Planet and sun

[juvenissun]

Here's one.

Where in this abst does it say the planets are made AFTER the sun? This work, in fact, suggested that planets can be made without the sun at the center.

Why should the cloud be thinned in order to have the planetary making process to start? Thicker cloud should be more favorable to make gravitational collection.

 

[Sphinx777]

(dark matter)

In astronomy and cosmology, dark matter is matter that is inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation. Its existence was hypothesized to account for discrepancies between measurements of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and measurements based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic medium.

According to observations of structures larger than galaxies, as well as Big Bang cosmology interpreted under the Friedmann equations and the FLRW metric, dark matter accounts for 23% of the mass-energy density of the observable universe. In comparison, ordinary matter accounts for only 4.6% of the mass-energy density of the observable universe, with the remainder being attributable to dark energy. From these figures, dark matter constitutes 80% of the matter in the universe, while ordinary matter makes up only 20%.

Dark matter was postulated by Fritz Zwicky in 1934 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe; these observations include the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies.

Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.

As important as dark matter is believed to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theoretical approaches have been developed which broadly fall into the categories of modified gravitational laws, and quantum gravitational laws.



Dark matter - Wikipedia, the free encyclopedia

 

[juvenissun]

One reason why it's impossible for the planets to for first (that I didn't think about until now) is that the largest object in the dust cloud would have the most gravity, and therefore have the most influence on what the center of the planetary system would be, and also be the first thing in the cloud to become a star. So the planets can't form first, because the first and largest object formed out of the stellar dust would end up becoming the star anyway.

If so, it would be possible to have a star orbiting a planet, which sits at the center of a solar system. The initial largest clump does not have to be at the center of the nebula.

 

[Itinerant Lurker]

Where in this abst does it say the planets are made AFTER the sun? This work, in fact, suggested that planets can be made without the sun at the center.

No it doesn't.

One of the many hypotheses about the formation of the solar system postulates that the planets were formed by the aggregation of particulate matter within a cloud of dust and gas surrounding the newly-formed sun. A test of the validity of one version of this hypothesis was obtained in a computerized Monte Carlo simulation of the process.
In the model used, nuclei are “injected” into the cloud one at a time, on elliptical orbits. ​

If there was no sun there would be no orbits to test. The answers to all your questions here seem like they're going to be pretty much the same: Gravity.




Lurker

 

[Itinerant Lurker]

If so, it would be possible to have a star orbiting a planet, which sits at the center of a solar system.

No.

The initial largest clump does not have to be at the center of the nebula.

It doesn't need to be at the center of the nebula to eventually form the center of a solar system.

The explanation for both of these answers is the same: Gravity.



Lurker

 

[Wiccan_Child]

I wonder when and how we will be able to understand how dark matter is involved in the formation of galaxies.

Dark matter is rather distressing, in that the nice discrete galaxies end up being one huge blob of dark matter with little specks of 'real' matter

I could be wrong here, but my understanding is that dark matter isn't some mysterious complicated mass we can't explain without a formula, but rather it's just all the stuff in the universe that doesn't emit it's own light.

It also doesn't absorb light, so it doesn't cast a shadow either. It's just literally 'dark'. But because we don't know of anything that could have mass, yet not have absorption or emission spectra, it's probably something cool and exotic... I hope

 

[Wiccan_Child]

No it doesn't.

One of the many hypotheses about the formation of the solar system postulates that the planets were formed by the aggregation of particulate matter within a cloud of dust and gas surrounding the newly-formed sun. A test of the validity of one version of this hypothesis was obtained in a computerized Monte Carlo simulation of the process.
In the model used, nuclei are “injected” into the cloud one at a time, on elliptical orbits. ​

If there was no sun there would be no orbits to test. The answers to all your questions here seem like they're going to be pretty much the same: Gravity.

Danke

 

[juvenissun]

No it doesn't.

One of the many hypotheses about the formation of the solar system postulates that the planets were formed by the aggregation of particulate matter within a cloud of dust and gas surrounding the newly-formed sun. A test of the validity of one version of this hypothesis was obtained in a computerized Monte Carlo simulation of the process.
In the model used, nuclei are “injected” into the cloud one at a time, on elliptical orbits. ​

If there was no sun there would be no orbits to test. The answers to all your questions here seem like they're going to be pretty much the same: Gravity.


Lurker

The sun and the orbit in their work are only unrelated statement. If there were no sun, the dust will still orbit around the center of the nebula. The assumed sun does not have any effect to their model.

 

[Delphiki]

It also doesn't absorb light, so it doesn't cast a shadow either. It's just literally 'dark'. But because we don't know of anything that could have mass, yet not have absorption or emission spectra, it's probably something cool and exotic... I hope

Here's what I don't get: How do we know it doesn't absorb light? What would there be to cast a shadow on that we could actually see at this distance? And if it's something that appears to have mass (because we see it's gravitational effects) but light doesn't effect it, isn't it also possible that it's just another fundamental force -- like a second gravity? Or maybe gravity doesn't actually follow an inverse square once you approach certain distances.

My guess is just that we've underestimated the amount of non-luminous matter in the universe and that dark matter isn't all that mysterious. I'll need to look further into why scientists think it's mysterious.

 

[juvenissun]

No.



It doesn't need to be at the center of the nebula to eventually form the center of a solar system.

The explanation for both of these answers is the same: Gravity.



Lurker

How would that work? The center should mean the center of the rotation.

In fact, this raised another good question, why should the sun always be the center of a solar system? If the nebula was rotating at the beginning, could the sun which is formed off the center change the center of rotation? My guess is not.

If so, why don't we have orbiting sun?

 

[Itinerant Lurker]

How would that work? The center should mean the center of the rotation.

Nope. All that needs to happen is for one part of the pre-solar system nebula to be more dense than any other part. As more and more dust particles accumulate via gravity, the central mass increases and thus generates more gravity which pulls in more and more dust particles. . .so on and so forth. "Center of gravity" and "center of a nebula" are not the same things.

In fact, this raised another good question, why should the sun always be the center of a solar system?

Is this a serious question? What do you think gravity is? What has over 95% of the mass in our solar system?

If the nebula was rotating at the beginning, could the sun which is formed off the center change the center of rotation? My guess is not.

My understanding is that the nebula itself wasn't rotating at the beginning, but that as stars formed and began generating more and more gravity the nebula began to collapse. The "spin" is simply the result of the conservation of angular momentum produced by that collapsing nebula.

If so, why don't we have orbiting sun?

Gravity.



Lurker

 

[Nostromo]

How would that work? The center should mean the center of the rotation.

In fact, this raised another good question, why should the sun always be the center of a solar system? If the nebula was rotating at the beginning, could the sun which is formed off the center change the center of rotation? My guess is not.

If so, why don't we have orbiting sun?

All the bodies in the solar system orbit their common centre of mass, and because most of the solar system's mass is in the Sun that centre of mass is very close to the Sun and often inside it. The Sun isn't completely still while the planets orbit around it.

Like so:

 

[Itinerant Lurker]

The sun and the orbit in their work are only unrelated statement. If there were no sun, the dust will still orbit around the center of the nebula. The assumed sun does not have any effect to their model.

Oh dear heavens no. Are you entirely sure you understand what nebula are? Surely you realize that these giant clouds of dust and gas are way, way, way bigger than a solar system. Here's a group of nebula,

You seem to be thinking that the bright spot in the middle will eventually form into a single solar system, but that's not what we're saying. See that blue part that's highlighted? That's the Trifid Nebula, a proposed "stellar nursery", and as we can see it's not at all at the "center" of anything. Let's zoom in:

Pretty amazing right? Yet even here we're not done. When we talk about nebulas collapsing into solar systems we start with the dense clumps of matter scattered throughout the nebula itself like this:

(source)

These little pockets of density start collapsing the nebula around them via gravity until they form proto-stars:

(Note: this is an actual image of several proto-stars surrounded by dust and gas in the Orion Nebula)

As stars continue to form conservation of angular momentum will increase the spin of the clouds of dust and gas which surround them and form an accretion disc:

Without a star or proto-star there won't be anything to pull the surrounding dust and gas into this formation. Is it really so unreasonable to conclude that the gravity needed to form a solar system would come from the most massive object in that system?




Lurker

 

[juvenissun]

The one knows a little stopped. Those know less keep talking.

Would I get any more useful info on my OP?

 

[Wiccan_Child]

The one knows a little stopped. Those know less keep talking.

Which means what, exactly?

Would I get any more useful info on my OP?

Doubtful. Your question's been answered many times over.

 

[Nostromo]

He wishes we'd all shut up so you can carry on.

 

[Naraoia]

Because without a star to orbit, they wouldn't meet the definition of a planet. That's not to say that enough matter can't come together under the pull of gravity to make planet-sized objects before the center of the accretion disk starts a fusion process... But something like this has never been observed (as far as I'm aware of, anyway) because, well, there'd be no visible star to find it.

That made me think of brown dwarfs. Though they are kind of visible, and not planet-sized by definition...

 

[Wiccan_Child]

Here's what I don't get: How do we know it doesn't absorb light? What would there be to cast a shadow on that we could actually see at this distance?

Background nebulae and galaxies and things. We can see right through it, which is contrary to normal matter - something that massive, if made of normal matter, would blot out the sky.

And if it's something that appears to have mass (because we see it's gravitational effects) but light doesn't effect it, isn't it also possible that it's just another fundamental force -- like a second gravity?

It affects light gravitationally, just not spectrographically - it may, for instance, cause lensing.

Or maybe gravity doesn't actually follow an inverse square once you approach certain distances.

My guess is just that we've underestimated the amount of non-luminous matter in the universe and that dark matter isn't all that mysterious. I'll need to look further into why scientists think it's mysterious.

Because we have no idea what it could be. It's not that there are many candidates for what it is, but rather, there are no candidates.

 

[Standing_Ultraviolet]

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