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Extrasolar Carbon Planets Marc J Kuchner1 arXiv astro ph 0504214v2 2 May 2005 Princeton University Department of Astrophysical Sciences Peyton Hall Princeton NJ 08544 S Seager Carnegie Institution of Washington 5241 Broad Branch Rd NW Washington DC 20015 seager dtm ciw edu ABSTRACT We suggest that some extrasolar planets 60 M will form substantially from silicon carbide and other carbon compounds Pulsar planets and low mass white dwarf planets are especially good candidate members of this new class of planets but these objects could also conceivably form around stars like the Sun This planet formation pathway requires only a factor of two local enhancement of the protoplanetary disk s C O ratio above solar a condition that pileups of carbonaceous grains may create in ordinary protoplanetary disks Hot Neptunemass carbon planets should show a significant paucity of water vapor in their spectra compared to hot planets with solar abundances Cooler less massive carbon planets may show hydrocarbon rich spectra and tar covered surfaces The high sublimation temperatures of diamond SiC and other carbon compounds could protect these planets from carbon depletion at high temperatures Subject headings astrobiology planets and satellites individual Mercury Jupiter planetary systems formation pulsars individual PSR 1257 12 white dwarfs 1 INTRODUCTION The recent discoveries of Neptune mass extrasolar planets by the radial velocity method Santos et al 2004 McArthur et al 2004 Butler et al 2004 and the rapid development 1 Hubble Fellow 2 of new technologies to study the compositions of low mass extrasolar planets see e g the review by Kuchner Spergel 2003 have compelled several authors to consider planets with chemistries unlike those found in the solar system Stevenson 2004 such as water planets Kuchner 2003 Leger et al 2004 Here we describe a new possibility extrasolar planets in which carbon is the most abundant component by number carbon planets Recently Lodders 2004 argued that



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