Orcus: The AntiPluto Dwarf Planet
In this video we will deal with Orcus, a transneptunian celestial body.
As the adjective "transneptunian" says, Orcus is a celestial body that is located beyond the
orbit of Neptune in a portion of space known as the Kuiper belt or Edgeworth - Kuiper.
The Kuiper belt or Edgeworth - Kuiper is an outer region of the Solar System that extends from the
orbit of planet Neptune, about 30 astronomical units from the Sun, up to 50 astronomical units
from the Sun; its overall shape is somewhat reminiscent of a donut. It takes its name
from the astronomers Gerard Kuiper and Kenneth Edgeworth, who in a 1951 article hypothesized
the presence of objects beyond Neptune's orbit, later known as trans-Neptunian objects (TNO).
Unlike the main asteroid belt between the orbits of Mars and Jupiter,
essentially composed of rocky bodies, the Kuiper belt is composed of small icy bodies
(mainly water ice, ammonia and methane), whose orbits are arranged on the plane of the ecliptic,
which is the same plane on which the orbits of the planets also lie.
The presence of Neptune has a profound effect on the structure of the Kuiper belt due to orbital
resonances: an orbital resonance occurs when two orbiting bodies have periods of revolution
such that their relationship can be expressed in fractions of small whole numbers.
For example Pluto and Neptune are in 2:3 orbital resonance:
this means that every three revolutions of Neptune around the Sun, Pluto makes two.
Over a period of time comparable to the age of the Solar System,
it can happen that Neptune's gravity destabilizes the orbits of objects in orbital resonance,
sending them into the inner solar system, or pushing them outwards, towards interstellar space.
The gravitational influence of Neptune means that there are pronounced gaps in the current
structure of the belt, that is areas in which there are very few or no objects:
these are precisely zones of gravitational instability produced by orbital resonances.
Over 2,000 Kuiper belt objects (or KBOs) have been discovered in the belt and it is thought
that there may be over 100,000 with a diameter greater than 100 km, and that the total mass
of all objects in the Kuiper belt could be a tenth, or even one hundredth, of the land mass.
In total, there may be millions of celestial bodies in the Kuiper Belt.
Kuiper belt objects can be roughly divided into "classical" and "resonant".
In the first case (classical Kuiper belt) the objects can maintain their orbits unchanged
over time: here we find about two thirds of the KBOs observed to date. (15760) 1992 QB1,
the first modern KBO discovered, Pluto and Charon apart, is considered the prototype of this group
and from it derives the term Cubewani, a term that indicates the classical KBOs and which
is the phonetic translation of the English acronym " QB1-os "(cubewanos). According
to an IAU guideline, the name to be given to classical KBOs is that of
mythological beings associated with creation. The classical KBOs are mainly concentrated in two
areas with orbital resonance with Neptune 2:3 and 1:2, between 42 and
48 astronomical units from the Sun, where the gravitational influence of Neptune is negligible.
The classical Kuiper belt appears to be composed of two different populations. The first, known as
the "dynamically cold" population, is composed of objects with orbits very similar to those
of the planets, ie almost circular, with an orbital eccentricity less than 0.1,
and orbital inclinations not exceeding 10 degrees. The second, the "dynamically hot" population,
has much more inclined orbits than the ecliptic, up to 30 degrees. The two populations were named
in this way not because of a difference in temperature, but because of the analogy
with the particles of a gas, which increase their relative velocity with temperature.
The hot population is believed to have formed near Jupiter and been ejected by the movements
and gravitational interactions of the gas giants. Furthermore,
the cold population is believed to have formed more or less in its current position,
although it may have been flung outwards later, during the migration of Neptune, described by
the Nizza model (which owes its name to the Observatoire de la Côte d'Azur of Nice, where
it was formulated): the Nice model is a physical model that simulates the dynamic evolution of the
Solar System in the phases immediately following its formation, and according to it Neptune and
Uranus were originally much closer to the Sun than they are today. Would you have liked to
see this important moment in the evolution of the Solar System? Write it in the comments!
As for the resonant Kuiper Belt Objects, they are in orbital resonance with Neptune.
Objects in resonance 2:3 with Neptune have a semi-major axis of about 39.4 AU
and about 200 of this type are known, including Pluto and its moons,
prototype of the members of this class known as plutinos. Plutinos have high
orbital eccentricities, suggesting that they did not originate in their current positions,
but were hurled towards the outer solar system by the migration of Neptune. IAU guidelines dictate
that all Plutinos must, like Pluto, be named after deities associated with the underworld.
The 1: 2 orbital resonance zone, whose objects complete one orbit while Neptune makes two
and have half-axes greater than around 47.7 AU, is sparsely populated. Members of this
class are known as twotini. There are other resonances, with ratios 3:4, 3:5, 4:7 and 2:5.
Neptune also has a number of Trojan asteroids occupying its Lagrangian points L4 and L5;
these are often in 1:1 resonance with Neptune and have generally stable orbits.
The largest body in the Kuiper belt is Pluto and the most massive is the dwarf planet Eris,
discovered in 2005, although some scientists consider Eris to be part of the diffuse disk
rather than the Kuiper belt. Since the year 2000 other objects of considerable size have been
found: 50,000 Quaoar, discovered in 2002, half the size of Pluto and larger than Ceres, the
largest of the traditional asteroids. The exact classification of these objects is unclear, as
they are likely very different from the innermost asteroids. Some satellites of the planets of the
Solar System appear to come from the Kuiper belt, such as Triton, the largest of Neptune's moons,
and the Saturnian moon Phoebe. It is believed that short-period comets, that is, those that
take less than 200 years to travel their orbit around the Sun, come from the Kuiper belt.
In addition to the classic and resonant Kuiper Belt objects, we should also mention
the diffuse disk, a sparsely populated region, the innermost part of which overlaps the Kuiper Belt,
and which extends up to 100 AU from the Sun and beyond. Unlike what happens to the objects of the
innermost Kuiper belt that we have spoken now, the objects of the diffuse disk travel in very
elliptical and often very inclined orbits with respect to the ecliptic. Most models of solar
system formation show that both KBOs and disk objects were part of a belt of primordial comets
and that subsequent gravitational interactions, particularly with Neptune, sent these objects
outward, some in stable orbits (KBOs) and others in unstable orbits, forming the diffuse disk.
Among the objects of the Kuiper belt, although present mainly in its outermost part,
there are also the "detached objects", whose orbit is almost entirely included
between 40 and 50 astronomical units from the Sun. Given their gigantic distance from Neptune,
it is unlikely that these objects will suffer the gravitational perturbations.
An example of a detached Kuiper belt object is Sedna, whose orbit has perihelion at 76
astronomical units from the Sun, and aphelion at about 1200 astronomical units from our star.
The Centauri asteroids deserve a separate mention: their orbits extend between Jupiter and Neptune,
and undergo their intense gravitational attraction. Many of them, due to these
gravitational perturbations, end up being ejected from the Solar System or pushed towards the
innermost areas of the Solar System where they become comets or crash into the Sun and planets.
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Among the many celestial bodies of the Kuiper belt we find Orcus, discovered on February 17th, 2004
by Michael E. Brown of the California Institute of Technology, Chad Trujillo of the Gemini
Telescopes and David Lincoln Rabinowitz of Yale University. But the celestial body was
later identified in some photographic images in the archive dating back to December 8th, 1951.
Possessing a size and an orbit similar to that of Pluto, Orcus has been given the name
of a deity of the underworld. In accordance with the astronomical naming conventions of
the International Astronomical Union, the discoverers suggested the name "Orcus",
which was approved and published on November 22nd, 2004. Orcus is both an
alternate name for the Greek deity Hades and an abstract god of the death in Roman mythology.
Orcus is a large plutino and takes 247 years to travel its orbit around the Sun:
although Orcus has an orbital period very similar to that of Pluto,
it has a different orientation. Thanks to 1951 images from the Siding Spring Observatory in
Australia and 1954 from Palomar Sky Survey, it was possible to determine that the distance of Orcus
from the Sun varies between 30.9 astronomical units and 48.1 astronomical units. Orcus's orbit
is inclined by 20.6 degrees on the ecliptic, more than that of Pluto, which is about 17 degrees.
The last passage in aphelion of Orcus took place in 1989, while the next passage in perihelion
will take place in 2113. Would you like to visit Orcus? Write it in the comments!
Due to its 2:3 resonance with Neptune, Orcus and Pluto are in opposite positions:
for this reason Orcus is also known as "anti-Pluto".
Every 14,000 years Orcus comes within 18 astronomical units of Neptune.
The apparent magnitude of the dwarf planet is equal to 19.1, which is roughly equivalent to
that of 50000 Quaoar: in any case it is a value well below the threshold
of visibility to the naked eye, which is around the sixth magnitude. Assuming
an albedo of 0.09, Trujillo estimates its diameter to be approximately 1600 km.
Recall that the albedo represents the fraction of incident solar radiation reflected from the
surface of a celestial body in all directions. Orcus would therefore be among the largest
objects in the Kuiper Belt so far, along with Eris, Pluto, Makemake, Haumea, Sedna and Gonggong.
According to observations in the infrared band made by the four Very Large Telescopes in Chile
of the European Southern Observatory with a diameter of 8 meters, on the surface of Orcus
there would be water ice and carbon residues. Observations made by the Hubble Space Telescope
from November 13th, 2005 onwards, made it possible to discover a satellite orbiting around Orcus.
The discovery was made by Mike Brown. The discovery of the Orcus satellite was
reported by the International Astronomical Union on 22nd February 2007. The satellite
thus discovered was initially indicated with the provisional designation S/2005 (90482) 1;
later it was given the official designation of Vanth as the Etruscan deity of the underworld.
Its orbit is nearly circular, with an orbital eccentricity of 0.0036,
an orbital period of 9.53 days, and a distance of just 8980 ± 20 km from Orcus. It also has a
spectrum very similar to Orcus, a characteristic that can determine its surface structure.
In 2005, the satellite was extimated to be separated of 0.25 arcseconds
from Orcus with a magnitude difference of 2.7 ± 1.
Assuming an albedo similar to that of the planet, the magnitude suggested a diameter
of about 250 km, equal to about one quarter / one third of the diameter of Orcus.
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