From dream to reality

Are there inhabited worlds elsewhere in the Universe? The question is as old as humanity. We can trace such debates back to antiquity, in texts written by Greek philosophers such as Epicurus (341–270 BCE) in particular. At the time of the Copernican revolution, a new dimension was reached, this time more on astronomical and physical grounds: since the Earth was no longer seen as the centre of the Universe, other planetary systems could exist around other stars. Giordano Bruno (1548–1600) was among the first to express his support for this new astronomical theory, in opposition to the Catholic church, a conviction for which he paid with his life. Many scientists such as Galileo (1564–1642) and Huygens (1629–1695) supported this hypothesis. Closer to our times, philosophers such as Fontenelle (1657–1757) and Kant (1724–1804), scientists such as Laplace (1749–1827) and later Flammarion (1842–1925) raised the question of the plurality of worlds.

The search for planets around other stars – also called ‘extrasolar planets’ or ‘exoplanets’ – did not start in earnest, however, until the twentieth century, because of our inability to observe them. Indeed, it is extremely difficult to detect the intrinsic visible light of such a planet, hidden in the blinding brightness of its host star, which is about ten million times brighter. Imaging extrasolar planets directly, in a few very favourable cases, has only become possible during the past decade, thanks to the development of techniques such as coronagraphy (which blocks light from the centre of a telescope in order to image the fainter surroundings) and adaptive optics (see Subsection 2.4.3). During the twentieth century, indirect methods had to be developed. The idea is the following: the light of the exoplanet is too weak to be detectable, but the presence of the planet induces a small motion of the host star around the centre of gravity of the combined star–planet system. The first method used by astronomers to detect this motion was astrometry, the measurement of stellar positions relative to their background. It was successfully applied by Bessel (1784–1846) who first detected a low-mass companion around Sirius A, the brightest star in our skies. The companion turned out to be a white dwarf, named Sirius B. A century later, the same technique was used to search for exoplanets.

The search for habitable worlds in the Universe entails our understanding of the conditions in which life appeared, survived and developed on Earth. This understanding has been growing consistently since the first geological, atmospheric, oceanographic and biological studies. As stated in The Limits of Organic Life in Planetary Systems, put together by the Committee on the Origins and Evolution of Life of the National Research Council (NRC, 2007):

Our planet is not blessed everywhere with conditions favourable to human life, but in spite of the harsh and extreme chemical and temperature ranges that living species have to deal with, we have proof today that life thrives on Earth wherever liquid water and energy sources are available. However, other lifeforms may well exist, as has been suggested by some scientific studies. In what follows in this chapter we try to give an overview of terrestrial life and what it requires, touch upon other possibilities and focus on the environmental conditions necessary for the sustainability of life of the standard definition (Earth-like), before we begin our trip across the Solar System and elsewhere in quest of habitable places.

Looking for habitable conditions in the outer Solar System leads us to the natural satellites rather than the planets themselves. Although the theoretical conditions under which life might be sustained on natural satellites are similar to those of planets, there are key environmental differences which can make moons of particular interest in the search for extraterrestrial life. The gaseous giant planets cannot provide even the minimal conditions of a surface or interior with suitable pressures and temperatures to sustain life. But the moons around these planets offer a great range of possibilities for exploring habitability conditions and furthermore studying the question of the emergence and evolution of habitable worlds in our Solar System, in some cases more so than any other object closer to the Sun. Scientists generally consider the probability of life on natural satellites within the Solar System to be remote, though the possibility has not been ruled out.

Within the Solar System’s traditional habitable zone, the only candidate satellites are the Moon, Phobos and Deimos, and none of these has an atmosphere or water in liquid form. But, as discussed in Chapter 2, the habitable zone may be larger than originally conceived. The strong gravitational pull caused by the giant planets may produce enough energy to sufficiently heat the cores of orbiting icy moons. This could mean that some of the strongest candidates for harbouring extraterrestrial life are located outside the solar habitable zone, on satellites of Jupiter and Saturn. The outer Solar System satellites then provide a conceptual basis within which new theories for understanding habitability can be constructed. Measurements from the ground and also from the Voyager, Galileo and Cassini spacecraft have revealed the potential of these satellites in this context, and our understanding of habitability in the Solar System and beyond can be greatly enhanced by investigating several of these bodies together.

Preface

Life in space, whether strange beings on distant worlds, or an expansion of our own species into the Solar System and beyond, is a very exciting idea. Humankind may currently stand on the verge of major discoveries and exciting progress in both areas. The discoveries of possibly life-related artefacts in a Martian meteorite, in a subsurface ocean on Europa, Titan or Enceladus, and in the atmospheres of extrasolar planets, for example, show how close we are to finding out at last whether the life that teems on our own planet is unique. Some increasingly sophisticated space missions are currently under way, such as Cassini, which has been exploring the Saturnian system and Titan, the Earth-like moon, since 2004; others are in preparation, such as the Mars Sample Return and the Jupiter Icy Moons Explorer missions. Plans to return to Venus, Mars, the Moon and Titan, to orbit Europa and to place giant planet-seeking telescopes in space are thus on the table. These and other advances promise rapid progress in the coming years.

This is a book that deals with possible habitats in our Solar System and beyond. We will define which places might be harbouring past, present or future life, or can be considered as ‘habitable’ in the sense that human life could survive, adapt or continue to evolve therein. The book will include a necessarily brief but pertinent definition of life as we know it on Earth and review it as a phenomenon, considering its origins, properties and potential; we will combine a discussion of present knowledge with informed speculation, bounded by scientific realism but using non-technical language. We will briefly review the origin of life in the Universe, the reasons for thinking it may be unique and reasons, in contrast, for believing it could be commonplace. We will also offer some thoughts on its destiny and on scientific discoveries yet to be made in areas we can barely apprehend at present. The main goal is to update the reader on the current situation in our Solar System and beyond, in terms of exploration for traces of past or present life and of the existence of conditions for habitable worlds. We also aim to provide and provoke thoughts about our distant horizons in this respect.

Exploring the Solar System remotely and in situ

Our exploration of Solar System habitats will continue with our usual astronomical means: ground-based telescopes, space observatories and in situ missions. Following the present generation of 10-metre class telescopes, astronomers are now working on the next step: a 30–40-m telescope. Three projects are currently being studied, two in the United States and one in Europe. On the American side, the GMT (Giant Magellan Telescope), made of seven 8-m telescopes, to be installed at Las Campanas in Chile, will reach an equivalent diameter of 21 m; the Thirty Meter Telescope (TMT), to be installed at Mauna Kea Observatory in Hawaii, will consist of a primary mirror composed of 492 hexagonal segments of 1.45 m diameter. Finally, the European ELT (E-ELT; Figure 6.1), to be installed at Cerro Armazones in Chile, will reach a diameter of 39 m by using about 800 hexagonal elements of 1.45 m diameter each. The first light of the E-ELT is planned for 2021. The E-ELT will be of special interest for the spectroscopy of transiting exoplanets, but will also be very useful for Solar System exploration, in particular the study of comets and trans-Neptunian objects.

As a follow-up to the HST, the New Generation Space Telescope, now renamed the James Webb Space Telescope (JWST; Figure 6.2) is built by NASA in partnership with ESA. This 6-m diameter telescope will be dedicated to infrared astronomical observations, from 0.6 to 28 μm. The spacecraft will be located at a special orbital position known as the L2 Lagrangian point of the Sun–Earth system, beyond the Earth on the Sun–Earth axis, a stable position much favoured by astronomical spacecraft. Its main scientific objectives are cosmology and exoplanets, but will also be a prime tool for Solar System exploration, especially for small bodies in the outer Solar System. It is expected to be launched in 2018 on an Ariane 5 rocket.

The search for life in the Universe, from theoretical concept to actual exploration, has never ceased to interest and amaze humanity. After the first ideas had arisen on cosmology (the structure of the cosmos) and cosmogony (its creation), early civilizations and philosophers turned their minds towards living beings and how they came to be. Once some basic principles had been set – for instance in the biblical book of Genesis or in Hesiodos’ Theogony, which both basically define the creation of Earth and Heavens from nothing (or Chaos) – the first ‘scientific’ minds set to work all over the world, and new ideas were sparked in Egypt, in the Indies, in the Americas, in China and in Europe. Thus, in Greece for instance, Aristarchus conceived the idea of the heliocentric Solar System; Eratosthenes proved that the Earth was spherical and determined the distance to the Moon; and Anaximander had a structure worked out for the whole Universe.

Some of the early thinkers had already advocated a Universe consisting of ‘many worlds’. Thales, from Miletus, and his students in the seventh century BCE argued for a Universe full of other planets, teeming with extraterrestrial life. They also proposed the idea with which we are all familiar today (through Drake’s equation, Carl Sagan’s musings, and the contributions of many other scientists): that a Universe so full of stars must also have a large number of populated worlds. This proposal was defended by Epicurus and other Greek atomists who countered the geocentric models put forward later by Aristotle. In the cosmogony developed by Plato’s famous disciple, the mythological separation of Earth from the Heavens was put into more modern words and widely promoted, as was his geocentric perception of the cosmos and the limited and well-defined sphere of stars in which matter and space were confined and interconnected. Aristotle’s philosophical attempt at modern physics took strong roots, caused the ancient open-minded theories to be forgotten and hindered scientific progress in this domain for quite a long period. The Copernican revolution in the sixteenth century gave a boost to the concept of life’s emergence and possible existence elsewhere in the Universe, because Earth was no longer the privileged and unique place where this could occur.

In the original definition of the habitable zone, its boundaries first encompassed the orbits of Venus to Mars, planets close enough to the Sun for solar energy to drive the chemistry of life – but not so close as to boil off water or break down the organic molecules on which life depends. These planets and their neighbourhood have been observed from the ground since the earliest times and explored in situ since the beginning of the space age. What have we found so far?

Although they are all members of the terrestrial planet family, Mercury, Venus, the Earth and Mars have very distinct properties. These differences are basically the result of two factors: their heliocentric distance (and hence their temperature) and their mass. Mercury, Venus and Mars, all easily visible with the naked eye, have been known since antiquity and celebrated in every mythology. But their analysis as physical objects has mostly developed since the beginning of the twentieth century, with the advent of spectroscopy and photography. A new era started in the 1960s with the advent of space exploration. The space adventure, marked with many failures, continues today. Two orbiters, Messenger and Venus Express, are currently observing Mercury and Venus, respectively; several orbiters are operating around Mars and a sophisticated rover, Curiosity, is exploring the nature of the Martian surface.

Introduction

Water is known to be ubiquitous in the Universe, from the dark spots of the Sun up to the most distant galaxies. It is also a major component of Solar-System objects, especially in the outer parts beyond heliocentric distances of a few astronomical units (one astronomical unit = AU = average Earth–Sun distance = 149.6 × 106 km). It should be mentioned, however, that the Earth is the only place in the Solar System where water can be present in its three states: solid, liquid and vapour. So far, outside the Earth, water has always been found in the form of vapour or ice, although there are some indications that liquid water might be – or have been – present elsewhere in the Solar System. Liquid water was probably present in the past on the surface of Mars and also possibly Venus; it might currently exist in the interiors of some satellites of giant planets.

The presence of large amounts of water, vapour or ice in the Universe is a natural outcome of the large cosmic abundances of the hydrogen and oxygen elements which form the water molecule. In addition, the large abundance of water in the outer Solar System is also a natural consequence of the formation scenario of the Solar System, which led to the accretion of two classes of planets, the terrestrial and the giant ones, separated by the ‘snow line’, which basically corresponds to the heliocentric distance of water condensation in the primordial solar nebula.