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concentrations of excess @xmath0 mg , the decay product of the short - lived radionuclide @xmath0al [ mean life = 1.03 myr @xcite ] , show that the solar system formed with @xmath1(@xmath0al)/@xmath1(@xmath2al ) = 5.2 @xmath3 10@xmath4 @xcite . although there is evidence that there may have been deviations from this canonical " ratio across the solar protoplanetary disk by as much as a factor of 2 @xcite , the overall concentration of @xmath0al in the solar disk was more than a factor of 10 greater than the current average value in the interstellar medium of 3.0 @xmath3 10@xmath5 @xcite . while some @xmath0al may have been produced within the early solar system , most of it was not @xcite ; there must have been a significant external source of this short - lived nuclide . commonly , the natal @xmath0al is taken as a signature of a nearby supernova that may have triggered the collapse of the molecular cloud from which the sun formed @xcite . alternatively , winds from massive stars may have supplied the bulk of the @xmath0al @xcite . a major consequence of large amounts of @xmath0al in the early solar system was substantial internal heating of young planetesimals which therefore melted and subsequently experienced igneous differentiation . iron meteorites are thought to be modern fragments of iron - rich cores formed during this era @xcite . if other planetary systems formed with considerably less @xmath0al , then their asteroids may not be differentiated . we can test this scenario by examining the elemental compositions of extrasolar minor planets . evidence is now compelling that some white dwarfs have accreted some of their own asteroids @xcite . in some instances , we have detected excess infrared emission from circumstellar disks composed of dust @xcite where gas also is sometimes evident @xcite . these disks lie within the tidal radius of the white dwarf and are understood to be the consequence of an asteroid having been shredded after its orbit was perturbed so it passed very close to the star @xcite . accretion from these disks supplies the orbited white dwarf s atmosphere with elements heavier than helium where they are normally not found because the gravitationally settling times are very short compared to the cooling age of the star . estimates of the amount of accreted mass argue that we are witnessing the long - lived evolution of ancient asteroid belts @xcite . in the most extreme case , the accreted parent body may have been as massive as ceres @xcite which has a radius near 500 km . however , the required mass more typically implies parent bodies with radii near 200 km @xcite . externally - polluted white dwarfs provide a means for placing the solar concentration of @xmath0al in context .    as a first approximation , extrasolar asteroids resemble bulk earth being largely composed of oxygen , magnesium , silicon and iron and deficient in volatiles such as carbon and water @xcite as expected in simple models for planet formation from a nebular disk . when eight or more polluting elements are detected , it is possible to tightly constrain the history and evolution of the parent body @xcite . recent studies of such richly polluted stars have shown abundance patterns that can be best explained if the accreted planetesimal evolved beyond simple condensation from the nebula where it formed . for example , nltt 43806 is aluminum rich as would be expected if the accreted planetesimal largely was composed of a crust @xcite while pg 0843 + 516 is iron rich which can be explained by the accretion of a core @xcite . @xcite found that the abundance pattern of the object accreted onto gd 362 resembles that of a mesosiderite  a rare kind of meteorite that is best understood as a blend of core and crustal material @xcite .    here , we first revisit the current sample of extrasolar planetesimals with well - measured abundances and reconfirm that igneous differentiation is widespread @xcite . we then present a model to explain this result . finally , we consider our solar system from the perspective of extrasolar environments . the evidence for igneous differentiation among extrasolar planetesimals can be presented in a variety of ways @xcite . here , we display in figure 1 the abundance ratios by number , @xmath1(fe)/@xmath1(al ) vs. @xmath1(si)/@xmath1(al ) , for all seven externally - polluted white dwarf atmospheres where these three elements have been reported . we see that @xmath1(fe)/@xmath1(al ) varies by more than a factor of 100 , a much greater range than shown among main - sequence planet - hosting stars , solar system chondrites and even @xmath1(si)/@xmath1(al ) among these same polluted stars .    the large range in @xmath1(fe)/@xmath1(al ) among extrasolar planetesimals must be the result of some powerful cosmochemical process . one possibility is that unlike in the solar system , some extrasolar planetesimals were formed largely of refractory elements @xcite resulting in low values of @xmath1(fe)/@xmath1(al ) because al is highly refractory . however , this scenario is not supported by available observations @xcite , and can not explain why some systems have relatively high values of @xmath1(fe)/@xmath1(al ) . because there is no viable nebular model to explain the observed range in @xmath1(fe)/@xmath1(al ) , the abundance variations must have been produced within the planetesimals themselves . abundance patterns in extrasolar planetesimals reproduce those in familiar rocks . the lowest value of @xmath1(fe)/@xmath1(al ) is comparable to the ratio in morb ( mid ocean ridge basalt ) , a characteristic crustal rock @xcite . the highest value of @xmath1(fe)/@xmath1(al ) exceeds that of dunite , a mantle rock , implying sampling of iron - rich core material @xcite . figure 1 shows that the range of @xmath1(fe)/@xmath1(al ) among extrasolar asteroids is even greater than the difference found between bulk moon @xcite and bulk mercury @xcite , two solar system objects which are understood as having a small and large iron core , respectively . we understand the variety of elemental compositions among extrasolar planetesimals as the consequence of a familiar three - step process . first , planetesimals form within the disk ; in this environment , volatiles such as water may be excluded . second , differentiation results in iron being concentrated in the core and aluminum being concentrated in the crust . third , collisions lead to stripping and blending of cores and crusts with a consequent dramatic variation in @xmath1(fe)/@xmath1(al ) in the end - product planetesimals , blends of different portions of core , mantle and crustal material . as with solar system asteroids , the heat source for igneous differentiation of extrasolar planetesimals most likely was from radioactive decay of @xmath0al @xcite . other possibilities do not seem viable . the gravitational potential energy released by forming a body of radius , @xmath6 , and mass , @xmath7 , can raise the temperature an amount , @xmath8 t given by : @xmath9 where @xmath10 is the specific heat ( j kg@xmath11 k@xmath11 ) and @xmath12 is the gravitational constant . for a typical object with @xmath6 @xmath13 200 km and @xmath7 @xmath13 1.0 @xmath3 10@xmath14 kg @xcite and with @xmath10 @xmath13 1000 j kg@xmath11 s@xmath11 @xcite , then @xmath15 @xmath13 20 k , much too small to be of importance . although mutual collisions can produce local heating , it seems unlikely that most of the material is melted during the period of planetesimal growth by collisions @xcite . within the average interstellar medium , @xmath16fe/@xmath17fe = 2.8 @xmath3 10@xmath18 @xcite , and if this ratio prevails within star forming regions , then heating from the radioactive decay of @xmath16fe can not be an important heating source within extrasolar planetesimals . it is possible that some stars form near supernovae that produce large amounts of @xmath16fe @xcite , and in these environments newly formed planetesimals could be significantly heated by radioactive decay of this radionuclide . however , because by number there is more @xmath0al than @xmath16fe within the entire galaxy @xcite and because @xmath0al is readily produced within massive stars and then injected into the local molecular interstellar medium where new stars form @xcite , it is probable that the majority of young stellar disks are similar to our own solar system where radioactive decay of @xmath0al was the dominant source of planetesimal heating . the usual expression @xcite governing the time ( @xmath19 ) variation of internal temperature , @xmath20 , as a function of radius , @xmath21 , of a spherical rocky body is : @xmath22 where @xmath23 ( m@xmath24 s@xmath11 ) is the thermal diffusivity and @xmath25 ( j kg@xmath11 s@xmath11 ) is the heating energy per unit mass per unit time . the typical timescale for the loss of internal heat is @xmath26 . for a 200 km radius object with a thermal diffusivity of 10@xmath5 m@xmath24 s@xmath11 @xcite , the outward diffusion of heat represented by the first term on the right hand side of equation ( 2 ) typically requires more than 1 gyr and has a negligible effect on the body s central temperature during the era of heating from @xmath0al . to compute the maximum internal temperature , @xmath27 , we integrate over a time scale much longer than the average decay time , @xmath28 , and consider the total released energy , @xmath29 , defined as : @xmath30 from the decay of @xmath0al . consequently , if the planetesimal originates at temperature , @xmath31 ( k ) , then : @xmath32 where @xmath33 is the initial fraction of the mass of the planetesimal that is @xmath0al . we assume that igneous differentiation is only possible if the internal temperature exceeds the solidus temperature , @xmath34 , @xcite and then derive the minimum aluminum isotope ratio by number , @xmath35al)/@xmath36al ) , required to achieve this temperature . we assume that the extrasolar planetesimal will have formed at some time , @xmath37 , after inheritance of @xmath0al from the molecular cloud . subsequently , no fresh @xmath0al enters the star - forming cloud ; instead , there is only radioactive decay with mean life , @xmath28 . validated by our detailed calculations not shown here , we take @xmath10 to be constant and independent of temperature . if @xmath38 is the fraction of mass of the planetesimal which is aluminum , and if @xmath31 @xmath39 @xmath34 , then : @xmath40    using cv chondrites with their relatively high al , thus providing a minimum for equation [ 5 ] , we take @xmath38 = 0.0175 @xcite . we adopt @xmath34 = 1500 k @xcite , and , for @xmath0al , we take @xmath29 = 1.2 @xmath3 10@xmath41 j kg@xmath11 @xcite . we assume two contributions to @xmath37 . first , there is free - fall gravitational collapse of a cloud core with an initial radius of 0.1 parsec that requires @xmath13 0.5 myr @xcite . second , planetesimals must assemble within the disk which , by analogy with the solar system , probably takes @xmath421 myr @xcite . adding both terms , @xmath37 = 1.5 myr . consequently , we compute from equation ( 5 ) that in extrasolar environments where planetesimals internally melted , @xmath35al)/@xmath36al ) @xmath43 3 @xmath3 10@xmath4 , approximately its value in the early solar system . this result is inexact . if , for example , we take @xmath38 = 0.0086 as found in ci chondrites @xcite , then the minimum values of @xmath35al)/@xmath36al ) should be doubled . as has been previously suggested qualitatively , a general enrichment of @xmath0al in protoplanetary disks might occur if this radionuclide is not distributed evenly throughout the milky way but , instead , is confined to regions of star formation @xcite . a plausible model to explain why @xmath0al would be so concentrated is that this species is largely injected into the interstellar medium from rotating massive stars @xcite . these massive stars are so short lived , that they all reside near their birth sites within molecular clouds . such a model can also explain why the solar system has a relatively high concentration of @xmath0al and a relatively low concentration of @xmath16fe @xcite . however , winds from wolf - rayet stars might shred cloud cores and prevent the formation of planets @xcite . while some recent models for supernova ejecta into molecular clouds also predict that solar mass stars commonly form with elevated amounts of @xmath0al @xcite , they do not naturally explain the solar system s simultaneously depressed value of @xmath16fe/@xmath17fe .    in the entire milky way , the mass of hydrogen in h@xmath44 is 8.4 @xmath3 10@xmath45 m@xmath46 @xcite . the amount of interstellar @xmath0al is measured from the intensity of the @xmath47-ray line at 1.8 mev that results from its radioactive decay . including foreground emission , there is somewhere between 1.5 and 2.2 m@xmath46 of @xmath0al within the galaxy @xcite . if we assume the solar aluminum abundance of @xmath1(@xmath2al)/@xmath1(h ) = 3.5 @xmath3 10@xmath5 @xcite , and if all measured interstellar @xmath0al is confined only to molecular clouds , then in these locations @xmath35al)/@xmath36al ) @xmath13 2.0 - 3.0 @xmath3 10@xmath4 , nearly the same as the minimum ratio we infer for the birth environment of extrasolar planetesimals . consider not only the entire milky way but also observations of the orion region , the nearest molecular cloud where large numbers of high - mass stars currently are being formed . orion s @xmath47-ray line emission is explained with 5.8 @xmath3 10@xmath48 m@xmath46 of @xmath0al @xcite from a region where the total mass of h@xmath44 is approximately 2 @xmath3 10@xmath49 m@xmath46 @xcite . the implied value of @xmath35al)/@xmath36al ) in the orion star - forming region is therefore 3 @xmath3 10@xmath4 , again substantially elevated over the average interstellar value . remarkably , the apparent fraction of @xmath0al within star - forming molecular clouds agrees with the value required to explain the widespread occurrence of differentiated extrasolar planetesimals . it follows that the solar system s initial complement of @xmath0al was essentially normal .      99 bond , j. c. , obrien , d. p. , & lauretta , d. s. 2010 , , 715 , 1050 bonsor , a. , mustill , a. j. , & wyatt , m. c. 2011 , , 414 , 930 boss , a. p. & keiser , s. a. 2013 , , 770 , 51 brown , s. m. , & elkins - tanton , l. t. 2009 , e&psl , 286 , 446 castillo - rogez , j. , lee , m. h. , turner , n. j. , et al . 2009 , icar . , 204 , 658 davison , t . m. , collins , g. s. , & ciesla , f. j. 2010 , icar . , 208 , 468 debes , j. h. , & sigurdsson , s. 2002 , , 572 , 556 debes , j. h. , walsh , k. j. , & stark , c. 2012 , , 747 , 148 desch , s. j. , morris , m. a. , connolly , h. c. , & boss , a. p. 2010 , , 725 , 692 draine , b. t. 2011 , physics of the interstellar and intergalactic medium ( princeton : princeton univ . press ) dufour , p. , kilic , m. , fontaine , g. et al . , 2012 , , 749 , 6 duprat , j. & tatischeff , v. 2007 , , 671 , l69 farihi , j. , jura , m. , & zuckerman , b. 2009 , , 694 , 805 gaensicke , b. , t. , koester , d. , farihi , j. , et al . , 2012 , , 424 , 323 gaenscike , b t. , marsh , t. r. , southworth , j. , & rebassa - 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recently acquired evidence shows that extrasolar asteroids exhibit over a factor of 100 variation in the iron to aluminum abundance ratio . this large range likely is a consequence of igneous differentiation that resulted from heating produced by radioactive decay of @xmath0al with an abundance comparable to that in the solar system s protoplanetary disk at birth . 
 if so , the conventional view that our solar system began with an unusually high amount of @xmath0al should be discarded .