P-type asteroids are

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

s that have low

albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

and a featureless reddish

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

. It has been suggested that they have a composition of organic rich

silicate In chemistry, 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 al ...

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

and

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

silicates, possibly with water

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

in their interior. P-type asteroids are found in the outer

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

and beyond. There are in the neighborhood of 33 known P-type asteroids, depending on the classification, including 46 Hestia, 65 Cybele, 76 Freia,

87 Sylvia Sylvia (minor planet designation: 87 Sylvia) is the one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of the Sylvia family and member of Cybele group located beyond the m ...

153 Hilda Hilda ( minor planet designation: 153 Hilda) is a large asteroid in the outer main belt, with a diameter of 170 km. Because it is composed of primitive carbonaceous materials, it has a very dark surface. It was discovered by Johann Palisa ...

476 Hedwig Hedwig (minor planet designation: 476 Hedwig) (1901 GQ) is a main-belt asteroid discovered on August 17, 1901, by Luigi Carnera at Heidelberg. Named in honour of the wife of Swedish-Danish astronomer Elis Strömgren. See also * List of Solar Sy ...

and, in some classifications,

107 Camilla Camilla ( minor planet designation: 107 Camilla) is one of the largest asteroids from the outermost edge of the asteroid belt, approximately in diameter. It is a member of the Sylvia family and located within the Cybele group. It was discovere ...

Taxonomy

An early system of asteroid taxonomy was established in 1975 from the

doctoral thesis A thesis ( : theses), or dissertation (abbreviated diss.), is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings.International Standard ISO 7144: ...

work of

David J. Tholen David James Tholen (born 1955) is an American astronomer at the Institute for Astronomy of the University of Hawaii. He holds a 1984 PhD from the University of Arizona, and specializes in planetary and Solar System astronomy. He is a discovere ...

. This was based upon observations of a group of 110 asteroids. The U-type classification was used as a miscellaneous class for asteroids with unusual spectra that didn't fit into the C and

S-type asteroid S-type asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are of this type, making it the second ...

classifications. In 1976, some of these U-type asteroids with unusual moderate

levels were labeled as M-type. Around 1981, an offshoot of the M-type asteroid branch appeared for minor planets that have spectra that are indistinguishable from M-type, but that also have low albedo not consistent with the M type. These were initially labeled X-type asteroids, then type DM (dark M) or PM (pseudo-M), before acquiring their own unique classification as P-type asteroids (where the P indicates "pseudo-M").

Properties

The P-type asteroids are some of the darkest objects in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

with very low albedos (pv<0.1) and appear to be organic-rich, similar to carbonaceous chondrites. Their colors are somewhat redder than

s and they do not show spectral features. The red coloration may be caused by organic compounds related to

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

. The reflectance spectra of P-type asteroids can be reproduced through a combination of 31% CI and 49% CM groups of

carbonaceous chondrite Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...

meteorites, plus 20% Tagish lake meteorites, after undergoing thermal metamorphism and

space weathering Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres (including the Moon, Mercury, the asteroids, comets, and most of the moons of other planets) take on ...

. The density of the only two well-characterized P-type asteroids,

and

P-type asteroids appears to be low, at – lower even than

C-type asteroid C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...

s. It is not clear what this tells us about their compositions. Both Sylvia and Camilla have moons and indications that they have been be disrupted, but they are also quite massive, at over kg, and so are unlikely to have much internal porosity affecting their densities.P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56 The outer part of the main asteroid belt beyond 2.6 AU from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

is dominated by low-albedo C, D and P-type asteroids. These are primitive asteroids that may have had their materials chemically altered by liquid water. There are 33 known P-type asteroids. In addition to this, P-type asteroids are thought to be found in the outer asteroid belt and beyond. The distribution of P-type asteroids peaks at an orbital distance of 4 AU.

Taxonomy

Properties

References

See also