Rhawn Joseph (Joseph 2009) offers a valiant effort to synthesise the disparate fields of biology, chemistry and physics in order to mount the case that life on Earth has its origins beyond the Earth. As a viewpoint, it is certainly one that many scientists in these disciplines find attractive, myself included. But as a comprehensive case for the argument it is still lacking in several key steps. I would like to respond to a few instances in Joseph's essay where further evidence is needed before the conclusions reached can be sustained.

The principal issue I have is with the direction of the argument made through invoking the principle of "life begat life". While certainly there is no direct evidence, as of the present time, for life being engendered in any other way, this does not provide a rationale either for, or against, the argument that life began off the Earth. The fundamental problem is of the origin of life in the first place. Placing that origin off our planet does not move us any closer to finding an answer to the problem. The best available evidence from Cosmology is that our Universe has an origin, and our best understanding of its physical state at early times implies that life could not have existed then. Hence life, somehow, somewhere, must have been begat from non-life, at least once in the Universe. From an astrophysical perspective there is no fundamental difference between the idea that life originated on the Earth to that of an external origin, which then seeded the Earth. They both present the same problem of forming life from non-life. The central idea underpinning Joseph's paper cannot yet be sustained. Moving life's origin off our planet simply has the philosophical advantage of providing an extra 5 billion years or so in which to solve the origin problem, and of increasing the phase space in which this genesis might have occurred. However, you then have the formidable problem of transporting those seeds to the Earth, the evidence for which remains circumstantial and relies on the assumption that the seeds really were adapted for survival in interstellar space before that transport occurred. Of course, there is a test to distinguish between these possibilities, and that is that the latter hypothesis predicts life to be a Galaxy-wide phenomenon, supported by some underlying, common chemical phenomena. The data to support such a supposition do not yet exist, however, despite considerable interdisciplinary effort devoted to the issue.

A more detailed objection to the argument concerns the timescales for the pre-solar stage of the process. Stars which develop Red Giant winds have lasted for timescales that are measured in the billions of years. Stars which explode as Supernovae, on the other hand, have lifetimes measured in the millions of years.

Our understanding of the lifetimes of the molecular clouds, in which are found the stellar nurseries where these stars are formed, is that they last no more than ~10 million years. This, then, provides an upper limit to the timescale before an interaction with a Supernova could have occurred. Hence the disruption of any protoplanetary disk would have taken place in a system that was no more than a few million years old.

It is now a harder problem to invoke a biogenic origin for material in this system than in the proto-earth, for which the relevant timescales are an order of magnitude larger.

On the other-hand, if the biogenic material is hypothesised to arise from dispersed material from a Red Giant (including its planetary system), there are other problems inherent in the argument presented. Firstly, billion year-old Red Giant stars themselves are not the progenitors of supernovae. Secondly, if organic material associated with their planetary systems is the source of the biogenic material for our Earth, ejected from the stellar system by some process, this is simply moving the genesis problem away from our Earth to another site in the Galaxy. Application of Occam’s Razer would then argue this is less likely to be the case than simply having the biogenic material form in our Solar System in the first place.

A specific objection to this hypothesis presented concerns the references to detections of glycine in the interstellar medium. No claims for such detections have yet withstood detailed scrutiny. The paper quoted by Gómez et al. (2008) is actually a conference paper and concerns the detection of the HCO+ ion in a planetary nebula. The paper referenced by Ehrenfreund and Menten (2002) discusses the possible organic environment in interstellar space, but does not claim a detection of glycine. Two recent papers by Cunningham et al. (2007) and Jones et al. (2007) placed stringent limits on both glycine and the chiral molecule propylene oxide in the two astronomical objects which are the richest sources of organic molecules currently known in interstellar space (SgrB2 and Orion-KL), also putting their results in context with claims that had been made on the detection of glycine in these objects.

The paper by Greaves et al. (2008) presents evidence for a disk of dust around a young, low mass star (HL Tau), a disk which also contains a compact feature which might be a massive planet forming within it. However, even if it is, these authors cannot date the age of this planet from their data. The modelling of the spectral energy distribution of the stellar system they undertook shows it is consistent with a young, pre-main sequence star (< 100,000 years old), and hydrodynamic models for a gravitational interaction within the disk suggest an encounter with another star took place around 600 years ago. But neither of these theoretical arguments provides evidence for a planet that formed ~2,000 years ago, as interpreted by Joseph¹. This particular set of observations cannot be used to provide evidence for the hypothesis of an external biogenic seeding for life on the Earth.

These last two paragraphs aside (on matters not central to the argument under discussion), I would like to finish this essay with some personal thoughts on how we might advance understanding of the Genesis question. There is, of course, the obvious way, by making direct communication with life elsewhere, whether accidentally or though a search program. Such an event would be so paradigm shifting it would change human culture almost immediately. However, we might wish to consider a more outcome-based approach to the task, taking account of the biased sample of one object that we have (the Earth!). Then our best method to find life elsewhere appears to be to search for, and classify, other planetary systems, in the hope that one may turn up evidence of biogenic activity. Certainly, we have the means to begin such an endeavour. It may even be argued that it has led to a distortion of funding programs in some science agencies, for the discovery of a new exo-planet has already reached the stage where it is no longer big news! We can expect the number of known exo-planets to continue to rise in the years ahead, and to extend their parameter space down to lower masses and to longer orbital periods. The recent successful launch of the Kepler spacecraft, and its demonstration of reduced photometric errors in the measurement of stellar fluxes over previous work in this arena, promises to greatly extend the number of exo-solar systems discovered by the transit technique. At the same time this will provide a sample of planetary objects whose atmospheres might be sampled. However, even when they then are, these objects are unlikely to represent “Earth-analogues”, as the techniques are still not sensitive to finding such systems. Our hope here may be in detect the non-equilibrium chemistry in the atmosphere of a planet where life is rampant, which has a clear spectral signature in the infrared that can be distinguished from the host star, and might be detected by an interferometer (a network of telescopes linked together) in space. However, while the specifications for the kind of experiment that is required can be given, the technology to implement it still lies some years away, let alone the political will to fund such a search program.

There are less ambitious projects, however, which might bear more fruit in the near term. While the simplest amino acid glycine has yet to be detected in the interstellar medium, there is no reason to believe it does not exist there. Certainly, molecules nearly as complex have been found (e.g. the sugar glycolaldehyde, CH2OHCHO; Hollis et al., 2000; glycine is NH2CH2COOH). The detection of glycine, if it comes, may end up being statistical result rather than a direct detection, based on positive correlations of spectral features with known glycine transitions, embedded within a forest of molecular lines from many species. While its detection could not be directly taken to imply that biogenic activity takes place in the interstellar environment, it would be a step on the route to demonstrating that it does, if this is the case. Equally interesting is the investigation of the origin of the glycine found in several meteorites that fell to Earth, and to determine whether this event really occurred elsewhere in the Solar System other than on our planet. The most intriguing suggestion made by Joseph though, to me at least, is that there exist microbes which could be adapted for survival in space. This may simply be a by-product of their need to survive in a harsh environment of the early Earth, in which case it would have no significance to the external seed hypothesis. But if it could be demonstrated, unequivocally, that space survivability was a specific evolutionary adaption then that is another matter, and would provide strong ancillary evidence for the external origins hypothesis.

Life begat life, at least as far as we know, but this does not help us solve the genesis question of where life first began (or indeed, whether life began independently more than once). There is a scientific process we may follow to pursue the question, however. Moving genesis away from the Earth, to a time before the Sun was born, may buy us another 5 billion years to solve the origin problem (essentially the lifetime of the Galaxy). If this really is the case, then surely the seeding of the Earth cannot have been an unusual case? Life must then exist elsewhere in our Galaxy, and display some common chemical traits. This is a hypothesis that can be tested.

REFERENCES

Cunningham, M.R., Jones, P.A., Godfrey, P.D., Cragg, D.M., Bains, I., Burton, M.G., Calisse, P., Crighton, N.H.M., Curran, S.J., Davis, T.M., Dempsey, J.T., Fulton. B., Hidas, M.G., Hill, T., Kedziora-Chudczer, L., Minier, V., Pracey, M.B., Purcell, C., Shobbrook, J. & Travouillon, T., 2007. A search for propylene oxide and glycine in Sagittarius B2 (LMH) and Orion. Monthly Notices Royal Astronomical Society, 376, 1201-1210.

Ehrenfreund, P., Menten, K. M. 2002. From Molecular Clouds to the Origin of Life. In G. Horneck & C. Baumstark-Khan. Astrobiology, Springer.

Gómez Y, Tafoya, D., Anglada, G., Loinard, L., Torrelles, J. M., Miranda, L. F., Osorio, M., Franco-Hernández, R., Nyman, L., Nakashima, J., Deguchi, S. 2008. HCO+ emission possibly related with a shielding mechanism that protects water molecules in the young PN K 3-35. Proceedings IAU Symposium No. 251, In S. Kwok & S. Sandford, eds. International Astronomical Union.

Greaves, J. S., Richards, A. M. S., Rice, W. K. M., Muxlow, T. W. B. 2008. Enhanced dust emission in the HL Tau disc: a low-mass companion in formation? Monthly Notices of the Royal Astronomical Society: Letters 391, L74-L78.

Hollis, J.M., Lovas, F.J. & Jewell, P.R. 2000. Interstellar Glycolaldehyde; the First Sugar. Astrophysical Journal, 540, L107-L110.

Jones, P.A., Cunningham, M.R., Godfrey, P.D. & Cragg, D.M., 2007. A search for biomolecules in Sagittarius B2 (LMH) with the Australia Telescope Compact Array. Monthly Notices Royal Astronomical Society, 374, 579-589.

Joseph, R. 2009. Life on Earth came from other planets. Journal of Cosmology, 1, 1-56.

¹Editor's note: Dr. Joseph based this statement on the interpretations publicized by Professor Greaves about her data (e.g, National Geographic, (2008). Newborn planet is youngest ever found).