20000 Varuna
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20000 Varuna (provisional designation ) is a large trans-Neptunian object in the
Kuiper belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
. It was discovered in November 2000 by American astronomer Robert McMillan during a
Spacewatch The Spacewatch Project is an astronomical survey that specializes in the study of minor planets, including various types of asteroids and comets at University of Arizona telescopes on Kitt Peak near Tucson, Arizona. The Spacewatch Project has be ...
survey at the
Kitt Peak National Observatory The Kitt Peak National Observatory (KPNO) is a United States astronomy, astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, west-southwest of Tucson, Arizona. With ...
. It is named after the Hindu deity
Varuna Varuna (; , ) is a Hindu god. He is one of the earliest deities in pantheon, whose role underwent a significant transformation from the Vedic to the Puranic periods. In the early Vedic era, Varuna is seen as the god-sovereign, ruling the sky ...
, one of the oldest deities mentioned in the
Vedic texts upright=1.2, The Vedas are ancient Sanskrit texts of Hinduism. Above: A page from the '' Atharvaveda''. The Vedas ( or ; ), sometimes collectively called the Veda, are a large body of religious texts originating in ancient India. Composed i ...
. Varuna's
light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...
is compatible with the body being a Jacobi ellipsoid, suggesting that it has an elongated shape due to its rapid
rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...
. Varuna's surface is moderately red in color due to the presence of complex
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s on its surface. Water
ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...
is also present on its surface, and is thought to have been exposed by past
collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great for ...
s which may have also caused Varuna's rapid rotation. Although no
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
s have been found or directly imaged around Varuna, analysis of variations in its light curve in 2019 suggests the presence of a possible satellite orbiting closely around Varuna. Assumptions that the body is in
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...
(and thus a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
) result in a calculated density too low for it to be a dwarf planet.


History


Discovery

Varuna was discovered by American astronomer Robert McMillan using the
Spacewatch The Spacewatch Project is an astronomical survey that specializes in the study of minor planets, including various types of asteroids and comets at University of Arizona telescopes on Kitt Peak near Tucson, Arizona. The Spacewatch Project has be ...
0.9-meter telescope during a routine survey on 28 November 2000. The Spacewatch survey was conducted by McMillan at the
Kitt Peak National Observatory The Kitt Peak National Observatory (KPNO) is a United States astronomy, astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, west-southwest of Tucson, Arizona. With ...
near Tucson, Arizona. At the time of discovery, Varuna was located at a moderately dense star field close to the northern galactic equator. Although Varuna was not detected by McMillan's real-time computer software, he was able to identify Varuna moving slowly among the background stars by manually comparing multiple scans of the same region using the blinking technique. After McMillan's observing shift, follow-up observations of Varuna were conducted by astronomer Jeffrey Larsen in order to confirm the object. By the end of Larsen's observing shift, both McMillan and Larsen had made a total of 12 observations that spanned three nights. The discovery of Varuna was formally announced in a Minor Planet Electronic Circular on 1 December 2000. It was given the provisional designation , indicating that it was discovered during the second half of November 2000. Varuna was the 2667th object observed in the latter half of November, as indicated by the last letter and numbers in its provisional designation. At the time, Varuna was thought to be one of the largest and brightest
minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s in the Solar System due to its relatively high
apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...
of 20 for a distant object, which implied that it might be around one-fourth the size of
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
and comparable in size to the dwarf planet . Subsequently, after the announcement of Varuna's discovery, precovery images of Varuna were found by German astronomers Andre Knofel and Reiner Stoss at the
Palomar Observatory The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...
. One particular precovery image, which was taken with the Palomar Observatory's Big Schmidt telescope in 1955, showed that Varuna was located three  degrees away from its extrapolated location based on the approximate circular orbit determined in December 2000. The oldest known precovery image of Varuna was taken on 24 November 1954. These precovery images along with additional observations from Japan, Hawaii, and Arizona helped astronomers refine its orbit and determine Varuna's proper classification. In January 2001, Varuna was assigned the minor planet number 20000 by the Minor Planet Center as its orbit was well determined from precovery images and subsequent observations. The minor planet number 20000 was particularly chosen to commemorate Varuna's large size, being the largest classical Kuiper belt object known at that time and was believed to be as large as Ceres. The number 20000 was also chosen to commemorate the coincidental 200th anniversary of the discovery of Ceres, which occurred in the same month as the numbering of Varuna.


Name

Varuna is named after the eponymous Hindu deity
Varuna Varuna (; , ) is a Hindu god. He is one of the earliest deities in pantheon, whose role underwent a significant transformation from the Vedic to the Puranic periods. In the early Vedic era, Varuna is seen as the god-sovereign, ruling the sky ...
, following the
International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
naming convention for non-resonant Kuiper belt objects after creator deities. The name was proposed by Indian choreographer Mrinalini Sarabhai, and was approved by the IAU in March 2001. Varuna is one of the oldest
Vedic upright=1.2, The Vedas are ancient Sanskrit texts of Hinduism. Above: A page from the '' Atharvaveda''. The Vedas ( or ; ), sometimes collectively called the Veda, are a large body of religious texts originating in ancient India. Composed ...
deities of Hindu literature, being mentioned in the earliest
hymn A hymn is a type of song, and partially synonymous with devotional song, specifically written for the purpose of adoration or prayer, and typically addressed to a deity or deities, or to a prominent figure or personification. The word ''hymn'' d ...
s of the ''
Rigveda The ''Rigveda'' or ''Rig Veda'' (, , from wikt:ऋच्, ऋच्, "praise" and wikt:वेद, वेद, "knowledge") is an ancient Indian Miscellany, collection of Vedic Sanskrit hymns (''sūktas''). It is one of the four sacred canoni ...
''. In Hindu literature, Varuna created and presided over the waters of the heaven and of the ocean. Varuna is the king of gods and men and the universe, and has unlimited knowledge.
Planetary symbol Planetary symbols are used in astrological symbol, astrology and traditionally in astronomical symbol, astronomy to represent a classical planet (which includes the Sun and the Moon) or one of the modern planets. The classical symbols were also use ...
s are no longer much used in astronomy, so Varuna never received a symbol in the astronomical literature. There is no standard symbol for Varuna used by astrologers either. Denis Moskowitz, a software engineer in Massachusetts who designed the symbols for most of the dwarf planets, proposed a symbol for Varuna (): it derives from the
Devanagari Devanagari ( ; in script: , , ) is an Indic script used in the Indian subcontinent. It is a left-to-right abugida (a type of segmental Writing systems#Segmental systems: alphabets, writing system), based on the ancient ''Brāhmī script, Brā ...
letter ''va'' व and Varuna's snake-lasso. This symbol is occasionally mentioned on astrology websites, but is not broadly used. Another sometimes seen is a variant of Neptune ( 16px with a globe and outward-facing tines), as Varuna is the Hindu equivalent of Neptune.


Orbit and classification

Varuna orbits the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
at an average distance of , taking 279 years to complete a full orbit. Its orbit is nearly circular, with a low
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...
of 0.056. Due to its low orbital eccentricity, its distance from the Sun varies slightly over the course of its orbit. Varuna's minimum distance possible ( MOID) from Neptune is 12.04 AU. Over the course of its orbit, Varuna's distance from the Sun ranges from 40.3 AU at
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
(closest distance) to 45.1 AU at
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
(farthest distance). Varuna's orbit is inclined to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
by 17 degrees, similar to Pluto's orbital inclination. Varuna had passed its perihelion in 1928 and is currently moving away from the Sun, approaching aphelion by 2071. With a nearly circular orbit at around 40 to 50 AU, Varuna is classified as a classical Kuiper belt object (KBO). Varuna's semi-major axis of 42.8 AU is similar to that of other large classical KBOs such as Quaoar ( a=43.7 AU) and
Makemake Makemake ( minor-planet designation: 136472 Makemake) is a dwarf planet and the largest of what is known as the classical population of Kuiper belt objects, with a diameter approximately that of Saturn's moon Iapetus, or 60% that of Pluto. It ...
(a=45.6 AU), although other orbital characteristics such as inclination widely differ. Varuna is a member of the "dynamically hot" class of classical KBOs, meaning that it has an orbital inclination greater than 4 degrees, the imposed maximum inclination for dynamically cold members of its population. As a classical KBO, Varuna is not in
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
with Neptune and is also free from any significant perturbation by Neptune.


Rotation

Varuna has a rapid
rotation period In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the '' sidereal rotation period'' (or ''sidereal day''), i.e., the time that the objec ...
of approximately 6.34 hours, derived from a double-peaked solution for Varuna's rotational
light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...
. Varuna's rotation was first measured January 2001 by astronomer Tony Farnham using the McDonald Observatory's 2.1-meter telescope, as part of a study on the rotation and colors of distant objects. CCD
photometry Photometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electr ...
of Varuna's light curve in 2001 revealed that it displays large brightness variations with an
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of about 0.5 magnitudes. The measured rotational light curve of Varuna provided two ambiguous rotation periods of 3.17 and 6.34 hours, for a single-peaked and a double-peaked solution, respectively. Additional possible rotation periods of 2.79 and 3.66 hours were also obtained by Farnham, although these values could not be ruled out at the time. A single-peaked interpretation of Varuna's rotational light curve (3.17 h) would assume a spherical shape for Varuna, with albedo features on its surface that would account for its brightness variations. However, in order for this interpretation to be valid, Varuna must have a density much greater than (roughly the density of water), otherwise it would deform and break apart as the given rotation period exceeds the critical rotation rate of ~3.3 hours for a body with a density of . A double-peaked interpretation of Varuna's rotational light curve (6.34 h) would assume that Varuna's shape is an elongated
ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a Surface (mathemat ...
, with an estimated ''a''/''b''
aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
of 1.5–1.6. The rotational light curve of Varuna was later investigated by astronomers David Jewitt and Scott Sheppard during February and April 2001, and concluded that the double-peaked interpretation for Varuna's light curve is the most plausible solution due to the absence of rotational variation in Varuna's color in the
visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...
. Examination of past photometric observations of Varuna's light curve has shown that its light curve amplitude had increased by roughly 0.13 magnitudes from 2001 to 2019. This increase in amplitude is due to the combined effects of Varuna's ellipsoidal shape, rotation, and varying phase angle. Geometric models for Varuna's changing amplitude have provided several possible solutions for the orientation of Varuna's rotational poles in ecliptic coordinates, with the best-fit solution adopting a spin axis
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...
and
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
of 54° and −65°, respectively. The best-fit pole orientation of Varuna implies that it is being viewed at a near-edge on configuration, in which Varuna's equator nearly faces directly toward Earth. Varuna's rapid rotation is believed to have resulted from disruptive
collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great for ...
s that have sped up its rotation during the formation of the Solar System. The present collision rate in the trans-Neptunian region is minimal, though collisions were more frequent during the formation of the Solar System. However, Jewitt and Sheppard calculated that the rate of disruptive collisions among large trans-Neptunian objects (TNOs) during the Solar System's formation is extremely uncommon, contradictory to the current abundance of binary and rapidly rotating TNOs that are believed to have originated from such collisions. To explain the abundance of binary and rapidly rotating TNOs, the rate of collisions among TNOs had likely increased as a result of Neptune's outward
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
perturbing the orbits of TNOs, thus increasing the frequency of collisions which may have led to Varuna's rapid rotation.


Physical characteristics


Size and shape

As a result of its rapid rotation, the shape of Varuna is deformed into a triaxial ellipsoid. Given the rapid rotation, rare for objects so large, Varuna's shape is described as a Jacobi ellipsoid, with an ''a''/''b'' aspect ratio of around 1.5–1.6 (in which Varuna's longest semi-axis ''a'' is 1.5–1.6 times longer than its ''b'' semi-axis). Examination of Varuna's light curve has found that the best-fit model for Varuna's shape is a triaxial ellipsoid with the semi-axes ''a'', ''b'', and ''c'' in ratios in the range of ''b''/''a'' = 0.63–0.80, and ''c''/''a'' = 0.45–0.52. Due to Varuna's ellipsoidal shape, multiple observations have provided different estimates for its diameter, ranging from . Most diameter estimates for Varuna were determined by measuring its thermal emission, although size estimates have been constrained to smaller values as a result of higher albedos determined by space-based thermal measurements. Observations of stellar
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...
s by Varuna have also provided varying size estimates. An occultation by Varuna in February 2010 yielded a chord length of , inferred to be across its longest axis. Later occultations in 2013 and 2014 yielded mean diameters of and respectively. Since the discovery of Varuna, , another larger rapidly rotating (3.9 h) object over twice the size of Varuna, has been discovered and is also thought to have an elongated shape, albeit slightly less pronounced (estimated ratios of ''b''/''a'' = 0.76~0.88, and ''c''/''a'' = 0.50~0.55, possibly due to a higher estimated density approximately ).


Unlikely to be a dwarf planet

Astronomer Gonzalo Tancredi considered Varuna likely to be a dwarf planet and a Jacobi ellipsoid in shape. Based on a best-fit Jacobi ellipsoid model for Varuna, Lacerda and Jewitt estimate that Varuna has a quite low density, of , just under Tancredi's minimum density criterion. Despite this, they assumed Varuna was in hydrostatic equilibrium for their calculations. Astronomer William Grundy and colleagues propose that dark, low-density TNOs around the size range of approximately are likely to be uncompressed, partially porous bodies. While the larger objects in this range, such as Varuna, may have fully collapsed into solid material in their interiors, their surfaces likely remain uncompressed. That is, they would not be in hydrostatic equilibrium and not dwarf planets.


Thermal measurements

Ground observations of Varuna's thermal emission from 2000 to 2005 yielded large diameter estimates ranging from to , making it comparable to the size of Ceres. Contrary to the ground-based estimates, space-based thermal observations from the Spitzer Space Telescope provided a smaller diameter range of . The discrepancy between ground-based and space-based size estimates are due to the limited observable wavelengths for ground-based observations, as a result of absorption of
Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...
. Distant trans-Neptunian objects such as Varuna intrinsically emit thermal radiation at longer wavelengths due to their low temperatures. However, at long wavelengths, thermal radiation cannot pass through Earth's atmosphere and ground-based observations could only measure weak thermal emissions from Varuna at
near-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
and
submillimeter Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers plac ...
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s, hindering the accuracy of ground-based thermal measurements. Space-based observations provided more accurate thermal measurements as they are able to measure thermal emissions at a broad range of wavelengths that are normally interfered by Earth's atmosphere. Preliminary thermal measurements with Spitzer in 2005 provided a higher albedo constraint of 0.12 to 0.3, corresponding to a smaller diameter constraint of . Further Spitzer thermal measurements at multiple wavelength ranges (bands) in 2007 yielded mean diameter estimates around and for a single-band and two-band solution for the data, respectively. From these results, the adopted mean diameter was . Follow-up multi-band thermal observations from the Herschel Space Observatory in 2013 yielded a mean diameter of , consistent with previous constraints on Varuna's diameter.


Occultations

Previous attempted observations of stellar occultations by Varuna in 2005 and 2008 were unsuccessful due to uncertainties in Varuna's
proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
along with undesirable conditions for observing. In 2010, an occultation by Varuna was successfully observed by a team of astronomers led by Bruno Sicardy on the night of 19 February. The occultation was observed from various regions in southern Africa and north-eastern Brazil. Although observations of the occultation from
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
and
Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...
had negative results, observations from Brazil, particularly at São Luís in
Maranhão Maranhão () is a States of Brazil, state in Brazil. Located in the country's Northeast Region, Brazil, Northeast Region, it has a population of about 7 million and an area of and it is divided into 217 municipalities. Clockwise from north, it ...
, successfully detected a occultation by Varuna of an 11.1 magnitude star. The occultation yielded a chord length of , quite large compared to mean diameter estimates from thermal measurements. Because the occultation occurred near Varuna's maximum brightness, the occultation was observing the maximum apparent surface area for an ellipsoidal shape; the longest axis of Varuna's shape was observed during the occultation. São Luís was also located very close to the predicted centerline of Varuna's shadow path, meaning the chord length was close to the longest measurable during the event, closely constraining the possible maximum equatorial diameter. Results from the same event from Camalaú,
Paraíba Paraíba ( , ; ) is a states of Brazil, state of Brazil. It is located in the Brazilian Northeast, and it is bordered by Rio Grande do Norte to the north, Ceará to the west, Pernambuco to the south and the Atlantic Ocean to the east. Paraíba i ...
, approximately south (and on what was predicted to be the very southern extent of the shadow path), showed a 28-second occultation, corresponding to an approximately chord, much longer than might otherwise have been expected. However, Quixadá, south of São Luís–between it and Camalaú–paradoxically had a negative result. To account for the negative Quixadá results, the apparent oblateness (flattening) of Varuna was imposed at a minimum value of approximately 0.56 (aspect ratio ''c''/''a'' ≤ 0.44), corresponding to a minimum polar dimension of approximately , based on the given chord length of . The resulting lower bound on Varuna's polar dimension is approximately equal to Lacerda and Jewitt's lower bound ''c''/''a'' aspect ratio of 0.45, which they previously calculated in 2007. A preliminary conference presentation, given before the Camalaú results were fully analyzed, concluded that the São Luís and Quixadá results together suggested a significantly elongated shape is required for Varuna. Later occultations in 2013 and 2014 yielded mean diameters of and , respectively. The mean diameter of , calculated from both chords from the occultations, appears seemingly consistent with the Spitzer and Herschel thermal measurement of . While the apparent oblateness of Varuna could not be determined from the single chord obtained from the 2014 occultation, the 2013 occultation yielded two chords, corresponding to an apparent oblateness of approximately 0.29. The imposed oblateness for the 2013 chord length of as Varuna's diameter corresponds to a polar dimension of approximately , somewhat consistent with the calculated 2010 minimum polar dimension of .


Spectra and surface

Varuna's
spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
was first analyzed in early 2001 with the Near Infrared Camera Spectrometer (NICS) at the Galileo National Telescope in
Spain Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...
. Spectral observations of Varuna at near-infrared wavelengths revealed that the surface of Varuna is moderately red and displays a red spectral slope between the wavelength range of 0.9 and 1.8  μm. Varuna's spectrum also exhibits strong absorption bands at wavelengths of 1.5 and 2 μm, indicating the presence of water ice on its surface. The red color of Varuna's surface results from the
photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...
of
organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
being irradiated by sunlight and
cosmic rays Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...
. The irradiation of organic compounds such as
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
on Varuna's surface produces tholins, which are known to reduce its surface reflectivity (
albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
) and are expected to cause its spectrum to appear featureless. Compared to , which was observed along with Varuna in 2001, it appears less red and displays more apparent water ice absorption bands, suggesting that Varuna's surface is relatively fresh and had maintained some of its original material in its surface. The fresh appearance of Varuna's surface may have resulted from collisions that have exposed fresh water ice beneath Varuna's layer of tholins above its surface. Another study of Varuna's spectra at near-infrared wavelengths in 2008 yielded a featureless spectrum with a blue spectral slope, contrary to earlier results in 2001. The spectra obtained in 2008 showed no clear indication of water ice, contradictory to the 2001 results. The discrepancy between the two results was interpreted as an indication of surface variation on Varuna, though this possibility was later ruled out by a 2014 study of Varuna's spectra. The 2014 results closely matched the previous spectra obtained in 2001, implying that the featureless spectra obtained in 2008 is likely erroneous. Models for Varuna's spectrum suggest that its surface is most likely formed of a mixture of amorphous
silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
s (25%), complex organic compounds (35%),
amorphous carbon Amorphous carbon is free, reactive carbon that has no crystalline structure. Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. As with other amorphous solids, some short-range order can be observed. Amo ...
(15%) and water ice (25%), with a possibility of up to 10% methane ice. For an object with a size similar to Varuna, the presence of volatile methane could not be primordial as Varuna is not massive enough to retain volatiles on its surface. An event that had occurred subsequently after Varuna's formation–such as an energetic impact–would likely account for the presence of methane on Varuna's surface. Additional near-infrared observations of Varuna's spectra were conducted at the
NASA Infrared Telescope Facility The NASA Infrared Telescope Facility (NASA IRTF) is a telescope optimized for use in infrared astronomy and located at the Mauna Kea Observatory in Hawaii. It was first built to support the Voyager program, Voyager missions and is now the US n ...
in 2017 and have identified absorption features between 2.2 and 2.5 μm that might be associated with
ethane Ethane ( , ) is a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...
and
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...
, based on preliminary analysis. For mid-sized bodies such as Varuna, volatiles such as ethane and ethylene are more likely to be retained than lighter volatiles such as methane according to volatile retention theories formulated by astronomers Schaller and Brown in 2007.


Brightness

Varuna's
apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...
, its brightness as seen from Earth, varies from 19.5 to 20 magnitudes. At opposition, its apparent magnitude can reach up 20.3 magnitudes. Combined thermal measurements from the Spitzer Space Telescope and the Herschel Space Observatory in 2013 obtained a visual absolute magnitude (''HV'') of 3.76, comparable to that of the similarly-sized Kuiper belt object ('' HV''=3.83). Varuna is among the twenty brightest trans-Neptunian objects known, despite the Minor Planet Center assuming an absolute magnitude of 3.6. The surface of Varuna is dark, with a measured
geometric albedo In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle (astronomy), phase angle) to that of an ''idealized'' flat, fully reflecting, diffuse reflection, d ...
of 0.127 based on thermal observations in 2013. Varuna's geometric albedo is similar to that of the possible dwarf planet , which has a geometric albedo of 0.109. Varuna was initially thought to have a much lower geometric albedo, as early ground observations of Varuna's thermal emissions from 2000 to 2005 estimated albedo values ranging from 0.04 to 0.07, around eight times darker than
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
's albedo. Later thermal measurements of Varuna with space-based telescopes refuted these previous albedo measurements: Spitzer measured a higher geometric albedo of 0.116 while further thermal measurements from Spitzer and Herschel in 2013 estimated a geometric albedo of 0.127. Photometric observations of Varuna in 2004 and 2005 were carried out to observe changes in Varuna's light curve caused by opposition surges when the phase angle of Varuna approaches zero degrees at opposition. The photometry results showed that Varuna's light curve amplitude had decreased to 0.2 magnitudes at opposition, less than its overall amplitude of 0.42 magnitudes. The photometry results also showed an increase in asymmetry of Varuna's light curve near opposition, indicating variations of
scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...
properties over its surface. The opposition surge of Varuna differs from those of dark
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s, which gradually becomes more pronounced near opposition in contrast to Varuna's narrow opposition surge, in which its light curve amplitude sharply changes within a phase angle of 0.5 degrees. The opposition surges of other Solar System bodies with moderate albedos behave similarly to Varuna, indirectly suggesting that Varuna might have a higher albedo in contrast to ground-based albedo estimates. This implication of a higher albedo for Varuna was confirmed in subsequent thermal measurements from Spitzer and Herschel.


Internal structure

Varuna is estimated to have a
bulk density In materials science, bulk density, also called apparent density, is a material property defined as the mass of the many particles of the material divided by the bulk volume. Bulk volume is defined as the total volume the particles occupy, includ ...
of , marginally less than that of water (). Varuna's low bulk density is likely due to a porous internal structure composed of a nearly proportional ratio of water ice and rock. To explain its porous internal structure and composition, Lacerda and Jewitt suggested that Varuna may have a
granular Granularity (also called graininess) is the degree to which a material or system is composed of distinction (philosophy), distinguishable pieces, granular material, "granules" or grain, "grains" (metaphorically). It can either refer to the exten ...
internal structure. Varuna's granular internal structure is thought to have resulted from fractures caused by past collisions likely responsible for its rapid rotation. Other objects including
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
's moons Tethys and
Iapetus In Greek mythology, Iapetus (; ; ), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other sources. Iapetus was linked ...
are also known to have a similarly low density, with a porous internal structure and a composition that is predominantly water ice and rock. William Grundy and colleagues proposed that dark, low-density TNOs around the size range of approximately are transitional between smaller, porous (and thus low-density) bodies and larger, denser, brighter and geologically differentiated planetary bodies (such as dwarf planets). The internal structures of low-density TNOs, such as Varuna, had only partially differentiated, as their likely rocky interiors had not reached sufficient temperatures to melt and collapse into pore spaces since formation. As a result, most mid-sized TNOs had remained internally porous, thus resulting in low densities. In this case, Varuna may not be in hydrostatic equilibrium.


Possible satellite

Photometric observations of Varuna's light curve, led by Valenzuela and colleagues in 2019, indicate that a possible satellite might be orbiting Varuna at a close distance. By using the
Fourier analysis In mathematics, Fourier analysis () is the study of the way general functions may be represented or approximated by sums of simpler trigonometric functions. Fourier analysis grew from the study of Fourier series, and is named after Joseph Fo ...
method of combining four separate light curves obtained in 2019, they derived a lower quality light curve amplitude with a greater amount of residuals. Their result indicated that Varuna's light curve experiences subtle changes over time. They plotted the residuals of the combined light curve in a Lomb periodogram and derived an orbital period of 11.9819 hours for the possible satellite. The satellite varies in brightness by 0.04 magnitudes as it orbits Varuna. Under the assumption that Varuna's density is and the satellite is tidally locked, the team estimates that it orbits Varuna at a distance of , just beyond the estimated
Roche limit In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal force ...
of Varuna (~). Due to the satellite's close proximity to Varuna, it is not yet possible to resolve it with space-based telescopes such as the
Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
as the
angular distance Angular distance or angular separation is the measure of the angle between the orientation (geometry), orientation of two straight lines, ray (geometry), rays, or vector (geometry), vectors in three-dimensional space, or the central angle subtende ...
between Varuna and the satellite is smaller than the resolution of current space-based telescopes. Although direct observations of Varuna's satellite are unfeasible with current telescopes, Varuna's equator is being directly viewed at an edge-on configuration, implying that mutual events between the satellite and Varuna could possibly occur in the future.


Exploration

Planetary scientist Amanda Zangari calculated that a flyby mission to Varuna could take just over 12 years using a Jupiter
gravity assist A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby (spaceflight), flyby which makes use of the relative movement (e.g. orbit around the Sun) and gra ...
, based on a launch date of 2035 or 2038. Alternative trajectories using gravity assists from Jupiter, Saturn, or Uranus have been also considered. A trajectory using gravity assists from Jupiter and Uranus could take just over 13 years, based on a launch date of 2034 or 2037, whereas a trajectory using gravity assists from Saturn and Uranus could take under 18 years, based on an earlier launch date of 2025 or 2029. Varuna would be approximately 45 AU from the Sun when the spacecraft arrives before 2050, regardless of the trajectories used.


Notes


References


External links

*
(20000) Varuna – Large TNO precovered on several old plates
Precovery images of Varuna. Retrieved 22 September 2019.
Beyond Jupiter: The World of Distant Minor Planets – (20000) Varuna
* {{DEFAULTSORT:020000 Classical Kuiper belt objects Discoveries by the Spacewatch project
Varuna Varuna (; , ) is a Hindu god. He is one of the earliest deities in pantheon, whose role underwent a significant transformation from the Vedic to the Puranic periods. In the early Vedic era, Varuna is seen as the god-sovereign, ruling the sky ...
Objects observed by stellar occultation 20001128