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Formation of the Solar System Chapter 8 Formation of the Solar System Chapter 8 To understand the formation of the solar system one has to apply concepts such as: • Conservation of angular momentum • Conservation of energy The theory of the formation of the solar system need to explain: • The pattern of the sense of rotation of planets • 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 Sun 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 planet rotates on its axis. But there are some exceptions • Venus rotates backwards (Rotational axis tilted close to 189 degrees) • Uranus rotates on its side (Rotational axis tilted close to 90 degrees) • Most small moons do not share the orbital plane of the planet • Triton (Satellite of Neptune) orbit the planet in opposite sense • Earth is the only terrestrial planet with a large moon A model for the formation of the solar system has to account for: • Different composition of planets (rocky, gaseous, icy) • Existence of many asteroids and comets Angular Momentum • 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 nebular hypothesis (and the rejected collision theory) • The idea that the solar system was born from the collapse of a cloud of dust and gas for proposed by Immanuel Kant (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 star. Planets formed from debris of the collision. But we know now that collision (or near collisions) between two stars are very, very rare. • Considering that collision are rare, the proposed idea of the collision may explain a unique event on how our planetary system 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 exoplanet (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 Galaxy The nebular theory • Stars are born from the collapse of an interstellar cloud 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 nebula 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 life in a spectacular explosion called supernova. • 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 evolution of these stars continues enriching the material in the clouds of dust and gas. The Orion nebula 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 gravitational collapse (collapse due to its own gravity) 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 temperature 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). protoplanetary disk. • 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 accretion) • The presence of dust is a key element in the formation of small particles which will stick together and form planetesimals. 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 temperatures in the inner part of the disk This is why the inner planets are rock-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 planetesimal. • 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 metals 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 metal became small compared with the gas. Formation of Jovian planets • Beyond the frost line, planetesimals could accumulate ice • Hydrogen 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.
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