The Omega Point

The Omega Point is a term coined by the French Jesuit Pierre Teilhard de Chardin (1881–1955) to denote the state of the maximum organized complexity (complexity combined with centrality), towards which the universe is evolving.

Definition:
Teilhard's universe is subdivided into matter and love, which are the tangential (rotational) and the radial (centripetal) components of the same spiral flow of fundamental psychic energy:

"We shall assume that, essentially, all energy is psychic in nature; but add that in each particular element this fundamental energy is divided into two distinct components: a tangential energy which links the element with all others of the same order (that is to say, of the same complexity and the same centricity) as itself in the universe; and a radial energy which draws it towards even greater complexity and centricity—in other words forwards." <...>

"Driven by the forces of love, the fragments of the world seek each other so that the world may come to being. This is no metaphor; and it is much more than poetry. Whether as a force or a curvature, the universal gravity of bodies, so striking to us, is merely the reverse or shadow of that which really moves nature. To perceive cosmic energy 'at the fount' we must, if there is a within of things, go down into the internal or radial zone of spiritual attractions. Love in all its subtleties is nothing more, and nothing less, than the more or less direct trace marked on the heart of the element by the psychical convergence of the universe upon itself."
—Chardin, Pierre Teilhard de, The Phenomenon of Man

Under the centripetal attraction of cosmic love, the universe's matter undergoes involution (in-formation) from the state of disorganized complexity (uniformly distributed particles of matter) to the state of singularity (pure love-energy, without any particles):

"Reduced to its ultimate essence, the substance of these long pages can be summed up in this simple affirmation: that if the universe, regarded sidereally, is in process of spatial expansion (from the infinitesimal to the immense), in the same way and still more clearly it presents itself to us, physicochemically, as in process of organic involution upon itself (from the extremely simple to the extremely complex)—and, moreover this particular involution 'of complexity' is experimentally bound up with a correlative increase in interiorisation, that is to say in the psyche or consciousness."
—Chardin, Pierre Teilhard de, The Phenomenon of Man

The Omega Point (the Millennium, "the end of the world as we know it") is the period immediately preceding the singularity (the end of the world proper). During the Omega Point, the universe is in the state of organized complexity, being neither uniformly distributed nor completely singular (essentially already singular, formally still complex). For the sake of simplicity, Teilhard visualizes the Omega-universe as a single spiral galaxy,[1] whose nucleus is self-reflective (turned in upon itself) and plays the role of a conscious observer, quantum-mechanically orchestrating the rest of the "galaxy." Currently, the role of the conscious quantum-mechanical observer is played by mankind, acting as a collective Christ:

"One might say that, by virtue of human reflection (both individual and collective), evolution, overflowing the physico-chemical organisation of bodies, turns back upon itself and thereby reinforces itself (see note following) with a new organising power vastly concentric to the first—the cognitive organisation of the universe. To think the world (as physics is beginning to realise) is not merely to register it but to confer upon it a form of unity it would otherwise (i.e. without being thought) be without."
—Chardin, Pierre Teilhard de, The Phenomenon of Man

Having reached the limit of its organized complexity, the collective Christ—mankind—will make a leap to a higher degree of singularity by dying and delegating all of its universe-orchestrating power to the single survivor, who will be automatically promoted to the rank of Christ personal:

"The end of the world: the wholesale internal introversion upon itself of the noosphere, which has simultaneously reached the uttermost limit of its complexity and its centrality.
The end of the world: the overthrow of equilibrium, detaching the mind, fulfilled at last, from its material matrix, so that it will henceforth rest with all its weight on God-Omega."
—Chardin, Pierre Teilhard de, The Phenomenon of Man

An orchestra with multiple conductors cannot produce anything but an incoherent cacophony. When the number of the conductors becomes reduced to a single man, the orchestra shifts from cacophony to symphony, turning into the conductor's "extended body." Analogously, when the universe is quantum-mechanically orchestrated by billions of human observers, it is incoherent (objective)—every part exists by itself, obeying the principle of locality. Having become orchestrated by a single human observer (Christ personal), the universe will shed its incoherence (objectivity) and turn into the observer's "cosmic body":

"Christ has a cosmic body that extends throughout the universe."
—Chardin, Pierre Teilhard de, Cosmic Life ♦ 1916

"Through the incarnation, God descended into nature in order to super-animate and take it back to him."
—Chardin, Pierre Teilhard de, Mysticism of Science ♦ 1939

The timing of the Omega Point:

According to Telhard, mankind will collapse into Christ personal upon reaching the limit of its organized complexity (information). That moment is coming apace and hastening:
- In 2005, information was doubling every 36 months.
- In June 2008, information was doubling every 11 months.
- On 4 August 2010, Google CEO Eric Schmidt said: "Every two days now we create as much information as we did from the dawn of civilization up until 2003."
- By the end of 2010, information will be doubling every 11 hours.

But it is another aspect of the holographic bound that is truly astonishing. Namely, that the maximum possible entropy depends on the boundary area instead of the volume. Imagine that we are piling up computer memory chips in a big heap. The number of transistors— the total data storage capacity—increases with the volume of the heap. So, too, does the total thermodynamic entropy of all the chips. Remarkably, though, the theoretical ultimate information capacity of the space occupied by the heap increases only with the surface area. Because volume increases more rapidly than surface area, at some point the entropy of all the chips would exceed the holographic bound. It would seem that either the generalized second law or our commonsense ideas of entropy and information capacity must fail. In fact, what fails is the pile itself: it would collapse under its own gravity and form a black hole before that impasse was reached.
—Bekenstein, Jacob D., Information in the Holographic Universe Scientific American, August 2003

Five attributes of the Omega Point:
Teilhard de Chardin's The Phenomenon of Man states that the Omega Point must possess the following five attributes. It is:

- Already existing. Only thus can the rise of the universe towards higher stages of consciousness be explained.
- Personal – an intellectual being and not an abstract idea or a human collective. The increasing complexity of matter has not only led to higher forms of consciousness, but accordingly to more personalization, of which human beings are the highest attained form in the known universe. They are completely individualized, free centers of operation. It is in this way that man is said to be made in the image of God, who is the highest form of personality. Teilhard expressly stated that in the Omega Point, when the universe becomes One, human persons will not be suppressed, but super-personalized. Personality will be infinitely enriched. This is because the Omega Point unites creation, and the more it unites, the increasing complexity of the universe aids in higher levels of consciousness. Thus, as God creates, the universe evolves towards higher forms of complexity, consciousness, and finally with humans, personality, because God, who is drawing the universe towards Him, is a person.
- Transcendent. The Omega Point cannot be the result of the universe's final complex stage of itself on consciousness. Instead, the Omega Point must exist even before the universe's evolution, because the Omega Point is responsible for the rise of the universe towards more complexity, consciousness and personality. Which essentially means that the Omega Point is outside the framework in which the universe rises, because it is by the attraction of the Omega Point that the universe evolves towards Him.
- Autonomous. That is, free from the limitations of space (nonlocality) and time (atemporality).
- Irreversible. That is attainable and imperative; it must happen and cannot be undone.

Technological singularity as a rival concept:
Some transhumanists argue that the accelerating technological progress inherent in the Law of Accelerating Returns will, in the relatively near future, lead to what Vernor Vinge called a technological singularity or "prediction wall." These transhumanists believe we will soon enter a time in which we must eventually make the transition to a "runaway positive feedback loop"[4] in high-level autonomous machine computation. A result will be that our technological and computational tools eventually completely surpass human capacities. Some transhumanist writings refer to this moment as the Omega Point, paying homage to Teilhard's prior use of the term, though Teilhard himself denounces the belief in a collective technological singularity as a form of cowardice. He foresees that at the approaches to the Omega Point, mankind will, for the last time, become split over the concept of its final state. The overwhelming majority will erroneously imagine the final state as a collective technological singularity within the framework of the current physical laws. A tiny minority will remain loyal to the idea of a supernatural singularity in Christ, "transcending the dimensions and the framework of the visible universe":

"Enormous powers will he liberated in mankind by the inner play of its cohesion: though it may be that this energy will still be employed discordantly tomorrow, as today and in the past. Are we to foresee a mechanising synergy under brute force, or a synergy of sympathy? Are we to foresee man seeking to fulfil himself collectively upon himself, or personally on a greater than himself? Refusal or acceptance of Omega? A conflict may supervene. In that case the noosphere, in the course of and by virtue of the process which draws it together, will, when it has reached its point of unification, split into two zones each attracted to an opposite pole of adoration. Thought has never completely united upon itself here below. Universal love would only vivify and detach finally a fraction of the noosphere so as to consummate it—the part which decided to ‘cross the threshold’, to get outside itself into the other. Ramification once again, for the last time." <...>

"The death of the materially exhausted planet; the split of the noosphere, divided on the form to be given to its unity; and simultaneously (endowing the event with all its significance and with all its value) the liberation of that percentage of the universe which, across time, space and evil, will have succeeded in laboriously synthesising itself to the very end. Not an indefinite progress, which is an hypothesis contradicted by the convergent nature of noogenesis, but an ecstasy transcending the dimensions and the framework of the visible universe." <...>

"The idea is that of noogenesis ascending irreversibly towards Omega through the strictly limited cycle of a geogenesis. At a given moment in the future, under some influence exerted by one or the other of these curves or of both together, it is inevitable that the two branches should separate. However convergent it be, evolution cannot attain to fulfilment on earth except through a point of dissociation. With this we are introduced to a fantastic and inevitable event which now begins to take shape in our perspective, the event which comes nearer with every day that passes: the end of all life on our globe, the death of the planet, the ultimate phase of the phenomenon of man."
—Chardin, Pierre Teilhard de, The Phenomenon of Man

Faint Young Sun Paradox

The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan and George Mullen in 1972.^[1] Explanations of this paradox take into account greenhouse effects, astrophysical deliberations, or a combination of the two.

Early solar output

Early in the Earth's history, the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. In the current environmental conditions, this solar output would be insufficient to maintain a liquid ocean. Astronomers Carl Sagan and George Mullen pointed out in 1972 that this is contrary to the geologic and paleontological evidence.^[1]

According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their main sequence life time.^[2] However, with the predicted solar luminosity 4 billion (4 × 10⁹) years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. However, the geological record shows a continually relatively warm surface in the full early temperature record of the Earth, with the exception of a cold phase about 2.4 billion years ago. Water-related sediments have been found that date to as early as 3.8 billion years ago.^[3] Hints of early life forms have been dated from as early as 3.5 billion years,^[4] and the basic carbon isotopy is very much in line with what is found today.^[5] A regular change between ice ages and warm periods is only to be found since one billion years.^{[citation needed]}

Greenhouse hypothesis

When it first formed, Earth's atmosphere may have contained more greenhouse gases. Carbon dioxide concentrations may have been higher, with estimated partial pressure as large as 1,000 kPa (10 bar), because there was no plant photosynthesis to convert the gas into oxygen. Methane, a very active greenhouse gas which reacts with oxygen to produce carbon dioxide, may have been more prevalent as well, with a mixing ratio of 10⁻⁴ parts per million by volume.^[6][7]

Based on a study of geological sulfur isotopes, in 2009 a group of scientists including Yuichiro Ueno from the University of Tokyo proposed that carbonyl sulfide (OCS) was present in the Archean atmosphere. Carbonyl sulfide is an efficient greenhouse gas and the scientists estimate that the additional greenhouse effect would have been sufficient to prevent the Earth from freezing over.^[8]

Following the initial accretion of the continents after about 1 billion years,^[9] geo-botanist Heinrich Walter and others believe that a non-biological version of the carbon cycle provided a negative temperature feedback. The carbon dioxide in the atmosphere dissolved in liquid water and combined with metal ions derived from silicate weathering to produce carbonates. During ice age periods, this part of the cycle would shut down. Volcanic carbon emissions would then restart a warming cycle due to the greenhouse effect.^[10][11]

According to the Snowball Earth hypothesis, there may have been a number of periods when the Earth's oceans froze over completely. The most recent such period may have been about 630 million years ago.^[12] Afterwards, the Cambrian explosion of new multicellular life forms started.

Astronomical considerations

A minority view, propounded by the Israeli-American physicist Nir Shaviv, uses climatological influences of solar wind, combined with a hypothesis of Danish physicist Henrik Svensmark for a cooling effect of cosmic rays, to explain the paradox.^[13] According to Shaviv, the early Sun had emitted a stronger solar wind that produced a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain an ice-free Earth.

The temperature minimum around 2.4 billion years goes along with a cosmic ray flux modulation by a variable star formation rate in the Milky Way Galaxy. The reduced solar impact later results into a stronger impact of cosmic ray flux (CRF), which is hypothesized to lead to a relationship with climatological variations.

An alternative model of solar evolution has been proposed as an explanation for the faint young sun paradox. In this model, the early Sun underwent an extended period of higher solar wind output. This caused a mass loss from the Sun on the order of 5−10% over its lifetime, resulting in a more consistent level of solar luminosity. (As the early Sun had more mass, resulting in more energy output than was predicted.) In order to explain the warm conditions in the Archean era, this mass loss must have occurred over an interval of about one billion years. However, records of ion implantation from meteorites and lunar samples show that the elevated rate of solar wind flux only lasted for a period of 0.1 billion years. Observations of the young Sun-like star π¹ Ursa Majoris matches this rate of decline in the stellar wind output, suggesting that a higher mass loss rate can not by itself resolve the paradox.^[14]

Oort Cloud

The Oort cloud (pronounced /ˈɔrt/ ort, alternatively the Öpik-Oort cloud IPA: [ˈøpik]) is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.[1] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest extrasolar star. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.[2]

The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. Astronomers believe that the matter comprising the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution.[1]

Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the Centaurs and Jupiter-family comets as well.[3] 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 Galaxy itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.[1] Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.[1][3] Although the Kuiper belt and the farther scattered disc have been observed and mapped, only four currently known trans-Neptunian objects—90377 Sedna, 2000 CR105, 2006 SQ372 and 2008 KV42—are considered possible members of the inner Oort cloud.[4][5]

In 1932, Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.^[6] In 1950, the idea was independently revived by Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox:^[7] over the course of the Solar System's existence, the orbits of comets are unstable; eventually, dynamics dictate that a comet must either collide with the Sun or a planet, or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence. If comets have been a part of the solar system throughout its history, the presence of such a reservoir is necessary.^[7][8][9]

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 short orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie. Nearly all isotropic comets have very long orbits, on the order of thousands of AU, and appear from every corner of the sky.^[9] Oort noted that there was a peak in numbers of nearly isotropic comets with aphelia—their farthest distance from the Sun—of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.^[9] Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.^[9]

Structure and composition

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU^[9] to as far as 50,000 AU^[1] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU.^[9] The region can be subdivided into a spherical outer Oort cloud (20,000–50,000 AU), and a doughnut-shaped inner Oort cloud (2,000–20,000 AU). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.^[1] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.^[10] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;^[10][11][12] it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.^[13]

The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km^[1] (with many billions with absolute magnitudes^[14] brighter than 11 - corresponding to ~20 km diameter), with neighboring objects typically tens of millions of kilometres apart.^[3][15] Its total mass is not known with certainty, but, assuming that Halley's comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3 × 10²⁵ kilograms, or roughly five times the mass of the Earth.^[1][16] Earlier it was thought to be more massive (up to 380 Earth masses),^[17] but improved knowledge of the size distribution of long-period comets has led to much lower estimates. The mass of the inner Oort Cloud is not currently known.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of various ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.^[18] However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also be home to rocky objects.^[19] Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and Jupiter-family comets shows little difference between the two, despite their vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,^[20] a conclusion also supported by studies of granular size in Oort cloud comets^[21] and by the recent impact study of Jupiter-family comet Tempel 1.^[22]

Origin

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago.^[1] The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giant planets such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits.^[1][23] Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.^[1]

Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward towards the Oort cloud, while a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.^[24] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.^[25]

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.^[26] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.^[1]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.^[1] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.^[27]

Comets

Comets are believed to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from the Kuiper belt or scattered disc, two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU from the Sun. Long-period comets, such as comet Hale-Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are believed to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets.^[9] Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.^[28] These centaurs are then sent farther inward to become the short-period comets.^[29]

There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley's Comet, are unusual in that while they are short-period comets, their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is believed they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.^[8] This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.^[3]

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No known dynamical process can explain this undercount of observed comets. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.^[30] Dynamical studies of Oort Cloud comets have shown that their occurrence in the outer planet region is several times higher than in the inner planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker-Levy 9 in 1994.^[31]

Tidal effects

Most of the comets seen close to the Sun are believed to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way Galaxy. Just as the Moon's tidal force bends and deforms the Earth's oceans, causing the tides to rise and fall, so the galactic tide also bends and distorts the orbits of bodies in the outer Solar System, pulling them towards the galactic centre. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun. At the outer reaches of the system, however, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides can deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes. These small galactic perturbations may be enough to dislodge members of the Oort cloud from their orbits, sending them towards the Sun.^[32] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.^[9]

Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia—closest distances to the Sun—of planetesimals with large aphelia.^[33] The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.^[34] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.^[35]

Star perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is believed to be interaction between the Sun's Oort cloud and the gravitational fields of near-by stars^[1] or giant molecular clouds.^[31] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, during the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.^[36] This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.^[36][37]

In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf or gaseous giant planet, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, is hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, no direct evidence of Nemesis has been found.^[38]

A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the Oort Cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause is a Jupiter-mass object in a distant orbit.^[39]

Oort cloud objects (OCOs)

Apart from long-period comets, only four known objects have orbits which suggest that they may belong to the Oort cloud: 90377 Sedna, 2000 CR₁₀₅, 2006 SQ₃₇₂ and 2008 KV₄₂. The first two, unlike scattered disc objects, have perihelia outside the gravitational reach of Neptune, and thus their orbits cannot be explained by perturbations from the gas giant planets.^[40] If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive.^[41] Their present-day elliptical orbits can be explained by a number of hypotheses:

1. These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster.^[4]
2. Their orbits could have been disrupted by an as-yet-unknown planet-sized body within the Oort cloud.^[42]
3. They could have been scattered by Neptune during a period of particularly high eccentricity or by the gravity of a far larger primordial trans-Neptunian disc.
4. They could have been captured from around smaller passing stars.

Of these, the stellar disruption and “lift” hypothesis appears to agree most closely with observations.^[4] Some astronomers prefer to refer to Sedna and 2000 CR₁₀₅ as belonging to the "extended scattered disc" rather than to the inner Oort cloud.^[41]