Radiation astronomy/Oort clouds

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This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in astronomical units (AU). The scale is logarithmic, with each specified distance ten times further out than the previous one.
An artist's rendering is of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility.

The Oort cloud or the Öpik–Oort cloud[1] is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.[2] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.[3]

The Oort cloud is divided into two regions: a circumstellar disc-shaped inner Oort cloud (or Hills cloud) and a circumstellar envelope, spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space.[4][5]

Voyager 1, the fastest[6] and farthest[7][8] of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years[5][9] and would take about 30,000 years to pass through it.[10][11] However, around 2025, the radioisotope thermoelectric generators on Voyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by Voyager 1.

Theoretical Oort clouds[edit]

Stars closest to the Sun include Barnard's Star (25 April 2014).[12] Credit: NASA/Penn State University.

Def. a "roughly spherical region of space composed of comet-like bodies and other minor planets and asteroids that orbit distantly in planetary systems"[13] is called an Oort cloud.

Def. a "roughly spherical region of space from 50,000 to 100,000 astronomical units (approximately 1 light year) from the sun; supposedly the source of most comets around the Solar System"[14] is called an Oort Cloud.


The comet Hale–Bopp in the night sky. Credit: Philipp Salzgeber.

Def. a "celestial body consisting mainly of ice, dust and gas in a (usually very eccentric) orbit around the Sun and having a "tail" of melted matter blown away [back][15] from it by the solar wind when [as][15] it is close to [approaches][15] the Sun"[16] is called a comet.

Def. a "comet which orbits the Sun and which returns to the innermost point of its orbit at known, regular intervals"[17] is called a periodic comet.

Def. "any periodic comet with an orbital period of less than 200 years"[18] is called a short-period comet.

Most of the comets lay at the distant reaches of our system in a hypothesized Oort cloud. At the very edge of the solar system, these comets orbit in very large loops around the distant reaches of our solar system. The passing of nearby stars, or other objects can alter their orbit, sending them speeding towards the inner reaches of our solar system. These comets typically retain very large orbits such that they will not return (once seen in the inner solar system) for many thousands of years.

Cosmic "ray protons at energies up to 10 GeV [may be] able to build-up large amount of organic refractory material at depth of several meters in a comet during [its] long life in the Oort cloud (~4.6 x 107 yr). Ion bombardment might also lead to the formation of a substantial stable crust (Johnson et al., 1987)."[19]

Long-period comets[edit]

Orbits of Comet Kohoutek (red) and the Earth (blue), illustrating the high orbital eccentricity of its orbit and its rapid motion when close to the Sun. Credit: NASA.

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[20] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[21]

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[20] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[22] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[23] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[24] that using a heliocentric unperturbed two-body curve fitting, best-fit suggests they may escape the Solar System.

As of 2018, 1I/ʻOumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While ʻOumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet.[25] Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[26]

If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[27]


This image shows the IBEX (photo cells forward) being surrounded by its protective nose cone. Credit: NASA (John F. Kennedy Space Center).
A hot plasma ion 'steals' charge from a cold neutral atom to become an Energetic Neutral Atom (ENA).[28] Credit Mike Gruntman.
The ENA leaves the charge exchange in a straight line with the velocity of the original plasma ion.[28] Credit: Mike Gruntman.
This image is an all-sky map of neutral atoms streaming in from the interstellar boundary. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

"The sensors on the IBEX spacecraft are able to detect energetic neutral atoms (ENAs) at a variety of energy levels."[29]

The satellite's payload consists of two energetic neutral atom (ENA) imagers, IBEX-Hi and IBEX-Lo. Each of these sensors consists of a collimator that limits their fields-of-view, a conversion surface to convert neutral hydrogen and oxygen into ions, an electrostatic analyzer (ESA) to suppress ultraviolet light and to select ions of a specific energy range, and a detector to count particles and identify the type of each ion.

"IBEX–Lo can detect particles with energies ranging from 10 electron–volts to 2,000 electron–volts (0.01 keV to 2 keV) in 8 separate energy bands. IBEX–Hi can detect particles with energies ranging from 300 electron–volts to 6,000 electron–volts (.3 keV to 6 keV) in 6 separate energy bands. ... Looking across the entire sky, interactions occurring at the edge of our Solar System produce ENAs at different energy levels and in different amounts, depending on the process."[29]

Proton–hydrogen charge-exchange collisions [such as those shown at right] are often the most important process in space plasma because [h]ydrogen is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities involving little exchange of momentum.

"Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange."[30]

"The IMAGE mission ... High Energy Neutral Atom imager (HENA) ... images [ENAs] at energies between 10 and 60 keV/nucleon [to] reveal the distribution and the evolution of energetic [ions, including protons] as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre‐storm levels."[31]

"In 2009, NASA's Interstellar Boundary Explorer (IBEX) mission science team constructed the first-ever all-sky map [at right] of the interactions occurring at the edge of the solar system, where the sun's influence diminishes and interacts with the interstellar medium. A 2013 paper provides a new explanation for a giant ribbon of energetic neutral atoms – shown here in light green and blue -- streaming in from that boundary."[32]

"[T]he boundary at the edge of our heliosphere where material streaming out from the sun interacts with the galactic material ... emits no light and no conventional telescope can see it. However, particles from inside the solar system bounce off this boundary and neutral atoms from that collision stream inward. Those particles can be observed by instruments on NASA’s Interstellar Boundary Explorer (IBEX). Since those atoms act as fingerprints for the boundary from which they came, IBEX can map that boundary in a way never before done. In 2009, IBEX saw something in that map that no one could explain: a vast ribbon dancing across this boundary that produced many more energetic neutral atoms than the surrounding areas."[32]

""What we are learning with IBEX is that the interaction between the sun's magnetic fields and the galactic magnetic field is much more complicated than we previously thought," says Eric Christian, the mission scientist for IBEX at NASA's Goddard Space Flight Center in Greenbelt, Md. "By modifying an earlier model, this paper provides the best explanation so far for the ribbon IBEX is seeing.""[32]


"Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM)."[33]

Flux spectra have been derived "from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM."[33]

"The ‘bubble’ of solar wind plasma and frozen-in magnetic fields expanding out from the solar corona, within a few radii of the Sun, to boundaries with the local interstellar gas and plasma near about 100 AU is called the heliosphere. Dependent on points of origin at the Sun, and on time phase during the eleven year cycle of solar activity, the solar wind plasma expands radially outward at speeds of 300–800 km/s. Neutral atoms flowing into the heliosphere from the Very Local Interstellar Medium (VLISM) can be ionized by solar UV, and by charge exchange with solar wind ions, then picked up by magnetic fields in the outward plasma flow. Due to inverse-square fall-off of solar wind ion density with distance from the Sun, these interstellar pickup ions increasingly contribute to the plasma pressure and become the dominant component beyond the orbit of Saturn (Burlaga et al., 1996; Whang et al., 1996). Further out near 90–100 AU (Stone, 2001; Stone and Cummings, 2001; Whang and Burlaga, 2002) the outflowing plasma is expected to encounter the solar wind termination shock where flow speeds abruptly transition to sub-sonic values ∼100 km/s. The shock position is dependent in part on the plasma and neutral gas density in the Local Interstellar Medium (LISM) and could move into the giant planet region, or even nearer to the Earth’s orbit, if the Sun passed through a region of much higher LISM density (Zank and Frisch, 1999; Frisch, 2000). Further out at 120 AU or more should be the heliopause, the contact boundary between the diverted solar wind plasma flows and the in-flowing interstellar plasma. The intervening region between the termination shock and the heliopause is called the heliosheath. In this latter region the previously radial flow of the solar wind is diverted into a direction downstream from the ∼26 km/s flow of the interstellar gas to form a huge teardrop-shaped structure called the heliotail which extends hundreds to perhaps thousands of AU from the Sun into the VLISM."[33]

"Within the heliosphere the interplanetary environment of solar wind plasma, solar (SEP) and interplanetary energetic particles, and galactic cosmic rays (GCR) has long been surveyed in-situ beyond Neptune’s orbit at 30 AU, since 1983 and 1990 by the Pioneer 10 and 11 spacecraft, and since 1987 and 1989 by Voyager 1 and 2. Of these, the Pioneers are no longer transmitting data and the Voyagers are now respectively at 89 and 71 AU, far beyond the 48 AU semi-major axis (a) cutoff of the Classical KBO population but within the range of aphelia 48 < Q < 103 AU for known Centaurs (perihelia at 5 < q < 35 AU) and Scattered KBOs (q > 35 AU). Voyager 1 is expected to cross the termination shock, later followed by Voyager 2, within the next several years and possibly to exit the heliosphere across the heliopause within its remaining ∼17 + years of operational lifetime. Both spacecraft will have been silent for millennia before reaching the Oort Cloud region at 104 to 105 AU. Within the next quarter century NASA may launch an interstellar probe (e.g., Mewaldt et al., 2001a) moving outward at 10 AU/year with the ultimate goal of surveying the VLISM environment out to several hundred AU. Until then, the next mission to the outer solar system is planned to be New Horizons (Stern and Spencer, 2003), which will fly by the Pluto/Charon system in 2015 and thereafter attempt several flybys of accessible KBOs. Enroute to Pluto this mission may attempt at least one Centaur flyby after swinging by Jupiter in 2007."[33]

"The initial solar wind conditions at the inner boundary at 1 AU are radial outward speed V = 441 km/s, solar wind proton density N = 7.0/cc and temperature T = 9.8 × 104 K, and interplanetary magnetic field = 7.0 × 10−5 Gauss. The interstellar hydrogen atoms at the solar wind termination shock are taken to have speed 20 km/s and temperature 1 × 104 K, while H0 density, and the energy partition ratio for ions, are varied to give good fits to radial speed and temperature profiles measured by the operational plasma spectrometer on Voyager 2. Good fits are obtained for a neutral density of 0.09/cc and a partition ratio of 0.05, which means that five percent of the total energy from the pickup process goes into solar wind protons. For the LISM plasma ions, which are not included in the Wang and Richardson model, we compute convecting maxwellian (Vasyliunas, 1971) distributions for the LISM parameters T ∼ 7000 K, u ∼ 26 km/s, and N ∼ 0.1/cc of interstellar protons as derived from Wood and Linsky (1997)."[33]

"For the present work we define ‘cosmic ray’ protons as being those with energies above 0.1 MeV from sources within and outside the heliosphere. Sources include solar energetic particle (SEP) events, acceleration by interplanetary shocks and the solar wind termination shock, and inward diffusion through the heliosheath of galactic cosmic rays thought mostly to be accelerated by interstellar shocks from supernova explosions. Protons and heavier ions accelerated at the termination shock, after pickup from photo-ionization of interstellar gas neutrals, are called anomalous cosmic rays (ACR)."[33]

"Near solar minimum the ACR ions, including protons, are dominant components of radiation dosage outward from ∼40 AU to the outer heliosphere, while these ions largely disappear at solar maximum. There is a 22-year cycle in the polarity of the solar dipole magnetic field, which is frozen into the solar wind plasma within several radii of the Sun and thereby carried outward into the heliosphere. Due to sign-dependent transport effects, the ACR ions accelerated at the termination shock have larger fluxes, and more positive radial gradients, at 40 to 85 AU near the Ecliptic when the solar dipole moment is directed southward (qA < 0 polarity) than when it is northward (qA > 0 polarity)."[33]

"For protons the primary radiation dosage process is deposition of energy within the volume of material as a function of depth. This deposition occurs either by electronic ionization of target atoms or by direct collisions with nuclei within the atoms. Nuclear collisions are purely elastic, as for billiard balls, up to some threshold energy for inelastic collisions, which can also excite or break up the struck nucleus with increasing effect at higher energies."[33]

"For the 85-AU termination shock location the times at 0.1-μm depth drop to 107 to 108 years, while in the LISM the electronic time scale even at 1 cm is below the 109-year limit. Flux and dosage rates increase by orders of magnitude in this depth range from 40 AU out into the LISM. From 40 AU to the termination shock this trend reflects the positive radial intensity gradient for ACR protons diffusing inward from the shock acceleration source."[33]

"Oort Cloud comets, and possibly Scattered KBOs with aphelia near the heliosheath and VLISM, are maximally irradiated, while Classical KBOs near 40 AU are minimally irradiated. Radial intensity gradients ≾􏰀 +10%/AU of ACR ions might account for spatial variations in color within this latter population, e.g., redder objects with increasing perihelia in the 32 < q < 45 AU range as reported by Doressoundiram et al. (2002) and at this conference by Doressoundiram (2003)."[33]


"Considering photon bombardment first, interstellar and solar ultraviolet (that is, h𝛎 > 3 eV) photons have copious fluxes in the Oort cloud and Kuiper belt, providing the energy necessary to break bonds and initiate substantial chemical change in cometary surfaces. Ultraviolet photosputtering is capable of eroding away the uppermost few micrometres of icy surfaces35. But more importantly, in a classic series of laboratory experiments and theoretical studies, M. Greenberg showed that ultraviolet photons would produce significant alteration of the composition, colour, and volatility of the upper several to few tens of micrometres of cometary surfaces36. Others37,38 confirmed and extended these results, showing that ultraviolet photons promote surface darkening (to albedos of only a few per cent) and devolitalization that becomes progressively more severe with dosage, and therefore age. Because of their much closer proximity to the Sun, Kuiper belt comets experience a much (~105 times) higher ultraviolet and solar cosmic ray (SCR) surface dose, greatly increasing the total deposited charged-particle energy incident on the surfaces of these bodies, relative to Oort cloud comets, but their ~10 times lower average surface age somewhat mitigates this effect."[34]


Plot shows the decreased detection of solar wind particles by Voyager 1 starting in August 2012. Credit: NASA.

Def. the region of space where interstellar medium is blown away by solar wind; the boundary, heliopause, is often considered the edge of the Solar System is called the heliosphere.

The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself.

On September 12, 2013 it was announced that the previous year, starting on August 25, 2012, Voyager 1 entered the interstellar medium.[35] Outside the heliosphere the plasma density increased by about forty times.[36]

Def. the boundary of heliosphere where the Sun's solar wind is stopped by the interstellar medium is called the heliopause.

Def. a zone between the termination shock and the heliopause, in the heliosphere, at the outer border of the Solar System, where the solar wind is dramatically slower than within the termination shock is called a heliosheath.

The heliosheath is the region of the heliosphere beyond the termination shock. Here the wind is slowed, compressed and made turbulent by its interaction with the interstellar medium. Its distance from the Sun is approximately 80 to 100 astronomical units (AU) at its closest point.

The flow of ISM into the heliosphere has been measured by at least 11 different spacecraft as of 2013.[37] By 2013, it was suspected that the direction of the flow had changed over time.[37] The flow, coming from Earth's perspective from the constellation Scorpius, has probably changed direction by several degrees since the 1970s.[37]

Fermi glow[edit]

The Fermi glow are ultraviolet-glowing[38] particles, mostly hydrogen,[39] originating from the Solar System's Bow shock, created when light from stars and the Sun enter the region between the heliopause and the interstellar medium and undergo Fermi acceleration[39], bouncing around the transition area several times, gaining energy via collisions with atoms of the interstellar medium. The first evidence of the Fermi glow, and hence the bow shock, was obtained with the help from Voyager 1[38] and the Hubble Space Telescope[38].

Local Interstellar Mediums[edit]

In 2009, Voyager 2 data suggested that the magnetic strength of the local interstellar medium was much stronger than expected (370 to 550 picotesla (pT), against previous estimates of 180 to 250 pT). The fact that the Local Interstellar Cloud is strongly magnetized could explain its continued existence despite the pressures exerted upon it by the winds that blew out the Local Bubble.[40]

Local Interstellar Clouds[edit]

Diagram shows the local clouds of matter that the Solar System is moving through, with arrows indicating cloud motion. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.{{free media}}
Map showes the Sun located near the edge of the Local Interstellar Cloud and Alpha Centauri about 4 light-years away in the neighboring G-Cloud complex. Credit: Jet Propulsion Lab staff.{{free media}}

The Local Interstellar Cloud (LIC), also known as the Local Fluff, is the interstellar cloud roughly 30 light-years (9.2 pc) across through which the Solar System is moving. It is unknown if the Sun is embedded in the Local Interstellar Cloud, or in the region where the Local Interstellar Cloud is interacting with the neighboring G-Cloud.[41]

The Solar System is located within a structure called the Local Bubble, a low-density region of the galactic interstellar medium.[42]

The cloud has a temperature of about 7,000 K (6,730 °C; 12,140 °F),[43] about the same temperature as the surface of the Sun. However, its specific heat capacity is very low because it is not very dense, with 0.3 atoms per cubic centimetre (4.9/cu in). This is less dense than the average for the interstellar medium in the Milky Way (0.5/cm3 or 8.2/cu in), though six times denser than the gas in the hot, low-density Local Bubble (0.05/cm3 or 0.82/cu in) which surrounds the local cloud.[42][44] In comparison, Earth's atmosphere at Kármán line, or the edge of space has around 1.2×1013 molecules per cubic centimeter, dropping to around 50 million (5.0×107) at 450 km (280 mi).[45]

The cloud is flowing outwards from the Scorpius–Centaurus Association, a stellar association that is a star-forming region.[46][47]

The Local Interstellar Cloud's potential effects on Earth are prevented by the solar wind and the Sun's magnetic field.[43] This interaction with the heliosphere is under study by the Interstellar Boundary Explorer (IBEX), a NASA satellite mapping the boundary between the Solar System and interstellar space.

Local hot bubbles[edit]

The Local Hot Bubble is hot X-ray emitting gas within the Local Bubble pictured as an artist's impression. Credit: NASA.

The 'local hot bubble' is a "hot X-ray emitting plasma within the local environment [the ISM] of the Sun."[48] "This coronal gas fills the irregularly shaped local void of matter (McCammon & Sanders 1990) - frequently called the Local Hot Bubble (LHB)."[48]

The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.[49][50]


This is an XMM Newton image of the Gemini gamma-ray source. Credit: P.A. Caraveo (INAF/IASF), Milan and ESA.
This all-sky view from GLAST reveals bright gamma-ray emission in the plane of the Milky Way (center), including the bright Geminga pulsar. Credit: NASA/DOE/International LAT Team.

Geminga may be a sort of neutron star: the decaying core of a massive star that exploded as a supernova about 300,000 years ago.[51]

"Geminga is a very weak neutron star and the pulsar next to us, which almost only emits extremely hard gamma-rays, but no radio waves. ... Some thousand years ago our Sun entered this [Local Bubble] several hundred light-years big area, which is nearly dust-free."[52]

The nature of Geminga was quite unknown for 20 years after its discovery by NASA's Second Small Astronomy Satellite (SAS-2). In March 1991 the ROSAT satellite detected a periodicity of 0.237 seconds in soft x-ray emission. This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble.[53] Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.[54] Geminga is the first example of a radio-quiet pulsar, and serves as an illustration of the difficulty of associating gamma-ray emission with objects known at other wavelengths: either no credible object is detected in the error region of the gamma-ray source, or a number are present and some characteristic of the gamma-ray source, such as periodicity or variability, must be identified in one of the prospective candidates (or vice-versa as in the case of Geminga).

Interstellar comets[edit]

Path of the hyperbolic, extrasolar object ʻOumuamua, the first confirmed interstellar object, discovered in 2017. Credit: Tomruen.
Comet Hyakutake (C/1996 B2) might be an interstellar object captured by the Solar System. Photographed at its closest approach to Earth on 25 March 1996. The streaks in the background are stars. Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria.
Comet Machholz 1 (96P/Machholz) is viewed by STEREO-A (April 2007). Credit: NASA. NRL.
The Solar apex, the direction of the sun's motion in the Local Standard of Rest, is towards a point between Hercules and Lyra, R.A. 18h28m and Dec. 30°N (Epoch J2000.0). Credit: Tomruen.

An interstellar object is an astronomical object, other than a star or substar, that is located in interstellar space and is not gravitationally bound to a star. The term can also be applied to objects that are on an interstellar trajectory but are temporarily passing close to a star, such as certain asteroids and comets (including exocomets).[55][56]

Due to present observational difficulties, an interstellar object can usually only be detected if it passes through the Solar System, where it can be distinguished by its strongly hyperbolic trajectory, indicating that it is not gravitationally bound to the Sun.[56][57]

In early 1980s C/1980 E1, initially gravitationally bound to the Sun, passed near Jupiter and was accelerated sufficiently to reach escape velocity from the Solar System. This changed its orbit from elliptical to hyperbolic and made it the most eccentric known object at the time, with an of 1.057.[58]

Asteroid 514107 Kaʻepaokaʻawela may be a former interstellar object, captured some 4.5 billion years ago, as evidenced by its co-orbital motion with Jupiter and its retrograde orbit around the Sun.[59]

Current models of Oort cloud formation predict that more comets are ejected into interstellar space than are retained in the Oort cloud, with estimates varying from 3 to 100 times as many.[56] Other simulations suggest that 90–99% of comets are ejected.[60] There is no reason to believe comets formed in other star systems would not be similarly scattered.[55]

If interstellar comets exist, they must occasionally pass through the inner Solar System.[55] They would approach the Solar System with random velocities, mostly from the direction of the constellation Hercules because the Solar System is moving in that direction, called the solar apex.[61] Until the discovery of 'Oumuamua, the fact that no comet with a speed greater than the Sun's escape velocity[62] had been observed was used to place upper limits to their density in interstellar space. A paper by Torbett indicated that the density was no more than 1013 (10 trillion) comets per cubic parsec.[63] Other analyses, of data from LINEAR, set the upper limit at 4.5×10−4/AU3, or 1012 (1 trillion) comets per cubic parsec.[56] A more recent estimate, following the detection of 'Oumuamua, predicts that "The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~1×104, each with a residence time of ~10 years."[64]

Computer simulations show that Jupiter is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years.[63] Comets Machholz 1 and Hyakutake C/1996 B2 are possible examples of such comets. They have atypical chemical makeups for comets in the Solar System.[62][65]

There should be hundreds of 'Oumuamua-size interstellar objects in the Solar System, based on calculated orbital characteristics, and there are several centaur candidates such as 2017 SV13 and 2018 TL6.[66]

On 8 January 2014 a bolide which has been identified as a potentially interstellar object originating from an unbound hyperbolic orbit exploded in the atmosphere over northern Papua New Guinea.[67] It had an orbital eccentricity of 2.4, an inclination of 10°, and a speed of 43.8 km/s when outside of the Solar System. This would make it notably faster than ʻOumuamua which was 26.3 km/s when outside the Solar System. The meteor is estimated to have been 0.9 meters in diameter. Other astronomers doubt the interstellar origin because the meteor catalog used does not report uncertainties on the incoming velocity.[68]

Hills clouds[edit]

The Hills cloud (also called the inner Oort cloud and inner cloud[69]) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 AU from the Sun, and whose inner border, less well-defined, is hypothetically located at 250-1500 AU, well beyond planetary and Kuiper Belt object orbits - but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.[70]

Objects ejected from the Hills cloud are likely to end up in the classical Oort cloud region, maintaining the Oort cloud.[71]

The existence of the Hills cloud is plausible, since many bodies have been found already. It would be denser than the Oort cloud.[72][73]

Comets may be rooted in a cloud orbiting the outer boundary of the Solar System.[74]

Comets are usually destroyed after several passes through the inner Solar System, so if any had existed for several billion years (since the beginning of the Solar System), no more could be observed now.[75] The distribution of the inverse of the semi-major axes showed a maximum frequency which suggested the existence of a reservoir of comets between 40,000 and 150,000 AU (0.6 and 2.4 ly) away.[75] This reservoir, located at the limits of the Sun's sphere of astrodynamic influence, would be subject to stellar disturbances, likely to expel cloud comets outwards or inwards.[75]

Most estimates place the population of the Hills cloud at about 20 trillion (about five to ten times that of the outer cloud), although the number could be ten times greater than that.[76] The orbits of most cloud comets have a semi-major axis of 10,000 AU, much closer to the Sun than the proposed distance of the Oort cloud.[72] Moreover, the influence of the surrounding stars and that of the galactic tide should have sent the Oort cloud comets either closer to the Sun or outside of the Solar System. The presence of an inner cloud, which would have tens or hundreds of times as many cometary nuclei as the outer halo was proposed.[72]

The majority of comets in the Solar System were located not in the Oort cloud area, but closer and in an internal cloud, with an orbit with a semi-major axis of 5,000 AU.[77]

It is likely that the Hills cloud is the largest concentration of comets across the Solar System.[78] The Hills cloud is much denser than the outer Oort cloud; it is somewhere between 5,000 and 20,000 AU in size. In contrast, the Oort cloud is between 20,000 and 50,000 AU (0.3 and 0.8 ly) in size.[79]

The mass of the Hills cloud may be five times more massive than the Oort cloud.[80] Or, the mass of the Hills cloud to be 13.8 Earth masses, if the majority of the bodies are located at 10,000 AU.[77]

The vast majority of Hills cloud objects consists of various ices, such as water, methane, ethane, carbon monoxide and hydrogen cyanide.[81] However, the discovery of the object 1996 PW, an asteroid on a typical orbit of a long-period comet, suggests that the cloud may also contain rocky objects.[82]

The carbon analysis and isotopic ratios of nitrogen firstly in the comets of the families of the Oort cloud and the other in the body of the Jupiter area shows little difference between the two, despite their distinctly remote areas, which suggests that both come from a protoplanetary disk,[83] a conclusion also supported by studies of comet cloud sizes and the recent impact study of Comet Tempel 1.[84]


The orbits of the three known sednoids with Neptune's 30 AU circular orbit is in blue. Credit: Tomruen.

A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU.[85][86] Only three objects are known from this population, 90377 Sedna, 2012 VP113, and 2015 TG387, all of which have perihelia greater than 64 AU,[87] but it is suspected that there are many more. These objects lie outside an apparently nearly empty gap in the Solar System starting at about 50 AU, and have no significant interaction with the planets. They are usually grouped with the detached objects. Some astronomers, such as Scott Sheppard,[88] consider the sednoids to be inner Oort cloud objects (OCOs), though the inner Oort cloud, or Hills cloud, was originally predicted to lie beyond 2,000 AU, beyond the aphelia of the three known sednoids.

This definition also applies for 2013 SY99 which has a perihelion at 50.02 AU, far beyond the Kuiper cliff, but it is thought not to belong to the Sednoids, but to the same dynamical class as 474640 2004 VN112, 2014 SR349 and 2010 GB174.[89] With these high eccentricities > 0.8 they can easily be distinguished from the high-perihelion objects with moderate eccentricities which are in a stable resonance with Neptune, that is 2015 KQ174, 2015 FJ345, 2004 XR190, 2014 FC72 and 2014 FZ71.[90]

The sednoids' orbits cannot be explained by perturbations from the giant planets,[91] nor by interaction with the galactic tides.[85] 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.[92]

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.[93][94]

Their orbits could have been disrupted by an as-yet-unknown planet-sized body beyond the Kuiper belt such as the hypothesized Planet Nine.[95][96]

They could have been captured from around passing stars, most likely in the Sun's birth cluster.[91][97]

Sednoids and Sednoid candidates[87][98]
Number Name Diameter
Perihelion (AU) Semimajor axis (AU) Aphelion (AU) Heliocentric
distance (AU)
Argument of perihelion (°) Year discovered (precovered)
90377 90377 Sedna 995 ± 80 76.06 506 936 85.1 311.38 2003 (1990)
2012 VP113 300–1000[99] 80.50 261.00 441.49 83.65 293.78 2012 (2011)
2015 TG387[100] 200–600 64.94 1094 2123 77.69 118.17 2015 (none)


Here, the presumed distance of the Oort cloud is compared to the rest of the Solar System using the orbit of Sedna. Credit: NASA / JPL-Caltech / R. Hurt.
Sedna, a possible inner Oort cloud object, is a discovery in 2003. Credit: NASA/Caltech.
Sedna is imaged by the Hubble Space Telescope. Credit: NASA.{{free media}}

Sedna was discovered from an image dated 2003-11-14 at coordinates 03 15 10.09 +05 38 16.5. The 3 overexposed stars are apparent magnitude 13. The "bright star" near Sedna is apmag 14.9 and about the same magnitude as Pluto. (Wikisky image of this region) The picture shows an area of the sky equal to the area covered by a pinhead held at arm's length. Sedna is too faint to be seen by all but the most powerful amateur telescopes.

Spectroscopy has revealed that Sedna's surface composition is similar to those of some other trans-Neptunian objects, being largely a mixture of water, methane, and solid nitrogen volatiles, ices with tholins. Its surface is one of the reddest among Solar System objects. It is most likely a dwarf planet. Among the eight largest trans-Neptunian objects, Sedna is the only one not known to have a moon.[101][102]

For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU[103] (31 times Neptune's distance), making it one of the most distant-known objects in the Solar System other than long-period comets. Sedna was about 86.3 AU from the Sun;[104] Eris, the most massive-known dwarf planet, and 225088 2007 OR10, the largest object in the Solar System without a name, are currently farther from the Sun than Sedna at 96.4 AU and 87.0 AU, respectively.[105] Eris is near its aphelion (farthest distance from the Sun), whereas Sedna is nearing its 2076 perihelion (closest approach to the Sun).[106] Sedna will overtake Eris as the farthest known large object in the Solar System in 2114, but the probable dwarf planet 225088 2007 OR10 has recently overtaken Sedna and will overtake Eris by 2045.[106]

Sedna compared to some other very distant orbiting bodies including 472651 2015 DB216 (orbit wrong), 87269 2000 OO67, 474640 2004 VN112, 2005 VX3, 308933 2006 SQ372, 2007 TG422, 2007 DA61, 418993 2009 MS9, 2010 GB174, 336756 2010 NV1, 2010 BK118, 2012 DR30, 2012 VP113, 2013 BL76, 2013 AZ60, 2013 RF98, 2015 ER61. Credit: Exoplanetaryscience.

1996 PW[edit]

Def. an "asteroid (such as 5335 Damocles) that exhibits long-period, highly eccentric orbits typical of periodic comets without showing a coma"[107] is called a damocloid, or Damocloid.

1996 PW is an exceptionally eccentric trans-Neptunian object and damocloid on an orbit typical of long-period comets but one that showed no sign of cometary activity around the time it was discovered.[108] The unusual object measures approximately 10 kilometers (6 miles) in diameter and has a rotation period of 35.4 hours and likely an elongated shape.[109]

1996 PW orbits the Sun at a distance of 2.5–504 AU once every 4,033 years (semi-major axis of 253 AU), an eccentricity of 0.99 and an inclination of 30° with respect to the ecliptic.[110]

Simulations indicate that it has most likely come from the Oort cloud, with a roughly equal probability of being an extinct comet and a rocky body that was originally scattered into the Oort cloud. The discovery of 1996 PW prompted theoretical research that suggests that roughly 1 to 2 percent of the Oort cloud objects are rocky.[111][112]

1996 PW was first observed on 9 August 1996 by the Near-Earth Asteroid Tracking (NEAT) automated search camera on Haleakala Observatory, Hawaii. It is the first object that is not an active comet discovered on an orbit typical of long-period comets.[111]

1996 PW has a rotation period of 35.44±0.02 hours and a double-peaked lightcurve with a high amplitude of 0.44±0.03 magnitude (LCDB quality code).[109][108] Its spectrum is moderately red and featureless,[113] typical of D-type asteroids and bare comet nuclei.[108][112][113] Its spectrum suggests an extinct comet.[113] The upper limit on 1996 PW's dust production is 0.03 kg/s.[108]

2015 TG387[edit]

2015 TG
(nicknamed The Goblin for the letters TG and because its discovery was near Halloween),[114][115] is a trans-Neptunian object (TNO) and sednoid in the outermost part of the Solar System.[116] It was first observed on October 13, 2015, with the Subaru Telescope at Mauna Kea Observatories, and publicly announced on October 1, 2018.[117][118]

2015 TG
is the third sednoid to be discovered, following 90377 Sedna and 2012 VP
.[119][87] It is estimated to be 300 km (190 mi) in diameter.[119]

Along with the similar orbits of other distant TNOs, the orbit of 2015 TG
suggests, but does not prove, the existence of a hypothetical Planet Nine in the outer Solar System.[119][118]

As of 2018, the object is 80 AU from the Sun; about two-and-a-half times farther out than Pluto’s orbit.[115] As with Sedna, it would not have been found had it not been on the inner leg of its long orbit, which suggests that there may be many similar objects, most too distant to be detected by contemporary technological methods, and implies a population of about 2 million Hills cloud, or inner Oort cloud, objects larger than 40 km (25 mi), with a combined total mass of 1022 kg, which is several times the mass of the asteroid belt.[119]

(308933) 2006 SQ372[edit]

Hubble Space Telescope image shows 2006 SQ372 taken in 2009. Credit: Hubble Space Telescope/Michael E. Brown.{{free media}}
Diagram shows the orbit of 2006 SQ372. Credit: NASA.{{free media}}

(308933) 2006 SQ372 is a trans-Neptunian object and highly eccentric centaur on a cometary-like orbit in the outer region of the Solar System, approximately 123 kilometers (76 miles) in diameter, discovered through the Sloan Digital Sky Survey on images first taken on 27 September 2006 (with precovery images dated to 13 September 2005).[120][121][122][123]

(308933) 2006 SQ372 has a highly eccentric orbit, crossing that of Neptune near perihelion but bringing it more than 1,500 AU from the Sun at aphelion.[124] It takes about 22,500 years to orbit the barycenter of the Solar System.[125] The large semi-major axis makes it similar to 87269 2000 OO67 and Sedna.[125] With an absolute magnitude (H) of 8.1,[126] it is estimated to be about 60 to 140 km in diameter.[127] It has an albedo of 0.08 which would give a diameter of around 110 km.[128]

The object could possibly be a comet,[125] could come from the Hills cloud,[125] or "it may have formed from debris just beyond Neptune [in the Kuiper belt] and been 'kicked' into its distant orbit by a planet like Neptune or Uranus".[129]

The orbit of 2006 SQ372 currently comes closer to Neptune than any of the other giant planets.[120] More than half of the simulations of this object show that it gets too close to either Uranus or Neptune within the next 180 million years, sending it in a currently unknown direction.[130] This makes it difficult to classify this object as only a centaur or a scattered disc object. The Minor Planet Center, which officially catalogues all trans-Neptunian objects, lists centaurs and SDOs together.[131] (29981) 1999 TD10 is another such object that blurs the two categories.[132]

Loop I Bubbles[edit]

3D representation of the Local Bubble (white) with neighbouring Molecular Clouds (pink) and a section of the Loop I Bubble (blue). Credit: Raydekk.{{free media}}

The Loop I Bubble is a cavity in the interstellar medium (ISM) of the Orion Arm of the Milky Way. From our Sun's point of view, it is situated towards the Galactic Center. Two conspicuous tunnels connect the Local Bubble with the Loop I Bubble cavity (the Lupus Tunnel).[133] The Loop I Bubble is a supershell.[134]

The Loop I Bubble is located roughly 100 parsecs, or 330 light years, from the Sun. The Loop I Bubble was created by supernovae and stellar winds in the Scorpius–Centaurus Association, some 500 light years from the Sun. The Loop I Bubble contains the star Antares (also known as Alpha Scorpii). Several tunnels connect the cavities of the Local Bubble with the Loop I Bubble, called the "Lupus Tunnel".[133]


The G-Cloud (or G-Cloud complex) is an interstellar cloud located next to the Local Interstellar Cloud. The G-Cloud contains the stars Alpha Centauri (a triple star system that includes Proxima Centauri) and Altair (and possibly others).[135][136][137][138][139][140]


Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[141] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[142][143] A total of 10 such exocomet systems have been identified as of 2013, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[141][142]


Clouds of material are along the paths of the Voyager 1 and Voyager 2 spacecraft through interstellar space. Credit: NASA, ESA, and Z. Levay (STScI).

The Voyager 1 spacecraft is a 722 kg (1,592 lb) space probe launched by NASA on September 5, 1977 to study the outer Solar System and interstellar medium.

The Cosmic Ray System (CRS) determines the origin and acceleration process, life history, and dynamic contribution of interstellar cosmic rays, the nucleosynthesis of elements in cosmic-ray sources, the behavior of cosmic rays in the interplanetary medium, and the trapped planetary energetic-particle environment.

Measurements from the spacecraft revealed a steady rise since May in collisions with high energy particles (above 70 MeV), which are believed to be cosmic rays emanating from supernova explosions far beyond the Solar System, with a sharp increase in these collisions in late August. At the same time, in late August, there was a dramatic drop in collisions with low-energy particles, which are thought to originate from the Sun.[144]

"It's important for us to be aware of what kinds of objects are present beyond our solar system, since we are now beginning to think about potential interstellar space missions, such as Breakthrough Starshot."[145]

At "least two interstellar clouds [have been discovered] along Voyager 2's path, and one or two interstellar clouds along Voyager 1's path. They were also able to measure the density of electrons in the clouds along Voyager 2's path, and found that one had a greater electron density than the other."[146]

"We think the difference in electron density perhaps indicates a difference in composition of overall density of the clouds."[145]

A "broad range of elements [were detected]] in the interstellar medium, such as electrically charged ions of magnesium, iron, carbon and manganese [and] neutrally charged oxygen, nitrogen and hydrogen."[146]

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External links[edit]

{{Radiation astronomy resources}}