Talk:Oort cloud

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Oort Cloud: not a mere "hypothesis" to the majority of scientists[edit]

I question your (user Serendipodous) use of "hypothetical" when referring to the Oort Cloud, and your systematic deletions of attempts to correct this.

At the very least, you are presenting a one-sided version of the scientific opinions on this matter. All scientists I know and certainly the majority of scientists I have read or heard, consider the Oort Cloud a "real region of space extending thousands of AU's beyond the orbits of known planetary bodies".

While there is no consensus about some of the evidence found of the inner Oort Cloud (object 2006 SQ372, discovered and analysed from 2006 to 2008, being the first with a direct connection to the inner cloud), the fact remains that a significant portion of scientists consider it serious and conclusive. Furthermore, you do not need to actually travel to or be in the Oort Cloud for it to be real - we know of three separate regions of space that are sources of comets, each characterized by what we call the "Tisserand parameter". Calculations give us precise trajectories of the objects considered, therefore finding where they come from is not difficult.

In order for this article to have any validity you ought to either remove the qualifier (hypothetical, which goes against the accepted views among scientists) or, at the very least, refer to it in a different manner (more intellectually honest). "The Oort Cloud, widely believed to be a spherical region of space (though it remains "hypothetical" to a fraction of scientists)..." would be a proper way of introducing the Cloud. There are several descriptions at NASA and ESA webpages which present this view.

MikeLousado 22:54, 18 April 2014 (UTC+1)

Minor planets → objects[edit]

The above change got me wondering: Could there be objects that are part of the Oort cloud and are not minor planets? In other words, would a hypothetical brown dwarf or super-jupiter out there be considered part of it? --JorisvS (talk) 16:36, 2 January 2013 (UTC)

Not really. Any more than an airplane passing through a cloud could be considered part of it. Serendipodous 17:46, 2 January 2013 (UTC)
But think in the opposite direction, size-wise. The bulk of the Oort cloud probably consists of objects that are too small to be classified as minor planets. — Quondum 18:30, 2 January 2013 (UTC)
Like meteoroids and dust? The same would hold for the Kuiper belt and the asteroid belt. --JorisvS (talk) 20:37, 2 January 2013 (UTC)
Mostly comets, I expect. — Quondum 20:46, 2 January 2013 (UTC)
Which are, if far enough from the Sun, minor planets. Most objects in the Kuiper belt are icy and would become comets if brought closer to the Sun. --JorisvS (talk) 21:00, 2 January 2013 (UTC)
Would dust particles also be considered "objects"? --JorisvS (talk) 23:10, 2 January 2013 (UTC)
I see I've been confusing minor planet with dwarf planet, going by the respective articles. I guess we need someone more familiar with the literature, and in particular with the modern accepted usage of the respective terms. I guess a dust particle would be considered an object; whether the average reader would consider it to be a minor planet is more doubtful. — Quondum 05:59, 3 January 2013 (UTC)
Technically an object can not be a listed as a comet until it has been seen to generate a coma (cometary). But in laymen speak I still think it is better to call Oort Cloud objects comets instead of minor planets so that readers such as Quondum do not get confused. All known near-Earth asteroids are also minor planets. -- Kheider (talk) 09:08, 3 January 2013 (UTC)
Even though we should try not to confuse lay readers, we should not blatantly violate scientific usage of terms either (and only in some places, at that! We don't call the majority of KBOs 'comets', even though the majority is just as icy as the majority of OCOs). --JorisvS (talk) 11:34, 3 January 2013 (UTC)

Couldn't we circumvent the issue by phrasing the lead similarly to that of Kuiper belt? --JorisvS (talk) 11:34, 3 January 2013 (UTC)

This certainly has merit. In the interest of accessibility (and in particular the correct interpretation by the typical interested reader), it may be sensible to forgo the brevity of jargon terms (e.g. minor planet) for a more descriptive approach – either by using a jargon term along with its definition in terms of more everyday terms, or by simply using a more involved description. — Quondum 12:49, 3 January 2013 (UTC)

The Oort Cloud a spherical cloud?[edit]

I find it difficult to understand how a shell of "icy objects" or any other matter could remain stable over a period of some 15 billion years. It seems to me, that because of collisions and gravitational interaction, the cloud would eventually evolve into a disk, such as that around Saturn. Are there computer models that support the globular cloud idea? Paul venter (talk) 10:19, 15 January 2013 (UTC)

It's only been 4.5 billion years but regardless, the only way it could form into a disk is if it were more gravitationally tied to the Sun. The inner Oort cloud is hypothesised to be disk-shaped. Serendipodous 10:41, 15 January 2013 (UTC)
I have to disagree with the "only way it could form into a disk is if it were more gravitationally tied to the Sun". Transforming a globular cloud into a disk is clearly a function of gravitational field strength and time - the weaker the field the longer it will take, but all other things being equal, it will happen. Paul venter (talk) 12:21, 15 January 2013 (UTC)
Yes, but if you calculate it, it's like a trillion years or so... which is far longer than it takes for the Oort cloud to be attritted. --JorisvS (talk) 12:35, 15 January 2013 (UTC)
Is that a calculation or a thumbsuck? Serendipodous (just above) states "The inner Oort cloud is hypothesised to be disk-shaped" - an effect which no doubt will move outward in time, but nothing like a trillion years. Paul venter (talk) 14:07, 15 January 2013 (UTC)
I didn't actually calculate it here, just gave a rough, conservative estimate. Similarly long time scales would hold for the inner Oort cloud. The reason that the inner Oort cloud is hypothesized to be doughnut-shaped, like the Kuiper belt, is because of its origins: scattered out of the plane of the Solar System into elongated orbits that are not very strongly affected by the gravity of other stars but still somewhat. The reason why the outer Oort cloud is hypothesized to be spherical is basically that the original doughnut distribution of orbits has been randomized by the galactic tide. --JorisvS (talk) 14:42, 15 January 2013 (UTC)
Yes, the Oort cloud is not nearly 15 billion years old...obviously. You cannot compare it to the rings of Saturn. I think one can compare it to the irregular moons of the giant planets: they too form spheres and are too far from their planet for tidal effects to affect the spherical form. Also, the Oort cloud is not quite stable, nowadays attrition is higher than replenishment, so the number of objects in the Oort cloud diminishes somewhat. --JorisvS (talk) 10:44, 15 January 2013 (UTC)
Glad that is cleared up. Perhaps the article can be amended to include those thoughts. Another issue is that "The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself". I tried to clarify this but was summarily reverted. The prime moving force here is due to the resultant gravitational fields of stars in the neighbourhood, whether they are "passing" or not is irrelevant. One also must seriously question the magnitude of the gravitational field contribution from the Milky Way which is nothing more than the resultant field of the stars and other matter comprising it. Has a serious measurement of this ever been done or computed from a model? cheers Paul venter (talk) 11:06, 15 January 2013 (UTC)
To appreciate that stars are 'passing by': In about 1.4 million years Gliese 710, a star currently some 64 ly from the Sun (i.e. not really a nearby star today), will pass within about a light-year from the Sun. --JorisvS (talk) 11:52, 15 January 2013 (UTC)
Quite - and in 1.4 million years its effect can be taken seriously - but not today. Paul venter (talk) 12:14, 15 January 2013 (UTC)
No, but that's not my point: Stars have been passing by like that for the past 4.5 billion years. --JorisvS (talk) 12:35, 15 January 2013 (UTC)
I understand that, but why is the article dismissing the effect of stars which at the moment are nearby (and rushing past)? We don't have to wait a few million years. Paul venter (talk) 13:07, 15 January 2013 (UTC)
Is it? These stars are also 'passing by', the only difference with Gliese 710 is that they don't near quite as closely. --JorisvS (talk) 14:42, 15 January 2013 (UTC)
Mentioning only stars that pass by places undue emphasis on the "passing" neglecting to inform the reader that the gravitational effect starts long before the "passing by" stage and carries on long after it has "passed by" - it would improve the article to make this clear by rewording. Paul venter (talk) 14:58, 15 January 2013 (UTC)
Yes, that's where the galactic tides come in. --JorisvS (talk) 15:59, 15 January 2013 (UTC)
Yes, you're probably right - gravitational fields are far too complex a notion for the average reader - 'galactic tides' and 'passing stars' are much simpler cocepts. Paul venter (talk) 17:28, 15 January 2013 (UTC)

Copied from Kheider's talk page: I note with interest your revert of my edit with the note "No, Oort cloud objects do not bounce "star to star"", which was not what I had written. Since the existence of the Oort Cloud is inferred a lot of its qualities are the subject of speculation. The extent of the Oort Cloud around a star (as I understand it) includes the matter at the very limits of its gravitational dominance, a dominance which automatically passes to the next nearest star and its gravitational field. I don't understand why the effects of the "Galactic tide", which I presume to mean the collective gravitational force of all the matter within the Galaxy, should be larger than that of stars in the neighbourhood. My view of the movement of the outermost parts would be more of a slow drift between the stars. I would love to know more about the reasons behind your emphatic rebuttal. cheers Paul venter (talk) 07:59, 15 January 2013 (UTC)

I reverted it because when there is not another nearby star temporarily dominating a given region of the Oort Cloud, the galactic tide can assume dominance when an object reaches the very edges of the Sun's hill sphere. -- Kheider (talk) 12:15, 15 January 2013 (UTC)
The stars Proxima Centauri, and the binary Alpha Centauri AB have a collective mass of about twice that of our Sun, and are all just over 4 light years distant, certainly qualifying as nearby stars, and having a collective Hill sphere radius far greater than the Sun's. Has any study put a definite outer radius to the Sun's Oort cloud, or does it merge seamlessly with the Oort cloud around Alpha Centauri AB? Paul venter (talk) 12:57, 15 January 2013 (UTC)
The size of the Oort cloud around any star is largely a function of the estimated mass of the Milky Way and thus the galactic tide. The hill sphere of Alpha Centauri only affects a small Oort cloud region between the two stars. Any lead that ignores the effects of the galactic tide and numerous passing stars over millions/billions of years is missing the big picture. Objects perturbed form the Oort cloud towards the Sun can take millions of years to get to the inner Solar System. -- Kheider (talk) 15:01, 15 January 2013 (UTC)
If you read my edits with more care I'm sure it will become clear that I'm not suggesting that the effects of galsctic tides and passing stars are ignored, but rather that the text is rephrased to more accurately reflect what is happening so that readers don't miss the Big Picture. And obviously objects perturbed from the Oort Cloud away from the Sun don't even bear thinking about. Paul venter (talk) 17:28, 15 January 2013 (UTC)
OCOs perturbed away from the Sun? They become unbound (rogues). --JorisvS (talk) 18:06, 15 January 2013 (UTC)

"the objects comprising the Oort cloud should experience accelerations of the order of 10^−10 m s^−2" Are you sure about that? I have never seen acceleration measured in inverse seconds squared. Acceleration as I have always seen it is measured in ms^2, not ms^-2. Are you sure it is acceleration? Are you sure you don't have an extra -? — Preceding unsigned comment added by (talk) 07:41, 23 April 2013 (UTC)

No, the unit of acceleration is m/s2, which is m·s−2. Compare the unit of velocity: m/s = m·s−1 (which is not m·s). --JorisvS (talk) 09:34, 23 April 2013 (UTC)

Inconsistency in sentence[edit]

The article states, "Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and..." implying that the Oort cloud is a known entity whose properties are being currently directly measured, yet the beginning of the article states that the Oort cloud is hypothesized. I would think the sentence should perhaps be re-written to indicate that these measurements are thought to be from the Oort cloud, although the existence of the Oort cloud has yet to be proven.

Laurence Klein — Preceding unsigned comment added by (talk) 13:58, 9 October 2013 (UTC)

Revised. Serendipodous 14:36, 9 October 2013 (UTC)

Moved from article[edit]

Aside from the incorrect formatting of references, can Zachariah1978's contribution be properly integrated into this featured article with less soapboxy-sounding text?

It should be noted that no one has ever actually detected the Oort cloud with a telescope or any other sensing device. "Many scientific papers are written each year about the Oort Cloud, its properties, its origin, its evolution. Yet there is not a shred of direct observational evidence for its existence.” Sagan and Druyan, p. 210. However, Sagan and Druyan believed that the Oort cloud exists, and went on to predict (p. 211) that “with the refinement of our scientific instruments, and the development of space missions to go far beyond Pluto,” the cloud will be seen, measured, and studied. Also, there are critical mathematical errors in the theory which led to the misplaced belief that a cloud of cometary material, called the Oort cloud, surrounds our solar system. Raymond A. Lyttleton, “The Non-Existence of the Oort Cometary Shell,” Astrophysics and Space Science, Vol. 31, December 1974, pp. 385–401. Assuming the Oort cloud exists helps preserve the belief in a multibillion-year age for the solar system. u “Recently, Lyttleton (1974) confirmed our conclusion of 1954: the Oort’s hypothetical cloud of comets cannot exist.” S. K. Vsekhsvyatsky, “Comets and the Cosmogony of the Solar System,” Comets, Asteroids, Meteorites, editor A. H. Delsemme (Toledo, Ohio: The University of Toledo, 1977), p. 470. Vsekhsvyatsky estimated (p. 470) that considerably more than 1020 gm/yr of cometary matter are lost from the solar system. Over the supposed age of the solar system (4.5 billion years), lost comet mass would “nearly correspond to the total present mass of the planets.” He believed this was unreasonable. “... many people would be happier if there were more objective evidence for the reality of the Oort Cloud.” John Maddox, “Halley’s Comet Is Quite Young,” Nature, Vol. 339, 11 May 1989, p. 95.

--NeilN talk to me 04:13, 15 December 2013 (UTC)

I'm not sure what you mean by soapboxy-sounding text. It is only recently that I learned about this Oort cloud hypothesis. I checked out some of the lines in that block of text and they all point to creationist websites (Young Earth Creationists, YEC). The issue seems to be on the same playing field as radiometric dating vs creationism and evolution theory vs creationism, junk DNA vs creationism, abiogenesis vs creationism, Big Bang Theory vs creationism, Plate Tectonic theory vs creationism, etc. There isn't anything fantastic about the Oort cloud proposition but for some people, it is a very difficult proposition to accept due to religious reasons. Vmelkon (talk) 01:06, 10 December 2014 (UTC)
The text is arguing for the creationist position. Apart from which, crank positions are generally not included in main articles, because doing so wildly unbalances the article. Anybody who could realistically be considered a reliable source of information on the Oort cloud will assert that it exists (and hell, we have directly observed it, but only the small amount close to the Sun. We've only observed the surface of the Earth (well, plus down a few holes), but we don't mention in the Earth article that some people might believe the Earth doesn't exist, or whatnot. WilyD 11:06, 10 December 2014 (UTC)

"Nearly all isotropic"[edit]

From the Hypothesis section: "There are two main classes of comet, short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie. Nearly all isotropic comets have very large orbits..."

Although it's been there a long time, I suspect that the word all should be deleted from the second sentence here, but I don't know enough about astronomy to be sure. From the context I think the phrase "Nearly isotropic comets" is intended. Dave.Dunford (talk) 14:53, 14 February 2014 (UTC)

It says that long-period comets are also called "nearly isotropic comets", because their distribution is nearly isotropic in the sky. It seems it was originally "All nearly isotropic comets ..." and that it was changed when it was misinterpreted as a grammatical error. Saying "nearly isotropic comet" a bit illogical anyway, because there is nothing isotropic about each of these comets individually. "Long-period comets" is logically more precise anyway. --JorisvS (talk) 16:26, 14 February 2014 (UTC)

Oort clouds around other stars[edit]

Some of the talk above refers to Oort clouds around other stars. The article does not mention this, so is the theory applicable to other stars, and so should be included? John a s (talk) 22:01, 18 February 2014 (UTC)

There's no reason to assume that other stars do not have Oort clouds; however, since we haven't even found ours yet, it's a bit too early to speculate. Serendipodous 22:03, 18 February 2014 (UTC)
Since other stars likely have comets, other stars likely have Oort Clouds. -- Kheider (talk) 22:12, 18 February 2014 (UTC)
New telescopes will be looking. -- Charles Edwin Shipp (talk) 19:11, 8 May 2014 (UTC)
There're two very recent papers on Oort clouds around other stars ( ) and ( ); and some older work ( & & ). I'm not aware of any work specifically about Oort clouds around Main sequence stars. WilyD 13:27, 27 October 2014 (UTC)

Hazard to future travel[edit]

We'll just wait and see whether the "nonsensical OR ignorant of the vast amount of space" claim in the article edit history turns out to be accurate. Given the sheer number as well as the size of objects of which they seem to be ignorant (have you read the article? I guess not), regardless of the amount of space between, probability says it is highly plausible and it's just going to happen. Any thoughts, please?

I accept the need for verifiable content, but one cannot just remove stuff because, to them, it is "nonsensical or ignorant."—An Sealgair (talk) 09:03, 19 February 2015 (UTC)

OK, let's do the math: Let's assume that the Oort cloud is at its closest (and therefore densest) probable distance of 0.8 ly. Let's make the (absurd) assumption that every Oort cloud object is the size of Pluto. Let's also round up insanely and say there there are about a quadrillion Oort cloud objects out there, and that they are all at 50,000 AU. A sphere with a radius of 0.8 ly means a surface area of 8.04 ly^2, which is 7.19814558 × 10^26 km^2. Pluto has a radius of 1184 km, which means it has a cross-section of 4,404,060 km^2. Multiplied by 1 quadrillion and you get 4.4040605*10^21 km^2. Even with this absurd setup therefore, a spacecraft would have about 1 chance in 200,000 of hitting anything. The fact that most Oort cloud objects are not going to be the size of Pluto but more likely the size of Halley's Comet would push that chance up to about 1 in a billion. And that's assuming they're all in one sphere; the real Oort cloud is likely to be 50-100,000 AU in radius.Serendipodous 09:14, 19 February 2015 (UTC)
Re: your addition. Yes we can, and we do, remove content because it is nonsensical and ignorant. Hence why creationists don't post here anymore. Serendipodous 09:29, 19 February 2015 (UTC)
The asteroid belt is ten thousand times less mass in about a trillion times less volume; you can ballpark that voyager, pioneer, etc. were about a hundred million times more likely to be destroyed crossing the asteroid belt than the oort cloud. It's actually worse, because collision velocities are higher in the asteroid belt, but no matter. The way we avoid destructive collisions between probes and asteroids is to not worry about it, since it's so unlikely. So no, you'll never find a reliable source for collisions in the oort cloud. Although in this paper [1] Roman and his student argue it's plausible for Oort cloud comets near the inner edge of the cloud to collide with a timescale of a few billion years. WilyD 09:50, 19 February 2015 (UTC)
This all is exactly why I removed it as "nonsensical OR (that is) ignorant of the vast amount of space". The vastness of space is beyond anything human minds can comprehend. Only by actually doing the math, we can know the facts. --JorisvS (talk) 11:56, 19 February 2015 (UTC)
The thing about space is that there's so much of it and it's almost completely empty. Jonathunder (talk) 23:08, 19 February 2015 (UTC)


For too long this article has been claiming that the Oort cloud only extends to 50,000 AU. If a=50000AU, Q can equal 100,000AU. The Oort cloud easily extends to 70,000+ AU from the Sun. And as objects are perturbed outward, they can be perturbed back inwards by other passing objects and galactic tides. The lower-density transient edges of the Oort Cloud could easily extend 120,000+ AU from the Sun as it can take millions of years for an object to leave the vicinity of the Sun. It might be time to find a more recent (more direct) source than 2005. The Sun has been swapping Oort cloud material with other stars for billions of years. -- Kheider (talk) 10:14, 19 February 2015 (UTC)

Is that maybe the dominant process out there: swapping Oort cloud material that typically remains far from the stars? If then an object is perturbed inwards it may have been swapped between Oort clouds (or should is it maybe closer to a vast continuous distribution of minor bodies?) for a long time.
Is the ejection ratio of objects (of all stars) consistent with the upper limit to the density of interstellar comets? --JorisvS (talk) 12:05, 19 February 2015 (UTC)
Material bound to the galaxy but not to the Sun encounters the Sun with relative velocities of tens of kilometers per second. The galactic tide and stellar flybys won't be able to bind it to the Sun; barring extremely close encounters with the planets, it will sail in and sail out without any howdy doody. Levison has a paper about grabbing comets during cluster dissolution, when relative velocities are much smaller (though no one has followed up on it, and I ain't sure even Hal believes it). We don't know anything about Oort clouds around other stars (except, perhaps, some unclear information from white dwarfs), so it's unclear how meaningful our knowledge of interstellar comets is (though see Moro-Martin's paper here: - which I believe is probably the most useful ref for this.) WilyD 13:47, 19 February 2015 (UTC)
I'm not talking about actually capturing interstellar comets that were already not bound to any star (say 'galactic comets' for clarity). We have never observed an object that was on a hyperbolic trajectory (relative to the Sun) before encountering any planets, which sets an upper limit to the density of galactic comets. This is distinct from objects that originated near another star and were scattered into that star's Oort cloud and later came to orbit in the Sun's Oort cloud when its star had a "close" encounter with the Sun (i.e. it was part of the swapped material). --JorisvS (talk) 16:05, 19 February 2015 (UTC)
The considerations for capturing galactic comets or comets from another Oort cloud are essentially identical - both are basically impossible, except for stars that are binaries/higher multiples, or still in the cluster they were born in. Otherwise the relative velocities are too high. WilyD 16:16, 19 February 2015 (UTC)
To capture an external object in the Oort cloud at ~50,000AU from the Sun requires a relative speed to the Sun of less than (0.185 km/s), so now-a-days captures should be rare. But the Sun's birth cluster could have been an orgy where up to 90% of the Oort cloud got swapped. -- Kheider (talk) 17:19, 19 February 2015 (UTC)

This may be nitpicking, but to say that 100,000 AU is 2 light years, or is half the distance to Proxima Centauri, is a quite imprecise. 100K AU is 1.58 light years. Yes, 2 is the nearest integer to 1.58, but that's still quite a rounding error, 21% off. And Proxima is about 4.24 Light Years away...half would be 2.12. To call 100K AU "half" the distance to Proxima Centauri is an error of more than 25%. Applejuicefool (talk) 15:23, 7 July 2015 (UTC)

That'd only be true if we were talking about 100 000. au, but we're talking about 10^5 au. We know the distance to Proxima quite well for everyday numbers. But we know the outer extent of the Oort cloud pretty well for astronomy numbers - i.e., to order unity. The most recent estimate I'm aware of is - but you shouldn't believe it's accurate at the 20% level. WilyD 16:01, 7 July 2015 (UTC)
Sorry, I'm a little confused by your reply. Are you saying that the lines in the article: "...icy planetesimals believed to surround the Sun at a distance of up to around 100,000 AU (2 ly).[3] This places it at half of the distance to Proxima Centauri..." are correct because of the uncertainty in things such as the actual boundary of the Oort Cloud, and the actual distance to Proxima Centauri? That's not really my main point here. The article implies that 100,000 AU (not 10^5, according to the article) is the same thing as - or at least "around" - 2 light years. It's not. You might be closer to a point with the "half the distance to Proxima" thing, but still 25% is *way* off, whether you're talking everyday numbers or astronomy numbers. Applejuicefool (talk) 19:15, 9 July 2015 (UTC)
There is no clean boundary to the outer Oort cloud. It will simply become more diluted as you get further from the Sun. Proxima Centauri is 4.24 ly (268,000 AU), so 100,000-120,000 is very crudely half that distance. Using 2 sigfigs with a conversion of 100,000 AU (1.6 ly) may make the reader think the boundary is better determined than it is. -- Kheider (talk) 19:50, 9 July 2015 (UTC)
Apart from the primary discussion of the Oort Cloud for a second, the article strongly implies that 100,000 AU is the same measurement as 2 light years. That's my main quibble here. 2 light years is ~126,500 AU. 26,500 AU might not be far where you come from, but it's even further than my daily commute, and that's saying something. Applejuicefool (talk) 06:40, 10 July 2015 (UTC)
I see no reason to use excessive sigfigs for an approximation that is only good to the order of magnitude. I have seen people claim that the Oort Cloud extends to 3 ly. Comparing astronomical scales it to your daily commute is just being silly. -- Kheider (talk) 15:40, 10 July 2015 (UTC)
Putting on my professional astronomer hat, yes, 100 000 au is around two light years (though, of course, I'd never use a light year, a hundred thousand au is half a parsec). If we meant 100 000 ± 1, we'd write 100 000. or some such nonsense. (And by writing 2 ly, we mean somewhere between 1.5 and 2.5 ly, or such, so we hit 10^5 au rather easily. We know the size of the Oort cloud to within a factor of three-ish, and the mass to within a factor of ten-ish? Certainly to within a factor of a hundred or so. Order ten percent errors are too small to be concerned about. WilyD 16:00, 10 July 2015 (UTC)

Alpha Centauri and the Oort Cloud?[edit]

Alpha Centauri system involves a star very much like the sun. If our solar system's Oort Cloud extends to roughly 2 l.y., so roughly half-way to Alpha Centauri, and if Alpha has an Oort Cloud of its own, could it be possible that the two clouds would overlap with one another? (talk) 13:55, 14 July 2015 (UTC)

Probably. But keep in mind that the Oort cloud is very porous and that the Sun makes a close approach to another Star every million years or so. -- Kheider (talk) 14:09, 14 July 2015 (UTC)
I thought the Oort cloud was inside the heliopause? As in, the Oort Cloud is part of our solar system, and the heliopause marks the end of the solar system where interstellar space begins. But that diagram has the oort cloud all the way to Centauri almost. Am I missing something? Leitmotiv (talk) 20:11, 21 July 2015 (UTC)
The heliopause is just where the Sun's solar wind blends into the background. File:PIA17046_-_Voyager_1_Goes_Interstellar.jpg is a Logarithmic scale. -- Kheider (talk) 20:25, 21 July 2015 (UTC)
The heliopause is where the Sun's solar wind is stopped by the interstellar medium (around 120 AU, give or take). This has exactly nothing to do with where objects are still gravitationally bound to the Sun (up to 2 ly / 100,000 AU), which limits the outer edges of the Oort cloud. --JorisvS (talk) 20:30, 21 July 2015 (UTC)
Yup and interstellar medium is further defined as "space between the star systems" but the Oort cloud is commonly referred to as orbiting our solar system... so it seems like a conflict in definitions here. Leitmotiv (talk) 21:54, 21 July 2015 (UTC)
The Oort cloud is largely empty as it is a ridiculously large volume with a radius of 100,000AU. The edge of the interstellar medium is not a decisive boundary. NASA's marketing department should have not have claimed that Voyager has left the Solar System. -- Kheider (talk) 23:23, 21 July 2015 (UTC)
@Leitmotiv: It is the ionic, atomic, and molecular matter that is travelling between the stars, not "the space between the stars". If it weren't for the solar wind, it would reach all the way to the inner Solar System. It has exactly nothing to do with whether bodies are gravitionally bound to the Sun, which is the defining feature of the Solar System: all bodies that are gravitationally bound to the Sun. --JorisvS (talk) 08:50, 22 July 2015 (UTC)

In the 2014 AO for the Discovery program[edit]

What does this mean? What is an AO? Should "program" be capitalized? Should there be a wlink for "Discovery" or "Discovery program"? TomS TDotO (talk) 09:26, 16 November 2015 (UTC)

  • AO is short for "Announcement of Opportunity" - NASA's jargon for a call for proposals. The link is correct - Programme probably should be capitalised, since it appears to be a proper name, yeah. WilyD 11:34, 17 November 2015 (UTC)