The hypothesis of the explosion of a number of planets and moons of our Solar System during its 4.6-billion-year history is in excellent accord with all known observational constraints, even without adjustable parameters or ad hoc helper hypotheses. Many of its boldest predictions have been fulfilled. In most instances, these predictions were judged highly unlikely by the current standard models. Moreover, in several cases, the entire exploded planet model was at risk of being falsified if the predictions failed. The successful predictions include: (1) satellites of asteroids; (2) satellites of comets; (3) salt water in meteorites; (4) ‘roll marks’ leading to boulders on asteroids; (5) the time and peak rate of the 1999 Leonid meteor storm; (6) explosion signatures for asteroids; (7) the strongly spiked energy parameter for new comets; (8) the distribution of black material on slowly rotating airless bodies; (9) splitting velocities of comets; (10) the asteroid-like nature of Deep Impact target Comet Tempel 1; and (11) the presence of high-formation-temperature minerals in the Stardust comet dust sample return. In physics and astronomy, hypotheses are either falsified if their predictions fail, or proved to be of value if they succeed. By all existing evidence, the exploded planet hypothesis has proved far more useful than the half-dozen or so hypotheses it would replace. Among the many important corollaries are these. (a) Perhaps as many as six former planets of our Solar System have exploded over its 4.6-billion-year history. (b) In particular, Mars is not an original planet, but a former moon of an exploded planet. (c) As a major player in Solar System evolution, the exploded planet scenario must be considered as a likely propagation vehicle for the spread of biogenic organisms. We conclude with a brief mention of three possible planetary explosion mechanisms.

Microbes, particularly Archaea, are well-known for their superb adaptation to extreme environments. However, amazing adaptations to extreme stresses do not only occur in microbes, but also in many multicellular organisms. Examples include tardigrades and their capability to survive freezing to near absolute zero, the Pompeii worm withstanding temperatures of up to 105°C, the Crucian carp remaining active in anoxic conditions for several months, and resurrection plants, which can survive near-complete desiccation. Here, we review some of the cryptobiotic strategies and adaptations exhibited in multicellular organisms, and point out their relevance to astrobiology.

The photolysis of dilute solutions of octacyclosulphur or hexacyclosulphur in n-hexane with 253.6 nm UV radiation produces S and possibly S2. The ‘ring-opening’ yields of these sulphur molecules range from 0.2 to 0.7. When the hydrogen end-capped polyyne C10H2 is irradiated in n-hexane, it transforms into unidentified products with a quantum yield of 3×10−5. When octacyclosulphur is added to the solution, the yield rises to 7×10−3. The putative sulphur-bearing product(s) could not be identified. It is suggested that sulphur-bearing molecules might be formed in astronomical settings by reactions of carbon molecules having triple or double C—C bonds with photolytically produced S and/or S2.

The Solar System passes within 5 pc of star-forming nebulae every ∼50–100 million years, a distance which can be bridged by protected micro-organisms ejected from the Earth by impacts. Such encounters disturb the Oort cloud, and induce episodes of bombardment of the Earth and the ejection of microbiota from its surface. Star-forming regions within the nebulae encountered may thus be seeded by significant numbers of microorganisms. Propagation of life throughout the Galactic habitable zone ‘goes critical’ provided that, in a typical molecular cloud, there are at least 1.1 habitable planets with impact environments similar to that of the Earth. Dissemination of microbiota proceeds most rapidly through the molecular ring of the Galaxy.

Panspermia, the dissemination of life through space, would require resistance to the conditions found in space, including UV light. All known life forms depend on DNA to store information. In an effort to understand the liabilities of DNA to UV light and modes of DNA protection in terrestrial life forms, we established UV–VUV (125–340 nm) absorption spectra for dry DNA and its polymerized components and mononucleotides, as well as for a selection of potential UV screens ubiquitous in all organisms, including proteins, selected amino acids and amines (polyamines and tyramine). Montmorillonite clay was included as a potential abiotic UV screen. Among the potential screens tested, adenosine triphosphate (ATP) appeared to be particularly attractive, because its UV absorption spectrum was similar to that of DNA. We suggest that the use of ATP in UV protection could have pre-dated its current role in energy transfer. Spectroscopy also showed that UV absorption varied according to nucleotide content, suggesting that base pair usage could be a factor in adaptation to given UV environments and the availability of UV screens.

We discuss whether it is possible to test the universality of biology, a quest that is of paramount relevance for one of its most recent branches, namely astrobiology. We review this topic in terms of the relative roles played on the Earth biota by contingency and evolutionary convergence. Following the seminal contribution of Darwin, it is reasonable to assume that all forms of life known to us so far are not only terrestrial, but are descendants of a common ancestor that evolved on this planet at the end of a process of chemical evolution. We also raise the related question of whether the molecular events that were precursors to the origin of life on Earth are bound to occur elsewhere in the Universe, wherever the environmental conditions are similar to the terrestrial ones. We refer to ‘cosmic convergence’ as the possible occurrence elsewhere in the Universe of Earth-like environmental conditions. We argue that cosmic convergence is already suggested by observational data. The set of hypotheses for addressing the question of the universality of biology can be tested by future experiments that are feasible with current technology. We focus on landing on Europa and the broader implications of selecting the specific example of the right landing location. We have previously discussed the corresponding miniaturized equipment that is already in existence. The significance of these crucial points needs to be put into a wider scientific perspective, which is one of the main objectives of this review.

The synthesis of physical information on early Earth (or Mars) with recent knowledge arising from microbial genomic, proteomic and phylogenetic studies, strongly indicates that there was insufficient time (∼600 000 years) for life to arise and evolve to reach the biochemical complexity evident within the Last Common Community (LCC). If recent strong evidence of fossil cyanobacteria in carbonaceous meteorites is accepted, then the LCC would have existed prior to the origin of life on Earth and the planet would then have been seeded with representatives of the three domains once it became habitable. The existence of intermittently active cyanobacteria in comets opens the possibility for the evolution of microaerobic bacterial metabolism, elements of which appear at a deep level of the microbial phylogeny, at or below the depth of the LCC. It is also notable from a panspermia perspective that recent phylogenetic evidence indicates that the Gram-positive lineage (representatives of which are endowed with long-lived radiation-resistant spores) lies at the deepest level of domain Bacteria, with Archaea and Eukarya evolving from this lineage probably before 3.6 Gigayears ago (Gya).