Secrets of the Universe

by Paul Murdin

  Life in the Universe

  Are we alone?

  My Reason is convinc’d, said the Marquiese, but my Fancy is confounded with the infinite Number of living Creatures, that are in the Planets; and my thoughts are strangely embarras’d with the variety that one must of Necessity imagine to be amongst them.

  Bernard le Bovier de Fontenelle, A Discovery of New Worlds, translated by Aphra Behn, 1688

  No one knows whether astronomers will ever discover life on other planets. If they do, it will prove that we are not alone in the Universe. Most people seem to like this idea, but the Universe has yet to reveal its secrets on the matter.

  Life is based on carbon. Carbon is made in stars and is everywhere in the Universe. It is the only element that makes complex ‘organic’ molecules that form a rich ‘vocabulary’ that can be used in the ‘script’ of life. Radio astronomers have discovered about 150 sorts of organic molecules in interstellar space. Comets and asteroids have organic ‘tars’ and ‘crusts’ on their surface.

  In 1969 a meteorite broke up over Murchison, near Melbourne, Australia, and pieces fell across the town. There were many eyewitnesses to the fall (church-goers on a Sunday morning), who promptly collected about 80 kilograms of fragments. The meteorite was found to contain more than ninety amino acids, a variety of organic compounds essential for life, as well as evidence that they were made in space, rather than being terrestrial contaminants. The implication of all this is that basic biochemicals are made by natural processes in the Universe, and move from place to place.

  The French chemists Jean-Baptiste Biot and Louis Pasteur discovered in the early half of the nineteenth century that certain biochemical molecules have a property called ‘chirality’: they occur in both ‘left-handed’ and ‘right-handed’ forms, only one of which is used by life. Thalidomide is a notorious example, in which one chirality, used as a drug, is benign and the other actually very harmful, causing birth defects. Inorganic chemical processes usually produce equal amounts of both chiralities. But if you shine strongly polarized light into a ‘soup’ in which there are equal numbers of left- and right-handed molecules, some can switch from one sort to the other, and the soup develops an excess of one form of chirality, and may favour biochemical rather than inorganic chemical reactions. Some astronomers think that chirality can be generated in the organic molecules on comets when they pass a source of strongly polarized light, such as a pulsar, transforming them into ‘seeds for life’.

  Astronomers discovered the processes that delivered the chiral chemicals to the Earth, but geologists identified the ‘fire’ that simmered them into life. Energy is required to fuel the chemical reactions that change the pre-biotic chemicals into more complex life-forming molecules. On Earth, sunlight is the usual energy source – plants photosynthesize chemicals to create cells – but it is not the only source. ‘Black smokers’ are geothermal vents in the deepest, darkest parts of the ocean, which are colonized by oceanic life that feeds on geothermal energy rather than sunlight. The microbial forms of life that flourish in such extreme environments are called archaea, and they are the oldest living things on Earth. In fact life seems to have started under the oceans, and only later evolved to harvest energy from sunlight on the surface. Some regions of the satellites of Jupiter and Saturn – Europa, Callisto, Ganymede and Enceladus – mimic this environment of water warmed by internal energy sources, and are regarded as potential sites in which to search for extraterrestrial life.

  Biochemistry needs a medium in which to operate: water. Although water is made of simple elements (hydrogen and oxygen) that are abundant everywhere in the Universe, liquid water can only survive on the surface of a planet in the so-called ‘Goldilocks Zone’ of a planetary system: that is, where the temperature is neither so cold so that water freezes, nor so hot so that it evaporates, but just right. This is mainly a matter of the planet being at the right distance from its sun, but can be helped or hindered by any greenhouse effect in the planet’s atmosphere. It is also possible for a planet outside the Goldilocks Zone to have liquid water because of geothermal heating, as on Mars or Europa.

  The idea that organic chemicals developed in space and were brought to Earth by cosmic processes, to evolve in the oceans, was first proposed by Isaac Newton and given modern form in 1908–11 by the father-and-son team Thomas C. Chamberlin and Rollin T. Chamberlin. The Chamberlins suggested that ‘planetesimals,’ the small bodies that merged into the planets, were the source of organic material from which life evolved, creating organic compounds as they collided and accreted with each other. The basic ideas of the theory were revived by Soviet astrobiologist Alexander Oparin in 1938, and reformulated with the aid of modern scientific knowledge by Joan Oró of the University of Texas and Chris Chyba, director of the SETI Institute (Institute for the Search for Extraterrestrial Intelligence) and a student of Carl Sagan.

  Another possible scenario for the early formation of life on Earth and elsewhere was discovered in the laboratory. In 1953 Stanley Miller, a PhD student at the University of Chicago, put simple organic chemicals into a flask of water from which oxygen gas had been removed, and then passed an electric spark in through the vapour. Miller did not have deep-sea geothermal vents in mind; he was guided by the idea that electric discharges (lightning) created biological molecules in the atmosphere. Miller found amino acids in the black sludge produced by his experiment. No one knows precisely how life first formed on Earth, but hundreds of similar experiments have since been carried out with a variety of chemicals and energy sources, showing similar results.

  Scientists believe that the right chemical ingredients, a source of energy and the presence of water as a solvent at some point combined to transform inanimate organic chemicals into life on Earth, perhaps also on Mars and Europa. In 1996 David McKay and his co-workers discovered what appeared to be traces of bacteria in the Martian meteorite ALH84001, which was found in Antarctica. The meteorite had been knocked off the surface of Mars by an asteroid impact 16 million years ago, and after a period in orbit round the Solar System, fell to Earth 12,000 years ago. It has curious surface shapes that look like fossil bacteria, and contains molecules that were clearly made in liquid water, as well as mineral grains and molecules that are similar to some made organically here on Earth. McKay cautioned that ‘none of these observations is in itself conclusive for the existence of past life’, but asserted that ‘when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on Mars’. This is a strong claim that needs stronger evidence.

  What of life in the Universe beyond the Solar System? There are few, if any, Earth-like planets among the thousands of exoplanets identified by our current technology, but stars are numerous, planets orbit around many of them, and there has been a lot of time for complex life to develop. So why are we not in contact with intelligent extraterrestrials from all these suitable planets? This contradiction is called the Fermi Paradox after the Italian-American physicist Enrico Fermi, who, speaking of extraterrestrials in 1950, asked ‘Where are they all?’

  Fermi’s question was posed expecting the answer that there have been no interstellar visitors, but there has been speculation about an object that, in 2017, visited the Solar System from interstellar space, passing within the orbit of Mercury. Named ‘Oumuamua, Hawaiian for ‘first messenger from afar’, it was our first known interstellar visitor, albeit an inanimate one. Probably a comet-like object from another planetary system, from its variability as it reflected sunlight it proved to be unusually long and thin, and its orbit was not exactly the orbit that it would have followed under the force of gravity alone. The extra force seems to have come from the push of solar radiation on the comet. However, the characteristics of the push suggest that the comet had the structure of an artificially constructed solar sail. This led some to claim that ‘Oumuamua was not a comet but an interstellar spaceship, indicating that intelligent extraterrestrials do live be
yond the Solar System.

  Although we have no firm evidence for it, we can assume that many places in the Universe have the right conditions and ingredients for life. In these circumstances, single-celled life seems to evolve fairly quickly: the oldest fossils of primitive life on Earth are stromatolites, mats of blue-green algae (cyanobacteria) fossilized in rocks as much as 3.5 billion years old. The algae thus represent what happened after the first 20% of the timeline of the history of the Earth. Simple life may well be abundant in the Universe.

  However, multicellular life forms and advanced intelligence need – crucially – time to evolve, and time of the right quality may be a much rarer commodity in the Universe than the right conditions and ingredients for single-celled life. Primates have evolved only in the last 85 million years, hominids less than 20 million years ago, and the earliest tool-making hominids 2 million years ago – less than 0.1% of the age of the Earth. Judging by the one example that we know, evolution needs to try out lots of experiments before it can come up with complex life, and requires a platform that is relatively stable for billions of years by way of a laboratory – a planet where temperature and climate change little, and where any sudden change is not too severe. The Earth has had several lucky escapes from catastrophic events. A freak accident (the creation of the Moon) gave the Earth an extra-large iron core and therefore a strong magnetosphere that defends it against the scouring and irradiating effects of the solar wind. The inner Solar System has also survived the migration of Jupiter-sized planets that rampage through other planetary systems.

  Your planet has survived all this and produced you. Other planets may not be so lucky, stable or fertile, so while simple life forms may be common in the Universe, complex, and therefore intelligent, life may be rather rare. The distances between the planets that have intelligent life, if there are indeed others, may consequently be so great that it is almost impossible for them to communicate with each other. This may be the explanation for the Fermi Paradox. We may like to take comfort in thinking that there is life elsewhere in the Universe, but we also have to face the possibility that, at least in practical terms, we are indeed alone.

  Glossary

  accretion The process by which an astronomical body increases in size, by gathering particles of matter to itself, either by means of its gravitational pull or by the adherence of particles with which it collides. The additional matter orbits the main body in a flat ‘accretion disc’.

  active galaxy A galaxy with a powerful and energetic nucleus (that is, an ‘active galactic nucleus’ or AGN).

  Algol paradox (In a binary star, such as the prototype, Algol) The anomaly that the less massive star is the more advanced in its evolution.

  ansae (Latin: ‘handles’) Ear-like protrusions from a celestial body.

  aperture synthesis interferometry A radio-astronomy technique in which a line of stationary radio telescopes depends on the rotation of the Earth to gather radio waves in a way that simulates the activity of a much larger radio telescope.

  asteroid A minor planet, of a size in the range of 1 metre to 1,000 kilometres, typically orbiting in the Solar System between Mars and Jupiter. An asteroid is not a dwarf planet, a comet, a meteoroid, or a moon or satellite. See also asteroid belt and Kuiper belt. Synonymous with minor planet.

  asteroid belt The main band of asteroids of the inner Solar System, lying between the orbits of Mars and Jupiter; compare Kuiper belt.

  aurora Light produced by atoms and molecules in the atmosphere and caused by the impact of ionized particles accelerated in a magnetic field. The term is applied especially to the Earth’s Northern and Southern Lights.

  Becklin–Neugebauer Object (BN Object) A source of intense infrared radiation found in the Orion Nebula and thought to be a newly formed star; it is named after the two astronomers who first observed it.

  Big Bang The dense, high-energy, explosive event that occurred at the beginning of the Universe.

  Big Crunch The hypothetical high- energy event that will occur if the Universe eventually implodes.

  binary In astronomy, a system composed of two bodies in orbit around each other: for example, a binary planet, binary pulsar or binary star.

  binary star A double star, particularly a close pair.

  bipolar nebula A symmetrical nebula that has two lobes, pointing in opposite directions; often, but not exclusively, a planetary nebula of that shape.

  black hole An astronomical body that is both small and massive, thus exerting such a strong force of gravity that no light or other radiation can leave the surface.

  blink comparator A viewing device used in astronomy to compare two photographs: the viewing optics switch (‘blink’) quickly between the two images to reveal changes, such as the successive positions of a moving star or planet, or the different appearance of a variable star.

  Bode’s Law (Titius–Bode Law) The law, popularized by Johann Bode (first formulated by Johann Titius), by means of which the distances of the planets from the Sun can be calculated.

  caldera (Spanish: ‘cauldron’) A volcanic crater, created when magma sinks from the subsurface, which weakens the surface so that it can no longer support the weight of the material above and the volcano collapses inwards.

  canals of Mars (from Italian ‘canali’) A network of long, straight markings apparently covering the surface of Mars; thought originally to be irrigation canals, they are, in fact, an optical illusion.

  catastrophism The theory that geological features are caused by unpredictable, large-scale events (catastrophes) such as floods or meteor impacts. Compare gradualism.

  celestial sphere (In Ptolemaic astronomy) The imaginary sphere centred on the Earth (or the observer); the fixed stars appear to lie on its surface.

  Cepheid variable star A pulsating variable star that varies in brightness in a regular cycle, as in the case of the star δ Cephei.

  Chandrasekhar mass The maximum possible mass of a white dwarf before it collapses under its own weight (to become a black hole); by extension, the maximum mass of a neutron star.

  chaos A mathematical effect: the property of certain equations that predict future behaviour (such as the weather or planetary positions), which are so sensitive to the initial conditions that even the slightest change in the starting point eventually produces a completely different outcome.

  chirality Asymmetry of the type in which the mirror image of an object cannot be superimposed on the object itself, no matter how the image is rotated. It is a property of the human hand, and of some of the molecules that are key to biochemistry.

  chondrite A type of meteorite made of ‘chondrules’ (near-spherical globules) and other material, which have not been subject to melting, as has happened, for example, in the interior of a planet.

  chromosphere (from Greek: ‘colour’ and ‘sphere’) The pink-coloured lower atmosphere of the Sun, visible at a solar eclipse; it extends to a height of about 2,000 kilometres and has a temperature of up to 20,000 K.

  circum-nuclear disc The outer part of an accretion disc in orbit around the nucleus of an active galaxy.

  CNO cycle The nuclear fusion cycle in massive stars, in which carbon, nitrogen and oxygen nuclei progressively combine with protons and eject alpha particles, thus turning hydrogen into helium.

  coma (Latin: ‘hair’) The ‘atmosphere’ surrounding the solid nucleus of a comet; viewed through a telescope, the coma makes the comet look fuzzy, as if it has hair.

  comet A small body in orbit in the Solar System; a comet resembles an asteroid but is made of icy material, the surface of which melts, surrounding the nucleus with a coma and producing a tail.

  continuous creation A (no longer widely accepted) theory that material is continuously produced in space at such a rate as to ensure that the density of matter in the Universe is constant, even though space is expanding.

  Copernican theory The heliocentric theory, attributed to Nicolaus Copernicus.

  core The innermost region of a planet,
usually a dense sphere. The core of a terrestrial planet is surrounded by the mantle, or outer solid layers.

  Coriolis force The apparent force (named after Gaspard-Gustave de Coriolis) that deflects objects moving on a rotating frame. Gives rise to the Coriolis effect – for example, in the Northern Hemisphere, winds are deflected to the right because the Earth is rotating.

  corona (Latin: ‘crown’) The atmosphere of the Sun above the chromosphere, revealed in a solar eclipse.

  cosmic fireball The extremely intense radiation that occurred at the Big Bang, the relic of which is the Cosmic Microwave Background.

  Cosmic Microwave Background (CMB; cosmic background) An almost uniform source of infrared and microwave radiation, of cosmic origin (see cosmic fireball).

  cosmic rays High-energy particles from the Sun and the Galaxy that permeate outer space.

  Cosmological Constant A term added by Einstein to his General Relativity equations describing the repulsive force between galaxies in the Universe to counter the gravitational force and to prevent the Universe from collapsing.

  crater A hole in the ground caused by an explosion or impact.

  cubewano A type of object in the Kuiper belt that orbits undisturbed beyond Neptune, in the same plane as the major planets.

  dark ages In the early history of the Universe, the period after the Big Bang but before galaxies became visible.

  dark energy (vacuum energy) A hypothetical constituent of the Universe, which, released gradually into space, causes the expansion of the Universe to accelerate.

  dark matter A hypothetical, unseen constituent of the Universe, which produces a gravitational attraction on galaxies and stars.

  dark nebula A nebula of interstellar dust that makes its presence known by absorbing light from the stars that happen to lie behind it.