It shrank because of all the boys going there to get stupider.
Jupiter was formerly twice its current size, had a much stronger magnetic field
61–70 of 78 posts
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#62Earlier quoted context omitted.
Gravity changes little over that distance - it's more because of the compounding effect of atmospheric pressure (the deeper you go, the more air you have above you which raises the pressure, raising the density and meaning that pressure increases exponentially faster).
What makes that curve exponential?
Newtonian gravity (classical mechanics).
Two-body gravitational attraction is observed to be an inverse square power law; gravitational attraction decreases with the square of the distance.
g, the gravitational constant of Earth, is observed to be exponential; 9.8 m/s^2.
Atmospheric pressure: https://en.wikipedia.org/wiki/Atmospheric_pressure#:~:text=P... :
> Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#63That seems at odds, with something I previously learned about Jupiter:
"As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve."
https://en.wikipedia.org/wiki/Jupiter
So is this a significant new finding, changing previous assumptions, or is it part of the "evolutionary history" meaning it was assumed before, that in early times it was bigger?
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#64Earlier quoted context omitted.
The density falls off pretty steeply at the “edge”, so the exact definition only makes little difference for the radius: https://www.researchgate.net/figure/Density-vs-radius-for-a-...
This is because of Newtonian gravity being inversely proportional to the square of the radius, right?
It's related to the Boltzmann distribution being an exponential. But there are all kinds of effects that make a planet's atmosphere deviate from an ideal gas at rest in a homogeneous container.
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#65Presuming that Jupiter got its start from a cloud of gas and dust, far far larger, at what point did its gradual compression make it a planet?
As its rotation slows, it will shrink even further.
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#66That seems at odds, with something I previously learned about Jupiter: "As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve." https://en.wikipedia.org/wiki/Jupiter So is this a significant new finding, changing previous assumptions, or is it part of the "evolutionary history" meaning it was assumed before, that in early times it was big…
The new part is probably the precise details about size and strength of the magnetic sphere in the past and that they used a different mechanism to fill in gaps in existing theories.
> Importantly, these insights were achieved through independent constraints that bypass traditional uncertainties in planetary formation models—which often rely on assumptions about gas opacity, accretion rate, or the mass of the heavy element core
> The results add crucial details to existing planet formation theories, which suggest that Jupiter and other giant planets around other stars formed via core accretion, a process by which a rocky and icy core rapidly gathers gas. These foundational models were developed over decades by many researchers, including Caltech's Dave Stevenson, the Marvin L. Goldberger Professor of Planetary Science, Emeritus. This new study builds upon that foundation by providing more exact measurements of Jupiter's size, spin rate, and magnetic conditions at an early, pivotal time
https://www.caltech.edu/about/news/jupiter-was-formerly-twic...
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#67Earlier quoted context omitted.
Venus would get a slight radius buff, too, if we applied that metric.
But Venus has a solid surface.
Yes, that's why I said buff. The radius as defined would increase, because surface pressure is 90 bar, so at 1 bar, you're pretty high in the atmosphere. I can see merit in such a definition because that is the level at which we wouldn't have to pressurize our space stations to be comfortable. (Really 1/3 bar is fine too.)
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#68Earlier quoted context omitted.
Gravity changes little over that distance - it's more because of the compounding effect of atmospheric pressure (the deeper you go, the more air you have above you which raises the pressure, raising the density and meaning that pressure increases exponentially faster).
What makes that curve exponential?
Starting at an initial density of air, suppose you descend a distance D such that the air density doubles. Now your air is twice as dense, which doubles the pressure underneath it, meaning if you descend a further D the density will double again. Continue ad infinitum (or at least until the ideal gas law stops being a good approximation).
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#69Earlier quoted context omitted.
It was hotter, a lot hotter. Like stars, radius for a gas giant is increased by heat, and decreased by increased mass. These two factors are rarely completely independent, of course, so it gets complicated. Especially in a star where masses are large enough to result in densities sufficient to cause fusion - and large releases of heat, which then cause decreased density, etc. But all other factors being constant, the…
> ... decreased by increased mass. I don't think this is in general true for planets or stars. You're confounding multiple effects. For a fixed number of particles, increasing metallicity , which follows average particle mass, should reduce radius, but for a fixed metallicity and temperature, increasing particles will increase radius. Temp has the effects stated. You can roughly validate this by the fact that massive…
In the sense that higher gravity increases density all other things being equal. yes, absolute size will slowly increase - until it collapses, anyway.
Re: Jupiter was formerly twice its current size, had a much stronger magnetic field
#70I've always wondered about the core of these gas giants. I assume it is some liquid form of light elements. What is puzzling is the presence of the gas giants in the middle of solar system's planetary line up: why are they in the middle and the ones closer or further away from the central star are not like them? Is it the temperature gradient?
Because of solar wind. After the sun formed from the material of the proto solar system it started producing solar winds. This pushes light elements to the edge of the solar system but heavy elements stay. So rocky planets form. Then the light elements collect as well and reenter the interior solar system as comets which redeposit light elements on the surface of rocky planets. In the mean time the light elements that collected together in great quantities formed the gas planets.
This is all a very traditional view afaik and doesn’t explain where mantle light elements come from. For example there is a great deal of water that is in the mantle that drives geochemical changes in the mantle rocks. Was that there originally? Or was it put their after plate tectonics started and subduction sucked water into the mantle? I don’t know but I would assume there are plenty of geodynamics people who would have opinions more deeper than mine on the topic.