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The Oort Cloud
Oort Cloud Facts The Oort Cloud is a theorized shell of icy objects that lie beyond the Kuiper Belt, as such the facts detailed on this page are hypothetical.
What is the Oort Cloud? The Oort Cloud is an extended shell of icy objects that exist in the outermost reaches of the solar system. It is named after astronomer Jan Oort, who first theorised its existence. The Oort Cloud is roughly spherical, and is thought to be the origin of most of the long-period comets that have been observed. This cloud of particles is theorized to be the remains of the disc of material that formed the Sun and planets. Astronomers now refer to those primeval objects as a protoplanetary disk. The most likely theory is that the material now in the Oort Cloud probably formed closer to the young Sun in the earliest epochs of solar system formation. As the planets grew, and in particular as Jupiter coalesced and migrated to its present position, its gravitational influence is thought to have scattered many icy objects out to their present position in the Oort cloud. The Oort Cloud is very distant from the Sun and it can be disrupted by the nearby passage of a star, nebula, or by actions in the disk of the Milky Way. Those actions knock cometary nuclei out of their orbits, and send them on a headlong rush toward the Sun.
Oort Cloud Location The inner limits of the Oort Cloud begin at about 2,000 AU from the Sun. The cloud itself stretches out almost a quarter of the way to the nearest star, Proxima Centauri. It is spherically shaped and consists of an outer cloud and a torus (doughnut-shaped) inner cloud.
Facts about the Oort Cloud• Objects in the Oort Cloud are also referred to as Trans-Neptunian objects. This name also applies to objects in the Kuiper Belt.• Some astronomers theorize that the Sun may have captured Oort Cloud cometary material from the outer disks of other stars that were forming in the same nebula as our star.• The Oort Cloud is a reserve of cometary nuclei that contain ices dating back to the origin of the solar system.• No one knows for sure how many objects exist in the Oort Cloud, but most estimates put it at around 2 trillion.• The planetoid Sedna, discovered in 2003, is thought to be a member of the inner Oort Cloud.• Astronomers think that long-period comets (those with orbital periods longer than 200 years) have their origins in the Oort Cloud. © Space Facts.com
What is the Oort Cloud? The Oort Cloud is an extended shell of icy objects that exist in the outermost reaches of the solar system. It is named after astronomer Jan Oort, who first theorised its existence. The Oort Cloud is roughly spherical, and is thought to be the origin of most of the long-period comets that have been observed. This cloud of particles is theorized to be the remains of the disc of material that formed the Sun and planets. Astronomers now refer to those primeval objects as a protoplanetary disk. The most likely theory is that the material now in the Oort Cloud probably formed closer to the young Sun in the earliest epochs of solar system formation. As the planets grew, and in particular as Jupiter coalesced and migrated to its present position, its gravitational influence is thought to have scattered many icy objects out to their present position in the Oort cloud. The Oort Cloud is very distant from the Sun and it can be disrupted by the nearby passage of a star, nebula, or by actions in the disk of the Milky Way. Those actions knock cometary nuclei out of their orbits, and send them on a headlong rush toward the Sun.
Oort Cloud Location The inner limits of the Oort Cloud begin at about 2,000 AU from the Sun. The cloud itself stretches out almost a quarter of the way to the nearest star, Proxima Centauri. It is spherically shaped and consists of an outer cloud and a torus (doughnut-shaped) inner cloud.
Facts about the Oort Cloud• Objects in the Oort Cloud are also referred to as Trans-Neptunian objects. This name also applies to objects in the Kuiper Belt.• Some astronomers theorize that the Sun may have captured Oort Cloud cometary material from the outer disks of other stars that were forming in the same nebula as our star.• The Oort Cloud is a reserve of cometary nuclei that contain ices dating back to the origin of the solar system.• No one knows for sure how many objects exist in the Oort Cloud, but most estimates put it at around 2 trillion.• The planetoid Sedna, discovered in 2003, is thought to be a member of the inner Oort Cloud.• Astronomers think that long-period comets (those with orbital periods longer than 200 years) have their origins in the Oort Cloud. © Space Facts.com
10 Interesting Facts about the Oort Cloud
Named after the Dutch astronomer Jan Oort, but sometimes also known as the Öpik–Oort cloud, this hypothetical cloud of small, icy planetesimals is thought to surround the solar system out to a distance of more than 3 light years. Refer to the image above for some details of the structure of the Oort cloud, and also note that by way of comparison, the Kuiper belt that exists beyond the orbit of Neptune is about one thousand times closer to the Sun than the inner wall of the Oort cloud is to the Sun. Below are 10 more interesting facts about the Oort cloud you may not have known.
The Oort cloud defines the solar system’s boundary In practical terms, the outer edge of Oort cloud defines the boundary of the solar system, and the limit of the Sun’s Hill sphere. In simple terms, the limit of Sun’s Hill sphere (named after American astronomer George William Hill, who defined this limit) can be seen as the point where the Sun’s gravity no longer dominates in the face of the gravitational effects of a more massive body, which in this case would be either the Milky Way galaxy, or the gravitational effects of a star passing close to the outer limit of the Sun’s Hill sphere.
Long-period comets may originate in the Oort cloud Although the question around the origin of the Oort cloud is far from settled, astronomers have used the observed orbits of long period comets such as Halley’s Comet as the basis for the notion that all long period comets, as well as “centaurs” and Jupiter-family comets have their origin in the Oort cloud. However, even though most short-period comets are thought to originate in the scattered disc (NOT a part of the Oort cloud) it is entirely possible that the ultimate origin of at least some short-period comets may have been in the outer parts of the Oort cloud.
The Oort cloud is really, really big Although the Oort cloud has not been directly observed, it is thought to resemble a spherical ball with a wall thickness that starts at about 2,000 – 5,000 AU (0.03 – 0.08 light years) from the Sun, and stretches to about 100,000 – 200,000 AU (1.58 – 3.16 light years) from the Sun. This is really big, considering that Proxima Centauri, closest star closest to the Sun, is only 4.22 light years away.
The Oort cloud is only about 5 times as massive as Earth Based on complex computer models, it is estimated that the Oort cloud contains at least several trillion objects that are bigger than 1 km (0.62 mile) in diameter, and a further several billion objects that are around 20 km (12 miles) in diameter, with the typical distance between objects being on the order of a few tens of millions of km. Although the total mass of the Oort cloud is not known, calculations based on the mass of Halley’s Comet (a suspected Oort-cloud comet) yield a total mass of the objects in the Oort cloud of about 3×1025 kilograms, which is roughly 5 times the mass of Earth. Note though that the mass of the inner Oort cloud (a torus-shaped part of the principal structure) has to date not been calculated.
The Oort cloud contains material from other stars While conventional wisdom holds that the Oort cloud represents the remains of the original protoplanetary disc out of which the solar system had formed about 4.6 billion years ago, new research has shown that the proto solar system once existed as part of a star cluster that consisted of between 200 and 400 stars. These findings imply that the Oort cloud could therefore not have formed close to the Sun as has been suggested, and could also therefore not have been “puffed up” to its present location and size by the action of the giant gas planets as they migrated away from the Sun. Moreover, improved modelling techniques suggest that since the structure of the Oort cloud is largely compatible with the notion that other stars may have contributed to its formation, close stellar encounters with the cloud would have been far more frequent then than it is now. In fact, in 2010 Harold F. Levison et al used highly complex computer models to show that up to 90% or more of the material in the present-day Oort cloud originated in the protoplanetary discs of other stars in the cluster that once accommodated the Sun.
The Oort cloud is flexible Since the outer reaches of the Oort cloud falls in an area where the gravity of the Sun is in direct competition with the gravitational effects of the Milky Way, the Oort cloud is both stretched in one direction, and compressed in another direction by the tidal forces of the galactic gravitational field. This” kneading” effect is thought to be the principal mechanism that perturbs some objects out of their otherwise relatively stable orbits, to become long period comets when the Sun’s gravity overtakes that of the Milky Way galaxy, which happens at distance of between 100,000, and 200,000 AU from the Sun. In astronomical parlance, this point is known as the “tidal truncation radius” beyond which the Milky Way’s gravity is stronger than that of the Sun.
Stars sometimes pass through the Oort cloud Apart from the gravitational effects of the galactic tide, another mechanism that perturbs the Oort cloud sufficiently to send comets into the inner solar system is the passing of nearby stars through the cloud. For example, the dim binary star designated WISE J072003.20-084651.2 (Scholz’s star) passed through the outer reaches of the Oort cloud about 70,000 years ago, although its high speed and low mass limited the effects of its passing. One other star, Gliese 710, has the potential to dislodge large numbers of comets from the Oort cloud during the next 10 million years or so.
Oort-cloud comets can disappear Soon after Jan Oort developed a model that predicted how many long-period comets from the Oort cloud will return to the solar system, he noted that far fewer comets actually do so than his model had predicted. To date, no known purely dynamical process can account for this, and although the number of returning comets that return to the outer solar system far exceeds the number that return to the inner solar system, the issue remains unresolved. Possible explanations for the observed under count of comets include destruction of comets by collisions with outer planets and other bodies, fragmentation caused by tidal stresses, or the depletion of all volatile material in the cometary nucleus, which would render such comets invisible.
The Oort cloud contains both comets and asteroids If what is known about the composition of known comets is taken to be representative of all comets, most objects in the Oort cloud will consist of various ices such as frozen water, methane, ethane, carbon monoxide, and hydrogen cyanide. However, the discovery of an object dubbed 1996 PW that has an orbit that is similar to long-period comets, and which has a composition and appearance similar to D-type asteroids, suggests that between 1% and 2% of the Oort cloud population consists of asteroids.
Nobody has seen the Oort cloud-yet Everything that is known about the Oort cloud is based on inference, deductive reasoning, theoretical computer models, and some intelligent guesses about the origin of long-period comets, since nobody has actually observed the cloud yet. The space probe that is currently closest to the Oort cloud is Voyager 1, and although it is the fastest of all the current space probes, it will only reach the inner reaches of the Oort cloud in another 300 or so years, and will need about 30,000 years to pass through the cloud’s wall. Sadly, however, the nuclear reactors that supply Voyager 1 with power is expected to cease working in about 2025, and none of the other space probes now in service is expected to be functional by the time they reach the Oort cloud. © August 18, 2017 Peter Christoforou SPACE.COM
The Oort cloud defines the solar system’s boundary In practical terms, the outer edge of Oort cloud defines the boundary of the solar system, and the limit of the Sun’s Hill sphere. In simple terms, the limit of Sun’s Hill sphere (named after American astronomer George William Hill, who defined this limit) can be seen as the point where the Sun’s gravity no longer dominates in the face of the gravitational effects of a more massive body, which in this case would be either the Milky Way galaxy, or the gravitational effects of a star passing close to the outer limit of the Sun’s Hill sphere.
Long-period comets may originate in the Oort cloud Although the question around the origin of the Oort cloud is far from settled, astronomers have used the observed orbits of long period comets such as Halley’s Comet as the basis for the notion that all long period comets, as well as “centaurs” and Jupiter-family comets have their origin in the Oort cloud. However, even though most short-period comets are thought to originate in the scattered disc (NOT a part of the Oort cloud) it is entirely possible that the ultimate origin of at least some short-period comets may have been in the outer parts of the Oort cloud.
The Oort cloud is really, really big Although the Oort cloud has not been directly observed, it is thought to resemble a spherical ball with a wall thickness that starts at about 2,000 – 5,000 AU (0.03 – 0.08 light years) from the Sun, and stretches to about 100,000 – 200,000 AU (1.58 – 3.16 light years) from the Sun. This is really big, considering that Proxima Centauri, closest star closest to the Sun, is only 4.22 light years away.
The Oort cloud is only about 5 times as massive as Earth Based on complex computer models, it is estimated that the Oort cloud contains at least several trillion objects that are bigger than 1 km (0.62 mile) in diameter, and a further several billion objects that are around 20 km (12 miles) in diameter, with the typical distance between objects being on the order of a few tens of millions of km. Although the total mass of the Oort cloud is not known, calculations based on the mass of Halley’s Comet (a suspected Oort-cloud comet) yield a total mass of the objects in the Oort cloud of about 3×1025 kilograms, which is roughly 5 times the mass of Earth. Note though that the mass of the inner Oort cloud (a torus-shaped part of the principal structure) has to date not been calculated.
The Oort cloud contains material from other stars While conventional wisdom holds that the Oort cloud represents the remains of the original protoplanetary disc out of which the solar system had formed about 4.6 billion years ago, new research has shown that the proto solar system once existed as part of a star cluster that consisted of between 200 and 400 stars. These findings imply that the Oort cloud could therefore not have formed close to the Sun as has been suggested, and could also therefore not have been “puffed up” to its present location and size by the action of the giant gas planets as they migrated away from the Sun. Moreover, improved modelling techniques suggest that since the structure of the Oort cloud is largely compatible with the notion that other stars may have contributed to its formation, close stellar encounters with the cloud would have been far more frequent then than it is now. In fact, in 2010 Harold F. Levison et al used highly complex computer models to show that up to 90% or more of the material in the present-day Oort cloud originated in the protoplanetary discs of other stars in the cluster that once accommodated the Sun.
The Oort cloud is flexible Since the outer reaches of the Oort cloud falls in an area where the gravity of the Sun is in direct competition with the gravitational effects of the Milky Way, the Oort cloud is both stretched in one direction, and compressed in another direction by the tidal forces of the galactic gravitational field. This” kneading” effect is thought to be the principal mechanism that perturbs some objects out of their otherwise relatively stable orbits, to become long period comets when the Sun’s gravity overtakes that of the Milky Way galaxy, which happens at distance of between 100,000, and 200,000 AU from the Sun. In astronomical parlance, this point is known as the “tidal truncation radius” beyond which the Milky Way’s gravity is stronger than that of the Sun.
Stars sometimes pass through the Oort cloud Apart from the gravitational effects of the galactic tide, another mechanism that perturbs the Oort cloud sufficiently to send comets into the inner solar system is the passing of nearby stars through the cloud. For example, the dim binary star designated WISE J072003.20-084651.2 (Scholz’s star) passed through the outer reaches of the Oort cloud about 70,000 years ago, although its high speed and low mass limited the effects of its passing. One other star, Gliese 710, has the potential to dislodge large numbers of comets from the Oort cloud during the next 10 million years or so.
Oort-cloud comets can disappear Soon after Jan Oort developed a model that predicted how many long-period comets from the Oort cloud will return to the solar system, he noted that far fewer comets actually do so than his model had predicted. To date, no known purely dynamical process can account for this, and although the number of returning comets that return to the outer solar system far exceeds the number that return to the inner solar system, the issue remains unresolved. Possible explanations for the observed under count of comets include destruction of comets by collisions with outer planets and other bodies, fragmentation caused by tidal stresses, or the depletion of all volatile material in the cometary nucleus, which would render such comets invisible.
The Oort cloud contains both comets and asteroids If what is known about the composition of known comets is taken to be representative of all comets, most objects in the Oort cloud will consist of various ices such as frozen water, methane, ethane, carbon monoxide, and hydrogen cyanide. However, the discovery of an object dubbed 1996 PW that has an orbit that is similar to long-period comets, and which has a composition and appearance similar to D-type asteroids, suggests that between 1% and 2% of the Oort cloud population consists of asteroids.
Nobody has seen the Oort cloud-yet Everything that is known about the Oort cloud is based on inference, deductive reasoning, theoretical computer models, and some intelligent guesses about the origin of long-period comets, since nobody has actually observed the cloud yet. The space probe that is currently closest to the Oort cloud is Voyager 1, and although it is the fastest of all the current space probes, it will only reach the inner reaches of the Oort cloud in another 300 or so years, and will need about 30,000 years to pass through the cloud’s wall. Sadly, however, the nuclear reactors that supply Voyager 1 with power is expected to cease working in about 2025, and none of the other space probes now in service is expected to be functional by the time they reach the Oort cloud. © August 18, 2017 Peter Christoforou SPACE.COM
(c) Sedna by Adolf Schaller for NASA
SEDNA (Inner Oort Dwarf Planet)
There has been quite a bit of buzz about dwarf planets lately. Ever since the discovery of Eris in 2005, and the debate that followed over the proper definition of the word “planet”, this term has been adopted to refer to planets beyond Neptune that rival Pluto in size. Needless to say, it has been a controversial subject, and one which is not likely to be resolved anytime soon. In the meantime, the category has been used tentatively to describe many Trans-Neptunian objects that were discovered before or since the discovery of Eris. Sedna, which was discovered in the outer reaches of the Solar System in 2003, is most likely a dwarf planet. And as the furthest known object from the Sun, and located within the hypothetical Oort Cloud, it is quite the fascinating find.
Discovery and Naming: Much like Eris, Haumea and Makemake, Sedna was co-discovered by Mike Brown of Caltech, with assistance from Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University on November 14th, 2003. Initially designated as 2003 VB12, the discovery was part of a survey that commenced in 2001 using the Samuel Oschin Telescope at the Palomar Observatory near San Diego, California. Observations at the time indicated the presence of an object at a distance of approximately 100 AU from the Sun. Follow-up observations made in November and December of 2003 by the Cerro Tololo Inter-American Observatory in Chile and the W. M. Keck Observatory in Hawaii revealed that the object was moving along a distant highly eccentric orbit. It was later learned that the object had been previously observed by the Samuel Oschin telescope as well as the Jet Propulsion Laboratory’s Near Earth Asteroid Tracking (NEAT) consortium. Comparisons with these previous observations have since allowed for a more precise calculation of Sedna’s orbit and orbital arc. According to Mike Brown’s website, the planet was named Sedna after the Inuit Goddess of the sea. According to legend, Sedna was once mortal but became immortal after drowning in the Arctic Ocean, where she now resides and protects all the creatures of the sea. This name seemed appropriate to Brown and his team because Sedna is currently the farthest (and hence coldest) object from the Sun. The team made the name public before the object had been officially numbered; and while this represented a breach in IAU protocol, no objections were raised. In 2004, the IAU’s Committee on Small Body Nomenclature formally accepted the name.
Classification: Astronomers remain somewhat divided when it comes to Sedna’s proper classification. On the one hand, its discovery resurrected the question of which astronomical objects should be considered planets and which ones could not. Under the IAU’s definition of a planet, which was adopted on August 24th, 2006 (in response to the discovery of Eris), a planet needs to have cleared its orbit. Hence, Sedna does not qualify. However, to be a dwarf planet, a celestial body must be in hydrostatic equilibrium – meaning that it is symmetrically rounded into a spheroid or ellipsoid shape. With a surface albedo of 0.32 ± 0.06 – and an estimated diameter of between 915 and 1800 km (compared to Pluto’s 1186 km) – Sedna is bright enough, and also large enough, to be spheroid in shape. Therefore, Sedna is believed by many astronomers to be a dwarf planet, and is often referred to confidently as such. One reason why astronomers are reluctant to definitively place it in that category is because it is so far away that it is difficult to observe.
Size, Mass and Orbit: In 2004, Mike Brown and his team placed an upper limit of 1,800 km on its diameter, but by 2007 this was revised downward to less than 1,600 km after observations were made by the Spitzer Space Telescope. In 2012, measurements from the Herschel Space Observatory suggested that Sedna’s diameter was between 915 and 1075 km, which would make it smaller than Pluto’s moon Charon. Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe. Nevertheless, many astronomers think that Sedna is the fifth largest trans-Neptunian object (TNO) and dwarf planet – after Eris, Pluto, Makemake, and Haumea, respectively. Sedna has a highly elliptical orbit around the Sun, which means it ranges in distance from 76 astronomical units (AU) at perihelion (114 billion km/71 billion mi) to 936 AU (140 billion km/87 billion mi) at aphelion. Estimations on how long it takes Sedna to orbit the Sun vary, although it is known to be more than 10,000 years. Some astronomers calculate the orbital period could be as long as 12,000 years. Although astronomers believed at first that Sedna had a satellite, they have not been able to prove it.
Composition: At the time of its discovery, Sedna was the intrinsically brightest object found in the Solar System since Pluto in 1930. In terms of color, Sedna appears to be almost as red as Mars, which some astronomers believe is caused by hydrocarbons or tholins. Its surface is also rather homogeneous in terms of color and spectrum, which may the result of Sedna’s distance from the Sun. Unlike planets in the Inner Solar System, Sedna experiences very few surface impacts from meteors or stray objects. As a result, it does not have as many exposed bright patches of fresh icy material. Sedna, and the entire Oort Cloud, is freezing at temperatures below 33 Kelvin (-240.2°C). Models have been constructed of Sedna that place an upper limit of 60% for methane ice and 70% for water ice. This is consistent with the existence of tholins on its surface, since they are produced by the irradiation of methane. Meanwhile, M. Antonietta Barucci and colleagues compared Sedna’s spectrum to that of Triton and came up with a model that included 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol and 33% methane. The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may have a tenuous atmosphere. During a 200-year period near perihelion, the maximum temperature on Sedna would likely exceed 35.6 K (-237.6 °C), which would be just warm enough for some of the nitrogen ice to sublimate. Models of internal heating via radioactive decay suggest that, like many bodies in the Outer Solar System, Sedna might be capable of supporting a subsurface ocean of liquid water.
Origin: When he and his colleagues first observed Sedna, they claimed that it was part of the Oort Cloud – the hypothetical cloud of comets believed to exist a light-year’s distance from the Sun. This was based on the fact that Sedna’s perihelion (76 AUs) made it too distant to be scattered by the gravitational influence of Neptune. Because it was also closer to the Sun than was expected from on Oort cloud object, and has an inclination in line with the planets and Kuiper Belt, they described it as being an “inner Oort Cloud object”. Brown and his colleagues have proposed that Sedna’s orbit is best explained by the Sun having formed in an open cluster of several stars that gradually disassociated over time. In this scenario, Sedna was lifted into its current orbit by a star that was part of this cluster rather than it having been formed in its current location. This hypothesis has also been confirmed by computer simulations that suggest that multiple close passes by young stars in such a cluster would pull many objects into Sedna-like orbits. On the other hand, if Sedna formed in its current location, then it would mean that the Sun’s original protoplanetary disc would have extended farther than previously expected – approximately 75 AUs into space. Also, Sedna’s initial orbit would have been approximately circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible. Therefore, it must have been tugged into its current eccentric orbit by a gravitational interaction with another body – which could have been another planet in the Kuiper Belt, a passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed. Another possibility is the Sedna’s orbit is the result of influence by a large binary companion thousands of AU distant from our Sun. One such hypothetical companion is Nemesis, a dim companion to the Sun. However, to date no direct evidence of Nemesis has been found, and many lines of evidence have thrown its existence into doubt. More recently, it has also been suggested that Sedna did not originate in the Solar System, but was captured by the Sun from a passing extrasolar planetary system. Astronomers believe that they will find more objects in the Oort Cloud in years to come, especially as ground-based and space telescopes become more advanced and sensitive. Most likely, we will also see Sedna officially christened a “dwarf planet” by the IAU. As with other astronomical bodies that have been designated as such, we can expect some controversy to follow! © Matt Williams, 2013 © Astronomy Trek.com
Discovery and Naming: Much like Eris, Haumea and Makemake, Sedna was co-discovered by Mike Brown of Caltech, with assistance from Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University on November 14th, 2003. Initially designated as 2003 VB12, the discovery was part of a survey that commenced in 2001 using the Samuel Oschin Telescope at the Palomar Observatory near San Diego, California. Observations at the time indicated the presence of an object at a distance of approximately 100 AU from the Sun. Follow-up observations made in November and December of 2003 by the Cerro Tololo Inter-American Observatory in Chile and the W. M. Keck Observatory in Hawaii revealed that the object was moving along a distant highly eccentric orbit. It was later learned that the object had been previously observed by the Samuel Oschin telescope as well as the Jet Propulsion Laboratory’s Near Earth Asteroid Tracking (NEAT) consortium. Comparisons with these previous observations have since allowed for a more precise calculation of Sedna’s orbit and orbital arc. According to Mike Brown’s website, the planet was named Sedna after the Inuit Goddess of the sea. According to legend, Sedna was once mortal but became immortal after drowning in the Arctic Ocean, where she now resides and protects all the creatures of the sea. This name seemed appropriate to Brown and his team because Sedna is currently the farthest (and hence coldest) object from the Sun. The team made the name public before the object had been officially numbered; and while this represented a breach in IAU protocol, no objections were raised. In 2004, the IAU’s Committee on Small Body Nomenclature formally accepted the name.
Classification: Astronomers remain somewhat divided when it comes to Sedna’s proper classification. On the one hand, its discovery resurrected the question of which astronomical objects should be considered planets and which ones could not. Under the IAU’s definition of a planet, which was adopted on August 24th, 2006 (in response to the discovery of Eris), a planet needs to have cleared its orbit. Hence, Sedna does not qualify. However, to be a dwarf planet, a celestial body must be in hydrostatic equilibrium – meaning that it is symmetrically rounded into a spheroid or ellipsoid shape. With a surface albedo of 0.32 ± 0.06 – and an estimated diameter of between 915 and 1800 km (compared to Pluto’s 1186 km) – Sedna is bright enough, and also large enough, to be spheroid in shape. Therefore, Sedna is believed by many astronomers to be a dwarf planet, and is often referred to confidently as such. One reason why astronomers are reluctant to definitively place it in that category is because it is so far away that it is difficult to observe.
Size, Mass and Orbit: In 2004, Mike Brown and his team placed an upper limit of 1,800 km on its diameter, but by 2007 this was revised downward to less than 1,600 km after observations were made by the Spitzer Space Telescope. In 2012, measurements from the Herschel Space Observatory suggested that Sedna’s diameter was between 915 and 1075 km, which would make it smaller than Pluto’s moon Charon. Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe. Nevertheless, many astronomers think that Sedna is the fifth largest trans-Neptunian object (TNO) and dwarf planet – after Eris, Pluto, Makemake, and Haumea, respectively. Sedna has a highly elliptical orbit around the Sun, which means it ranges in distance from 76 astronomical units (AU) at perihelion (114 billion km/71 billion mi) to 936 AU (140 billion km/87 billion mi) at aphelion. Estimations on how long it takes Sedna to orbit the Sun vary, although it is known to be more than 10,000 years. Some astronomers calculate the orbital period could be as long as 12,000 years. Although astronomers believed at first that Sedna had a satellite, they have not been able to prove it.
Composition: At the time of its discovery, Sedna was the intrinsically brightest object found in the Solar System since Pluto in 1930. In terms of color, Sedna appears to be almost as red as Mars, which some astronomers believe is caused by hydrocarbons or tholins. Its surface is also rather homogeneous in terms of color and spectrum, which may the result of Sedna’s distance from the Sun. Unlike planets in the Inner Solar System, Sedna experiences very few surface impacts from meteors or stray objects. As a result, it does not have as many exposed bright patches of fresh icy material. Sedna, and the entire Oort Cloud, is freezing at temperatures below 33 Kelvin (-240.2°C). Models have been constructed of Sedna that place an upper limit of 60% for methane ice and 70% for water ice. This is consistent with the existence of tholins on its surface, since they are produced by the irradiation of methane. Meanwhile, M. Antonietta Barucci and colleagues compared Sedna’s spectrum to that of Triton and came up with a model that included 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen, 26% methanol and 33% methane. The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may have a tenuous atmosphere. During a 200-year period near perihelion, the maximum temperature on Sedna would likely exceed 35.6 K (-237.6 °C), which would be just warm enough for some of the nitrogen ice to sublimate. Models of internal heating via radioactive decay suggest that, like many bodies in the Outer Solar System, Sedna might be capable of supporting a subsurface ocean of liquid water.
Origin: When he and his colleagues first observed Sedna, they claimed that it was part of the Oort Cloud – the hypothetical cloud of comets believed to exist a light-year’s distance from the Sun. This was based on the fact that Sedna’s perihelion (76 AUs) made it too distant to be scattered by the gravitational influence of Neptune. Because it was also closer to the Sun than was expected from on Oort cloud object, and has an inclination in line with the planets and Kuiper Belt, they described it as being an “inner Oort Cloud object”. Brown and his colleagues have proposed that Sedna’s orbit is best explained by the Sun having formed in an open cluster of several stars that gradually disassociated over time. In this scenario, Sedna was lifted into its current orbit by a star that was part of this cluster rather than it having been formed in its current location. This hypothesis has also been confirmed by computer simulations that suggest that multiple close passes by young stars in such a cluster would pull many objects into Sedna-like orbits. On the other hand, if Sedna formed in its current location, then it would mean that the Sun’s original protoplanetary disc would have extended farther than previously expected – approximately 75 AUs into space. Also, Sedna’s initial orbit would have been approximately circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible. Therefore, it must have been tugged into its current eccentric orbit by a gravitational interaction with another body – which could have been another planet in the Kuiper Belt, a passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed. Another possibility is the Sedna’s orbit is the result of influence by a large binary companion thousands of AU distant from our Sun. One such hypothetical companion is Nemesis, a dim companion to the Sun. However, to date no direct evidence of Nemesis has been found, and many lines of evidence have thrown its existence into doubt. More recently, it has also been suggested that Sedna did not originate in the Solar System, but was captured by the Sun from a passing extrasolar planetary system. Astronomers believe that they will find more objects in the Oort Cloud in years to come, especially as ground-based and space telescopes become more advanced and sensitive. Most likely, we will also see Sedna officially christened a “dwarf planet” by the IAU. As with other astronomical bodies that have been designated as such, we can expect some controversy to follow! © Matt Williams, 2013 © Astronomy Trek.com