An alternative explanation is suggested by the expectation from particle physics that completely empty regions of space can still produce a “vacuum energy.” It is thought that pairs of particles and their associated anti-particles are continually created out of the vacuum, only to almost immediately annihilate each other and disappear. As they do so, they produce a minute outward pressure. Averaged over the entire voids of the universe, there would be sufficient such “vacuum fluctuations” to produce enough pressure to push the universe further apart. Since this is a property of empty space, as the universe grows larger and the voids grow bigger, the effect of vacuum energy thus becomes increasingly dominant over that of gravity. However, a better understanding of particle physics is required before the vacuum energy theory can be shown to be a reality—current estimates suggest it would be much more powerful than observed even in our accelerating universe. Another rival for dark energy is the suggestion of a new force field which goes by the name of quintessence. Scientists are debating a rigorous mathematical description of quintessence, but it may be a force whose strength and importance changes through the history of the universe. It could be linked to the very early inflationary period of expansion thought to have occurred immediately after the initial Big Bang; if quintessence then lay relatively dormant for a period, we could be in another active phase producing the current accelerated expansion.
Whatever the true nature of the dark energy, if it continues to exert its influence we can speculate on a bleak future for our universe. The galaxies will continue to fly further and further apart ever faster, perhaps leading to an eventual “Big Chill.” Even worse, if the effects of dark energy become increasingly important, it may begin to dominate over gravity on smaller scales, such as within galaxies—leading to a “Big Rip.”
23
PLANETS, MOONS AND THE SEARCH FOR LIFE
The story of creation does not end with the birth of the stars. Indeed, for many other bodies in the universe, the Big Bang was just the start. Until some stars had been born, and eventually died, there could be no planets. And until there were planets there could be no life on Earth.
At the present time we have no proof that any form of life exists in the universe other than on our own planet. But before we start hunting for planets that may support life, we must understand how the Earth itself was formed. The first stars, formed hundreds of millions of years after the Big Bang, had no solid rocky planets orbiting around them. It was not until some of the stars had ended their lives with massive supernovae explosions that the space dust and the atoms of the heavier elements appeared in abundance throughout the galaxies. Only then could the rocky planets be created. A star and its accompanying planetary system will have formed from within a giant molecular cloud which eventually collapses under gravity, and after a few million years will reach pressures and temperatures sufficient to ignite nuclear fusion; a young star has formed at the core of the cloud. But the young star does not comprise all the material in the cloud. During its formation it is surrounded by a “proto-planetary” disc that is also in the process of gravitational collapse subsequently to form the planetary system.
Individual planets form by a process called gravitational accretion. As the proto-planetary disc cools, particles of dust condense and form. As they pass close to each other, they are pulled together by their tiny gravitational attraction, until after a few millennia larger particles form. The larger lumps of matter—called planetesimals—are better able to attract additional particles and are thus more likely to grow than the smaller masses. The larger and more massive bodies continue to accumulate space debris and grow steadily. After more millennia of accumulation some have become a proto-planet, with a mass the size of a planet. Our solar system is an excellent example of planetary formation. Each early planetesimal had its own ring of space where it could gravitationally capture any smaller particle and grow a little larger. The exception is at the region of the asteroid belt, where the strong gravity of the proto-planet Jupiter disturbed the gravitational accumulation of the debris, preventing the formation of a single larger planet.
Solar Systems
After this evolutionary period, some of the stars had evolved planetary systems orbiting around them. Within these systems, planets known as gas giants—like Jupiter and Saturn in our own solar system—have been commonly identified. As yet, the smaller, rocky terrestrial planets are proving much harder to discover. Once a sizeable planet begins to form it can capture most of the matter within several million miles of its own orbit. Some of this matter itself condenses to form satellite moons around the proto-planet. We see very complex moon systems around all of the very massive gas giants but few in orbit around the inner rocky planets. The larger moons are true satellites that originated in the initial collapse; they are spherical in shape and they have orbits that are nearly circular. However, many other moons are smaller and irregularly shaped and often have very elongated orbits. They are not true satellites, but rather captured asteroids and comets that have succumbed to the gravitational field of the planet much later. Saturn has a spectacular ring system. Jupiter, Uranus and Neptune have fainter and less striking ring systems. Seen close up, the rings are found to be the remains of moons that have been broken into small pieces by the tidal forces of the planet.
How the Moon Was Formed
The Earth’s moon (which we call “the Moon”) is very large compared with the size of our planet. It also has a much more complex and violent history than any of the other moons in the solar system. It is possible for a planet the size of the Earth to capture small objects to be held as satellites in orbit around it, such as we see in the case of the two small moons of Mars, but it is impossible for the Earth to capture a passing object as large as the Moon and to retain it in orbit. Over the years many theories have been suggested about the formation of the Moon. One suggested that the Moon had somehow broken off from the Earth, leaving a twin planetary system with both planets orbiting about their common center of gravity. The truth is much more complex. At one time there were two planetesimals competing with each other for the matter in the space between Venus and Mars. The one destined to become the Earth was the larger and more successful, but the second planetesimal still managed to attract a substantial amount of the matter.
The two planetesimals both had elliptical orbits around the Sun. They avoided each other for millions of years, but then, billions of years before life began on the planet, a catastrophic collision took place between them. The impact was so great that the orbit of the Earth was considerably changed by the collision. The proto-Earth was greatly deformed, as the heat generated in the giant impact made the matter in the planetesimal molten and fluid, and a huge quantity of matter was thrown out into space into an orbit around the Earth. The other planetesimal disintegrated after the collision, except for its core, which had adhered to the proto-Earth during the collision.
After a time the two orbiting lumps of matter regained their spherical shapes and they evolved to become the Earth–Moon system. It happened that the Earth was very much the larger body. It is interesting to speculate what would have happened if the Moon had been larger and a twin planetary system had formed. Could this have created two life-supporting planets close to each other? What is not in doubt is that the tidal forces on the Earth would have been enormous, and a very different planet would have evolved. We know that the tidal forces of the Moon have played a major part in the evolution of life on our planet. It is also likely that the impact changed the Earth’s axis of rotation to create the angle of the ecliptic—and therefore our familiar seasons.
The newborn Moon was so hot after the great collision that it remained molten for several millennia. As it cooled down, lakes of lava began to form on its surface and these eventually solidified to leave a crust. About four billion years ago the Earth and the Moon were subjected to a great barrage of debris from space, forming large craters on their surfaces. The heavily cratered lunar surface is testimony to
this violent pounding. The Earth, however, although heavily scarred as well, was afforded some protection by the atmosphere and, over time, the weather has eroded the craters.
Are We Alone?
There is one major disappointment that has come about just because we now know so much about the solar system. Many different environments in the solar system have been discovered and explored, but there has been no positive sign of life anywhere other than on our own planet. There is one intriguing find in the form of the SNC meteorite discovered in Antarctica. It has been identified as a small piece of the planet Mars, thrown into space by a massive impact over a billion years ago with such velocity that it escaped the gravitational field of Mars and eventually landed on Earth. There is evidence to show that the rocky chunk had once been exposed to water and there is also evidence of fossilized primitive bacterial life, but scientists think the exposure to water was on Earth and not Mars. The Mars Global Surveyor and the Mars rovers Spirit and Opportunity have mapped the surface of Mars in detail. There is little doubt that at one time the planet had a much warmer climate and flowing water.
There is still a chance of finding primitive life forms, and the search for microscopic life continues. There has been speculation that some of the moons of the outer planets, in particular Saturn’s moon Titan that is known to harbor complex organic compounds, could be suitable sites for life but they need a much warmer environment.
As early as 1952 those hoping to find evidence of life elsewhere in the solar system received encouragement when the American scientists Stanley Miller (1930–2007) and Harold Urey (1893–1981) performed a classic experiment with the simplest of laboratory equipment. They showed that in a closed container, using heat and electric sparks to simulate lightning, simple chemical elements such as hydrogen and nitrogen with molecules of water and carbon dioxide can combine to form organic molecules. Later experiments along the same lines have produced a wide variety of organic compounds. It is safe to conclude that the DNA molecule, and therefore life itself, could form under primitive Earth-like conditions.
New Directions
The space age has provided many new directions in which to take astronomy. The skies are being mapped in almost every frequency of the spectrum and in more detail than ever before. Objects once studied only through theory, such as black holes and planetary formations, are now the focus of intense observational study. We have close-up images of planetary surfaces, moons, and asteroids and comets on their journeys around the Sun. There are pulsars like the one in the Crab Nebula, the remains of an exploding supernova where the heavy elements are synthesized. There are countless galaxies from the nearby Andromeda Galaxy to distant active galaxies harboring at their core black holes with a mass of over a billion suns. We can now observe so far back in the history of the universe that we will soon be able to see the first galaxies shortly after they formed, and we can map the distribution of galaxies throughout the firmament into the giant clusters and superclusters, themselves gathered to form the large-scale “walls” around the void.
Humans have been on Earth for a few million years, and our recorded observations of the universe go back only a few thousand years. We have seen how humans have always looked with wonder at the skies. We have seen how the uncovering of the secrets of the universe has gradually taken place. This knowledge has been passed on to future generations. We are the fortunate people that now inherit this knowledge, and we are able to appreciate the origins of the universe far better than our ancestors. We know that there is far more for us to discover in the skies, and that every generation will add new knowledge. It is important to remember also that we are the custodians of the Earth and we must look after it. For all we know, our fragile world may be unique in the universe.
GLOSSARY
Words in SMALL CAPITALS refer to other entries in the Glossary.
absolute magnitude The magnitude a star would have if it were located at ten PARSECS from the Earth.
accretion The gradual accumulation of matter by an astronomical body, usually by gravitation.
active galactic nucleus A GALACTIC NUCLEUS giving out strong emissions in the ELECTROMAGNETIC SPECTRUM.
active galaxy A very luminous GALAXY, usually containing an ACTIVE GALACTIC NUCLEUS.
aphelion The point of a planet’s or comet’s orbit at which it is farthest from the Sun. Opposite to PERIHELION.
apogee The point in the orbit of the Moon, or of any planet, at which it is at its greatest distance from the Earth; also the greatest distance of the Sun from the Earth.
apparent magnitude A measure of the brightness of a stellar object as seen from Earth.
asteroid A rocky object over a few hundred meters in diameter orbiting the Sun.
atmosphere The sphere of gases surrounding the Earth or any celestial body.
aurora Light radiated by atoms and IONS in the Earth’s upper atmosphere.
azimuth An arc of the heavens extending from the ZENITH to the horizon, which cuts it at right angles; the quadrant of a great circle of the sphere passing through the zenith and NADIR.
barred spiral galaxy A SPIRAL GALAXY but with the spiral arms attached to a bar running through the nuclear bulge.
Big Bang The event that created the universe about 13 billion years ago, creating space, time, energy and matter.
binary star Sometimes called a double star. Two STARS that revolve around each other. They are held together by the force of their mutual GRAVITY.
black hole A body with such a strong gravitational field that light cannot escape from it.
blazar A type of ACTIVE GALAXY with very powerful emissions.
celestial object Any object visible in the night sky.
celestial sphere The whole of the night sky mapped onto a sphere.
cepheid variable star A pulsating yellow SUPERGIANT star used to calculate stellar distances.
cluster of galaxies A collection of a few hundred to a few thousand GALAXIES held together by their own gravity.
coma A spherical diffuse cloud of gas seen around the nucleus of a COMET near the Sun.
comet A small body of ice and dust in orbit around the Sun. The ice vaporizes near the sun giving rise to a characteristic tail.
conjunction The lining-up of three or more bodies. For example Earth–Venus–Sun, which gives rise to the transit of Venus across the Sun.
cosmic background radiation (CMR) The radiation from the primordial fireball known as the BIG BANG that fills all space.
cosmic ray High-speed particles traveling through space.
dark energy A repulsive gravitational effect causing the UNIVERSE to expand outward.
dark matter Undetected missing matter from the universe with as yet unknown properties.
declination The angular distance of a heavenly body (north or south) from the celestial equator, measured on a meridian passing through the body. It corresponds to latitude on the Earth.
dwarf star Any star smaller than a giant, e.g. MAIN SEQUENCE stars and WHITE DWARFS.
eclipse The blocking of all (total eclipse) or part (partial eclipse) of the light from one celestial body by another.
ecliptic The plane of the Earth’s orbit extended to infinity from the Sun. So called because eclipses can happen only when the Moon is on or very near this plane.
electromagnetic radiation A very wide range of radiation including gamma rays, X-rays, the optical spectrum, microwaves and radio waves.
electromagnetic spectrum The whole array of possible electromagnetic emissions.
electron An atomic particle with negative charge, usually found orbiting an atom.
elliptical galaxy A galaxy that is elliptical in shape with no spiral arms.
emission nebula A gaseous NEBULA, glowing by the light from a nearby star.
ether A medium believed to occupy the whole of space, carrying light as a wave motion.
Euclidean geometry The classical geometry of space as described by Euclid in the ancient world.
&nb
sp; event horizon The boundary of a BLACK HOLE.
false-color image An image, usually outside the optical part of the spectrum, showing the radiation in false colors.
galactic disc A disc of gas and dust surrounding the nucleus of a galaxy.
galactic nucleus The central part of a galaxy inside the nuclear bulge.
galaxy An assembly of young stars, gas and dust kept together by their mutual gravity.
gas giant A star with a radius of 10 to a 100 times that of the Sun.
geocentric The system of the world with the Earth as the center.
gravitational energy The energy of a gravitational field such as is found around a BLACK HOLE.
gravity The property that all matter has an attraction for all other matter in the universe.
heliocentric The system of the universe with the Sun at the center.
Hertzsprung–Russell (H–R) diagram A diagram plotting the absolute magnitude of stars against their surface temperature or spectral class.
Hubble constant (H0) A constant relating the distance of a galaxy to its speed of recession. The reciprocal of H0 determines the age of the universe.
infrared Radiation with a wavelength greater than the red end of the visible spectrum.
intercluster medium Gas and dust between the galaxies in a cluster.
intergalactic medium Gas and dust between neighboring galaxies.
interplanetary medium The gas and dust in the space between the planets.
The Story of Astronomy Page 25