On any clear night you will see a faint luminous band arching across the night sky – our galaxy, the Milky Way. To the ancient Chinese and Arabs it was a river in the sky. It was a roadway to the Anglo-Saxons, an avenue to Valhalla for the Norse and spilt milk to the Greeks. (It is the Greek word for milk- gala - that is the root of galaxy.) Modern technology has enabled us to map out our galaxy in unprecedented detail, giving us an accurate picture of the Earth's place in this huge star city.
Imagine two fried eggs stuck back to back ! This relatively simple picture describes the essence of our galaxy. It is a rotating disk of gas, dust and stars with a central bulge. Seen from above it resembles water vanishing down a plug hole. It contains roughly 200 billion stars, divided into two populations. Most of these stars inhabit the spiral arms and tend to be young. These are Population I types. The stars at the centre tend to be older - Population II types. This means the spiral arms have a bluish tinge, whilst the core is more red. It is truly vast. If the solar system were the size of a saucer, the next star would be a city block away and the whole galaxy would be the size of North America !
To comprehend these distances we have to use a different measure of distance - light travel time. Light is the fastest thing we know - nature's "top speed". It travels at 186 282 miles (299 000 km) per second in perfectly straight lines. It takes light 1.25 seconds to get to the Moon; 8.3 minutes to reach us from the Sun and Pluto is 4.5 light hours away. Radio signals also travel at the speed of light in space, and so any course correction message to a probe in the vicinity of Pluto would have to be sent 4.5 hours before the actual manoeuvre. A telephone conversation with a Plutonian would be an unbelievably dull affair! Even at this incredible speed, light would take 4.3 years to reach the nearest star - Proxima Centauri. In 1 year, light travels 6 trillion miles (9.5 trillion km). Polaris is 650 ly. away. The centre of the Milky Way is 30 000 ly. away and the entire galaxy is about 100 000 ly. across. Its disk is roughly 1500 ly thick. [incidentally, a more common unit of distance among astronomers is the parsec (pc.). This is the distance at which 1 AU (astronomical unit or Earth-Sun distance) subtends 1 second of arc and is roughly equal to 3.26 ly, so most of the Sun's neighbours are about a parsec apart.]
Doppler measurements of nearby stars show that the Sun is moving towards Cygnus and away from Canis Major with a velocity of 230 km/s. This is due to the Sun's rotation in its spiral arm. A simple calculation yields a rotation period of 230 million years. However, different parts of the galaxy rotate at different speeds, the inner stars moving faster than the outer stars - so called differential rotation.
As well as the bright stars that dot our night sky, the galaxy contains clumps of both dark matter and luminescent matter, the latter divisible into nebulae and star clusters.
The space between the stars is not empty, rather it contains a thinly distributed fog of gas and fine dust. This is called the interstellar medium. This medium can be subdivided into four types:
Molecular Clouds - These are cold (10K), dense gas clouds containing many molecules. They are usually seen as dark regions obscuring background stars e.g. the Horsehead nebula in Orion or the Cone nebula in Monoceros. Many of these clouds contain organic molecules like CN, formaldehyde and even methylamine and formic acid, the "building blocks" of life. Hoyle and Wickramasinghe have even tried to match the spectrum of the larger particles in these clouds to freeze dried bacteria and have used this as evidence of their panspermia hypothesis – that life originated elsewhere, permeates the space between the stars and has come to Earth on the back of comets.
HI regions (pronounced "H one") - These are the most common regions in the galaxy and consist of neutral hydrogen at about 100K. They are transparent to most light, but emit radio waves at a wavelength of 21cm when the hydrogen atoms change their spins. This is the characteristic "hydrogen line" so favoured by early SETI scientists.
HII Regions (pronounced "H two") - Near the hotter blue/white stars, the hydrogen gas is strongly heated to about 10 000 K when it becomes ionised or stripped of electrons. When these electrons recombine they emit light of a characteristic frequency. For hydrogen this is mainly red (hydrogen alpha line). This process gives rise to emission nebulae, the most famous of which is M42 - the Great Nebula in Orion. The Trifid Nebula in Sagittarius is another fine example. The stars responsible for exciting the gas have short life times, so these types of nebulae are found surrounding stellar nurseries. A special subclass of emission nebula, called a bubble, occurs when a star near the end of its life throws off a halo of gas and excites it. M57, the Ring Nebula, in Lyra is a classic example of this type of nebula (designated planetary, because it looked like a planetary disk through early telescopes).
Superbubbles - In this type of region, the gas is heated to 1 million K by a local supernova explosion. This hot gas expands into the cooler surrounding gas creating a high velocity bubble of excited atoms. The Crab supernova remnant in Taurus is an example of a superbubble, as is the Vela remnant and the X-ray bubble called Cas-A in Cassiopeia.
Dust - The violent deaths of massive stars seed the interstellar medium with many heavy atoms, complex molecules and dust grains. Atoms and molecules tend to absorb starlight at discrete wavelengths, giving rise to absorption lines in the spectrum. Interstellar dust grains have a different effect. They cause scattering of light in a manner very similar to the way dust and small particles in our atmosphere scatter sunlight to give us a blue sky and red sunsets. Red light from distant stars can penetrate further through the dust than blue light, so all distant stars are affected by interstellar reddening. Radioastronomers need to take this into account when choosing suitable transmission frequencies.
The majority of stars that we observe are part of multiple systems, usually binaries, but sometimes containing many more components. Castor in the constellation Gemini is a good example. It is actually a 6 star system. There are basically three types of clusters:
Open Clusters - These are fairly loose groupings within the galaxy, numbering between 100 and 1000 stars and ranging in diameter from 15 to 70 ly. They tend to be young, blue stars, often still surrounded by the gas clouds that spawned them. The best naked eye examples are the Hyades and Pleiades, both in Taurus and Praesepe, the Beehive Cluster, in Cancer. Other fine open clusters visible in binoculars are M35 in Gemini, M36, M37, M38 in Auriga and the "Double Cluster" in Perseus. There are many more catalogued by Charles Messier in his comet hunters guide. The nearest open cluster is the Ursa Major cluster, of which the Sun is a part. All the stars in the Plough, bar two, are moving through space at the same speed and in the same direction.
Associations - These are larger in size than open clusters and tend to possess fewer stars. They may number 10 to 100 stars, but be as much as 300 ly wide. Again these are distributed within the plane of the galaxy and tend to be made up of younger blue stars.
Globular Clusters - These are different from other clusters for two reasons: Firstly they contain many more stars, typically 100 000, although some contain several million. These stars are arranged in a symmetrical sphere that becomes very densely packed towards the centre, so much so, that large telescopes cannot resolve their core stars. Secondly they are way outside the disk of the galaxy. Measurements made in the 1920s established that there are about 150 globular clusters distributed in a vast halo about the centre of our galaxy. The furthest globulars are 50 000 ly from the centre. In fact, this knowledge helped to pinpoint the centre of the Milky Way long before radio telescopes were able to peer through the dust obscuring the galactic core to visible astronomers. Globular clusters are some of the oldest objects in the galaxy, most of their stars are first generation (14 billion yr.) and red. The best examples are M13 in Hercules - a naked eye/binocular object comprising 100 000 stars and lying 42 000 ly away, M5 in Serpens, M15 in Pegasus and 47 Tucanae in the southern hemisphere.
1959 heralded a turning point in the scientific search for life on other worlds, for in that year the modern SETI era began. It began in fact with an article in the journal Nature written by two Cornell physicists Giuseppe Cocconi and Philip Morrison. This article was devoted to the possibility of using microwave radio to communicate between the stars. A young radio astronomer named Frank Drake had independently reached the same conclusion whilst working at the National Radio Astronomy Observatory. He formulated an equation that listed all the relevant factors contributing to the rise of an intelligent communicative civilisation within our galaxy. This formula has now become known as the Drake Equation. Although his estimates of upper and lower bounds are considered naive by today’s standards, it is still a standard approach used to justify any SETI programme. One current version of this equation is:
N = Rs x fP x nh x f L x fI x fC x L
where N is the expected number of civilisations at any one time in a typical galaxy (like the Milky Way) and is given by the multiplying factors:
The galaxy’s 200 billion stars come in a variety of luminosities, ages and sizes. These stars are classified according to their spectral type, which correlates well with their temperature. These spectral types fall into the categories O, B, A, F, G, K, M (remembered by the mnemonic Oh Be A Fine Girl and Kiss Me). The hottest bluest stars are class O, whereas the coolest, reddest stars are M. The Sun is a fairly average yellow star in class G. Stars have life cycles where they are born from condensation of the interstellar medium. This contraction process often gives rise to multiple star systems. In fact as many as 80% of all stars may be part of such binary systems, but more conservative estimates based on observation give this at 50%. For reasons given later on, multiple star systems are not very good candidates for planetary systems. Once the stars have "switched on" and settled onto the main sequence their lifetimes depend very sensitively on their masses. Heavy stars squander their fuel and exhaust their reserves in a few hundred million years, whereas Sun like stars last for billions of years. Stars that are stable for long periods of time are a necessity for the evolution of life if our own record is anything to go by. Bearing this in mind we should be looking at the rate of formation of single stars with classes F, G and K (M class stars are thought too small to have habitable zones). There is a stellar pyramid of abundances, where O,B and A stars represent the very tip and account for less than 1%, while smaller, cooler stars form the bulk of the pyramid (70%). Classes F,G and K however account for some 1, 4 and 10% respectively of the galactic population.
Current astrophysical modelling and observation gives this as approximately 1 suitable star per year.
The solar system consists of one G2 class star of middle age, eight and a half major bodies - the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn (all known to the ancients), Uranus (1781 Herschel), Neptune (1845 Leverrier/Couch-Adams), Pluto (1930 Tombaugh), the moons of these planets, swarms of rocky debris known as asteroids or minor planets, large chunks of dirty ice known as slow moving objects (SMOs) and comets and a general distribution of gas and dust. With the exception of the comets, all of these bodies are constrained to move within a narrow disk, and with few exceptions all move in an anticlockwise direction as seen from "above" (the Earth’s North Pole).
This is a vital clue to the origin of our solar system. The oldest rocks in our possession come from space in the form of meteorite debris (originally from the asteroid belt) and are some 4.6 billion years old. Analysis of radioisotopes within these indicates that prior to this time there was one, possibly two local supernova explosions. These massive cataclysms seeded local space with heavier elements and caused a shock compression of the interstellar medium which, according to the "standard model" of planetary formation, ultimately led to the collapse of a large cloud of cool dust and gas. As this cloud condensed under gravity it increased its rotation and spun out to become a disc. At the same time the gravitational energy lost in contraction was transformed into increasing kinetic energy of the particles within it and it heated up (Helmholz effect). The original make-up of the cloud was typical of the elemental composition of the universe i.e. 3/4 hydrogen, 1/4 helium and smaller traces of lithium, carbon, oxygen, silicon, iron, and heavier elements up to uranium. At the centre the temperature climbed high enough to initiate helium fusion and the proto-sun switched on. Further out the solar nebula started to cool and various elements and compounds "dropped out" i.e. solidified in a definite order depending on their melting/boiling points. The first to solidify were iron, nickel and the silicates. These quickly aggregated to form a travelling rubble pile which built up to form planetoids and finally planets (although the precise mechanism for this is still not fully understood). Gravity accelerated this process, since any accumulating body also has more gravitational attraction - a run away effect. The net effect when things had settled down was that the Sun had 8 major bodies orbiting it all composed primarily of heavier non-volatile elements and compounds. In the inner solar system the temperature of the surrounding gas was fairly high and the planets could only hold on to the densest of these gases as primordial atmospheres. Typical ingredients were nitrogen, methane, ammonia and carbon dioxide. Those inner or terrestrial planets have remained pretty much the same to this day (with one important exception). Further out beyond the orbit of Mars, the temperature dropped far enough for the gases water, ammonia and methane (all mainly hydrogen anyway) to condense into chunks of ice. These snowballs rained down on the rocky cores quickly increasing their masses and gravitational attractions to the point where they could hold on to even the nimble gases hydrogen and helium. Thus were born the massive gas giants which dominate the outer solar system. Pluto is thought to be one of a class of icy bodies stretching beyond the orbit of Neptune, most of which have been ejected to form the Oort cloud or captured (Triton) due to the chaotic orbits they possess. Pluto has been protected from this fate by a 3:2 orbital resonance with Neptune.
All the planets have nearly circular orbits (ellipses with the Sun at one focus) with two exceptions - Mars and Pluto , the latter has an eccentricity so great that its orbit dips inside that of Neptune. They are arranged in increasing distances from the Sun and corresponding increases in orbital period (Mercury takes 88 days, Pluto 248 years). The average distances from the Sun can be approximated by a simple number series called Bode’s rule. The reason for this may lie in chaotic dynamics which favoured this particular arrangement over other unstable arrangements.
The precise details of the formation of our solar system are still being worked out, yet mathematical modelling of collapsing clouds of dust and gas indicate that planets could be a natural accompaniment to single stars. The recent ability to simulate the evolution of such ensembles has led to an appreciation that chaotic dynamics can rapidly weed out planets in unstable orbits, possibly leading to the disruption of solar systems in only a few million years.
Chaotic orbits predominate in binary or multiple systems. It is dubious if any planets could form in such systems, or having formed, last longer than 100 million years unless they were either a) very close to one star and the stars were separated widely (in which case the planet would be very hot), or b) the planet was so far away from the multiple star system that it only effectively saw "one" gravitational source (in which case how did it get there and what energy source would be available to replace the starlight?)
The same chaotic process could lead to an evolution of a stable system which has fairly narrow constraints on the sizes and positions of the bodies - in fact the apparently random distribution in our solar system might be indicative of a route to stability followed by many other systems. Whilst these simulations are still theoretical, great advances have happened in observational astronomy which for the first time has revealed indirect evidence for planets around other stars. Optically, the task is well nigh impossible, since even the planet Jupiter is a billion times fainter than the Sun when viewed from the nearest star, and its feeble reflected light is lost in the glare of the parent.
Whilst individual planets might be invisible, the disk of gas and dust out of which they condensed/are condensing is visible in certain cases. Several stars have such disks, notably b Pictoris and Vega in Lyra. The Hubble Space Telescope has detected intriguing "clumping" in the b Pictoris disk.
Pulsars are a class of star which only came to light in 1964, and their regular and rapid radio pulses were tentatively attributed to LGM (little green men). The nature of these exotic objects excludes such an hypothesis - they are the remains of the most catastrophic events in the galaxy - supernovae. Any planets within the vicinity of such a violent event will be incinerated and stripped of an atmosphere. However the incredible stability of a pulsar’s beat (some can produce 2000 beats per second and only slow down by 1s in a billion years!) means that any perturbations in their frequency can be used to infer orbiting companions. This has been done for a number of pulsars now, and although the planets found seem to be very strange by solar system standards, it is nevertheless indirect evidence for extra-solar planets.
Since 1987, radioastronomers have been applying a sensitive Doppler shift technique to detect the reflex motion of stars. This reflex motion results from the orbit of the star and the planets surrounding it about a common centre of gravity which is slightly displaced from the star’s true centre. Thus the star appears to wobble in the sky. The frequency and extent of this wobble gives information about the invisible companions. The range of planetary sizes discovered by this method is limited - only large (Jupiter sized and above, or very close smaller sized) bodies have enough effect on the parent star to be measured, nevertheless it is an encouraging development and beyond the wildest dreams of astronomers just 15 years ago.
Recent (as of Jan 2004) findings may be summarised as follows:
The upshot of this information is that we consider planets to be frequent accompaniments to star systems with the fraction of stars possessing them varying from 0.001 to 0.3
For life as we know it to get started there are three basic requirements:
1 and 3 seem to be common. 2 is more problematic and the crucial issue in defining a habitable zone.
Given our understanding of planetary formation, in particular the expected composition of the atmospheres of such planets, it falls to ask the question: "How wide is the zone of possible orbits that will allow liquid water to exist?" This would depend on several properties of the planet itself e.g. spin rate, albedo, axial tilt, mass, but these are subservient to the energy flux due to solar heating from the local star. There is an equilibrium situation set up where sunlight illuminating half the planet’s surface is re-radiated by the planet as a whole. The temperature at which this stabilises must be between 273K and 373K at one atmosphere of pressure (lower pressures would yield a lesser upper limit, higher pressures a greater boiling point). Since we know the Earth to be in the habitable zone, one way of analysing the zone’s width is to run computer simulations of Earth’s at different distances from the Sun. In 1979 Michael Hart at NASA’s Goddard Spaceflight Centre ran such an ensemble of calculations with the following results:
If the Earth’s orbit were only 5% smaller than it actually is, during the early stages of its history there would have been a runaway Greenhouse Effect and temperatures would have increased to a point where the oceans would have boiled away completely (this echoed a similar conclusion by Rasool & de Bergh using a less detailed model in 1970)
If the Earth/Sun distance were only 1% larger there would be runaway glaciation about 2 billion years ago (similar conclusions were found by Budyko and Sellers (1969))
So it seems that the habitable zone is narrow - far narrower than previously thought (e.g. Huang in 1960.)
One can also speculate on the effect of the planetary mass on the width of the habitable zone. Hart’s model indicated that unless a planet has a mass between 0.85 and 1.33 times that of the Earth then it cannot, regardless of its distance from the Sun, maintain moderate surface conditions for more than 2 billion years.
Regarding the effect of the star’s mass on the habitable zone, the conventional wisdom prior to 1970 was that all stars of classes A,F,G,M and K should have a habitable zone and that this zone should be broader and more distant for the more luminous (and higher mass) F type stars, whereas the dimmer, cooler K type stars should have a narrower zone located much closer in. Hart’s simulations showed that no stars of mass less than 0.83 times that of the Sun could have any continuous habitable zone around them, so too any stars with masses greater than 1.2 solar masses. This restricts the possibilities tremendously, and indeed the simulations seem to be born out by the evidence from within our own solar system. Indeed the compounding effects of the planetary characteristics seem to impose even more restriction on the production of successful biospheres.
As previously discussed, the viability of liquid water is the overriding criterion of suitability. The stability of the planet’s orbit both in eccentricity and tilt are important factors, with near circular orbits and small tilts being preferred. The presence of a large moon helps to enhance the axial stability (cf. Earth and Mars) and also raises tides - an important factor in moving from marine to terrestrial life.
There are other possible sites outside the habitable zone where alternative energy sources may provide the heat necessary to keep water (or other solvents) fluid e.g. moons around large gas giants kept warm by tidal flexing. In our own solar system there are 3 or 4 such bodies, with Europa being the most likely candidate for biological processes.
Estimates for this range from 0.1 to 3.
The two schools of thought are:
Given the presence of liquid water, the usual elemental mix expected in primordial atmospheres and the injection of energy in the form of solar radiation, lightning or geothermal activity, then it is inevitable that amino acids, autocatalytic sets, primitive replicators and prokaryotic cells form. Some biochemists go so far to say that it is written into the laws of chemistry. If this is the case, then life should be abundant and ubiquitous. The rapidity of its formation on Earth lends credence to this idea.
On the other hand pessimists see that the delicate balance of conditions and environmental pressures that have shaped the origin of life and its subsequent development here on Earth seem to be so critically dependent on "chance" that the likelihood of all of these things occurring in the right order on any other world is vanishingly small. Earth may be the only place to have hosted such a process.
This gives a polarisation of probability, from near zero to 1
Similar arguments are repeated for this fraction as for the previous factor:
If life is present, given what we know about Darwinian evolution, then the rise of intelligence is inevitable given enough time. Our fossil record indicates that cranial capacity in dinosaurs was on the increase prior to the mass extinction event 65 million years ago and some fanciful extrapolations see an Earth dominated by intelligent bipedal lizards if this hadn’t occurred.
Contrariwise the fossil record shows that a tiny proportion of the billions of species on this planet took the route towards intelligence. The factors that give rise to intelligence have been outlined earlier, social carnivory in a 3D environment is a favoured route, although one not too encouraging in this context!
Again the factor ranges from near zero to 1
Once intelligent creatures have developed it is by no means inevitable that they will be tool users and build radio telescopes and spacecraft. Cetaceans here on Earth are generally assumed to be intelligent and have a rich social interaction, yet the limitations of evolution in water means they cannot smelt metals and have no "hands" to manipulate tools. Other factors may be important as well - an intelligent tool using civilisation that cannot see the stars due to permanent weather or multiple suns might not apply their skills to the exploration of space, simply because they don’t realise it is there. Again there is a delicate chain of causality that has allowed us to flower and carve out a niche for ourselves, but is this to be expected everywhere?
This fraction lies within narrower extremes though. Estimates vary from 0.001 to 0.1.
This is the most difficult question to answer and certainly no scientific answer can be forthcoming until we have a greater sample size than 1! However we may speculate on the possible future of our technical civilisation and look for clues to indicate a value for this. It seems that the future of humankind can go in five ways:
We have certainly been hanging on by the skin of our teeth through the Cold War, and we are by no means free of the possibility of global annihilation. It seems the biggest hurdle to overcome is the cultural barrier. Our technology has developed exponentially in the last 100 years, yet our emotional and social development has proceeded at a much slower rate (some would argue it doesn’t really develop, but is a cyclical process). We are, at root, aggressive territorial primates - an attribute that has led us this far, yet will it also destroy our species within a few decades of acquiring the ability to communicate over interstellar distances? Does this apply to all potential communicative civilisations out there? Only time will tell for us, but some sociologists believe that if we can survive 100 years of nuclear capability then we might have a persistent stable future lasting for the duration of our species (millions of years).
Of course if a civilisation becomes independent of its host planet, then it can populate many other worlds and the lifetime is substantially increased since that life is not as susceptible to planet wide catastrophe.
Putting all these factors together we can produce a best and worse case scenario for N.
Sagan et al (1976) made a more vigorous estimate of there being between 1 and 10 million communicative civilisations in the galaxy. If this were true then the average separation is of the order of only 100 ly in the best case, whereas in the worst case we could be separated by tens of thousands of ly.
Despite the fact that the sciences of astronomy, geology, chemistry and biology will allow us to make increasingly accurate predictions of the first four terms of Drake’s equation, we lack the experimental evidence to refine the last terms. This will not improve the wide variance of predicted outcomes in the near future, hence many scientists believe that the best way is to go ahead and search anyway, and base the number of civilisations currently active, not on hypothesis, but on the empirical results of such a search. In the words of Cocconi & Morrison (1959):
"The probability of success is difficult to estimate, but if we never search the chance of success is zero!"