collapse proceeded, the rotation speed of the cloud was gradually increasing due to conservation of angular momentum.
Going, going, gone
Gravitational collapse was much more efficient along the spin axis, so the rotating ball collapsed into thin disk with a diameter of 200 AU (0.003 light years) (twice Pluto's orbit), aka solar nebula (movie), with most of the mass concentrated near the center.
As the cloud contracted, its gravitational potential energy was converted into kinetic energy of the individual gas particles. Collisions between particles converted this energy into heat (random motions). The solar nebula became hottest near the center where much of the mass was collected to form the protosun(the cloud of gas that became Sun).
At some point the central temperature rose to 10 million K. The collisions among the atoms were so violent that nuclear reactions began, at which point the Sun was born as a star, containing 99.8% of the total mass.
What prevented further collapse? As the temperature and density increased toward the center, so did the pressure causing a net force pointing outward. The Sun reached a a balance between the gravitational force and the internal pressure, aka as hydrostatic equilibrium, after 50 million years.
Around the Sun a thin disk gives birth to the planets, moons, asteroids and comets. Over recent years we have gathered evidence in support of this theory.
Close-up of the Orion Nebula obtained with HST, revealing what seem to be disks of dust and gas surrounding newly formed stars. These protoplanetary disks span about 0.14 light years and are probably similar to the Solar Nebula.
II. The structure of the disk
The disk contained only 0.2% of the mass of the solar nebula with particles moving in circular orbits. The rotation of the disk prevented further collapse of the disk.
Uniform composition: 75% of the mass in the form of hydrogen, 25% as helium, and all other elements comprising only 2% of the total.
The material reached several thousand degrees near the center due to the release of gravitational energy --> it was vaporized. Farther out the material was primarily gaseous because H and He remain gaseous even at very low T. The disk was so spread out that gravity was not strong enough to pull material and form planets.
Where did solid seeds for planet formation come from? As the disk radiated away its internal heat in the form of infrared radiation (Wien's law) the temperature dropped and the heaviest molecules began to form tiny solid or liquid droplets, a process called condensation.
III. Formation of the planets
Inner structure of the Earth
Formation of terrestrial planets:
In the warmer inner solar system, planetesimals formed from rock and metal, materials cooked billions of years ago in cores of massive stars.
These elements made up only 0.6% of the material in the solar nebula (and the faster collisions among particles close to the Sun were more destructive on average), so the planets could not grow very large and could not exert large pull on hydrogen and helium gas.
Solar system is said to be formed after the birth of our Sun about 4.6 billion years ago.
1: The collapse of the solar nebula
2: Formation of Rotating protostellar disk.
3: Center of nebula fusion that results in the birth of star(Sun).
4: Planteseimals formations and planet formation.