Submitted to Current Science

Astronomical observations of planets orbiting stars other than our Sun, will inevitably lead to a more precise understanding of our own Solar System and as well, perhaps, of the Universe as a whole. The discovery of so-called “hot Jupiter” exo-planets, those with anomalously inflated size and low density relative to Jupiter, has evoked much discussion as to possible sources of internal heat production. But to date, no explanations have come forth that are generally applicable. For example, hot Jupiters are found with insufficient eccentricity to be heated internally by tidal dissipation as originally suggested [1]. Other ideas, such as internal conversion of incident radiation into mechanical energy [2] and on-going tidal dissipation due to a non-zero planetary obliquity [3] also appear to lack general applicability. Charbonneau et al. note that two cases [HD 209458b and HAT-P-1b] suggest at least “…there is a source of internal heat that was overlooked by theoreticians” [4].

One purpose of the present communication is to suggest a source of internal heat production for hot Jupiters exo-planets that indeed has been overlooked by theoreticians and which may be of general applicability. Another purpose is to suggest that the observation of hot Jupiter exo-planets may prove to be the first observational evidence of the correctness of my concept of the ignition of stellar thermonuclear fusion reactions by nuclear fission [5]. Yet another purpose is to discuss implications pertaining to the thermonuclear ignition of dark galaxies, and to suggest that the distributions of luminous stars in galaxies are reflections of the distributions of fissionable elements.

In the late 1960s, astronomers discovered that Jupiter radiates into space about twice as much energy as it receives from the Sun. Later, Saturn and Neptune were also found to radiate prodigious quantities of internally generated energy. That excess energy production has been described as “one of the most interesting revelations of modern planetary science” [6]. Stevenson [7], discuss­ing Jupiter, stated, "The implied energy source ... is apparently gravitational in origin, since all other proposed sources (for example, radio‑activity, accretion, thermonuclear fu­sion) fall short by at least two orders of magni­tude....” Similarly, more than a decade later, Hubbard [6] asserted, “Therefore, by elimi­nation, only one process could be responsible for the luminosities of Jupiter, Saturn, and Nep­tune. Energy is liberated when mass in a gravi­tationally bound object sinks closer to the center of attraction ... potential energy be­comes kinetic energy ....”

In 1990, when I first considered Jupiter’s internal energy production, that explanation did not seem appropriate or relevant because about 98% of the mass of Jupiter is a mixture of hydrogen and helium, both of which are extremely good heat transport media. Moreover, Neptune’s mass is only about 5% that of Jupiter. Having knowledge of the fossil natural nuclear fission reactors that were discovered in 1972 at Oklo, Republic of Gabon, in Western Africa, I realized a different possibility and proposed the idea of planetary‑scale nuclear fission re­actors as energy sources for the giant planets [8]. At first I demonstrated the feasibility for thermal neutron reactors in part us­ing Fermi’s nuclear reactor theory, i.e., the same calculations employed in the initial de­sign of commercial nuclear reactors and used by Kuroda [9] to predict condi­tions for the natural reactors that were later discovered at Oklo. Subsequently, I extended the concept to include planetocentric fast neutron breeder reactors, which are applicable as well to non-hy­drogenous planets, especially the nuclear georeactor as the energy source for Earth’s magnetic field [5, 10].

There is strong terrestrial evidence for the planetocentric nuclear reactor concept. In the 1960’s geoscientists discovered occluded helium in oceanic basalts which, remarkably, possessed a higher ³He/⁴He ratio than air. At the time there was no known deep-Earth mechanism that could account for the ³He, so it was assumed that the ³He was a primordial component, trapped at the time of Earth’s formation, which was subsequently diluted with ⁴He from radioactive decay. State-of-the-art numerical simulations of georeactor operation, conducted at Oak Ridge National Laboratory, yielded fission-product helium, as shown in Figure 1, with isotopic compositions within the exact range of compositions typically observed in oceanic basalts [11, 12]. For additional information, see Rao [13].

One might expect planetocentric nuclear fission reactors to occur within exo-planets of other planetary systems that have a heavy element component, provided the initial actinide isotopic compositions are appropriate for criticality. And indeed, planetocentric nuclear fission reactors may be a crucial component of hot Jupiter exo-planets. But it is unlikely that fission-generated heat alone would be sufficient to create the “puffiness” that is apparently observed. For example, as calculated using Oak Ridge National Laboratory nuclear reactor numerical simulation software, a one Jupiter-mass exo-planet without any additional core enrichment of actinide elements could produce a constant fission-power output of ~ 4 x 10²¹ ergs/s for only ~ 5 x 10⁸ years. Even with that unrealistically brief interval, the fission-power output is orders of magnitude lower than the 10²⁶ to 10²⁹ ergs/s hot Jupiter model-estimates [1]. 

Figure 1. Ratio of 3He/4He, relative to that of air, tabulated for oceanic basalts at 95% confidence level shown for comparison with similar values obtained from nuclear georeactor numerical calculations. In particular, note the distribution of calculated values at 4.5 gigayears, the approximate age of the Earth. Adapted from [11].

 At the beginning of the 20th century, under­standing the nature of the energy source that powers the Sun and other stars was one of the most important problems in physical science. Initially, gravitational potential energy release during protostellar contraction was consid­ered, but calculations showed that the energy released would only be sufficient to power a star for a few million years and life has existed on Earth for a longer time. The discovery of radio­activity and the developments that followed led to the idea that thermonuclear fusion reac­tions power the Sun and other stars.

Thermonuclear fusion reactions are called “thermonuclear” because temperatures on the order of a million degrees Celsius are required. The principal energy released from the detonation of hydrogen bombs comes from thermonuclear fusion reactions. The high temperatures necessary to ignite H-bomb thermonuclear fusion reactions comes from their A-bomb nuclear fission triggers. Each hydrogen bomb is ignited by its own small nuclear fission A-bomb.

In 1938, when the idea of thermonuclear fusion reactions as the energy source for stars had been reasonably well developed [14], nuclear fission had not yet been discovered [15]. Astrophysicists assumed that the million-degree-temperatures necessary for stellar thermonuclear ignition would be produced by the in-fall of dust and gas during star formation and have continued to make that assumption to the present, although clearly there have been signs of potential trouble with the concept. Proto-star heating by the in-fall of dust and gas is off-set by radiation from the surface which is a function of the fourth power of temperature. Generally, in numerical models of protostellar collapse, thermonuclear ignition temperatures, on the order of a million degrees Celsius, are not attained by the gravitational in-fall of matter without assumption of an additional shockwave induced sudden flare-up [16, 17] or result-optimizing the model-parameters, such as opacity and rate of in-fall [18].

After demonstrating the feasibility for planetocentric nuclear fission reactors, I suggested that thermonuclear fusion reactions in stars, as in hydrogen bombs, are ignited by self-sustaining, neutron induced, nuclear fission [5]. I now suggest the possibility that hot Jupiter exo-planets may derive much of their internal heat production from thermonuclear fusion reactions ignited by nuclear fission.

Unlike stars, hot Jupiter exo-planets are insufficiently massive to confine thermonuclear fusion reactions throughout a major portion of their gas envelopes. One might anticipate instead fusion reactions occurring at the interface of a central, internal substructure, presumably the exo-planetary core, which initially at least was heated to thermonuclear ignition temperatures predominantly by self-sustaining nuclear fission chain reactions. After the onset of fusion at that reactive interface, maintaining requisite thermonuclear-interface temperatures might be augmented to some extent by fusion-produced heat, which would as well expand the exo-planetary gas shell, thus decreasing the exo-planet’s density. Viewed in this context, hot Jupiter exo-planets appear to be stars in the process of ignition, but unable to fully ignite because their mass is almost, but not quite, sufficient for gravitational containment. Thus, observations of hot Jupiter exo-planets may stand as the first observational evidence for the correctness of my concept of stellar thermonuclear fusion ignition by nuclear fission chain reactions [5].

The idea that stars are ignited by nuclear fission triggers opens the possibility of stellar non-ignition, a concept which may have fundamental implications bearing on the nature of dark matter and, as suggested in the present communication, on the thermonuclear ignition of dark galaxies, and on the distribution of luminous stars in galaxies Universe-wide. As I noted in 1994, the corollary to thermonuclear ignition is non-ignition, which might result from the absence of fissionable elements, and which would lead to dark stars [5].

Observational evidence, primarily based on velocity dispersions and rotation curves, suggests that spiral galaxies have associated with them massive, spher­oidal, dark matter components, thought to reside in their galactic halos [19]. Interestingly, the luminous disc stars of spiral galaxies belong to the heavy‑element‑rich Population I; the luminous spheroidal stars of spiral galaxies belong to the heavy‑element‑poor Population Il. In spiral galaxies, the dark mat­ter components are thought to be associated in some manner with the spheroidal heavy‑element‑poor Population II stars [20, 21]. The association of dark matter with heavy‑element‑poor Population II stars is in­ferred to exist elsewhere, for example, surrounding elliptical galaxies [22, 23]. Because of the apparent association of dark matter with heavy‑metal‑poor Population II stars, I have suggested the possi­bility that these dark matter components are composed of what might be called Population III stars, zero metallicity stars or stars at least devoid of fissionable elements, and, consequently, unable to sustain the nuclear fission chain reactions necessary for the ignition of thermonuclear fusion reactions.

Although dark matter is thought to be more than an order of magnitude more abundant than luminous matter in the Universe, there has yet to be an unambiguous identification of a wholly dark, galactic-scale structure. There is, however, increasing evidence that VIRGOHI 21, a mysterious hydrogen cloud in the Virgo Cluster, discovered by Davies et al. [24] may be a dark galaxy. Minchin et al. [25] suggested that possibility on the basis of its broad line width unaccompanied by any responsible visible massive object. Subsequently, Minchin et al. [26] find an indubitable interaction with NGC 4254 which they take as additional evidence of the massive nature of VIRGOHI 21. If indeed VIRGOHI 21 turns out to be composed of dark stars having approximately the mass of stars found in luminous galaxies, it would lend strong additional support to my concept of stellar thermonuclear ignition by nuclear fission. 

The existence of a dark galaxy composed of non-brown-dwarf, solar-massive dark stars would certainly call into question the long-standing idea of gravitational collapse as the sole source of heat for inevitable stellar thermonuclear ignition, which after all has no laboratory support, unlike my idea of a nuclear fission trigger [5], which has been demonstrated experimentally with each H-bomb detonation. Population III stars comprising a dark galaxy might well be considered first-generation stars, devoid of metallicity. But, such a concept, although understandable, may lead to a seeming enigma, a paradox within the framework of current astrophysical models.

For half a century, the concept that elements are synthesized within stars [27] has become widely accepted. In the so-called B²FH model, heavy elements are thought to be formed by rapid neutron capture, the R-process, at the supernova end of a star’s lifetime. If actinide elements are required to ignite stars, but are formed at the end of a star’s lifetime, then how did the first stars ignite? The solution to that paradox may be that something like an R-process occurs in other circumstances. I suggest that heavy elements are formed by gravity-induced, massive explosions of nuclear matter at the galactic core. These heavy elements are ejected, jetted into the surrounding galaxy, where they tend to settle in the galactic plane, and seed dark stars, subsequently igniting stellar thermonuclear fusion reactions, and changing dark galaxies into luminous ones. The dynamics and nature of the galactic jet determines the distribution of fissionable matter. From that perspective, the distribution of luminous stars in a galaxy, and consequently the type of galaxy, for example, barred or spiral, may simply be a reflection of the distribution of the fissionable elements.

The variety of morphological galactic forms, especially the prevalence of “bars” can be understood in a logical and causally related way from my concept of heavy-elements being formed in galactic centers, jetted into space where they seed dark stars with fissionable elements, which in turn ignite thermonuclear fusion reactions. Astronomical observations provide support. Figure 2 shows a 10,000 light years long galactic jet observed by the Hubble Space Telescope. Figures 3-6 are four Hubble Space Telescope images of galaxies. The jet-like distribution of luminous stars is especially conspicuous in Figures 3 and 4, but that jet-like distribution is more-or-less a common feature of galaxies, usually referred to as bars, such as in the barred spiral galaxy shown in Figure 5. Even the “starfish” arms of a spiral galaxy, like Figure 6, may owe its origin to repeated jets occurring in the faster rotating galactic center. Bars are often found even in disc galaxies.

Since the 1930s, astrophysics has been built upon the concept that thermonuclear reactions in stars are ignited automatically by heat generated by the collapse of dust and gas during star formation. I challenge the validity of that concept, suggesting instead that stars, like hydrogen bombs, are ignited by nuclear fission, the consequences of which lead to fundamentally different interpretations of much of astrophysics and leads to a logical progression of understanding in which causal relationships become evident.

Figure 2. Hubble Space Telescope of a 10,000 light year long galactic jet. Galactic light was digitally removed for clarity.

Figure 3. Galaxy NGC 4676.

Figure 4. Tadpole Galaxy, UGC 10214.

Figure 5. Barred spiral galaxy, NGC 1300.

Figure 6. Spiral galaxy, M101.

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