ORIGIN OF THE SOLAR SYSTEM AND ITS ELEMENTS

O. K. MANUEL, J. T. LEE, D. E. RAGLAND, J. M. D. MACELROY^*, BIN LI^** AND W. K. BROWN^***

Nuclear Chemistry, University of Missouri, Rolla, MO 65401 (USA)

Correspondence author’s e-mail address: om@umr.edu

(Journal of Radioanalytical and Nuclear Chemistry, in press)

Formation of the Solar System from heterogeneous debris of a supernova (SN) that exploded 5 billion years ago was recorded as a) inter-linked chemical and isotopic heterogeneities in meteorites, b) higher levels of extinct nuclides in grains that trapped larger isotopic anomalies, c) the physical properties of grains mentioned in part b, and d) patterns of isotopic anomalies in meteorites, in the solar-wind, and in solar flare particles. The Sun formed on the SN core, and planets formed in a rotationally-supported, equatorial disk of SN debris. Interiors of the Sun and the inner planets accreted first in a central, Fe-rich region surrounding the SN core. These were layered as condensate from other parts of the SN fell toward the condensing Sun. Elements in outer SN layers formed low-density, giant Jovian planets. Intra-solar diffusion enriches hydrogen and lighter isotopes of individual elements at the Sun’s surface.

^*Chemical Engineering, Univ. College Dublin, Belfield, Dublin 4 (Ireland)

^**Lunar & Planetary Lab, University of Arizona, Tucson, AZ 85721 (USA)

^*** 5179 Eastshore Drive, Lake Almanor, CA 96137 (USA)

Introduction

This paper was presented at the Conference on Presolar Grains in St. Louis, MO (USA) on October 31, 1996 and at the International Conference on Isotopes in the Solar System in Ahmedabad (India) on November 13, 1997. Parts were also presented as papers #25.03 and #27.02 at the 189th and 191st Meeting of the American Astronomical Society on January 13, 1997 (Toronto, Canada) and January 7, 1998 (Washington, D.C., USA). This is a summary of recent data that compelled us to consider local element synthesis and formation of the Solar System from heterogeneous supernova debris, rather than remote synthesis and collection of our elements from vast regions of space. A few earlier observations and conclusions that are consistent with local element synthesis are noted below.

REYNOLDS¹ discovery of radiogenic ¹²⁹Xe in isotopically anomalous meteoritic Xe would be expected, since extinct ¹²⁹I and heterogeneities in the SN debris would diminish with time. SN debris is a natural site for a) heterogeneous accretion of cores of the inner planets^2-4 and the Sun^5,6 in a central Fe-rich region of the nebula and b) production of IAB iron meteorites and their inclusions by nebular, rather than igneous, processes^7,8.

Over 40 years ago, BURBIDGE et al.⁹ noted that “...the back sides of the abundance peaks of the r-process isotopes might suggest that the conditions obtaining in a single supernova were responsible for their synthesis.” After considering astronomical implications, they concluded that “It does not appear unreasonable from this point of view, therefore, that a single supernova has been responsible for all of the material built by the r-process currently in the solar system” (BURBIDGE et al.⁹ , p. 639).

Four years later, FOWLER et al.¹⁰ suggested local element synthesis, induced by the bombardment of planetesimals with charged particles from the condensing Sun, to explain the abundances of deuterium, lithium, beryllium, and boron in the Solar System. In addition, they noted that several short-lived nuclides, like ¹²⁹I, ¹⁰⁷Pd and ²⁶Al, might have been produced locally by this irradiation of planetesimals.

Inter-linkage of chemical and isotopic heterogeneities provided the first clue, over 20 years ago, that not only r-products but the entire Solar System came from a single supernova^11,12. Elemental abundances of He and Ne in meteorites were shown to be linked with specific isotopes of heavy noble gases, as expected of stellar debris. Noble gases are ideally suited to preserve the primordial linkage of elemental and isotopic heterogeneities before condensation: The inertness of these elements protected them from the chemical reactions that most elements underwent as they mixed and reacted to form condensable compounds.

Exotic material from a multiplicity of stellar sources is currently the accepted^13-16 explanation for isotopic anomalies seen in tiny grains of meteorites. In the following sections, we review observations which suggest that these grains are instead early condensate of a local supernova¹⁷ which exploded in the manner shown in Fig. 1. It has been suggested that such asymmetric supernovae explosions may produce bipolar nebulae¹⁸ and allow SN debris in the equatorial plane to retain heterogeneities of the parent star¹⁷.

We will show below how this explains inter-linked chemical heterogeneities, short-lived radioactivities, physical aspects of the matrix grains, and isotopic anomaly patterns in meteorites, in solar flare particles and in the solar-wind. In 1980, LAVRUKHINA¹⁹ also noted that formation of the Solar System from debris of one supernova might explain planetary scale variations in the isotopic compositions of some elements.

Short-Lived Radioactivities

Decay products of extinct ⁴¹Ca, ²⁶Al, ⁵³Mn, ¹⁰⁷Pd, ¹⁸²Hf, ¹²⁹I, ²⁴⁴Pu, or ¹⁴⁶Sm might be used to date the supernova event shown in Fig. 1. Of these, ²⁴⁴Pu is best suited because a) decay products of ²⁴⁴Pu occur in U,Th-Pb dated samples, i.e., the ²⁴⁴Pu-¹³⁶Xe and the U,Th-Pb chronometers can be linked, and b) ²⁴⁴Pu is an r-product that could not be made after the supernova by irradiation from the condensing Sun¹⁰. The volatile nature of the fission product, ¹³⁶Xe, is the major disadvantage.

KURODA²⁰ predicted the occurrence of ²⁴⁴Pu in nature, ROWE and KURODA²¹ discovered its decay products, and KURODA and MYERS²² used ²⁴⁴Pu to show that the supernova event shown in Fig. 1 occurred about 5 billion years ago. According to their calculations, the first carbonaceous chondrites started to retain fissiogenic ¹³⁶Xe about 100 My later. Their results are shown in Fig. 2, where data from the Kapoeta achondrite are used to link the ²⁴⁴Pu-¹³⁶Xe and the U,Th-Pb chronometers²³.

Significant mixing may occur over planetary distances in 100 My, although this is short for galactic mixing¹⁰. Grains that formed in this time might incorporate products from different SN regions, before ²⁴⁴Pu-rich minerals start to retain gaseous ¹³⁶Xe --- 100 My after the SN event. If mixing occurred, larger isotopic anomalies would be expected in grains that formed first and trapped even shorter-lived radioactivities, like ²⁶Al.

However, early results suggested meteorites trapped rather uniform initial levels of extinct radioactivities ≈ 10^-4 those of neighboring nuclides, in spite of a wide range in half-lives. FOWLER¹³ and WASSERBURG¹⁴ thus concluded that mixing 0.0001 parts exotic nucleogenetic material with 0.9999 parts normal solar system material could explain the decay products of extinct nuclides and isotopic anomalies found in meteorites.

Recent measurements have revealed levels of extinct radioactivities >>10^-4 that of neighboring nuclei. For example, radiogenic ²⁶Mg from the decay of ²⁶Al corresponds to initial values of ²⁶Al/²⁷Al up to 0.06 in graphite from the Murchison meteorite²⁴. Five SiC grains of that meteorite, labelled X grains²⁵, formed when 0.10 < ²⁶Al/²⁷Al < 0.60. These trapped very large isotopic excesses of ¹²C, ¹⁵N, ²⁸Si, ⁴⁹Ti and ⁴⁴Ca. The authors²⁵ note that different zones of supernovae can “...account for almost all the isotopic compositions observed in grains X....” (AMARI et al.²⁵, p. L46).

These results²⁵ confirm that high levels of radioactive ²⁶Al are associated with large isotopic anomalies, as expected of early condensate of supernova debris. They support neither the suggestion of a “canonical” value²⁶ of ²⁶Al/²⁷Al ≈ 5 x 10^-5 nor reports that short-lived radioactivities and isotopic anomalies in the solar system can be explained by the addition of 10^-4 parts exotic nucleogenetic material. These X grains also have the physical properties expected for early condensate of supernova debris.

KURODA and MYERS²⁷ recently reviewed data on extinct ²⁶Al. They note that, like radioactivity trapped in fresh atmospheric fallout particles from nuclear weapons testing, the ²⁶Al/²⁷Al ratios²⁵ in the X grains of SiC decrease with particle size. They calculate ²⁶Al/²⁷Al = 1.15 for the production ratio at the surface of an exploding supernova and conclude that X grains of SiC formed soon after a supernova. The ²⁶Al-²⁶Mg chronometer in Fig. 3 indicates that “X” grains formed within 1-3 My of the supernova. Early nucleation would form larger grains in local condensation, but this trend is unexpected if interstellar grains²⁸ brought this exotic nucleogenetic material into the solar system from multiple stellar sources^14-17.

More recently, AMARI et al.²⁹ note that isotopic anomaly patterns of titanium change systematically with grain size in eight other SiC grains from Murchison, samples KJA-KJH. They note that this might be explained by “... assuming that SiC grains with different sizes come from AGB stars with different metallicities ...” (AMARI et al.²⁹, p. 23). Even if grain size were preserved as a marker of the star-of-origin as interstellar grains traversed space, local condensation of heterogeneous supernova debris is a more simple, direct explanation for the observations.

The time scale shown in Fig. 3 indicates rapid condensation of solids following the supernova event. New measurements suggest an even earlier separation^30,31 of supernova products and a single stellar source for short-lived nuclides⁷⁹. Isotope ratios of r-products in Xe, Kr and Te in diamonds of the Allende meteorite suggest that chemical separation^30,31 of these elements occurred within 10⁴ seconds of the supernova event, even before the decay of precursor nuclei like ¹³¹I, ⁸³Br, ¹²⁵Sb, etc.

Inter-Linked Chemical and Isotopic Heterogeneities

Abundances of ordinary, planetary He and Ne in meteorites correlate with isotopically exotic components of Ar, Kr and Xe and extrapolate to isotopically normal Ar, Kr and Xe at the intercept³², where [He] ≈ [Ne] ≈ 0. This was the first clue that the solar system formed out of material with the inter-linked chemical and isotopic heterogeneities expected in supernova debris^11,12. One example of these correlations is shown in Fig. 4.

The ¹³⁶Xe/¹³²Xe isotope ratios are shown in the top section of Fig. 4, and the elemental ratios of ⁴He/¹³²Xe and ²⁰Ne/¹³²Xe are shown in the bottom. Similar correlations could be shown between elemental abundances of ordinary He and Ne with each exotic isotope ratio of Ar, Kr and Xe. This inter-linkage of ordinary He and Ne with exotic Ar, Kr and Xe is a common property of meteoritic noble gases³².

The correlations shown in Fig. 4, illustrate the difficulty in explaining isotopic anomalies by the addition of exotic nucleogenetic material^13,14. If exotic Ar, Kr and Xe were alien nucleosynthesis products, for example, why do they accompany isotopically normal He and Ne? Why do the correlations extrapolate to isotopically normal Ar, Kr and Xe as elemental abundances of isotopically normal He and Ne approach zero?

These correlations led to the suggestion^11,12 that the solar system condensed from heterogeneous SN debris: Normal He and Ne and exotic Ar, Kr and Xe came from outer layers that were rich in low-Z elements; normal Ar, Kr and Xe came from the interior where fusion reactions had depleted He, Ne and other low-Z elements. In the astrophysics community, it was suggested²⁸ that interstellar grains may have carried normal He and Ne and exotic Ar, Kr and Xe into the solar system. Fig. 1 of this manuscript and Fig. 4 of CLAYTON²⁸ offer alternative explanations for the observed association of normal He and Ne with exotic Ar, Kr and Xe.

Recently PEPIN et al.³⁴ suggested that the exotic heavy noble gas component in Allende accounts for 8% of the ¹³⁶Xe in the Sun. If so, the correlations of He and Ne with ¹³⁶Xe (See Fig. 4) suggest that large amounts of He and Ne in the Sun are from this exotic noble gas component.

CLAYTON et al.³⁵ provide additional evidence for large scale, inter-linked isotopic and elemental heterogeneities in the solar system. They found that fractional enrichments of an exotic oxygen component (monoisotopic ¹⁶O) can be used to group meteorites and planets into at least six categories. Objects of one category cannot be derived by fractionation or differentiation of material in another category. On the basis of exotic oxygen, material in the earth and its moon is like that in stony-iron meteorites, achondrites and enstatite chondrites, but isotopically distinct from material in carbonaceous and ordinary chondrites or urelites. Heterogeneous supernova debris (Fig. 1) offers a natural site for radial sorting of exotic oxygen, characteristic of the region where the material formed.

The occurrence of terrestrial-type Xe and its host minerals also provides a record of coupled chemical and isotopic heterogeneities related to radial distance from the sun. REYNOLDS¹ found that the isotopic composition of terrestrial Xe is distinct from that in chondrites. Terrestrial-type Xe occurs in iron sulfide of diverse meteorites³⁶, in atmospheres of Earth and Mars³⁷, and in the Sun³⁸, as expected if this were the dominant form of Xe in the inner Fe,S-rich region of the early solar nebula³⁶.

The matrix composition of carrier grains of exotic xenon and other noble gases also demonstrates linkage of chemical and isotopic heterogeneities at the source. Diamonds trapped Xe enriched in r- and p-products; SiC trapped Xe enriched in s-products. This coupling of chemical and isotopic heterogeneities so thoroughly permeated the solar nebula that measurements of the isotopic compositions of xenon and other noble gases in acid residues have recently been used to determine the abundances of diamonds and SiC grains in seven different types of chondrites³⁹.

LEE et al.⁴⁰ cite other indications that chemical and isotopic heterogeneities were coupled throughout the early solar nebula⁴⁰. A related discovery is the radial heterogeneity of ⁵³Mn (t_1/2 = 3.7 My), as evidenced by the correlation of excess ⁵³Cr with heliocentric distance for material from the Earth-Moon, Mars, ordinary chondrites and eucrites⁴¹.

Formation of the solar system from supernova debris (See Fig. 1) predicts both inter-linked chemical and isotopic heterogeneities in the early solar nebula^{11-12,19,32,35-37,39,40} and a dependence of composition on radial distance from the Sun^{2-4,11-12,19,32,35,36,40,41}.

Isotopic Anomaly Patterns

BEGEMANN⁴² offers compelling evidence that isotopic anomalies are unmixed products of the same nuclear processes that made our elements. Allende’s inclusion EK1-4-1 has excess (+) r- and p-products in Ba, Nd and Sm; Murchison’s SiC has deficits (-) of these same isotopes. These mirrorimage isotopic anomaly patterns are shown in Fig. 5. Smaller anomalies in Allende’s EK1-4-1 are multiplied by a factor of ≈10² to depict the mirror-image anomaly pattern. Although “quantitatively, the (anti)-match of the anomalies pattern is not perfect for all elements”, he concludes that it “---appears too good to be entirely serendipitous---” (BEGEMANN⁴², p. 524). This pattern is expected in poorly mixed products of the reactions that made our elements, but it is unlikely that exotic material, added to an otherwise homogeneous presolar nebula, would mimic the nucleosynthesis products required from one r-process and one s-process, so that the sum yields the isotopic composition of normal, solar-system material.

The sun accounts for 99% of the mass of the solar system. Isotopic compositions of solar elements provide additional evidence for large reservoirs of isotopically diverse material at the birth of the solar system. In general, planetary-type noble gases are enriched in the heavy isotopes relative to gases in the solar wind. KAISER³⁸ showed, for example, that isotopic ratios of solar-type Xe can be reproduced by a mass fractionation enrichment of 4.1% per atomic mass unit in lighter mass isotopes of terrestrial Xe. KURODA and MANUEL⁴³ noted a correlated fractionation effect in neon. However, early efforts to understand other solar isotopic ratios were frustrated by apparently large fractionation effects in the isotopes of solar Xe, Ne and He, accompanied by little or no fractionation effects across the isotopes of solar Ar and Kr.

Following the discovery of nucleogenetic isotopic anomalies in many elements, and linkage of primordial He and Ne with isotopically anomalous Ar, Kr and Xe in diverse meteorites³², MANUEL and HWAUNG⁴⁴ suggested that noble gases in the Sun are a mixture of the nucleogenetic components seen in planetary material. They noted a systematic mass fractionation pattern across isotopes of all five noble gases in the solar wind, assuming that terrestrial Xe is dominant in the bulk sun³⁸, that He, Ne and Ar there have the isotopic compositions of noble gases that accompany anomalous Xe in meteorites³³, and that solar Kr is a mix of these two components.

Their results, shown in Fig. 6, suggest that intrasolar diffusion enriches lighter isotopes of He, Ne, Ar, Kr and Xe at the solar surface by about 200%, 30%, 9%, 6% and 4% per amu, respectively⁴⁴. Light isotopes are less enriched in solar flares⁴⁵, as expected if energetic events disrupt diffusive fractionation in the Sun⁴⁶. Recent observations^47,48 show ¹H/⁴He in solar flares to be ≈10 times less than that in the solar photosphere.

If plasma diffusion⁴⁹ produced the fractionation pattern shown in Fig. 6, the Sun’s interior may be Fe-rich⁴⁴ like the inner planets. When this was first suggested⁴⁴, diffusional fractionation was considered relatively unimportant in the Sun and solar-type stars^50-52. However, we will show in the next section that these views are changing. It has been noted that heterogeneous accretion of the Sun and the four inner planets, beginning with core formation in a central Fe-rich region of the nebula⁵³, may have assisted diffusive fractionation in stratification of the Sun.

Recently, PEPIN et al.³⁴ also attempted to resolve the isotopic composition of solar Xe in terms of different isotopic components observed in planetary material. They concluded that the exotic Xe shown in Fig. 4 accounts for about 8% of the ¹³⁶Xe in the solar wind.

We agree that the primordial noble gas component with normal He and Ne and exotic Ar, Kr and Xe may constitute a significant part of the Sun’s noble gases^34,44. However, since “... the composition of condensed bits of interstellar matter would favor easily condensed elements over the noble gases” (ARNETT⁵⁴, p. 2) , this conclusion is incompatible with prevailing opinion that interstellar grains²⁸ brought the exotic Ar, Kr and Xe into the solar system from a multiplicity of stellar sources^13-16.

Intrasolar Diffusion: Does it Operate in the Sun?

Several recent articles^55-57 stress the importance of diffusional fractionation in the Sun. More than a decade ago, MACELROY and MANUEL⁵⁸ attempted to evaluate the contribution of intrasolar diffusion to isotopic anomalies in the solar wind. We will briefly review their conclusions, some of the more recent work on diffusion in the Sun, and provide a few comments on issues that must be addressed for a quantitative understanding of intrasolar diffusion.

Possible fractionation effects for primordial ⁴He/H mixtures are shown in Fig. 7 as a function of the effective depth, d, of the perfectly mixed outer layers of the sun. The value of d shown there is linearly proportional to the total mass within this mixed zone⁵⁸.

For the current solar surface ratio^59,60 of ⁴He/H = 0.04, it is found that values of d required to satisfy a primordial ⁴He/H ratio of 0.09-0.15, the currently accepted range for cosmic abundances, are ~ 43 - 65. This value of d corresponds to a temperature of about 10⁶ K for the transition region between the diffusion limited zone and the perfectly mixed zone.

One typical example of the possible influence of intrasolar diffusion on an isotopic ratio of xenon, ¹²⁴Xe/¹³⁰Xe, is shown in Fig. 8. From the results shown there, it is concluded that the enrichment of light isotopes of xenon is accurately predicted for intrasolar diffusion conditions very similar to those suggested by Fig. 7: A diffusion limited/perfectly mixed transition region corresponding to a temperature of approximately 10⁶ K in the standard solar model and a Xe ionization state of +15. This result, coupled with additional observations from numerical data reported earlier⁵⁸, provides independent support for the existence of significant diffusional fractionation occurring in the sun.

Further support for the influence of intrasolar diffusion and its relative importance in modelling the solar interior is seen in the very recent detailed numerical analyses conducted by a number of independent research groups.

(1) DAR and SHAVIV⁵⁵ have demonstrated that reliable prediction of the observed flux of solar neutrinos requires consideration of the effects of diffusion within the sun. When the standard solar model is modified to include diffusion, they find that in one of the models investigated the observed solar neutrino flux is only 50% higher than the predicted value. By comparison, the observed neutrino flux is 100%-150% higher than the predicted value when intrasolar diffusion is assumed to be insignificant.

(2) GUENTHER et al.⁵⁶ have shown that observed solar oscillations can be predicted with reasonable accuracy only when intrasolar diffusion is incorporated into the standard solar model.

(3) BAHCALL et al.⁵⁷ show that sound speeds for solar models that include diffusion agree with helioseismological measurements for almost the entire sun. They also note that helioseismological measurements effectively rule out solar models that do not include diffusion, or which assume that the interior of the sun is significantly mixed.

We have already noted that another observation, namely noble gas isotope ratios in the solar wind, also requires consideration of intrasolar diffusion. We are encouraged that these disparate fields of research may be gradually approaching a common consensus. There are a few remaining issues that need to be addressed.

An accurate hydrodynamic analysis of the interfacial region between the radiative zone and the convection zone within the sun is one major problem on the computational side which has yet to be properly addressed.

This could help to eliminate a number of the apparent inconsistencies in the predictions in all these areas of research. One particular inconsistency concerns the locations of the diffusion limited transition region and the base of the convection zone. These need not necessarily occur at the same point within the sun.

The mixing length theories employed in modelling of intrasolar diffusion are known to inadequately describe convection in the outer layers of the sun⁶¹. Advanced computational fluid mechanics has, in fact, shown that mixing within the convection zone is not as complete as implied in the classical mixing length approach. Detailed hydrodynamic modelling of the solar interior, coupled with consideration of elemental and isotopic diffusion, will probably be required to provide the agreement sought between theory and observations.

On the experimental side, there is a need for measurements on the isotopic compositions of refractory elements in the solar wind⁶² and for an understanding of the origin of the Fractionation part of FUN anomalies⁶³ in meteorites. We suspect that both may be the result of stellar diffusion. However, two fractionation processes, one acting on elements in the Sun and the other on elements present at the birth of the solar system, can be confusing. There were, for example, early indications of mass fractionation in the isotopes of Ne and Xe in the solar wind⁴³, but not for the intermediate elements, Ar and Kr. Recognition of a common fractionation effect⁴⁴ across the isotopes of solar Ar and Kr awaited discovery³³ of a primordial, exotic noble gas component in which Ar and Kr are enriched in the heavier isotopes.

Recently, the isotopic compositions of Mg have been reported in the solar wind⁶⁴ and in solar flare particles⁶⁵. These results are from direct measurements of particles coming from the Sun, and error limits are much larger than those on elements that have accumulated in lunar soil over geologic time periods. In spite of large uncertainties, systematic trends in the data suggest possible interference from MgH⁺ which increases with particle energy⁴⁶. Natural isotopic abundances cause this interference to be greatest at m/q = 25. The ²⁴Mg/²⁶Mg ratio is less effected. Shifts in the best values for ³He/⁴He, ²⁰Ne/²²Ne, ²⁶Mg/²⁴Mg and ³⁶Ar/³⁸Ar in solar flares and in the solar wind match⁴⁶ the pattern expected if solar flares are disrupting diffusional processes that enrich lighter nuclei at the solar surface. However, for magnesium isotope ratios these shifts are within experimental uncertainties reported on the measurements.

Finally, it should be noted that an experiment already completed may provide important information on isotopic anomalies in the solar system and the role of intrasolar diffusion in producing anomalous isotope ratios in the solar wind. Isotopic analyses of elements in Jupiter with the mass spectrometer on the Galileo probe may shed new insight on this matter. To date, only the isotope ratios of H, He and C have been reported⁶⁶ but we expect to have information on isotope ratios of Ne, Ar, Kr and Xe in Jupiter within the current calendar year.

From Supernovae to Planetary Systems

Until recently, formation of a planetary system from supernova debris was considered highly unlikely. However, recent astronomical observations on supernova remnants may be changing this view.

The Crab nebula, the remnant of a 1054 AD supernova, has long served as a model for explosions of supernovae, especially Type II SNs. Progress in this area of study is limited by the infrequency of such events. However, about 10 years ago, on February 23, 1987, Ian Shelton discovered a supernova explosion in the Large Megellanic Cloud. This is now known as Supernova 1987A or SN 1987A. GOLDSMITH⁶⁷ tells how this discovery has changed our understanding of supernovae. For example, the parent star was not a red giant star, as expected, but clearly blue. It was the first stellar explosion observed in the Local Group of galaxies since 1885, and the first one bright enough to be seen with the naked eye since Kepler’s supernova of 1604. The suspected pulsar in SN 1987A is rotating much more rapidly than expected of a newborn neutron star, suggesting the possibility of rapid rotation in the parent star.

CHEVALIER^68,69 reviewed observations on SN 1987A, five and ten years after the explosion. In the first⁶⁸, he noted that the light emitting region is a ring, rather than a shell. In the second, he reports three rings of light. The smaller ring is in the central plane of the supernova, with larger rings of light above and below that. He also notes little or no radiation from any central compact object associated with either SN 1987A or SN 1993J. In both cases, radiation from the central compact object is estimated to be < 0.01 of radiation from the Crab Nebula pulsar.

Another recent astronomical observation on supernova remnants has been even more surprising. The first planetary system discovered⁷⁰ and confirmed⁷¹ beyond our solar system was that orbiting the collapsed core of a supernova, the millisecond pulsar, PSR 1257+12.

There had been earlier reports that planets might be orbiting other pulsars. These were not confirmed, but they stimulated thinking about possible mechanisms for producing a planetary system from supernova debris. LIN et al.⁷² suggest that such a planetary system could be produced from a rotationally-supported disk of material that may fall back after a supernova explosion. The planetary system that WOLSZCZAN⁷¹ confirmed around PSR1257+12 consists of three, Earth like planets within 0.5 AU of the central pulsar. In the Appendix, we show that such a planetary system could be produced around PSR1257+12 from a rotationally-supported, disk of material (0.068 M_sun mass) that falls back after the SN explosion.

The computed mass distributions, with and without shear, are shown in Fig. 9, together with the positions and relative masses of the planets WOLSZCZAN⁷¹ reports orbiting PSR1257+12.

From these results, we conclude that the formation of a planetary system from supernova debris is clearly possible. In earlier sections of this paper, we have summarized observations which suggest that the solar system was in fact produced in this manner.

Conclusions

The record of inter-linked chemical and isotopic heterogeneities and short-lived radioactivities that has been preserved in meteorites and in the planetary system is not explained by the addition of small amounts of exotic material. These are products of the supernova event, outlined in Fig. 1, that gave birth to the solar system 5,000 My ago.

This paper is dedicated to the memory of Dr. Dwarka Das Sabu.

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Figure Captions

Fig. 1. The solar system formed directly from heterogeneous debris of a spinning supernova¹⁷ that exploded axially to produce a bipolar nebula¹⁸.

Fig. 2. Carbonaceous chondrites start to retain gaseous fission products of ²⁴⁴Pu about 100 My after the SN event^22,23.

Fig. 3. Radioactive ²⁶Al and large isotopic anomalies were trapped in graphite²⁴ and in X grains of silicon carbide²⁵ from the Murchison meteorite within a few My of the SN event²⁷.

Fig. 4. Isotopic ratios of Xe correlate with elemental abundances of He and Ne in mineral separates of the Allende meteorite³³. “Normal” Xe is at the intercept where [He] = [Ne] = 0.

Fig. 5. Mirror image (+ & -) isotopic anomalies that BEGEMANN⁴² noted in elements with 7 stable isotopes like Ba, Nd or Sm.

Fig. 6. Isotopic ratios of solar-wind-implanted gases⁴⁴ show a smooth, mass-dependent fractionation pattern, as expected if intrasolar diffusion enriches lighter elements and lighter isotopes of individual elements at the solar surface. Mass fractionation is shown relative to ²⁰Ne.

Fig. 7. Predicted primordial values of the ⁴He/H ratio are shown as a function of the effective mixing depth, d, for outer layers of the Sun. The lines marked a, b, and c correspond to predictions for current solar surface ⁴He/H values of 0.06, 0.04 and 0.02, respectively.

Fig. 8. Predicted solar wind values of the ¹²⁴Xe/¹³⁰Xe ratio (relative to the primordial value) are shown as a function of ionization state in the transition region between the diffusion-limited zone and the perfectly-mixed zone within the Sun. The curves from left to right correspond to ⁴He/H surface number ratios of 0.05, 0.03 and 0.01 for a primordial solar ratio of ⁴He/H = 0.15.

Fig. 9. Computed mass distributions, with and without shear, for fallback material⁷² rotating in the nebular disk, with Wolszczan’s three planets⁷¹ superimposed.

Fig. 10. A differential cylinder of radius, r₁, thickness dr₁, and height, h, inscribed in a sphere of radius R_FB, and rotating with an angular velocity w.

Fig. 11. The mass distribution in the planetary disk, with and without shear, are quite similar.