What is Planet formation: Definition and 19 Discussions

The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting the Sun. The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens (1755) and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular theory is the solar nebular disk model (SNDM) or solar nebular model. It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the original nebular theory are echoed in modern theories of planetary formation, but most elements have been superseded.
According to the nebular theory, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). These clouds are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk (proplyd) around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes approximately 1 million years to form, with the protoplanetary disk evolving into a planetary system over the next 10–100 million years.The protoplanetary disk is an accretion disk that feeds the central star. Initially very hot, the disk later cools in what is known as the T Tauri star stage; here, formation of small dust grains made of rocks and ice is possible. The grains eventually may coagulate into kilometer-sized planetesimals. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes approximately 100 million to a billion years.The formation of giant planets is a more complicated process. It is thought to occur beyond the frost line, where planetary embryos mainly are made of various types of ice. As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses (M⊕) it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. Ice giants such as Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.

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  1. M

    I Formation of rocky planets without a star

    After googling for many hours I am still not understanding why the formation of rocky planets shall be restricted to protoplanetary discs. Why can't they form from dustclouds far away from any star? Agglomeration should still happen over time. Jeans criterium should not play a role since it is...
  2. K

    I Planetary accretion model?

    Hi - having come up blank when searching, I figured I'd ask here: do you know of any (free, preferably open source) planetary accretion models/simulators available for download? I'm primarily looking for solar system formation / evolution rather than planetary geochemistry (although I'd have an...
  3. Guilherme Franco

    A Where can I find a text about heavy elements migration?

    Let me be more specific: I'm needing some source that talks, in a more broad way, about how heavier elements tend to "sink into the core" during Earth's formation (when we're talking about siderophile elements, that readily combine with molten iron). Wikipedia's page about the Goldschmidt...
  4. Q

    I Planetary Orbits in AU - Stability & Mass Limits

    All of the planets should have individual orbits, and should be between the mass of Mercury and Mars. What can their orbits be in AU's? Is there a way to find out how close they can be without destabilizing each other? If we assume the star is about the same size as Sol.
  5. STC4476

    Planets and Cell structure...

    This i know sounds like a really odd question, but thinking along the Lines that i am.. i guess it has a sense of validity. Could it be Possible, that Planets exist with Dual cores that internally orbit inside a Planet.. or partially joined. Thinking along the lines of Cell structure and how...
  6. C

    Lagrangian mechanics and planetary formation

    I am disappointed by my graduate-level classical mechanics course, and especially the treatment of Lagrangian/Hamiltonian mechanics. Now, I scanned my notes and some crazy idea popped into my head, further fueling my discontent towards this course, because all the problems covered in class were...
  7. JosephLa

    How is centrifugal force created in a cloud of gas in space?

    First, may I apologize in advance if this question is in the wrong section of the forums. Given a cloud of mixed gasses and rocky material, how is spin created when a star is born? I will also give the cloud of gas a random order of movement prior to star formation. Can the physics for the...
  8. Kelson Adams

    Calculating the Age of Planets without Radioactive Dating

    I understand that it's possible to calculate the age of terrestial planets through radioactive dating their soil. However, the gas planets present a different challenge since we cannot currently land on them. Any ideas on how to calculate their ages in a different manner?
  9. H

    Heavy water mismatch between comet 67P and Earth's oceans

    On the recent news announcement that finds a mismatch in the heavy-to-regular water ratio on 67P found by the Rosetta mission and Earth's oceans, I wonder if the passage of time has been considered? Could the several billion years that comets have been exposed to radiation have increased their...
  10. V

    Planet formation vs Sun collapse

    I understand how conservation of momentum leads to planet formation and planet rotation. However, after studying this model, I have ran into a point of confusion that I cannot find the answer to: Why don't the planets collapse into the sun just as dust particles collapsed inward via...
  11. M

    Star and planet formation according to the 2nd Law of Thermodynamics

    In the first part of Brian Cox's documentary series 'Wonders of the Universe', he explains how the entropy of the universe always increases, and that we are therefore headed for a state of total 'disorder' where all is left of the universe is photons and dying black holes. But wasn't this...
  12. KevinMWHM

    Planet Formation Question

    Is it common for rocky planets to form closer to a star and gas planets further? Why or why not is this?
  13. D

    Chance of Planet Formation without Stars

    What is the chance of planets forming from gas clouds without a star? has anyone calculated this? are they more likely to form than stars? could there be billions of rogue planets floating arround our galaxy?
  14. A

    Friction necessary for planet formation?

    For a gas and dust cloud to collapse under gravity to form a star and planets, is the concept of friction important at all? In other words: let's consider a large number of billiard balls, no internal degrees of freedom, participating in completely elastic collisions, with random initial...
  15. T

    Planet formation and star formation

    When our star the Sun finally accumulated enough matter for fusion to start what would that have done to the planets around it? Afterward what would have happened to the Earth for it to get from when the sun started fusion and blasted the Earth and now, with water and atmosphere?
  16. S

    Planet Formation: Unsolved Questions in Solar System Origin

    In a recent theological discussion, I came to realize that there is a point in question regarding planet formation in the current model of the origins of the Solar System. The question itself is not theological, but has some theological ramifications. It goes like this: Planets are believed...
  17. Q

    The Role of Gravity in Star and Planet Formation

    can u help me to explain the role of gravity in the equilibrium of stars in their production of enegry , and an explanation of the role of gravity in the formation of planets. thankssssssss
  18. G

    Gas Giant Ring Collapse Theory for Rocky Planet Formation

    Has there ever been a hypothesis or theory propsed (current or obsolete) proposing that the rocky planets formed from gas giants whose ring system collapsed and combined with the core of the gas giant forming the bulk of a new rocky planet? Just wondering if this has ever been proposed or...
  19. P

    Planet formation DOES happen in slow motion

    [FONT=Comic Sans MS] [COLOR=LightBlue] Okay, this is a general question.. i just read this thread (https://www.physicsforums.com/showthread.php?t=28343) and started wondering about it. THis planet is considered "young", and it is a million years old. SO, my question was why does planet formation...
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