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trent1
2011-May-29, 07:38 PM
Consider that you were on a planet, or moon, some 100ly away from our Sun. You point your telescope towards the Sun and you see a very large star about 2ly in diameter.

Well you're not actually seeing the star itself, but reflected light from the (relatively) high albedo number of the Oort Cloud. Could the same be possible of some large stars that we've documented from Earth? I mean, for example, could VY Canis Majoris be a smaller star, hiding under a spehrical layer of reflective dust and ice?

I haven't had time to consider the physics, and frankly it's such a 'shot in the dark' idea that I might find myself trying to calculate nonsense.

I was just wondering if anyone here had done the math on the idea? Or can tell me that I'm completely wrong by other means! :razz:

If there was any truth in my suggestion - It would make finding and identifying habitable planetary systems incredibly difficult - But would also, ironically, (almost) prove that they are there, 'hiding', since our very own Oort Cloud is an indicator of planetary accretion.

I've said it - Now make me a liar! :lol:

Grashtel
2011-May-29, 11:00 PM
No matter how reflective Oort cloud objects are (and they IIRC aren't though to be particularly reflective) they wouldn't produce the kind of effect you are thinking of because there are only a tiny number of them compared to the volume they are spread over. If the effect you are proposing did occour we would be able to see it from Earth as the night sky would be bright with reflected light from the Oort cloud.

WayneFrancis
2011-May-30, 01:56 AM
Consider that you were on a planet, or moon, some 100ly away from our Sun. You point your telescope towards the Sun and you see a very large star about 2ly in diameter.

Well you're not actually seeing the star itself, but reflected light from the (relatively) high albedo number of the Oort Cloud. Could the same be possible of some large stars that we've documented from Earth? I mean, for example, could VY Canis Majoris be a smaller star, hiding under a spehrical layer of reflective dust and ice?

I haven't had time to consider the physics, and frankly it's such a 'shot in the dark' idea that I might find myself trying to calculate nonsense.

I was just wondering if anyone here had done the math on the idea? Or can tell me that I'm completely wrong by other means! :razz:

If there was any truth in my suggestion - It would make finding and identifying habitable planetary systems incredibly difficult - But would also, ironically, (almost) prove that they are there, 'hiding', since our very own Oort Cloud is an indicator of planetary accretion.

I've said it - Now make me a liar! :lol:

If the Oort cloud was as you describe then we wouldn't be able to see out through it. IE if its properties made a planet 100ly away observe our sun to be as large as the Oort cloud is then we to would be able to see the Oort cloud. But we can't. Just like the "asteroid belt" is actually very sparse the space between items in the Oort cloud are VERY vast. The fact that we can't yet visually observe our own Oort Cloud means that it is doubtful we'd be able to observe another solar system's Oort cloud and there is no chance for another solar system with similar technology to observe our Oort cloud.

To give you an idea of the magnitude of what would need to be involved.

The Oort cloud is at about .03 light year from the sun.
The surface area of something at .03 light year is about 1.012241x1030m2
If the Oort cloud was just 1mm thick then the volume would be about 1.012241x1027m3
The volume of the sun is only ~1.412x1021m3
The mass of the Oort cloud is only estimated at about 3◊1025 kg or about 5x the Earth
The volume of the Earth is only 1.08321◊1015m3
So the The ort cloud would have a density at that point of 100,000,000,000 (100 billion times less)

To make things worse the Oort cloud isn't 1mm in thickness but 4.7302642◊1014m in thickness at the lower boundary.

Now it gets even worse if you want to use a Oort cloud that is 1ly in radius not .03ly in radius.

astromark
2011-May-30, 08:14 AM
Trent 1 .. I actually understand your thought but as WayneFrancis has just said NO.

Understanding that about 4.5 billion years ago there was a great cloud of stellar material that birthed the sun and the planets.

That some of that material did not get swept up by the gravity of other masses and become planets is what the Oort cloud is.

chornedsnorkack
2011-May-30, 09:17 AM
How can we observe light reflected by Oort cloud?

We do see zodiacal light, and gegenschein. But do we know where they come from?

Zodiacal light and gegenschein can be observed because they are concentrated in parts of the sky and can be contrasted against somewhat darker sky elsewhere. But Oort Cloud is supposed to be around us.

How much of the diffuse background illumination of the sky is the glow of the atmosphere of Earth in a few tens to a few hundreds of km away, how much is sunlight reflected by Oort cloud few tens to few tens of thousands of AU away, how much is unresolved stars and interstellar gas a few tens to a few tens of thousands of lightyears away in disc and crown of Milky Way, and how much is unresolved galaxies and intergalactic matter ten million to ten milliard lightyears away?

antoniseb
2011-May-30, 12:05 PM
At 100 ly, an observer sees about 30 million visible photons from our Sun per second per square meter of their telescope's collecting area.
Assuming that the Oort cloud is 300 million white spheres 20km in radius, 0.5 ly from the Sun, let's figure out how many photons per second your observer would see from the Oort cloud. ...
Each one has effectively 5 billion square meters of surface area. So each one receives 2x10-23 of the total bath of solar photons. Combined they receive 5x10-15 of the total solar output. So this Oort cloud would present one photon per square meter of collecting area every two months, and that light would be spread over a few square degrees of the sky.

Now the actual objects are much less white, but some of them are (as Wayne Francis (nice work Wayne!) indicated in his very conservative estimate) much closer to the Sun than 0.5 ly. So, these numbers are probably right within one or two orders of magnitude.

Net result is that your observer probably wouldn't see it.

chornedsnorkack
2011-May-30, 12:45 PM
We can and we do see zodiacal light and gegenschein.

Could an observer on, say, Epsilon Eridani see the zodiacal light from Solar System?

ngc3314
2011-May-30, 02:30 PM
How can we observe light reflected by Oort cloud?

We do see zodiacal light, and gegenschein. But do we know where they come from?

Yes. Pioneers 10 and 11 measured the disappearance of the zodiacal light past the main asteroid belt, and Doppler shifts in reflected sunlight tell us their dynamics in exquisite detail (that was Brian May's dissertation when he finally finished it).


Zodiacal light and gegenschein can be observed because they are concentrated in parts of the sky and can be contrasted against somewhat darker sky elsewhere. But Oort Cloud is supposed to be around us.

How much of the diffuse background illumination of the sky is the glow of the atmosphere of Earth in a few tens to a few hundreds of km away, how much is sunlight reflected by Oort cloud few tens to few tens of thousands of AU away, how much is unresolved stars and interstellar gas a few tens to a few tens of thousands of lightyears away in disc and crown of Milky Way, and how much is unresolved galaxies and intergalactic matter ten million to ten milliard lightyears away?

Gegenschein is concentrated because of its scattering behavior, so that by itself doesn't tell you its distance (its being part of the zodiacal light does).

We can separate scattered starlight from interstellar dust (emission from gas is confined to emission lines) because it also shows up in the far-IR, obviously much colder than the zodiacal light. Atmospheric airglow can be separated from both zodiacal light and the more distant background by simultaneously space- and ground-based observations at multiple angles from the Sun (I recall plans for a campaign involving HST and Las Campanas for that, which also gave a value for the integrated light of galaxies (http://thesis.library.caltech.edu/3745/)). The Oort Cloud must be below the errors in all the above, which is such a loose limit that it tells us nothing we didn't already think we knew from comet statistics.

Folks working on the Terrestrial Planet worried a lot about exo-zodiacal emission, which could be the major noise contributor for detection of Earth analogs around many stars. A few such clouds have been detected interferometrically (http://arxiv.org/pdf/1104.1382), but our detection limits currently are 200 times the SUn's zodiacal (to say nothing of Oort-cloud) level.

chornedsnorkack
2011-May-30, 04:45 PM
We can separate scattered starlight from interstellar dust (emission from gas is confined to emission lines) because it also shows up in the far-IR, obviously much colder than the zodiacal light.

Yes, and Oort cloud would also be much colder than inner disc in far-IR. Can we separate cold dust scattering and absorbing light of Sun within a fraction of lightyear from cold dust scattering and absorbing light of other stars tens or hundreds of lightyears away?

orionjim
2011-May-30, 04:48 PM
I always pictured the Oort cloud as a cloud surrounding our solar system that was dense with matter so I took a look at how much matter is estimated contained in it. I was surprised that the total was estimated to be 5 times the mass of the earth. So I thought I would compare that to the density of matter in the Kuiper belt and the Asteroid belt. I basically went through Wayne Francis did except I thought I would spread the mass of each across the surface area of the outside of a sphere defined by each ones size.

So using a formula of 4 x pi x Radius^2 to calculate the surface area of each then divide by each ones mass.

Oort cloud Radius = 2500 auís mass 5 earths = 15,707,963
Kuiper belt radius = 25 auís mass .1 earths = 78,539
Asteroid belt radius = 2 auís mass 1 earth = 50.3

With this you get the Kuiper belt 200 times denser than the Oort cloud
The Asteroid belt is 312,500 times denser than the Oort cloud and 1562 times denser than the Kuiper belt.

I donít know if what I did even makes sense, but to me it certainly changed my view of the Oort cloud.

Jim

Grashtel
2011-May-30, 07:36 PM
Asteroid belt radius = 2 au’s mass 1 earth = 50.3
The Asteroid belt doesn't have anything like that much mass, according to a quick and dirty calculation from Wikipedia data its mass is only about 0.0005 Earth masses.

trent1
2011-May-30, 08:01 PM
If the Oort cloud was as you describe then we wouldn't be able to see out through it. IE if its properties made a planet 100ly away observe our sun to be as large as the Oort cloud is then we to would be able to see the Oort cloud. But we can't. Just like the "asteroid belt" is actually very sparse the space between items in the Oort cloud are VERY vast. The fact that we can't yet visually observe our own Oort Cloud means that it is doubtful we'd be able to observe another solar system's Oort cloud and there is no chance for another solar system with similar technology to observe our Oort cloud.

All of the relevant answers are excellent! Thank you so much for sharing with me, the weight of your knowledge.

What I have learned most of all, however, is how easy it is to miss something obvious that is also of incredible value! :razz:

The quote I've included above, and especially the fact that I should have considered it a factor before I typed the first letter in my post, actually made me laugh at myself!

Sorry if our introduction makes me look like a braindead idiot! Although that may yet remain provable! :P

orionjim
2011-May-31, 02:00 AM
The Asteroid belt doesn't have anything like that much mass, according to a quick and dirty calculation from Wikipedia data its mass is only about 0.0005 Earth masses.

I tried to find where I found the 1 earth mass and couldnít find it. I did find a site the said the mass was about 4% of the Moonís mass, and I found a NASA site that lists the Moonís mass to be .0123 of earthís mass. .04 x .0123 is the .0005 that you figured. Using this number then the Asteroid belt would be about 78% less dense than the Kuiper belt but still 156 times greater than the Oort cloud. So yes youíre correct, I was way off on the Asteroid belt.

Even with all of that it still destroys the image I had in my mind of the Oort clouds density.

Thanks!

chornedsnorkack
2011-Jun-01, 07:55 AM
The zodiacal light is quoted as having 10ˇ-7 times the luminosity of Sun in reflected visible light and 10ˇ-5 times the luminosity in IR.

Do you notice that (since for example Earth intercepts just 2*10ˇ-9 times the light of Sun and reflects just 6*10ˇ-10 times the light of Sun) the zodiacal light still by far outshines all planets combined?