Astronomy:P-type asteroid

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P-type asteroids are asteroids that have low albedo and a featureless reddish spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interior. P-type asteroids are found in the outer asteroid belt and beyond. There are 33 known P-type asteroids,[1] including 46 Hestia, 65 Cybele, 76 Freia, 87 Sylvia, 153 Hilda and 476 Hedwig.[2][3]


An early system of asteroid taxonomy was established in 1975 from the doctoral thesis work of David J. Tholen. 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 classifications. In 1976, some of these U-type asteroids with unusual moderate albedo levels were labeled as M-type.[4]

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").[4]


The P-type asteroids are some of the darkest objects in the Solar System with very low albedos (pv<0.1) and appear to be organic-rich, similar to carbonaceous chondrites. Their colors are somewhat redder than S-type asteroids and they do not show spectral features. The red coloration may be caused by organic compounds related to kerogen.[5][6] The reflectance spectra of P-type asteroids can be reproduced through a combination of 31% CI and 49% CM groups of carbonaceous chondrite meteorites, plus 20% Tagish lake meteorites, after undergoing thermal metamorphism and space weathering.[2]

The outer part of the main asteroid belt beyond 2.6 AU from the Sun 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.[7] The distribution of P-type asteroids peaks at an orbital distance of 4 AU.[8]


  1. "JPL Small-Body Database Search Engine: spec. type = P (Tholen)". JPL Solar System Dynamics.;obj_kind=all;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=OBJ;c1_item=Ay;c1_op=%3D;c1_value=P;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsAiArApAxAy;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AyA. 
  2. 2.0 2.1 Hiroi, T. (March 15–19, 2004). "What are the P-type Asteroids Made Of?". League City, Texas. Bibcode2004LPI....35.1616H. 
  3. Ziffer, J.; Campins, H.; Licandro, J.; Fernandez, Y. R.; Bus, S. (August 2005). "Near-infrared Spectra of Two Asteroids with Low Tisserand Invariant". Bulletin of the American Astronomical Society 37: 644. Bibcode2005DPS....37.1529Z. 
  4. 4.0 4.1 Tholen, D. J.; Bell, J. F. (March 1987). "Evolution of Asteroid Taxonomy". Houston, Texas. pp. 1008–1009. Bibcode1987LPI....18.1008T. 
  5. Planetary Sciences. Cambridge University Press. 2001. p. 353. ISBN 0-521-48219-4. 
  6. Ehrenfreund, Pascale (2004). Astrobiology: Future Perspectives. Springer Science & Business. p. 159. ISBN 1-4020-2304-9. 
  7. Lazzarin, M.; Barbieri, C.; Barucci, M. A. (December 1995). "Visible Spectroscopy of Dark, Primitive Asteroids". Astronomical Journal 110: 3058. doi:10.1086/117747. Bibcode1995AJ....110.3058L. 
  8. McSween, Harry Y. (1999). Meteorites and their parent planets (2nd ed.). Cambridge University Press. p. 101. ISBN 0-521-58751-4. 

See also