Astrophysics and Space Science

, Volume 341, Issue 2, pp 295–299

Life-bearing primordial planets in the solar vicinity

Authors

    • Buckingham Centre for AstrobiologyThe University of Buckingham
  • Jamie Wallis
    • School of MathematicsCardiff University
  • Daryl H. Wallis
    • Buckingham Centre for AstrobiologyThe University of Buckingham
  • Rudolph E. Schild
    • Harvard-Smithsonian Center for Astrophysics
  • Carl H. Gibson
    • Buckingham Centre for AstrobiologyThe University of Buckingham
    • University of California San Diego
Original Article

DOI: 10.1007/s10509-012-1092-8

Cite this article as:
Wickramasinghe, N.C., Wallis, J., Wallis, D.H. et al. Astrophys Space Sci (2012) 341: 295. doi:10.1007/s10509-012-1092-8

Abstract

The space density of life-bearing primordial planets in the solar vicinity may amount to ∼8.1×104 pc−3 giving total of ∼1014 throughout the entire galactic disk. Initially dominated by H2 these planets are stripped of their hydrogen mantles when the ambient radiation temperature exceeds 3 K as they fall from the galactic halo to the mid-plane of the galaxy. The zodiacal cloud in our solar system encounters a primordial planet once every 26 My (on our estimate) thus intercepting an average mass of 103 tonnes of interplanetary dust on each occasion. If the dust included microbial material that originated on Earth and was scattered via impacts or cometary sublimation into the zodiacal cloud, this process offers a way by which evolved genes from Earth life could become dispersed through the galaxy.

Keywords

Primordial planetsPanspermiaHGD cosmologySolid hydrogenCometsStar formation

1 Introduction

Estimates of the grand total of planets in the galaxy, in particular the population of unbound, “free-floating” planets have been revised upwards over the past decade (Hurley and Sharma 2002; Sumi et al. 2011; Cassan et al. 2012). Cassan et al. (2012) estimate ∼1011 exoplanets in the galaxy and Sumi et al. (2011) have argued for a similar number of unbound planets. Gibson (1996) estimates at least 1018 primordial exoplanets per galaxy.

The most powerful method for detecting unseen planets was pioneered by Schild (1996) who studied the gravitational microlensing of quasars by planets along lines of sight that crossed globular clusters in the halos of distant lensing galaxies. From Schild’s data it can be inferred that “rogue planets” are indeed so abundant that they could account for a large fraction of the much debated “dark baryonic matter”—the missing mass of the galaxy (Gibson and Schild 1996, 2009).

From the application of hydrogravitational dynamics (HGD cosmology) Gibson and Schild (1996) have argued that globular cluster-mass clumps of planets condense due to instabilities in the cosmological plasma 300,000 years after the Big Bang. A fraction of these planets coalesce into massive stars that end as supernovae producing heavy elements (C, N, O) at a very early stage. The bulk of the primordial planets envisaged in the Gibson-Schild theory constitute a universal total of 1080 planetary bodies with interiors that remain warm and liquid for millions of years. It is within this set of “connected” primordial planets that we argued that life originated in the early Universe (Gibson et al. 2011).

After the lapse of a further few million years the Universe cools below the freezing point of H2 permitting extensive mantles of solid hydrogen to form (Wickramasinghe et al. 2010). Such frozen planets, containing an accumulating complement of heavy elements and life, are the building blocks of galaxies and stars that form at later cosmological epochs (Gibson and Schild 1996).

2 Chemical composition of primordial planets

The halos of galaxies, in the Gibson-Schild model, would represent large numbers of approximately spherical accumulations of primordial planets. Such condensations eventually evolve dynamically into galaxies such as our own Milky Way system. Our own galaxy is known to possess a nearly spherical halo of material of which the globular clusters (each comprised of 105–106 population II stars) probably form a minute component. Very similar configurations of globular clusters are observed for other galaxies, in particular, for the nearby Andromeda nebula, M31 (Galleti et al. 2004). Thus processes discussed in the present paper would apply to other spiral galaxies as well.

Stars in globular clusters of our galaxy have a median spectral type and F8 a total mass of some 107–108 M. The entire mass of visible stars and normal baryonic matter of the galaxy is ∼5×1011 M. A larger dark matter contribution from the halo of a comparable amount is needed to explain the observed rotation curve of stars. The “dark matter” must extend out to distances of ∼100 kpc from the galactic center, and is most likely to represent frozen primordial planets that failed to form population II stars. They might be envisaged to exist as loosely bound “open clusters” from which individual planets continue to escape at a slow rate and fall towards the galactic mid-plane. Such escaping planets will tend to be attracted to the larger molecular clouds in the spiral arms of the galaxy. GMC’s like the Orion complex with masses M>105 M would provide the main sites of accumulations of primordial planets by gravitational focusing, which would thus exert a major control over the processes of star formation (Gibson and Schild 1996). A significant fraction of the organic molecules and organic dust associated with GMC’s may well have their origin in infalling primordial planets rather than by in situ gas phase synthesis in clouds as is usually believed (Kwok and Zhang 2011; Wickramasinghe and Kwok 2012).

3 Primordial planet structure and evolution

Based on solar ratios of the relative abundances of elements as set out in Table 1 we envisage a stratified concentric shell structure of primordial planets, consistent with the condensation sequence of solids expected to form in a cooling gas cloud (Hoyle and Wickramasinghe 1968).
Table 1

Relative solar abundances by number (log10A) (Allen 1963)

Nuclide

1H

12C

14N

16O

56Fe

24Mg

28Si

Log A

12

8.56

8.05

8.93

7.54

7.58

7.55

Here we assume the specific gravity values of iron, silicate, and water-ice to be 7.9, 2.7 and 1 respectively. With the outer radius of the main ice-organic domain normalized to unity, the outer radii of the silicate and iron cores could be estimated as 0.4 and 0.25 respectively. Exterior to the main water-ice-organic layer there would be a frozen H2 mantle extending to ∼5 units on our relative scale (Fig. 1) that condensed and solidified a few million years after the Big Bang.
https://static-content.springer.com/image/art%3A10.1007%2Fs10509-012-1092-8/MediaObjects/10509_2012_1092_Fig1_HTML.gif
Fig. 1

Schematic depiction of stratified interior of a primordial planet assuming solar abundances: inner core is mainly iron; middle domain silicates, outer bulk water/organics

The role of solid hydrogen in astronomy and cosmology was originally discussed by Wickramasinghe and Reddish (1968) and by Hoyle et al. (1968) and recently revived by Pfenniger and Puy (2003) and Lin et al. (2011). In the present paper we shall discuss the behavior of large planetary bodies that are mainly comprised of H2 which condensed in the very early history of the Universe.

In the radiation environment of the galactic halo, outside the globular clusters, the ambient temperature would be very close to 3 K and we would expect the H2 mantle to remain thermodynamically stable and in tact. However, as the solid hydrogen dominated planets began to fall towards the galactic disc, particularly into star-forming regions, we would expect the H2 to sublimate.

The rate of reduction of planetary radius due to sublimation of H2 is given by
$$ \frac{dR}{dt}=-\frac{p_{sat}}{s}\biggl(\frac{m}{2\pi kT} \biggr)^{1/2} $$
(1)
where psat(T) is the saturation vapor pressure of solid H2 at temperature T, m is the mass of the H2 molecule and s is its density in the solid phase. From laboratory data for para-hydrogen the number density of saturated gas molecules nsat in equilibrium with the solid can be calculated (Van de Hulst 1949; Pfenniger and Puy 2003) thus
$$ n_{sat}=\frac{p_{sat}}{kT}\cong3\times10^{20}T^{3/2} \exp\biggl(-\frac{91.75}{T}\biggr) $$
(2)
setting s=0.1 g cm−3 for solid H2 and re-writing (1) in the form
$$ \frac{dR}{dt}\cong-2.57\times 10^{-14}n_{sat}T^{1/2}~\mathrm{km}/\mathrm{ky} $$
(3)
we obtain a life-time for a 5000 km solid H2 planet of
$$ t\cong\frac{1.2\times 10^{17}}{n_{sat}T^{1/2}}~\mathrm{ky} $$
(4)
Lin et al. (2011) have pointed out that the thermochemical data for solid H2 is unlikely to be exactly relevant for H2 that condensed in astronomical settings in the presence of high fluxes of ionizing radiation. Binding energies are likely to be significantly increased due to inclusion of ions such as \(\mathrm{H}_{6}^{+}\) and \((\mathrm{HD})_{3}^{+}\) in the crystalline lattice structure. To take account of such enhancement of binding we introduce a factor ξ in the exponent and replace (2) with
$$ n_{sat}=\frac{p_{sat}}{kT}\cong3\times10^{20}T^{3/2} \exp\biggl(-\frac{91.75\xi}{T}\biggr) $$
(5)
and vary the factor ξ. For ξ=1,1.5,2 we can solve (4) and (5) to give the characteristic sublimation time for a planet of radius 5000 km. The results are shown in Table 2.
Table 2

Time (ky) for sublimation of a solid H2 planet of radius 5000 km at various temperatures for various values of the binding enhancement ξ

Temperature (K)

ξ=1

t (ky)

ξ=1.5

t (ky)

ξ=2.0

t (ky)

3

7.3E8

3.2E15

1.4E22

10

3.3E–2

3.25E0

3.02E3

20

8.5E–4

8.5E–4

8.25E–3

30

8.0E–6

3.75E–5

1.7E–4

We see from Table 2 that solid H2-dominated primordial planets would have an indefinite persistence so long as they reside in radiation environments with effective temperatures only slightly in excess of 3 K. The temperature characterizing locations far from stars within the halo of the galaxy would satisfy this condition. Only when solid H2 planets enter globular clusters or approach the galactic disc would temperatures exceed the critical values for rapid sublimation for all values of ξ considered in Table 2. Sublimation will be followed by rapid dissociation on a timescale much less than 103 years, so H2 molecular densities observed in clouds need to be maintained by re-formation on grain surfaces (Solomon and Wickramasinghe 1969).

The extreme rapidity of the loss of H2 indicated from our calculations for T>10 K might be exaggerated, however, due to the neglect of H2 gas establishing a transient atmospheric reservoir. Let us assume now that the evaporated H2 forms a short-lived isothermal atmosphere of temperature T∼100–200 K. The escape velocity from the solid icy planet beneath the atmosphere is
$$ v_{esc}=\biggl(\frac{2GM}{R}\biggr)^{1/2}\cong7.4\biggl( \frac{R}{1000~\mathrm{km}}\biggr)\times10^4~\mathrm{cm}/\mathrm{s} $$
(6)
and the mean thermal speed of H2 molecules is
$$ v_{th}=\biggl(\frac{3kT}{m}\biggr)^{1/2}\cong1.13\times \biggl(\frac{T}{200}\biggr)^{1/2}~\mathrm{cm/s} $$
(7)
showing that vth<vesc. The scale height of an isothermal atmosphere for a uniform spherical planet of radius R, surface gravity g(=GM/R2) and average density s=1.2 is
$$ H=\frac{kT}{mg}\cong2.6\times10^{3}\biggl(\frac{T/200}{R/1000~\mathrm{km}} \biggr)~\mathrm{cm} $$
(8)
Hence the mean lifetime of the H2 atmosphere is
$$ \langle t\rangle=\frac{H}{v_{th}}\cong2.3\times10^3 \frac{(T/200)^{1/2}}{R/1000~\mathrm{km}}~\mathrm{s} $$
(9)
For T∼100–200 K and an icy planet (interior to the hydrogen) with radius 1000 km, the lifetime of the atmosphere is measured in thousands of seconds. This calculation indicates that the sublimation of solid H2 from primordial planets is essentially instantaneous. When molecules evaporate from the surface they are essentially lost into space. This would be the fate of all primordial planets once they leave the low-radiation-temperature environs of the galactic halo.

4 Radioactive heating of residual icy planet

We have already pointed out that approximately solar system abundances would be expected to prevail in the material from which our primordial planet condensed. In the heaviest elements, however, a diminution by a factor ∼10 might be reasonable, but no more. Solar abundance tables give (U+Th)/(C+N+O)=6×10−9 by number, corresponding to a mass ratio of ∼10−7. We consider it plausible that about 10 % of this ratio would be appropriate for primordial planet material.

With (U+Th)/(C+N+O)∼10−8 by mass and given the total energy released from the fission of U+Th to be H∼1014 J/kg we have an average heat release per unit mass of ∼106 J/kg. This is to be compared with ∼3.34×105 J/kg appropriate for the heat of fusion of water ice. The implication is that a substantial mass fraction of the planet’s ice content would be converted to liquid water on energetic arguments alone. This would have been the initial state at the time when the planetary object condensed at time t<106 yr post Big Bang. We next examine whether the initial heat content of the liquid water in the planet’s interior can be contained within an exterior frozen shell for cosmologically relevant timescales.

Consider an icy planet of radius R with a liquid core of radius 0.9R with an outer temperature of 300 K. Suppose this body is surrounded by a frozen ice/organic mantle of thickness 0.1R. The energy contained in the liquid core from the fusion of Uranium and Thorium (making up 10−8 by mass of the water) gives a total
$$ Q\cong10^3\frac{4}{3}\pi(0.9R)^{3} \bigl(10^6\bigr)\approx10^9\frac{4}{3}\pi R^3~J $$
(10)
In a 1-dimensional approximation the rate of conduction of heat energy Q through a distance x is given by
$$ \frac{dQ}{dt}=\kappa A\frac{dT}{dx} $$
(11)
where A is the area across which conduction occurs T is the temperature, x is the distance and κ is the thermal conductivity of ice. Now, regarding our thin spherical ice shell to be a plane slab of area A=4πR2 (10) and (11) yield:
$$ 10^9\frac{4}{3}\pi\frac{R^3}{t}\cong\kappa4\pi R^2\Delta T/(0.1R) $$
(12)
Here t is the characteristic cooling time and ΔT is the temperature difference from the inside to the exterior of the shell. We can suppose the inside of the shell to be at a temperature close to 273 K and the exterior close to 13 K giving ΔT=260 K. From (12) we obtain the cooling time, with κ=0.17 W m−1 K−1 for an ice-organic mixture we obtain
$$ \frac{t}{\mathrm{yr}}\approx2.5\times10^4\biggl(\frac{R}{\mathrm{km}} \biggr)^2 $$
(13)
For a planet with radius 1000 km the cooling time is thus estimated as t∼2.5×1010 yr, longer than the age of the Universe. This result could be verified using a more detailed heat transfer calculation.

5 Distribution of H2 depleted primordial planets

Primordial planets stripped of their hydrogen mantles would tend to fall towards minima of the galaxy’s gravitational potential, close to its mid-plane. Larger concentrations of such planets are also to be expected in the more massive GMC’s which are more powerful attractors and can be identified as sites of active star formation.

We next estimate the space density of H2 depleted primordial planets that might exist in the galactic disk. If the total population of pristine primordial planet material in the galactic halo is taken to account for the missing mass of the galaxy, it would be reasonable to suppose that the CNO-rich planetary cores in the galactic disc in the solar vicinity would be comparable to the total mass density of C, N, O in the form of gas and dust, ∼3×10−26 g cm−3. With planetary bodies each of radius 1000 km and mass ∼1025 g, we have a number density of such objects N given by the equation
$$ N~\bigl(\mathrm{cm}^{-3}\bigr)\times10^{25}~\mathrm{g} \approx3\times10^{-26}~\mathrm{g}\,\mathrm{cm}^{-3} $$
(14)
yielding
$$ N~\bigl(\mathrm{pc}^{-3}\bigr) \approx 8.1 \times 10^{4} $$
(15)
With a disc volume of ∼109 pc3, this corresponds to a total of 109×8.1×104≈1014 such objects occupying the galactic disc. The present estimate of unbound planets exceeds the values given by Sumi et al. (2011) by a factor of 103. The mean separation between these planetary bodies will thus be ∼N−1/3≈0.02 pc.

6 Ingress into the solar system

Although the number density of interstellar planets given by (15) is well below the limit determined for non-detection of hyperbolic comets which is ∼10−13 pc−3 (Valtonen and Innanen 1982), some effects of their ingress into the inner zodiacal cloud (R∼10 AU) are to be expected. The entire solar system moves through the local interstellar medium at a speed of ∼20 km/s and the zodiacal cloud of estimated radius R≈10 AU presents a target area σ=7.85×10−9 pc2. The distance L traveled between successive encounters with planetary/cometary bodies is given by
$$\sigma NL\approx1 $$
leading to a value L=1.57 kpc with N=8.1×10−4 pc−3. This distance will be traversed at 20 km/s in a time of 26 My.
On the average every 26 My, according to this estimate, the passing planetary body of assumed radius 1000 km would encounter a total mass of zodiacal dust
$$10^3\biggl(\frac{\rho}{10^{-22}~\mathrm{g}\,\mathrm{cm}^{-3}}\biggr)~\mathrm{tonnes} $$
where ρ is the space density of zodiacal grains. With an estimated dust density in the Earth’s vicinity ρ≈10−22 g cm−3 we find that a total of 103 tonnes of dust grains would have impacted and become implanted in the planetary ice surfaces.

7 Transfer of earth-life to passing primordial planets

An interesting implication of the foregoing discussion is the possibility that microbiota from Earth can be transferred to passing primordial planets. The transfer must involve, as a first step, the transportation of microorganisms in a viable form from Earth to the Zodiacal cloud. The latter could readily be achieved by means of comet or asteroid impacts.

Cometary bodies have impacted the Earth at more or less regular intervals after life became first established. Enhanced rates of impact can be shown to occur whenever the Oort shell of comets comes to be perturbed during encounters with molecular clouds (Wickramasinghe et al. 2010). A well-documented comet impact event occurred 65 million years ago resulting in the Chicxulub crater. In such an impact the total mass of crater ejecta would be expected to exceed the atmospheric mass by a large factor, and as a consequence a massive plume would be produced that blows a hole through the atmosphere (Melosh 1989). Terrestrial rocks and debris would be swept up in the plume along with high speed vapor to reach escape speeds.

A significant fraction of the expelled material would not have been either shocked or heated to sterilization levels, and entire ecologies of viable microorganisms could thus have reached the zodiacal cloud (Wallis and Wickramasinghe 2004). Napier (2004) has argued that fragmentation and erosion of ejected rocks into beta-meteoroids would occur on a timescale of 104 yr, thus ensuring their escape from the solar system by the action of radiation pressure. However, potentially life-bearing dust of micrometer sizes would remain within the zodiacal cloud for tens of millions of years (Wickramasinghe et al. 2010). With clusters of Chicxulub-type impacts occurring over a similar timescale, a steady-state distribution of life-bearing dust in the zodiacal cloud would be maintained.

Dust particles carrying Earth-life would be amongst those that collide with and become embedded in the ice surfaces of transiting planets as discussed in Sect. 6. It should also be noted in this context that impacts of bacteria and spores on ice surfaces at 20 km/s have actually been simulated in laboratory experiments by Burchell et al. (2001, 2004) and a significant viable fraction has been recovered. Also, as pointed by Wesson (2010) even partially destroyed/deactivated bacterial genomes could carry and thereby perpetuate the information of life.

Because the comet collision process spans the entire age of the Earth and because life has evolved and adapted locally from the time it was first introduced, the genes expelled into the zodiacal cloud would reflect the entire evolutionary history of life on our planet. It is such a broad sweep of evolved terrestrial genes that would be carried away by every primordial planet that crosses our solar system. And since we cannot regard, the Earth as being special in this regard, the same processes will have been repeated in every life-bearing planetary system similar to our solar system. One could thus envisage the entire galaxy to be a single connected biosphere where primordial planets play a crucial role in transmitting and mixing the genetic information needed for life to evolve and realize the fullest range of its evolutionary potential.

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© Springer Science+Business Media B.V. 2012