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Why is Earth so Dense ?

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Jun 1, 2020
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For the all comments about pin ball and giant impacts, some known exoplanet systems do not fit this view. Those exoplanets have densities like Earth too.
Yes, though I doubt anyone expects a working model that would fit every star system. That would be next to impossible. Physics almost always needs to know the exact initial conditions to make a model, or theory, in order to work for any specific case. The possible number of initial conditions that can occur when a cloud fragments and begins to collapse into a star system is close to infinite.

We might find something for our system since we may be able to get almost exact current conditions (vectors, mass, etc.) and rewind the clock. But it won't be able to address any prior disturbances from passing stars, etc. So I'm not optimistic we will have any great model, though the Nice model is not bad, considering.
 
Jun 1, 2020
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In English, nice is considered (sadly) almost a non word, being very 'flat'.
"Nice cakes and a nice cup of tea in the afternoon." "How nice" "Nice dress".
That's also true here, IMO. We used to say "good" but we recognize this should be used for more moral issues, so we seem to be stuck with "nice", though if you have a more inflated word, that would be, uh, nice. :)
 

rod

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Folks, keep in mind exoplanet studies continue and continue to show challenges to the MMSN for our solar system, example how gas giants form. Here is a newer report. Small Jupiter-like planets hint we need to rethink how gas giants form,
https://www.newscientist.com/article/2299870-small-jupiter-like-planets-hint-we-need-to-rethink-how-gas-giants-form/, “Two Jupiter-like planets that orbit a young star are much smaller than expected, which may suggest we need to rethink our ideas of the early evolution of gas giant planets."

NASA ADS Abstract, Rapid contraction of giant planets orbiting the 20 million-years old star V1298 Tau, https://ui.adsabs.harvard.edu/abs/2021arXiv211109193S/abstract, November 2021. “Current theories of planetary evolution predict that infant giant planets have large radii and very low densities before they slowly contract to reach their final size after about several hundred million years. These theoretical expectations remain untested to date, despite the increasing number of exoplanetary discoveries, as the detection and characterisation of very young planets is extremely challenging due to the intense stellar activity of their host stars..."

My observation. The 47-page arXiv paper (https://arxiv.org/pdf/2111.09193.pdf) “Table 2: Planetary parameters for the V1298 Tau system” shows V 1298 Tau b orbital period 24.1399 days, a=0.1719 au, e=0.134. Mass = 0.64 Mjup, radius = 0.868 Rjup. The distance to V 1298 Tau is 108.5 pc and the star mass reported is 1.17 solar masses and 20 million years old age. Using my calculations, V 1298 Tau b orbital period = 2.4061E+01 days or 24.06 days, very close to the published 24.1399-day period. In the postulated 20 million years age reported, the exoplanet could complete 3.0360E+08 revolutions around the parent star or more than 300 million revolutions. Applying the MMSN to the postulated protoplanetary disk that the exoplanets reported at V1298 Tau evolved from, total dust and gas mass = 3.895626E+03 earth masses, or nearly 3900 earth masses. V 1298 Tau is a 4-exoplanet system with a companion star, HD 284154. The arXiv paper reports, “2.1 Membership to Group 29. V1298 Tau is the low-mass companion of the warmer, G0-type star HD 284154 at a projected separation of 97.7 arcsec (or 10600 AU at the distance of the system). The pair belongs to the recently identified Group 29,12 which is a young, sparse association of coeval stars in the Taurus 16 region, all of which share very similar proper motions and distances based on the Tycho-Gaia astrometric catalog (TGAS).46, 47.”

Some of the gas giants at V 1298 Tau, challenge current evolutionary models on how gas giant planets form. Science continues to work on showing How Gas Clouds Evolve into People but with struggles ongoing :)
 
May 14, 2021
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Does make sense that if the earth was hit twice (Theia and Theia II), that it was more silicate crust and mantle and less iron/nickel core that was crunched out to make the moon, leaving earth with most of the core and less of the mantle, therefore denser.
 
Dec 3, 2021
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Hello ..
Somewhat Plausible Theory but hard to relate to the creation of many other moons around other planets in our solar system (over 200 and none for Mercury and Venus… the planets which are closest to Earth density).
No other moons were formed like our Moon, taking a big chunk of less dense mantle and crust into space. They were all captured or made from much less dense gas giants
 
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Does make sense that if the earth was hit twice (Theia and Theia II), that it was more silicate crust and mantle and less iron/nickel core that was crunched out to make the moon, leaving earth with most of the core and less of the mantle, therefore denser.
You only need one big collision to do the job (which appears to be the case), and two would likely have left some indication in the Earth-Moon system.
 
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May 14, 2021
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Whether it be one hit or two as one latest theory holds, would result in mantle and a bit of core to make the moon leaving denser earth.
 
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Whether it be one hit or two as one latest theory holds, would result in mantle and a bit of core to make the moon leaving denser earth.
I have not heard of a two hit theory, two hits is much less likely than one hit, and if one hit does the job of knocking out mantle to make the moon, I'll plead Occam's Razor and say one hit.
 
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I had read somewhere about multiple impacts forming the moon, but, I found this tonight. Haven't read it all, but, looks like where the theory came from. I can well imagine that there were dozens of almost planets in various orbits in the system, some were ejected by the large planets, some careened into the sun. It seems it may have taken several hits to knock Venus' rotation retrograde. It would not surprise me for multiple smaller hits to form the moon from the earth.
 
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Dec 3, 2021
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I had read somewhere about multiple impacts forming the moon, but, I found this tonight. Haven't read it all, but, looks like where the theory came from. I can well imagine that there were dozens of almost planets in various orbits in the system, some were ejected by the large planets, some careened into the sun. It seems it may have taken several hits to knock Venus' rotation retrograde. It would not surprise me for multiple smaller hits to form the moon from the earth.
Well, the paper looks reputable, so perhaps. Maybe they are right, maybe they are the only ones that think this. This is what real science does, rational discussion. I've always been a bit skeptical of numerical simulations, going back to my days of doing Monte Carlo simulations. It looks plausible, and many small collisions smells better than 2 quite big ones. It looks like the world experts are still discussing it, with presumably different viewpoints. It sure as hell is not going to be decided here; suffice to say both viewpoints will give an excess core size to Earth giving the slightly higher density compared to the other 3 rocky planets. Maybe the space.com world can let it rest at that.
 
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Jun 1, 2020
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I had read somewhere about multiple impacts forming the moon, but, I found this tonight. Haven't read it all, but, looks like where the theory came from. I can well imagine that there were dozens of almost planets in various orbits in the system, some were ejected by the large planets, some careened into the sun. It seems it may have taken several hits to knock Venus' rotation retrograde. It would not surprise me for multiple smaller hits to form the moon from the earth.
There is evidence that two large bodies merged to form the Moon. These bodies would have been in orbit with Earth and would have a far more gentle impact, a "splat", during their merger that would explain many features on the Moon, especially their differences between the near side and far side.

But a large single impact would also explain this, as well. I suspect that the probability of even one hitting Earth just right is very low, and two would be far more challenging to explain, IMO. If it were that easy, where are the moons due to impacts with other planets?

Perhaps there will be enough evidence found to favor one model over the other.
 
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We may never know how many major impactors there were, we just may find several various scenarios which could fit the picture.
 
Jun 1, 2020
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We may never know how many major impactors there were, we just may find several various scenarios which could fit the picture.
Yep, like detective work after the reported crime. :)

The evidence is mostly found in the Earth and Moon itself. The Moon has some unusual features associated with its formation that seem to favor a second but gentle impact upon it, hence one orbiting with it rather than coming from afar at high speeds.

Thanks to complex modeling efforts and experiments, an impact with a planet like Earth seems to have far more ways to avoid making a moon than otherwise.
 

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