To give an idea of the size of the Halo, we note that Pluto lies at an average distance of about 6 billion kilometers from the Sun. This is about forty astronomical units, where the astronomical unit, usually abbreviated to AU, is defined as the mean distance of the Earth from the Sun. The AU provides a convenient yardstick for measurements within the Solar System. One light-year is about 63,000 AU (inches in a mile, is how I remember it). So the volume of space in the Halo is 4 billion times as large as the sphere enclosing the nine known planets.
By Solar System standards, the Halo is thus a huge region. But beyond Neptune and Pluto, we know little about it. There are a number of “trans-Neptunian objects,” but no one knows how many. Some of them may be big enough to qualify as planets. The search for Pluto was inspired early this century by differences between theory and observation in the orbits of Uranus and Neptune. When Pluto was found, it soon became clear that it was not nearly heavy enough to produce the observed irregularities. The obvious explanation is yet another planet, farther out than the ones we know.
Calculations of the orbit and size of a tenth planet needed to reconcile observation and theory for Uranus and Neptune suggest a rather improbable object, out of the orbital plane that all the other planets move in and about seventy times the mass of the Earth. I don’t believe this particular object exists.
On the other hand, observational equipment and techniques for faint objects are improving rapidly. The number of known trans-Neptunian objects increases almost every month.
The other thing we know for sure about the Halo is that it is populated by comets. The Halo is often called the Oort cloud, since the Dutch astronomer Oort suggested thirty years ago that the entire Solar System is enveloped by a cloud of cometary material, to a radius of perhaps a light-year. He regarded this region as a “cometary reservoir,” containing perhaps a hundred billion comets. Close encounters between comets out in the Halo would occasionally disturb the orbit of one of them enough to divert it to the Inner System, where it would appear as a long-period comet when it came close enough to the Sun. Further interactions with Jupiter and the other planets would then sometimes convert the long-period comet to a short-period comet, such as Halley’s or Encke’s comet, which we observe repeatedly each time they swing by close to the Sun.
Most comets, however, continue their lonely orbits out in the cloud, never approaching the Inner System. They do not have to be small to be invisible to us. The amount of sunlight a body receives is inversely proportional to the square of its distance from the Sun; the apparent area it presents to our telescopes is also inversely proportional to the square of its distance from Earth. For bodies in the Halo, the reflected light that we receive from them thus varies as the inverse fourth power of their distance from the Sun. A planet with the size and composition of Uranus, but half a light-year away, would be seven trillion times as faint. And we should remember that Uranus itself is faint enough that it was not discovered until 1781, when high-quality telescopes were available. So far as present-day detection powers are concerned, there could be almost anything out there in the Halo.
One of the things that may be there is life. In a carefully argued but controversial theory developed over the past thirty years, Hoyle and Wickramasinghe have advanced the idea that space is the natural place for the creation of “pre-biotic” molecules in large quantities. Pre-biotic molecules are compounds such as carbohydrates, amino acids, and chlorophyll, which form the necessary building blocks for the development of life. Simpler organic molecules, such as methyl cyanide and ethanol, have already been observed in interstellar clouds.
Hoyle and Wickramasinghe go further. They state explicitly: “We shall argue that primitive living organisms evolve in the mixture of organic molecules, ices and silicate smoke which make up a comet’s head.”
The science fiction of the fourth chronicle consists of these two assumptions:
1. The complex organic molecules described by Hoyle and Wickramasinghe are located in a particular region of the Halo, a “life ring” that lies between 3,200 and 4,000 AU from the Sun;
2. The “primitive living organism” have evolved quite a bit further than Hoyle and Wickramasinghe expected, on at least one body of the Oort cloud.
Missing matter and the beginning of the Universe.
Today’s so-called “standard model” of cosmology suggests that the Universe began in a “Big Bang” somewhere between ten and twenty billion years ago. Since we have been able to study the Universe in detail for less than four hundred years (the telescope was invented about 1608), any attempt to say something about the origin of the Universe implies considerable extrapolation into the past. There is a chance of success only because the basic physical laws of the Universe that govern events on both the smallest scale (atoms and subatomic particles) and the largest scale (stars, galaxies, and clusters of galaxies) appear not to have changed since its earliest days.
The primary evidence for a finite age for the whole Universe comes from observation of distant galaxies. When we observe the light that they emit, we find, as was suggested by Carl Wirtz in 1924 and confirmed by Edwin Hubble in 1929, that more distant galaxies appear redder than nearer ones.
To be more specific, in the fainter (and therefore presumably more distant) galaxies, every wavelength of light emitted has been shifted toward a longer wavelength. The question is, what could cause such a shift?
The most plausible mechanism, to a physicist, is called the Doppler effect. According to the Doppler effect, light from a receding object will be shifted to longer (redder) wavelengths; light from an approaching object will be shifted to shorter (bluer) wavelengths. Exactly the same thing works for sound, which is why a speeding police car’s siren seems to drop in pitch as it passes by.
If we accept the Doppler effect as the cause of the reddened appearance of the galaxies, we are led (as was Hubble) to an immediate conclusion: the whole Universe must be expanding, at a close to constant rate, because the red shift of the galaxies corresponds to their brightness, and therefore to their distance.
Note that this does not mean that the Universe is expanding into some other space. There is no other space. It is the whole Universe — everything there is — that has grown over time to its present dimension.
And from this we can draw another immediate conclusion. If expansion proceeded in the past as it does today, there must have been a time when everything in the whole Universe was drawn together to a single point. It is logical to call the time that has elapsed since everything was in that infinitely dense singularity the age of the Universe. The Hubble galactic redshift allows us to calculate how long ago that happened.
Our estimate is bounded on the one hand by the constancy of the laws of physics (how far back can we go, before the Universe would be totally unrecognizable and far from the place where we believe today’s physical laws are valid?); and on the other hand by our knowledge of the distance of the galaxies, as determined by other methods.
Curiously, it is the second problem that forms the major constraint. When we say that the Universe is between ten and twenty billion years old, that uncertainty of a factor of two betrays our ignorance of galactic distances.
It is remarkable that observation of the faint agglomerations of stars known as galaxies leads us, very directly and cleanly, to the conclusion that we live in a Universe of finite and determinable age. A century ago, no one could have offered even an approximate age for the Universe. For an upper bound, most nonreligious scientists would probably have said “forever.” For a lower bound, all they had was the age of the Earth.