A stable orbit from something that is not a star at a light years distance? Really?!!

Also as in relation to Nemesis. Some people think it exists due to the irregular orbits of neptune and pluto. A brown Dwarf surely wouldnt be able to influence those orbits at a distance of just over a light year?

Yes. What makes you think it would be a problem? The laws of physics are the same a light year out as they are at 1 au. The gravitational atraction is smaller, so the tangential velocities are smaller, but as long as an object is in a location where the force of Sol is greater than the force of any other object, it is possible for that object to orbit Sol. I don't know the exact crossover distance, but in the direction of Alpha Centauri, it should be something like 1.5 lightyears. The system of 3 Centauri stars has roughly twice the mass of Sol, so I am guestimating the crossover is about 1/3 the distance from Sol. There may be other factors involved.

dirty_g,

As Tony said in his reply to me, comets in the Oort cloud are not so
much in circular orbits as in orbits which have perihelion far enough
from the Sun that they don't grow visible comas and tails, and so
are never noticed. Aphelion might be a light-year from the Sun,
while perihelion might be inside the orbit of Neptune.

Any nearby star so dim that it isn't immediately obvious from Earth
is too dim to change icy asteroids into comets with comas and tails.
Even if a comet did have a full-blown tail, the star would not be
bright enough to illuminate it. Even if such a comet were illuminated
as brightly as one near the Sun, at a distance of a light-year it
would almost certainly be invisible.

-- Jeff, in Minneapolis

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"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

The slow motions of such distanct comets are the main obsticle in detecting them. Consider Iris (Xena). It's the 4th brightest KBO, but over 500 KBOs were discovered before it. That's because it moved too slow for the detection schemes to pick it up.

Wasn't it Gould who called it the "Shiva Hypothesis" because he, too, recognized the apparent periodicity that is so hotly contested?

Binary Research Institute highlights what some consider evidence.

And then I myself include astronomers like Murray, Matese, Whitmire, Brown and a few others who've suggested such a perturber in the Oort cloud can not be ruled out.

In my mind, the perturbed highly elliptical and elongated orbits of various recently discovered outer system bodies, coupled with the sheer edge of the kuiper belt, the facts that not only are binaries more prevalent in the observable universe (brown drawfs too?) and planets have been found to be stable in these systems... these and various other puzzles in our system all constitute evidence for such a body.

I'd suggest we've not found it simply because it is a dim brown dwarf moving slowly and inclined to the ecliptic, generally toward the galactic center.

I suspect we'll discover it soon.

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It seems to me that the periodicity should be shakey at best. If Nemesis exists in a wide orbit, it is likely that nearby stars would be perturbing its orbit. I doubt it would trace the same orbit over and over again. Its period each orbit would likely be different.

The mispronunciation of the name of Halley's comet can be traced to the 50's band Bill Haley and the comets.

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T. Anderson

seems the Nemesis theory is gaining support recently. Still I am sure it has equal opposition. I suppose though that if you say it is a brown dwarf then that is an easy way of getting out of detecting it as it would be so hard to see.

With regards to the orrt cloud I still do not see why there has to be a cloud of them together?? Maybe im just being silly but why can't the comments all just orbit the solar system on long period orbits instead? Understand im not rguing with you all here I just am really interested as to why a cloud of them would group together?? Oh and if a Brown Dwarf was hiding behind them all (the apparent Nemesis) then wouldnt it of sent them all heading off towards us by now??

The point is, theories do not need "opposition" like political propositions (in principle anyway, in reality all human endeavors are political!). What they need is supporting evidence, and the supporting evidence for Nemesis is vitually nil. All you can say is that there yet exists a parameter space where you could "hide" one (or several) bound companions to the Sun, without our knowing about them. However, they would not have the almost magical an entirely unsupported quality of being able to have all kinds of effects on us during the perihelion of the orbit. Indeed, any orbit that highly elliptical is rather unlikely-- the only reason comets can have spectacularly elliptical orbits is that there are so bloody many of them in the Oort cloud that it would be almost impossible for some not to. Indeed, this is the entire reason for suspecting that there is an Oort cloud-- so where is the Oort cloud of stars orbiting the Sun that could explain why Nemesis is so elliptical?
Bingo. That's a calling card of bad astronomy: it's hiding in the shadows of what cannot be falsified, and it is given all kinds of extremely unlikely attributes (like excessive ellipticity) to allow it's originators to use it to explain things that are much more naturally explained in some more mundane way. Kind of like ghost stories, or UFOs, you see what I'm saying.
That's what they do, there is no reason to draw a distinction between "cloud" and a "whole bunch of very long period orbits in all directions". The cloud should surround the whole solar system over a huge volume, not a cloud like in the sky. One must be careful not to read too much into a word choice, but rather look at the intended meaning.
Yes it would, but that's one of the motivations behind Nemesis-- to explain alleged cyclical extinction patterns whose very existence is highly unclear. But there is of course no accompanying calculations that actually solve the equations of physics and predict such a periodic "rain" of comets. If there were, it would make it an interesting hypothesis worthy of further testing, but hardly a "theory".

so the Oort cloud surrounds the Sol System like a shell over an eggs yoke. If we are the yoke the cloud (the shell) totally surrounds us? Is it proposed that most Stars would have their own Orrt Clouds??

Yes on the first part, and as for the second part, I would say yes if they are single stars that contain massive planets in Neptunelike orbits. We really don't know how common that configuration is, we might be special or we might be typical.

Remembering that there is no hard evidence for the Oort cloud, I think it is highly likely that each stellar system (less likely between members of bound systems) has a similar distribution of cometary material surrounding it. The existence of such an environment does not seem to violate our concepts of star formation from the collapse of large clouds of gas and dust and the existence of planetary objects capable of slingshotting many chunks of material into independent galactic orbits. Can analysis of edge-on views of other galaxies showing dark bands of shadow forming material be of help?

When designing interstellar missions, one has to consider the likelyhood that an Oort cloud-like environment exists between each of the stellar systems (occupies all of interstellar space within the MW) with density falling off towards the mid-point between stellar systems. Such material would provide both hazards and a source for propellant for interstellar ships.

Has anyone computed how much mass could be contributed to that of the MW by such material and not have been detected either by visual observation or by its gravitational interaction with the observable objects?

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Negligible. For one thing, comets have way too much oxygen to be an important mass component, unless the universe had a huge amount of oxygen (and other choice constituents, like silicon) that didn't show up in stars for no apparent reason.