[evan_au (OP)]

Cheryl J asks Why are the planets such different sizes to begin with and in no particular size order?


What do you think?

[Ophiolite]

The answer is complex, but this is a simplified version.

Planets form from a collapsing cloud of gas and dust. They are almost incidental, since most of the mass goes into the proto-star. The rest of the dust and gas form a flat disc around the star in which particles condense and coalesce.

As the star warms up the temperature in the disc prohibits ices from forming in the inner part of the system, so that rocky planets are formed. Further out ices can condense and coat any rocky cores that are formed. Stellar activity also drives gas away from the inner part of the system (and eventually all of it), but the outer rocky cores can attract some of this gas.

Thus we end up with small rocky planets in the inner system and large gaseous and icy planets with rocky cores in the outer system. During this process planets can move around a lot as a consequence of interaction with the disc and mutual gravitational effects. Some are lost into the parent star, some are ejected from the system and some swap places.

[guest39538]

Planets form from a collapsing cloud of gas and dust.

By what physics does the cloud collapse?

[puppypower]

The Titius-Bode law was formulated around 1770 and correctly calculated the position of Uranus before it was even discovered. The law states that there is a certain ratio between the orbital periods of planets in a solar system. So the ratio between the orbital period of the first and second planet is the same as the ratio between the second and the third planet and so on.

Steffen Kjær Jacobsen is a PhD student in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen. Jacobsen said:

We decided to use this method to calculate the potential planetary positions in 151 planetary systems, where the Kepler satellite had found between three and six planets. In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets. Using T-B’s law we tried to predict where there could be more planets further out in the planetary systems. But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite.

In 27 of the 151 planetary systems, the planets that had been observed did not fit the T-B law at first glance.

The researchers then tried to place planets into the ‘pattern’ for where planets should be located. They added the planets that seemed to be missing between the already known planets and also added one extra planet in the system beyond the outermost known planet.

In this way, they predicted a total of 228 planets in the 151 planetary systems.

Why do planets align with orbital period ratios if the formation scenarios seems to imply something less ordered?

[JoeBrown]

There is a lot we don't know about the formation of "our" solar system.   There are some mechanics we've figured out.

• Heavier elements, the bulk of inner planets (Mercury, Venus, Earth & Mars), wasn't available in the early Universe.
  
  • These elements formed inside a larger star that existed before ours
  • That star exploded, scattering material, from which these planets and ourselves are we're made.
  • Most likely we are made of remnants of a previous generation star, ours could be a 3rd generation star, the larger the start the shorter it's life span
• Sun's raw material (hydrogen)
  
  • Was it a cloud of gas that collapsed due to shock wave of previous generation star's explosion?
  • Was the previous generation star large enough that remaining hydrogen supply was enough to form our sun?
  • We don't have data to make an informed determination
  • I suspect the first, but it's a guessing game
• Electromagnetic and gravity forces cause masses of material (comets, asteroids, meteors and gas) to swirl toward a centeral body of gravity (like a bathtub drain) of the sun.
• The distribution of material caused the size of planets, which evolved over a long period of time from proto-planets
• We believe the Earth and Moon were two different proto-planets that once collided, each likely had similar characteristics, but were two different bodies, than they are now.
• The past is fun to speculate, but there's many unknowns.  We have little data for how many other planets or proto-planets existed or how many were swallowed by the Sun, Juipiter, Saturn, Uranus or Neptune.
• Our solar system is ~4 billion years old, it's changed a lot over that period of time

[guest39538]

There is a lot we don't know about the formation of "our" solar system.   There are some mechanics we've figured out.

• Heavier elements, the bulk of inner planets (Mercury, Venus, Earth & Mars), wasn't available in the early Universe.
  
  • These elements formed inside a larger star that existed before ours
  • That star exploded, scattering material, from which these planets and ourselves are we're made.
  • Most likely we are made of remnants of a previous generation star, ours could be a 3rd generation star, the larger the start the shorter it's life span
• Sun's raw material (hydrogen)
  
  • Was it a cloud of gas that collapsed due to shock wave of previous generation star's explosion?
  • Was the previous generation star large enough that remaining hydrogen supply was enough to form our sun?
  • We don't have data to make an informed determination
  • I suspect the first, but it's a guessing game
• Electromagnetic and gravity forces masses of material (comets, asteroids, meteors and gas) to swirl toward a centeral body of gravity like a drain (the sun).

[/li]
[li]The distribution of material caused the size of planets, which evolved over a long period of time from proto-planets[/li]
[li]We believe the Earth and Moon were two different proto-planets that once collided, each likely had similar characteristics, but were two different bodies, than they are now.[/li]
[li]The past is fun to speculate, but there's many unknowns.  We have little data for how many other planets or proto-planets existed or how many were swallowed by the Sun, Juipiter, Saturn, Uranus or Neptune.[/li]
[li]Our solar system is ~4 billion years old, it's changed a lot over that period of time[/li] 
[/list]

Indeed there is a lot of ideas about the ''prequel'' before Human existence and the existence of matter.   I consider that starting ideas should resolve around space pre-existing before anything of substance, on the presumption that for anything to exist it needs space to exist in? 

added - to add further, if there is no unbounded n-dimensional exterior space it is seemingly obvious that a dimension of an object or an event could not be?

[chiralSPO]

The Titius-Bode law was formulated around 1770 and correctly calculated the position of Uranus before it was even discovered. The law states that there is a certain ratio between the orbital periods of planets in a solar system. So the ratio between the orbital period of the first and second planet is the same as the ratio between the second and the third planet and so on.

Steffen Kjær Jacobsen is a PhD student in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen. Jacobsen said:

We decided to use this method to calculate the potential planetary positions in 151 planetary systems, where the Kepler satellite had found between three and six planets. In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets. Using T-B’s law we tried to predict where there could be more planets further out in the planetary systems. But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite.

In 27 of the 151 planetary systems, the planets that had been observed did not fit the T-B law at first glance.

The researchers then tried to place planets into the ‘pattern’ for where planets should be located. They added the planets that seemed to be missing between the already known planets and also added one extra planet in the system beyond the outermost known planet.

In this way, they predicted a total of 228 planets in the 151 planetary systems.

Why do planets align with orbital period ratios if the formation scenarios seems to imply something less ordered?

Because the planets interact with one-another gravitationally.

The current orbits are not necessarily what they were as the planets were forming. Planets tug on each other when they reach the point closest in their orbit. If the tugs all happen at one point of the orbit, then each successive tug will change the orbits of both planets. If the orbital periods are whole-number ratios then these tugs will happen evenly (symmetrically) on multiple points on the orbit, and cancel out to have no net tug.

This is also observed in the Jovian and Saturnian moon systems.

See orbital resonance: https://en.wikipedia.org/wiki/Orbital_resonance

[Ophiolite]

By what physics does the cloud collapse?

Gravity, though pressure waves from proximal, or embedded supernovae may trigger the collapse.

[evan_au (OP)]

By what physics does the cloud collapse?

Another aspect of the collapse is that as you compress gas and dust into a protoplanetary disk, parts of it swirling in different directions collide and it heats up. This increased temperature resists further compression.

However, when you have a mixture of elements, they can radiate heat away from the disk, keeping it cool enough to compress more, under the influence of gravity.

There are two important scenarios in cosmology where this collapse mechanism fails:

• The first generation of stars formed after the Big Bang - the raw materials were mostly Hydrogen, with a bit of Helium, and a touch of Lithium. This is not enough heavy elements to radiate away the heat of compression. It is thought that it would take really massive gravitational field to overcome the expansion of the gas due to temperature rise. It is thought that this would have produced some truly massive stars, perhaps 100 times the mass of the Sun. These would have quickly burnt their fuel and exploded, seeding space with heavier elements, and allowing second-generation stars to form at lower masses, resulting in longer-lived stars.
  
• Many physicists believe that the effects of Dark Matter can be attributed to a currently-unknown neutral subatomic particle which interacts weakly with matter, and with itself. In particular, it would not interact with electric charge, and it could not produce photons (heat). Unable to radiate away their initial kinetic energy, these particles would not form a planetary or galactic disk, but would continue orbiting galaxies as a "Dark Matter Halo", resulting in the observed gravitational lensing of distant galaxies.