Planetoid

The terms minor planet and planetoid refer to a classification, currently out of use, that until 2006 included the bodies of the solar system that, not being satellites or comets, were be smaller than "traditional" planets but larger than meteoroids, commonly defined as having a maximum size of 10 meters.

Minor planets include asteroids (near-Earth objects, Mars-crossing asteroids, main belt asteroids, and Jupiter Trojans) as well as distant minor planets (centaurs and trans-Neptunian objects), most of which reside in the belt. Kuiper and the scattered disk. As of May 2022, there were 1,131,201 known objects, divided into 611,678 numbered (assured discoveries) and 519,523 unnumbered minor planets, with only five of them officially recognized as a dwarf planet.

The first minor planet to be discovered was Ceres in 1801. The term "minor planet" has been used since the 19th century to describe these objects. The term "planetoid" has also been used, especially for larger planetary objects, such as those that the IAU has called a dwarf planet since 2006. Historically, the terms "asteroid", "minor planet" and "planetoid" have been more or less synonymous. This terminology has been made more complicated by the discovery of numerous minor planets beyond the orbit of Jupiter, especially trans-Neptunian objects that are not generally considered asteroids. A minor planet seen releasing gas can be classified like a comet

Objects are called dwarf planets if their own gravity is sufficient to reach hydrostatic equilibrium and form an ellipsoid shape. All other minor planets and comets are called "minor bodies of the Solar System". The IAU stated that the term "minor planet" can still be used, but the term 'minor body of the Solar System' will be preferred. However, for the purposes of numbering and naming, the traditional distinction between minor planet and comet is still used.

Populations

Hundreds of thousands of minor planets have been discovered within the Solar System and thousands more are discovered every month. The Minor Planet Center has documented more than 213 million observations and 794,832 minor planets, of which 541,128 have orbits well enough known to be assigned permanent official numbers. Of these, 21,922 have official names. A As of November 8, 2021 2021, the unnamed minor planet with the lowest number is (4596) 1981 QB, and the named minor planet with the highest number is 594913 ꞌAylóꞌchaxnim.

There are several populations of minor planets:

• Asteroids; traditionally, most have been bodies of the inner Solar System.
  • Near-Earth asteroids, those whose orbits carry them within the orbit of Mars. Another subclassification of these, based on orbital distance, is used:
    • Asteroid Apoheles orbit within the perihelio distance of the Earth and therefore are fully contained within the orbit of the Earth.
    • Asteroid Atens, those who have lesser semieges than that of the Earth and shave (distance farther from the Sun) than 0,983 UA.
    • Apolo asteroids are those asteroids with a semieje greater than that of the Earth and have a perihelio distance equal to or less than 1.017 AU. Like Aten asteroids, Apollo asteroids are Asteroids that cross Earth's orbit.
    • Asteroid Love are those asteroids near Earth that approach Earth's orbit from beyond but do not cross it. The Love asteroids are subdivided in turn into four subgroups, depending on where your major semieje falls between the Earth orbit and the asteroid belt;
  • Earth Trojans, asteroids that share Earth's orbit and are gravitationally linked to it. In 2022 two terrestrial Trojans were known: 2010 TK7 and 2020 XL5.
  • Tuesday Trojans, asteroids that share the orbit of Mars and are gravitationally attached to it. In 2007, eight such asteroids were known.
  • Asteroid belt, whose members follow approximately circular orbits between Mars and Jupiter. They are the original and best-known group of asteroids.
  • Jupiter Trojans, asteroids that share Jupiter's orbit and are gravitationally linked to it. Numerically it is estimated to match the asteroids of the main belt.
• Minor distant bodies; generic term for minor planets of the outer solar system.
  • Hundreds, bodies of the outer solar system located between Jupiter and Neptune. They have unstable orbits due to the gravitational influence of giant planets, so they must proceed from another place, probably out of Neptune.
  • Neptune Trojans, bodies that share the orbit of Neptune and are gravitationally linked to it. Although only a few are known, there is evidence that Neptune's Trojans are more numerous than asteroids of the asteroid belt or that Jupiter's Trojans.
  • Transneptunian objects, bodies in or beyond the orbit of Neptune, the outermost planet.
    • The Kuiper Belt, objects within an apparent population drop of approximately 55 AU from the Sun.
      • Classic Kuipers belt object such as Makemake, also known as Cubewanos, are found in relatively circular primordial orbits that are not resonance with Neptune.
      • Kuiper belt resonant objects
        • Plutins, bodies like Template:Dp that are in resonance 2:3 with Neptune.
    • Disco scattered objects like Eris, with afelia outside the Kuiper belt. It is believed that they were scattered by Neptune.
      • Disband resonant objects.
    • Separate objects such as Sedna, with shaves and perihelios outside the Kuiper belt.
      • Sednoids, detached objects with perihelios higher than 75 UA (Sedna, 2012 VP113and Leleākūhonua.
    • The Oort cloud, a hypothetical population that is believed to be the source of long-term comets that can extend to 50,000 AU from the Sun.

Definition

This distinction was made based on visual appearance at the time of its discovery; comets had to show a comma, and were listed in their own catalogues. In contrast, minor planets appear as stars ("asteroid", from Greek αστεροειδές, asteroides = "star-like", "star-shaped", from ancient Greek Aστήρ, astēr i> = "star"), and receive an annual provisional name in the order of their discovery and a designation (consecutive number) and name if their existence is well established with a given orbit. After the meeting of the International Astronomical Union (IAU) in 2006, the categories of celestial objects were redefined into three types: planet, dwarf planet and minor body of the solar system, so the term "minor planet" lost its validity, and can now be used informally as the equivalent of a minor body in the solar system, that is, to generically designate all those bodies that, without being satellites, have not reached a sufficient size to adopt a shape essentially spherical, an identifying attribute of the planets, both the "traditional" or "major" ones and the "dwarf" ones.

Although the minor planet category is no longer official, the UAI catalog of minor planets has maintained that name. Since its own catalog has not been created for the recent category of dwarf planet, the IAU has continued to assign a number in the catalog of minor planets to this new category of celestial bodies, a reason that, together with tradition, causes some authors to continue using the term minor planet to refer to "dwarf planets", although they are not strictly such.

It is relatively common to use the names "asteroid", planetoid and minor planet interchangeably, although in scientific environments the term asteroid is usually reserved for objects located in the belt. of asteroids between Mars and Jupiter, while objects located in the Kuiper belt or, in general, beyond the orbit of Neptune are usually called trans-Neptunian objects.^{[citation needed]}

History

The first minor planet was Ceres, discovered on January 1, 1801 by the Italian Giuseppe Piazzi. Ceres was originally considered a new planet, although it was later downgraded to an asteroid or minor planet, and since 2006 it has been classified as a dwarf planet. William Herschel, discoverer of Uranus, coined the term "asteroid" for the first objects discovered in the century XIX, which orbit the sun between Mars and Jupiter, and generally in an orbit of low relative eccentricity. Since then have found minor planets in all planetary orbits from Mercury to Neptune, and an increasing number of trans Neptunian objects (TNOs) beyond Neptune's orbit.

The minor planets were classified into groups and families based on the characteristics of their orbits. In addition to these extensive divisions, it was customary to name a group of asteroids after the first member of the group discovered (usually the largest). While “groups” are relatively loose dynamic associations, “families” are more stable and coherent. "Families" are only recognized within the asteroid belt, and were first recognized by Kiyotsugu Hirayama in 1918, being called "Hirayama families" in his honor.

Physical properties of comets and minor planets

Commission 15 of the International Astronomical Union is dedicated to the Physical Study of Comets and Minor Planets.

Archival data on the physical properties of comets and minor planets is found in the PDS Asteroid/Dust Archive. Includes standard physical characteristics of asteroids such as properties of binary systems, occultation times and diameters, masses, densities, rotation periods, surface temperatures, albedos, spin vectors, taxonomy and absolute magnitudes and slopes. In addition, the European Asteroid Research Node (E.A.R.N.), an association of asteroid research groups, maintains a Database of Physical and Dynamic Properties of Near-Earth Asteroids.

Environmental properties

Environmental characteristics have three aspects: space environment, surface environment and internal environment, including geological, optical, thermal, radiological environmental properties, etc., which are the basis for understanding the basic properties of minor planets, carrying out scientific research, and are also an important reference base for designing the payload of exploration missions

Radiation environment

Without the protection of an atmosphere and its own strong magnetic field, the minor planet's surface is directly exposed to the surrounding radiation environment. In the cosmic space where the minor planets are located, the radiation on the surface of the planets can be divided into two categories according to its sources: one comes from the sun, including electromagnetic radiation from the sun, and ionizing radiation from the solar wind and energy particles. solar; the other comes from the sun outside the solar system, that is, galactic cosmic rays, etc.

Optical environment

Normally, during a period of rotation of a minor planet, the albedo of a minor planet will change slightly due to its irregular shape and uneven distribution of material composition. This small change will be reflected in the periodic change of the planet's light curve, which can be observed by ground-based equipment, in order to obtain the magnitude of the planet, the rotation period, the orientation of the axis of rotation, the shape, albedo distribution and dispersion properties. In general, the albedo of minor planets is usually low, and the global statistical distribution is bimodal, corresponding to minor planets of type C (mean 0.035) and type S (mean 0.15). In the exploration mission of minor planets, measuring the albedo and color changes of the planet's surface is also the most basic method to directly know the difference in the material composition of the planet's surface.

Geological environment

The geological environment of the surface of minor planets is similar to that of other unprotected celestial bodies, with the most widespread geomorphological feature present being impact craters: however, the fact that most minor planets are debris piles, loose and porous structures, gives the impact action on the surface of minor planets unique characteristics. On very porous minor planets, small impacts produce splash blankets similar to common impacts: while in large impacts compaction predominates and it is difficult for splash blankets to form, and the longer the planets receive these large impacts, the greater is the overall density. In addition, statistical analysis of impact craters is an important means of obtaining information about the age of a planet's surface. Although the Crater Size-Frequency Distribution (CSFD) dating method, commonly used on surfaces of minor planets, does not allow obtaining absolute ages, it can be used to determine the relative ages of different geological bodies for comparative purposes. In addition to the impact, there is a variety of other rich geological effects on the surface of minor planets, such as mass weathering on slopes and walls of impact craters, large-scale linear features associated with graben, and electrostatic transport of dust. analysis of the various geological processes on the surface of the minor planets, it is possible to know the possible internal activity at this stage and some of the key evolutionary information about the long-term interaction with the external environment, which may lead to some indication of the nature of the origin of the progenitor body. Many of the largest planets are typically covered by a layer of soil (regolith) of unknown thickness. Compared to other bodies without an atmosphere in the solar system (for example, the Moon), minor planets have weaker gravitational fields and are less able to retain fine-grained material, which results in a somewhat larger size of the layer of surface soil. Soil layers are inevitably subjected to intense space weathering that alters their physical and chemical properties due to direct exposure to the surrounding space environment. In silicate-rich soils, the outer layers of Fe are reduced to nanophase Fe (np-Fe), which is the main product of space weathering. For some small planets, their surfaces are more exposed. of boulders of various sizes, up to 100 meters in diameter, due to their lower gravitational attraction. These rocks are of great scientific interest, since they may be deeply buried material excavated by the action of an impact or fragments of the mother body of the planet that have survived. The rocks provide more direct and primitive information about the material inside the minor planet and the nature of its parent body than the soil layer, and the different colors and shapes of the rocks indicate different sources of material on the surface of the minor planet or different evolutionary processes.

Magnetic environment

Normally, inside the planet, convection of the conducting fluid will generate a large and strong magnetic field. However, the size of a minor planet is generally small, and most minor planets have a 'crushed stone pile' structure, and there is basically no 'dynamo' structure. inside, so it will not generate a self-generated dipolar magnetic field like the Earth. But some minor planets do have magnetic fields, because, on the one hand, some minor planets have Remnant Magnetism: if the parent body had a magnetic field or if the nearby planetary body has a strong magnetic field, the rocks of the parent body will become magnetized during the cooling process and the planet formed by the fission of the parent body will continue to retain remanence, which can also be detected in extraterrestrial meteorites from the minor planets; on the other hand, if the minor planets are composed of electrically conductive material and its internal conductivity is similar to that of meteorites containing carbon or iron, it is likely that the interaction between the minor planets and the solar wind is unipolar induction, giving rise to an external magnetic field for the minor planet. In addition, the fields Magnetics of the minor planets are not static; Impacts, space weathering, and changes in the thermal environment can alter the existing magnetic fields of minor planets. Currently, there are not many direct observations of magnetic fields of minor planets, and the few existing planet detection projects typically carry magnetometers, with some targets such as Gaspra and Braille having been measured to have strong near magnetic fields, while others like Lutetia do not have a magnetic field.