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Home , Accretion (astrophysics), Nebular hypothesis, Planetesimal

Formation of the Chapter 8 To understand the formation of the solar system one has to apply concepts such as: • Conservation of angular • Conservation of energy The theory of the formation of the solar system need to explain: • The pattern of the sense of of • The plane of the orbits of planets • The sense of rotation of satellites • The different composition of terrestrial, Jovian and dwarf planets

Some of the patterns we can find in the solar system: • All planets orbit the counterclockwise (as seen from the north pole) • Nearly all the planetary orbits lie in a plane • Almost all planets rotate in the same direction with their axes perpendicular to the orbital plane • Most satellites revolve around planets in the same direction that the rotates on its axis. But there are some exceptions • rotates backwards (Rotational axis tilted close to 189 degrees) • rotates on its side (Rotational axis tilted close to 90 degrees) • Most small do not share the orbital plane of the planet • Triton (Satellite of ) orbit the planet in opposite sense • is the only with a large A model for the formation of the solar system has to account for: • Different composition of planets (rocky, gaseous, icy) • Existence of many and

• Objects rotating around a point have angular momentum. • Simplest case ( a small sphere orbiting a larger mass) L = m x v x r L :angular momentum m: mass of small sphere v: velocity of the small sphere r :separation between the small sphere and the larger object • Conservation of angular momentum  if r changes, v must change (ice skaters) But the value of L remains constant

The (and the rejected collision theory)

• The idea that the solar system was born from the collapse of a cloud of and for proposed by (1755) and by Pierre Simon Laplace 40 years later. • During the first part of the 20th century, some proposed that the solar system was the result of a near collision of the Sun with another . Planets formed from debris of the collision. But we know now that collision (or near collisions) between two are very, very rare. • Considering that collision are rare, the proposed idea of the collision may explain a unique event on how our formed but not how other planetary systems formed. • During the rest of the 20th century, new ideas and theories about the formation of stars (and possible planets) made this collision theory obsolete and was discarded • In 1995, the first (planets orbiting other stars) was discovered • Many more planets have been found so far in the solar neighborhood ( close to 1000 confirmed and more than 2000 possible ones). It is clear now that formation of planets is not a rare event. • Any theory about the formation of planetary system must explain the formation of planets, not as a single unique and rare event but more like a common event in the

The nebular theory

• Stars are born from the collapse of an of dust and gas. • Planets form as part of the process of the formation of a star • As part of the formation of a star, a proto planetary disk forms around the star • Planets are formed from the collapse of the material of a proto planetary disk • The basic and simple idea suggested by Kant and Laplace needed a lot of modifications before it became the actual nebular theory for the formation of the solar system and other planetary system

Where did all the dust and gas that formed the originated? • All the dust and gas comes from “recycled” material in the Galaxy. • The first original stars in the Galaxy were formed from H and He. • The rest of the heavier elements were synthesized in the interior of the stars as part of the process of energy generation inside the star (fusion, conversion of lighter elements into heavier elements) • The most massive stars are able to generate the heaviest elements. They end their in a spectacular explosion called . • All that material is thrown into the interstellar space and contaminate the original clouds of H and He gas. • Heavier elements and lighter elements combine and form molecules. These molecules form aggregates which become dust particles. • Gas and dust form clouds of “contaminated” material. • From that material new stars are formed, already containing heavier elements. • The fusion process inside of these stars and the of these stars continues enriching the material in the clouds of dust and gas. The An example of an interstellar cloud of gas and dust where new stars are being born A schematic representation of the process of contamination of interstellar clouds

The cloud of gas that gave birth to our solar system resulted from the recycling of material through many generations of stars within our galaxy. Nebular Hypothesis

• Event such as the impact of a shock wave from the explosion of a supernova or the passage of a compression wave in the galaxy, (spiral density wave) will triggers the (collapse due to its own ) of the cloud Gravitational force

Radiation pressure

Gravitational force

Radiation pressure

Centrifugal force Nebular Hypothesis • The collapse cause an increase in density • Increased density -> increased gravity -> more material gets sucked in -> center heats up • Because of conservation of angular momentum, as the nebula collapses, it decreases it radius and it will spin faster • The density and in the center increases • Gravitational potential energy is converted into kinetic energy • The cloud collapse into a • As the temperature keeps increase in the center , it will reach disk the fusion point (It needs to reach about 10 million K in the • This is called a case of the Sun to convert H into He). . • Once the central body start generating energy, a new star is • Planets form from the material in the disk born • Outer, cooler particles suffer repeated collisions, building planet-sized bodies from dust grains ( Process called ) • The presence of dust is a key element in the formation of small particles which will stick together and form . Gas molecules by themselves will not stick and form planetesimals Nebular Hypothesis

• Accretion clears a gap near the new star • Most of the gas is accreted into the central star and into forming planets

•Young stellar activity produces high stellar winds, which blows off any remaining gas and leaves an embryonic solar system Nebular Hypothesis The sequence of the collapse and formation of a planetary system A better sequence of a collapsing cloud and the formation of planets A detailed view of the process of accretion of planetesimals and formation of planets Planetary Compositions The temperature decreases with distance from the Sun and regulates: • Which elements actually condense • Which compounds are formed from the elements • At what rates the compounds are formed.

Volatile species will only be stable beyond a point in which the temperature in the disk s low enough. Heavier elements (Like silicon and its compounds) can condense at higher in the inner part of the disk This is why the inner planets are -rich and the outer planets gas- and ice-rich Formation of terrestrial planets

• Terrestrial planets formed from the accretion of smaller bodies called planetesimals • The process started with small solid particles that condensed from the dust and gas in the nebula. Even with the high temperatures, the dust was able to condense. These particles were too small to “stick” together by gravitational forces. Electrostatics forces may be responsible for them to stick together . • Once they grew bigger, gravitation was responsible for them to attract more particles and began forming . • Collision were frequent during this stage. Some of the small planets and planetesimals may have been shattered. Only the largest ones may have survived and became planets and grew large to form the terrestrial planets.

The formation of Jovian planets

• The process of accretion also took place at the distance in which the Jovian planets formed. • But at that distance, condensation of ice was possible due to the lower temperature. • The planetesimals were formed from condensation of large amounts of ice, and some and rocks. • Planetesimals grew bigger and faster. They grew large enough to attract and retain H and He • They attracted so much H and He so that the original “seeds” of rock and became small compared with the gas. Formation of Jovian planets • Beyond the frost line, planetesimals could accumulate ice • and other low-mass compounds are more abundant (98%) than rock/metal (2%) so Jovian planets got bigger, faster. Outside the “Frost line”, there was more efficiently capture (by their bigger gravitational pull) of H/He gas before it was dispersed by the Sun’s radiation and solar wind

+ dust particles Nebular gas collapsed onto rock-ice cores of perhaps 10 Earth masses

Rocky, icy core • Each young Jovian planet formed its own “miniature” solar nebula out of the gas around them. • The satellites of the Jovian planets formed out of these gas and dust disks. What ended the process of planet formation?

• A large part of the H and He of the original nebula never became part of the solar system. • Once the Sun formed, it developed a strong solar wind. The solar wind are charged particles, electrons, and protons ejected by the Sun. Stellar winds is common in young stars. • The strong solar wind blew away into the interstellar space all the material including H and He that were not captured in planetesimals and planets. • Clearing of the gas in the disk sealed the fate of the planets: they did not have more material to accrete. • If gas may have remained longer, the composition of the planets, specially the terrestrial planets may be different. • This may be the case of other planetary systems! Lost of angular momentum of the Sun Magnetic field slow down the rotation of the Sun Three kinds of planets . . .

• Nebular material can be divided into “gas” (mainly H/He), “ice” and “rock” (including metals) • Planets tend to be dominated by one of these three end-members

Gas-rich

Rock-rich

Ice-rich Ratio 100:1:0.1 Terrestrial (silicate) planets Venus Earth

Ganymede Moon • Consist mainly of silicates (compounds of silicon oxide) and • Volatile elements (H, He) uncommon in the inner solar system because of the initially hot conditions. (some were supplied by comets) • Satellites like have similar structures but have an ice layer on top ( are more common in the outer nebula)

Gas and Ice Giants

• 90% H/He and consist mainly of He/H with a rock- ice core of ~10 Earth masses • Uranus and Neptune are primarily ices covered with a 75% H/He thick He/H atmosphere • Their cores grew more slowly 10% H/He and captured less gas

10% H/He Evidence of planet formation beyond our Solar System

• Early stages of a planetary system formation can be imaged directly • Dust disks have large surface area and radiate effectively in the infrared

Hubble image of a young solar system. Young star clearing part of the Thick disk gas A proto planetary disk () in the Orion nebula Proto-planetary Disks

-IR image of a disk around a star -Images in IR obtained at wavelengths from 8.7 to 24.3 micrometers - Dust disk around a young star -The gas has been blown out of the system - The star is an example An artist’s impression of a young star and its proto planetary disk in the process of forming planets

The young Sun gas/dust nebula

solid planetesimals How do we explain the existence of our moon? • Our moon is large compared with the size of the Earth •Its composition is not similar to the composition of the Earth. Its density is lower and it has less iron. •It did not form from the same material of the Earth •The most accepted theory about the existence of our moon is the impact of a large size body (Mars-size) with the Earth. •The material ejected from the impact may have accreted and condensed to form the moon. • Where did this object came from? Some proto planets may have been large (Mars- size) and one of them may have collided with the Earth •The composition of the moon is more similar to the composition of the Earth crust •Computer simulations support the impact theory for the formation of the Moon