IT HAS NOW been seventeen years since you left Earth, and in that time you have witnessed first-hand the awesome power of our nearest star, the Sun, and the effect it has on the unwary space traveller. You have journeyed to the inner planets, gazed upon the cratered landscape of Mercury and experienced the hostile conditions on Venus. You have walked on the surface of Mars, a world that has been visited by a number of unmanned space probes, and become the first human to leave footprints there. You then had plenty of time for reflection as Jupiter slowly grew bigger in the Kaldi’s viewing window. Seeing both it and Saturn close up was truly magnificent; the latter’s rings at close range were spellbinding. An intimate cruise past the mysterious icy worlds of the giants Uranus and Neptune was a fitting way to round off the most incredible trip, a true once-in-a-lifetime experience. But the journey has not quite ended yet.
Behind you lie the familiar planets of the Solar System, but what lies ahead? The discovery of Uranus and Neptune was a wonderful triumph for mathematics and for science in general. You will recall that Uranus was discovered by chance when William Herschel was studying stars in the constellation Taurus, and that Neptune was finally detected following mathematical analysis of the orbit of Uranus. Careful study of the orbits of Uranus and Neptune showed further perturbations. It seemed there might well be another planet out there in the depths of the Solar System gently tugging at these two icy giants.
It was left to astronomer Percival Lowell, founder of the Lowell Observatory in Flagstaff, Arizona, to try to find it. He set about the search and soon homed in on a number of possible locations for the unidentified ninth planet, which was soon dubbed ‘Planet X’. Sadly he died without knowing his team had captured images of it on two separate occasions. Clyde Tombaugh was also working at the Lowell Observatory and between 1929 and 1930 had been comparing photographic plates of the same part of the sky that were taken a few nights apart. After a year of painstaking work, Tombaugh spotted a faint moving object, which was eventually called Pluto. Its discovery was thought to complete the picture of the Solar System.
The case for Planet X was reopened in 1978 with the discovery of Charon, the faint moon of Pluto, which allowed astronomers to calculate accurately the mass of Pluto. When it was found to be just 0.2% of the mass of the Earth it was immediately clear that Pluto was far too small to cause the observed perturbations in the orbits of Uranus and Neptune. Then the Voyager fly-by of Neptune in 1989 allowed that planet’s mass to be calculated. The Voyager data reduced the giant’s mass by just 0.5%, and reapplying this new value to the orbit of Uranus nicely accounted for the perturbations. This finally laid to rest the need for a Planet X: Uranus and Neptune were moving exactly as they should be.
Finding Pluto, it seems, was just a stroke of luck. Diligent scientific observation shows that Planet X never existed and was proposed simply to account for unexplained variations in the orbit of Uranus. But does this mean that our Solar System is complete and we will never discover another major planet out in the depths? The chances of such a discovery seem very slim. The Voyager and Pioneer probes are now heading out into interstellar space and neither has detected the presence of a previously unidentified planet.
That said, in recent years there has been a slow revival of the Planet X concept due to a number of observed characteristics in the outer Solar System. The most recognized of these is the so-called Kuiper Cliff. The Kuiper Belt is found beyond the orbit of Neptune and can be thought of as a larger version of the asteroid belt between Mars and Jupiter, although its composition includes many more frozen elements such as water, ammonia and methane, owing largely to the greater distance from the Sun. Pluto and a number of the other ‘trans-Neptunian’ objects are now considered to be Kuiper Belt objects. It is reasonable to assume that the existence of Kuiper Belt objects would gently decrease with distance from the Sun, but in reality the belt seems suddenly to terminate at a distance of about 48 astronomical units (some 7 billion kilometres – as you may recall, 1 astronomical unit, or AU, is the average distance between the Earth and Sun). Some scientists believe that this ‘Kuiper Cliff’ is the result of an unidentified planet orbiting the Sun at this distance, perhaps the size of Earth. Such an object would not only explain the abrupt termination of the belt, it would also account for why a few objects seem to have been ejected into different orbits.
Far beyond the Kuiper Belt is another region of icy objects known as the Inner Oort Cloud, thought to be a part of the theorized Oort Cloud out of which comets originate. In 2003 the first suspected member of the Inner Oort Cloud was discovered and named Sedna. It’s an icy object that has a diameter of about 1,000 kilometres, but what makes it remarkable and of great interest to astronomers is that its highly elliptical orbit takes it from its closest point to the Sun, at just 11.3 billion kilometres, to an incredible 140 billion kilometres, which is 940 times further than the Earth–Sun distance. When you compare these figures to the average distance from the Sun to Pluto, which is 5.9 billion kilometres, then you realize just how far away Sedna is. At that distance it is only accessible for observation from Earth for a mere fraction of its 11,400-year-long orbit. Outside that short window, it cannot be seen. What has really excited the scientific community is the nature of its orbit and what may have caused it. Perhaps the passage of a nearby star dislodged it from its orbit within the Inner Oort Cloud; maybe it was exiled to the outer regions of the Solar System by other stars that formed with the Sun; or has its orbit been changed by the presence of another planet-sized object that is currently beyond the range of our detection?
The discovery of another suspected Inner Oort Cloud object, 2012 VP113, has brought astronomers a step closer in terms of understanding these objects with highly elliptical orbits. 2012 VP113 has been found to come as close as 11.9 billion kilometres to the Sun at its perihelion and as far away as 67 billion kilometres at aphelion. If a few more of this type of object can be found then close examination of their orbits will allow us to deduce more about their natures and where they have come from.
Unlike Sedna and 2012 VP113, Pluto is thought to be a Kuiper Belt object, and since 2006 has been classed as a minor planet because it is not gravitationally dominant in its orbit. It is joined on its 247-year orbit of the Sun by five natural satellites, the largest of which is Charon with a diameter of 1,200 kilometres, compared to the 2,368-kilometre diameter of Pluto. Charon is less massive than Pluto at 1.5 sextillion kilograms (Pluto’s mass is an estimated 13 sextillion kilograms – 0.2% of the Earth’s mass), but because their masses are of the same order of magnitude, the point at which their gravitational force balances is above the surface of Pluto. Where objects are an identical mass then this point, which is known as the barycentre, would sit exactly halfway between the two objects, but if one is more massive then the point moves towards the higher-mass object. For all the major planets in the Solar System, the mass of the planet is so high in comparison to the mass of the moon that the barycentre is, if not close to the centre of the planet, then at least somewhere within the body of the planet.
It is very difficult to learn more about Pluto from space exploration because of its mass. Sending a spacecraft so far out in the Solar System means it has to travel fast to oppose the gravitational pull of the Sun, but because Pluto has such a low mass, any spacecraft that does venture out that far cannot be captured by its gravity so instead continues straight past it. Missions like New Horizons, which launched in 2006, will only get a couple of days of closeup observation at best – a brief opportunity to study the surface detail. Thankfully, even at these huge distances the Hubble Space Telescope has been able to record enough details to give us a moderate understanding of the minor planet’s nature.
Before the HST, some of the first attempts to map Pluto’s surface involved very careful study during moments when Charon eclipsed Pluto. As the moon passed in front of the planet it slowly blocked out brighter and darker regions which caused a change in the overall brightness of the system. By studying this change in brightness, a very rough map could be produced of the surface features. The result was a surface that changed quite significantly, not just in brightness – or rather reflectivity – but also in colour, ranging from a dark grey through to a dark orange and even white. Over the first few years it was observed, the northern polar region brightened and the southern region darkened, suggesting seasonal changes as a result of its axial tilt. Its colour too seems to have changed, becoming a little more red than before, possibly as a result of chemicals from the surface sublimating into the atmosphere. Spectral studies of the surface reveal it is made up almost entirely of nitrogen ice with small portions of methane and carbon monoxide.
By studying the way Charon moves in its orbit around Pluto and knowing the minor planet’s volume, we can determine its overall density, which is around 2,000 kilograms per cubic metre. This suggests that it is composed of around 70% rock and 30% ice. Due to heat released from the radioactive decay of elements, the ice would melt, allowing them to separate from the rock and giving Pluto a differentiated structure. If it has evolved with a structure like this then the core will be around 1,700 kilometres in diameter and be surrounded by an ice mantle over 300 kilometres thick. Some scientists have suggested that any current radioactive decay may have caused the complete melting of the ice in the core–mantle border, which could produce a liquid sub-surface ocean, but further study is needed to understand if this could actually exist.
The surface is composed almost entirely of nitrogen ice with small amounts of carbon monoxide and methane. The observed seasonal changes have a relationship with the thin atmosphere of Pluto. While they are affected by the axial tilt of the planet, they are also affected by the distance between Pluto and the Sun. At its perihelion, Pluto is 29.6 times further from the Sun than the Earth; at its aphelion it is 48.8 times further away. When Pluto is nearer the Sun, more of the surface ice sublimates and thickens the atmosphere; when it is at its most distant and the temperatures are at their lowest, the gas in the atmosphere turns straight back into a solid in a process known as deposition. The transfer of material between the surface and the atmosphere is one of the main reasons why the surface appearance of the planet changes over time.
Much of what we know about the atmosphere of Pluto has come from the observation of stars as they disappear behind the planet. If there was no atmosphere then stars that pass behind Pluto would simply vanish in the blink of an eye; if there was an atmosphere the light from the star would gradually fade. The average atmospheric pressure at the surface of Pluto varies from between six hundred thousandths of the pressure experienced at the surface of the Earth to two hundred and forty thousandths, with the greater atmospheric pressure experienced when Pluto is nearer the Sun.
Arriving at Pluto fills you with a mix of emotions. The demoted planet, now only of minor status, will always hold a special place among the countless smaller chunks of rocks in the Solar System. With the exception of the New Horizons mission, Pluto has really not been explored much, so this is a great opportunity to take a look around this icy outpost.
As you take your first tentative steps on the surface you catch a glimpse of the Sun in the sky. Surprisingly from this distance the Sun is still quite bright, obviously much fainter than it is from Earth but still uncomfortable to look at – about 200 times brighter than the full Moon appears on Earth. You have arrived at Pluto while it is nearing its most distant point from the Sun and, because the ellipticity of its orbit is so great, its changing distance has an impact on its surface temperature. This ranges between minus 240 degrees and minus 218 degrees, which when compared with the coldest temperatures recorded on Earth of minus 92 degrees seems rather chilly. At this temperature many of the gases in the atmosphere will freeze on to the surface like frost. The scene that greets you, therefore, is like a cold, hard-frost morning back at home, the surface a glistening white.
Moving around is, as you might expect, not too dissimilar to moving around on the Moon. The easiest and most efficient method of manoeuvring in such low gravity (about a twelfth that of the Earth) is by hopping along. Unless you are feeling particularly brave, though, do not put too much effort into your leaps. Most people jump with an initial velocity of 4 metres per second, so with an escape velocity of around 1.2 kilometres per second there is no danger of you floating off into space, but you would certainly get to a decent height. If you can jump about a metre on Earth then a leap with the same effort on Pluto would get you soaring to a height of about 30 metres, so you’d need a head for heights.
And you need to be careful not to lose your footing on landing. The terrain is uneven with loose material, and there are craters dotted around; add that to the frostiness of the landscape and it’s easy enough to slip over. But if you do slip over you won’t fall as instantly as you do on Earth, it’ll be a more graceful event. You’ll have plenty of time to put your hand out to break your fall – which is essential if you want to ensure you do not damage your space suit on any of the jagged rocks that pepper the surface.
Of all the places you have visited and walked on, Pluto appears to be the most alien. The eerie light, the frostiness of the surface and your superhuman ability to jump over houses have made this one final excursion to remember. As you prepare to return to the ship you look back towards the Sun and pause, realizing that Earth is somewhere out there in the inky blackness. In a moment of homesickness a solitary tear wells in your eye, but the gravity is so weak here that it does not roll down your face.
There are a number of theories to explain Pluto’s origins. One of the earlier ideas suggested it used to be a moon of Neptune and that the arrival of Triton in the Neptunian system dislodged Pluto from its orbit. That now seems unlikely because, although Pluto gets closer to the Sun than Neptune, at no point do their orbits cross. The full story started to reveal itself in the early 1990s with the discovery of more small icy and rocky bodies beyond the orbit of Neptune. These trans-Neptunian objects seemed to share many properties with Pluto, not least in terms of approximate size, composition and even orbital properties, and Pluto is currently accepted as the largest member of the group.
We have already looked briefly at the Kuiper Belt and the Kuiper Cliff, which seems to mark a sudden decline in Kuiper Belt objects at a distance of about 48 astronomical units from the Sun. There are currently well over 1,000 known Kuiper Belt objects, most of which are made of ice and rock, and it is thought that these are tiny bodies left over from the formation of the Solar System that never quite formed into planets. As the outer Solar System was forming, a huge influence was exerted on the minor bodies by the large gas giants as they were settling into stable orbits, and it is this which is thought to have influenced the formation and current structure of the Kuiper Belt. Jupiter and Saturn eventually settled into orbits with a 5:2 resonance so the two mighty planets meet three times during every five orbits of Jupiter. The consequences of this planetary resonance were felt throughout the outer Solar System. They disturbed Uranus and Neptune. The orbit of Neptune in particular became a little more eccentric, sending it out further into interplanetary space. This had a big impact on the planetesimals that are now part of the Kuiper Belt, sending them out further into space and altering their orbits into much more eccentric ones, like that of Pluto. It is possible too that a great number were ejected off into space, perhaps even reducing the population to well under half of its original total.
In much the same way that Jupiter’s immense gravity dominated the evolution of the asteroid belt, then, Neptune has affected the development of the Kuiper Belt beyond it. The presence of Neptune in its current orbit seems to be maintaining certain features, including perhaps the inner and outer boundary of the main belt (and it extends from the orbit of Neptune for another 3.7 billion kilometres). Objects that orbit along the inner boundary tend to have an orbital resonance with Neptune of 2:3, so that for every three orbits of Neptune, the Kuiper Belt objects (KBOs) complete two orbits. Pluto is one such KBO which orbits the Sun twice for every three orbits of Neptune, and in recognition of this, any other object with the same resonance with Neptune is called a Plutino. Another resonance with Neptune may be keeping the outer boundary formed, the so-called Kuiper Cliff, so that KBOs at this distance complete two orbits for every one of Neptune.
Between the two boundaries, in the region known as the Classical Kuiper Belt, the gravitational effects of Neptune are not felt and the orbits of the KBOs are left largely undisturbed. Within the Classical Kuiper Belt there are two distinct groups of objects. The first is known as the ‘cold population’: the name does not reflect their temperature, it comes from the fact that their movement is somewhat analogous to the movement of molecules in a cloud of cool gas. They have nearly circular orbits which are constrained broadly to the plane of the ecliptic and have a different composition, making them appear more red than the other group. The ‘hot population’ have very different orbits which are much more elliptical and inclined to the ecliptic by as much as 35 degrees. The origin of the two groups is thought to be different, too: it is believed the cold population formed in their current position whereas the hot population may have formed closer to the Sun, perhaps in the vicinity of Jupiter, but were forced further out as the giant planets settled into their current orbits.
Beyond lies the Kuiper Cliff, which as we saw earlier is defined by a 2:1 resonance with Neptune. Its boundary is understood; what is not understood is why there seems to be few objects beyond it. We saw one possible explanation earlier: a large, currently unseen planet may be gravitationally restricting other objects. But it may simply be that there was insufficient material for the belt to form into a planet.
At your current speed, and with Pluto sitting at a point in its orbit where it is at the inner edge of the Classical Kuiper Belt, it takes us another eighteen months to transit the belt and reach the Kuiper Cliff – not that this would be noticeable to you. In fact you may not even have spotted a KBO in all the time you were in the belt: they would have been very dimly lit and almost undetectable. The void that seems to exist beyond the Kuiper Cliff is largely unknown. The two Voyager craft were directed out of the Solar System above and below its plane, as was Pioneer 11; Pioneer 10 was the only craft that was sent out along the plane. There is not a lot of direct evidence for objects at this distance from Earth simply because objects are not illuminated much by the Sun and are therefore difficult to detect. And if there were any more decent-sized planets out here, any craft or space traveller would be very lucky to be in the right place at the right time to see them.
From here on, the ion engine is going to be fired up to increase velocity and speed your journey through the outer reaches of the Solar System. The speed slowly increases, but it still takes twenty-three years to get to the edge of the Solar System, where interplanetary space ends and interstellar space begins, soon after which you’ll depart the Kaldi for the very last time. This edge is defined by the point where the influence of the Sun is matched by the influence of other stars, and understanding exactly where this is means understanding the nature of the interstellar medium – the matter that exists between the stars in our Galaxy. This matter is made up of a mixture of gas (mostly hydrogen and helium), dust and radiation, and it is within this medium that a bubble exists which surrounds the Sun. The bubble is the heliosphere, which is generated from the pressure exerted by the solar wind, and the point where the pressure from the solar wind is balanced by the ‘wind’ from other nearby stars marks the outer limit of the Solar System. On 25 August 2012, Voyager 1 became the first man-made object to pass through this point and enter interstellar space, at a distance from the Sun of 18.1 billion kilometres.
The exact shape and structure of the heliosphere is still not fully understood, and with the passage of the Voyager spacecraft many more questions were raised than were answered. The solar wind which generates the heliosphere leaves the Sun at speeds of up to 750 kilometres per second, and it is the interaction of the solar wind and the interstellar wind that drives the structure. The wind from the Sun is composed of a magnetic field and electrically charged particles known as ions, but because the Sun rotates on its axis it induces a spiral-shaped ripple through the Solar System. When you visited the Sun at the start of your journey we looked at the eleven-year solar cycle, but something else happens every eleven years too: the magnetic field of the Sun reverses, so that with a change in polarity the north pole becomes the south and vice versa. This produces disturbances in the spiral-shaped heliospheric current sheet which leave Earth susceptible to cosmic ray strikes. As the Earth orbits the Sun, it is usually protected from cosmic rays by the current sheet; it’s only vulnerable when it is moving through from one wave to another. During field reversal, Earth is likely to be much more at risk from cosmic ray strikes. This is a particular concern for space travellers and spacecraft outside the protection of the Earth’s atmosphere – although the Kaldi has some protection, of course, from the superconducting magnets producing your very own magnetic field.
As the Sun itself travels through the interstellar medium at a speed of about 83,700 kilometres per hour, it and the region of space around it slams into the interstellar medium causing the solar wind to slow to subsonic speeds, generating a shockwave. This point is known as the termination shock, and at this point the solar wind gets compressed like a group of people trying to walk through a dense crowd. The density of material in the interstellar medium is actually pretty low, with just 10 million molecules per cubic centimetre. But even with that low density it takes around 13.3 billion kilometres for the strength of the solar wind to drop sufficiently to be slowed to speeds lower than the speed of sound. This reduction in speed produces the shockwave. The distance of the termination shock from the Sun is not consistent: Voyager 1 measured its distance (from determination of the speed of the solar wind and its temperature) to be 94 astronomical units, while Voyager 2 measured it to be 84 astronomical units. This apparent discrepancy is a result of the motion of the Sun through the space.
Beyond the termination shock, in the heliosphere, the solar wind is slowed even more and the increased interaction with the interstellar medium causes turbulence. The turbulent nature of the heliosheath (the outer region of the heliosphere) means that the speed of the solar wind varies – indeed Voyager 1 detected regions where the solar wind unexpectedly dropped to zero, although it later increased again. The heliosheath is thought to be comet-shaped, extending to about 100 astronomical units from the Sun in the direction of its movement but stretching out many times further in the downwind direction. It is contained by the heliopause, the region that marks the edge of the Solar System, and it is here that the solar wind, which left the Sun travelling at hundreds of kilometres per second, is finally brought to a halt. Until August 2012 this was a purely theoretical boundary but its existence, as we saw in chapter 3, was confirmed by Voyager 1. Its presence was marked by an increase in cosmic rays which are usually blocked by the heliopause, a reduction in temperature, and a change in direction of the magnetic field.
Just like the shockwave that formed at the termination shock inside the heliosphere, one theory has suggested it may be possible for a shockwave to form in front of the heliosphere as it moves through the interstellar medium. However, unlike the solar wind, which travels at supersonic speed, the relative motion of the interstellar medium as we move through it is subsonic, at just 83,700 kilometres per hour. This sounds supersonic by normal standards, and indeed the speed of sound at ground level on Earth is only 1,234 kilometres per hour, but the speed of sound varies with temperature and density. It is much higher in the more rarefied interstellar medium and our Solar System is simply moving too slowly to form a bow shock. According to NASA’s Interstellar Boundary Explorer there is more likely to be a bow wave than a bow shock – more like the structures formed in front of a boat slowly moving through the water.
From this point on, you are in interstellar space. You have finally passed through the heliopause at a distance from the Sun of about 18.1 billion kilometres and it has taken you a little over forty-two years to get here. According to your flight plan the next port of call is, or more accurately might be, the Oort Cloud. This theoretical cloud surrounds the Solar System between 5,000 and 55,000 astronomical units from the Sun. Even with the slow but continual acceleration from the ion propulsion system it will take 1,500 years to get there, so the Kaldi will have to carry on without you. The time has finally come to return home.
Fortunately, thanks to the science-busting nature of the RSU, the journey home is nothing more than a flick of a switch. Once safely back on Earth the journey is not yet over, and before you can finally be reunited with your family and friends you must spend time in quarantine. Having journeyed around the Solar System and visited strange new worlds for the first time, great care must be taken to ensure that you have not brought back anything that could pose a risk to life on Earth. This same caution has been exercised ever since the astronauts returned from the very first Moon landing. Unlike them, however, your adjustment to life back on Earth is much more straightforward, as you have no issue reacclimatizing to the influence of our planet’s gravitational field because you have been experiencing simulated gravity for the majority of your journey. After a series of tests and examinations you can finally leave the quarantine facility for an emotional reunion with your loved ones. It’s been a long, fascinating journey but, as you take in your first breaths of fresh air, it feels great to finally be home.
The Kaldi continues on without you to the Oort Cloud, which is thought to be a massive halo of icy bodies that surrounds the entire Solar System, its outer members so distant that they are well on their way to being a quarter of the way towards the nearest star, Proxima Centauri. It is thought to have two distinct regions: a doughnut-shaped inner cloud known as the Hills Cloud and a spherical outer cloud which is only loosely gravitationally bound by the Sun. Within the two clouds there are believed to be several trillion icy planetesimals, each of them measuring about a kilometre in size. The only evidence for the existence of the cloud comes from the observation of comets, icy visitors to the inner Solar System that occasionally grace the skies of Earth.
Studies have shown that there are two groups of comets: the short-period comets that orbit the Sun over relatively brief periods of time (less than 200 years) and the long-period comets whose orbital period is greater. The short-period comets tend to come from the Kuiper Belt but the long-period comets, whose orbits can be many thousands of years, are believed to originate in the Oort Cloud.
Other than the time it takes the comets to orbit the Sun, there is no fundamental difference between the two. Both are a mixture of rock and ice, although the long-period comets tend to have more ice than their short-period counterparts. At the centre of a comet is a nucleus that usually measures just a few tens of kilometres across and is often likened to a dirty snowball. You may be able to remember as a child scooping up a handful of snow only to find you had picked up a fair amount of soil and stone with it. Just like your snowball, the nucleus of a comet is primarily ice but contains varying quantities of rock too. While the comet remains in the outer reaches of the Solar System its ice stays solid, but when some kind of disturbance sends it towards the Sun the temperature increases and things start to change. Because the nucleus of the comet is so small with an almost negligible gravitational force associated with it, the nuclei do not retain an atmosphere, so with the low pressures that come with an environment like this, ice that gets heated turns straight into a gas rather than a liquid. With the sublimation of ice, the nucleus gets surrounded by a halo of dust and gas. These halos are known as the coma of a comet, and when the pressure from the solar wind pushes against them they can extend for millions of kilometres, forming the comet’s trademark tail. It is a common misconception that the tail of a comet stretches out behind it as it whooshes through space. As we have just seen, the reality is that the tail always gets ‘blown’ downwind from the solar wind, which means that it always points away from the Sun.
Spectroscopic studies of comets and robotic space exploration have shown that many comets contain ammonia and other elements that are the building blocks for amino acids and proteins, which are key to the evolution of life. This important discovery reveals that the seeds that brought life to Earth may well have arrived by comet. When the Earth was young it would have suffered numerous bombardments from asteroids and cometary nuclei bringing with them a variety of life-supporting chemicals. When the theory was first suggested there was concern that the delicate compounds may well have been destroyed in such an impact. To counter any concern, experiments were conducted where high-velocity projectiles coated in organic compounds were fired at metal plates. The experiments simulated the forces and conditions experienced during a comet impact on Earth and found that the compounds survived. It is even just possible that the energy released during the impact may have become a catalyst for chemical changes to kick-start the evolution of life.
By studying the orbits of the long-period comets it has been possible to build a profile for the likely place they came from. The appearance of long-period low-inclination comets allows us to put an estimate on the inner doughnut-shaped Hills Cloud at between 2,000 and 20,000 astronomical units. Comets that appear with a very long-period orbit – such as Hale-Bopp, the so-called ‘Great Comet of 1997’, which had a highly inclined orbit of 2,537 years – suggest the outer cloud structure ranges from about 20,000 to 50,000 astronomical units. When these comets appear they can be big and spectacular because they spend much of their life in the deep freeze of the outer Solar System rather than making regular visits to the Sun like their short-period cousins who shed quantities of ice on their orbits around the Sun.
If the Oort Cloud does indeed exist then, like most things in the outer reaches of the Solar System, there is some uncertainty as to where it came from and how it formed. The most popular theory for a number of years explains that the cloud formed out of the remains of the protoplanetary disc that surrounded the young Sun. It is quite likely that the cloud could have formed nearer to the Sun and gravitational interaction with the giant planets ejected its members into highly elliptical and distant orbits. More recently, studies have suggested that the cloud may have been formed as a result of an exchange of material between the Sun and the stars it formed with, since most stars are believed to form in hot young stellar clusters; over time, they drift apart to lead either solitary lives with families of planets or as members of binary or multiple star systems. With the slow but gradual separation of the stars, this latest theory suggests that they swapped material which eventually led to the formation of the very distant cloud of objects that surrounds our Solar System. It is also likely that some of the outer members of the cloud could still be interacting with nearby stars as they pass through the Galaxy.
There are other influences experienced by the Oort Cloud, not least of which are gravitational forces from the Galaxy itself. Just like planets in orbit around the Sun or moons in orbit around a planet, they are all subject to gravitational tidal forces. At the distance of the Oort Cloud the gravitational influence of the Sun is substantially weakened, so much in fact that the gravitational force of the Galaxy has played a more significant role in the evolution of the cloud. During its early formation, many of the objects within it would have had highly elliptical orbits but the galactic tidal forces have led these orbits to become circularized into the spherical cloud. It is also possible that the tides could dislodge objects from within the cloud, sending them in towards the Sun. The Hills Cloud is closer to the Sun so the galactic influence there is less. There has not been sufficient time to make the orbits circular.
The Kaldi emerges from its long stay in the Oort Cloud and continues the remainder of its journey to Gliese 581, just over twenty light years away in the constellation Libra. The majority of the journey will be through interstellar space and will involve travelling through the interstellar medium. As we learned earlier, the density of molecules in the medium is low. In liquid water there are 10 sextillion (that’s a 1 with twenty-two zeros) atoms per cubic centimetre, but in the medium there is on average one atom per cubic metre. If the ship’s fuel runs out then the Kaldi will continue on at the same speed and in the same direction unless acted upon by another force, which might perhaps come from the gentle nudge of a nearby star.
On arrival at Gliese 581, the Sun will appear as a pretty insignificant fourth-magnitude star. All objects in the sky, including the planets, are described in terms of their brightness by their magnitude, with brighter objects having a negative number and the faintest objects visible to the naked eye assigned a value of 6. From Earth, the Sun has a magnitude of minus 26, the full Moon is minus 13, Venus is minus 5 at its brightest and the most distant visible object, a galaxy known as UDFj-39546284, has a magnitude of 29, which is 500 million times fainter than the human eye can detect.
From the surface of the Earth, Gliese 581 is not visible to the naked eye; it requires a pair of binoculars to be able to see it. It was chosen as your mission’s ultimate destination because of the possibility of alien civilization.
The star itself is nothing special. It’s a red star, cooler than our Sun but like our Sun is still fusing hydrogen to helium deep in its core. It is much smaller than the Sun, though, with about a third of its mass, which accounts for its lower temperature and redder colour. Until April 2007 it received very little attention, but that was until the discovery of Gliese 581c, the second planet found to be in orbit around the star. What particularly interested astronomers about this planet was that it was the first exoplanet to be found orbiting a star within the habitable zone. This is the zone within which any planet in orbit with a suitably dense atmosphere can sustain liquid water at the surface.
581c is a planet that has a mass about 5.6 times the mass of the Earth. Its distance from Gliese 581 is 11 million kilometres, compared to the Earth–Sun distance of 150 million kilometres, but the lower temperature of the star puts the planet right on the edge of the habitable zone. At that distance from the star, it takes only thirteen days for it to complete one orbit so its year is considerably shorter than ours, but unlike Earth it is tidally locked with Gliese 581. As we have seen before, this means that just one face of the planet stays facing the star with the other remaining permanently in the dark. This has quite significant implications for the likelihood of liquid water on the planet. Any water that does exist is very likely to evaporate in the high temperatures of the daytime side and then freeze on the night-time side through deposition. Given the tidal locking on the planet, the atmosphere could reach a state where the entire water content of the planet has frozen solid on the night-time side. One way that water can be detected on planets around other stars is to study the light from the star as it passes through the atmosphere of the planet. In this way the existence of water vapour in the atmosphere gives itself away through the light from the star that it absorbs. This extraction of the light from the star behind is seen as dark absorption lines in the spectrum of the star. Unfortunately in the case of Gliese 581c, the orientation of its orbit does not pass in front of the star when viewed from Earth.
There are three confirmed planets in the system. One of them, 581e, is just 1.7 times the mass of the Earth. Unfortunately, this one orbits the star at a distance of just 0.03 astronomical units, which equates to 4.5 million kilometres from the parent star – over 53 million kilometres closer than Mercury orbits the Sun. At that distance it completes one orbit of the star in just over three days and is bathed in searing heat and doses of radiation that make life on the planet very unlikely.
All the planets around Gliese 581 were discovered using the radial velocity method of extrasolar planet detection. The technique relies on one simple principle which we have looked at already, the movement of objects around the barycentre, the centre of gravity of the two objects in question. To recap, if two objects are of equal mass then they will orbit each other around a point directly between them. If one of the objects is more massive, then this point will lie closer to it, and if it is significantly more massive then the point will lie almost at the centre of the more massive object. Even in a case like this, the more massive object will still orbit around the point although its movement will be tiny and barely detectable. In the case of planets in orbit around a star, the presence of the planet in orbit will cause the star to wobble a tiny amount.
It is possible to detect the tiny wobble or movement of the star by studying its light through a spectroscope, which as we have seen splits the incoming light into its spectrum. As the star moves, the absorption lines in the spectrum will be moved first towards the red end of the spectrum and then towards the blue end of the spectrum in an effect known as the red and blue shift. This concept is something we have all experienced as the Doppler effect, such as when an emergency vehicle passes with its siren sounding. As the vehicle approaches, its motion causes the sound waves to get bunched up, causing the pitch of the siren to increase, but as it continues past, the pitch changes and starts to decrease as the sound waves are stretched out again. It is similar for the spectrum of the star: its movement causes the light to get bunched up as it moves towards you, making the absorption lines appear to shift towards blue; as it moves away again, the light waves get stretched out so the lines shift back towards the red end again. If the shift in position in absorption lines is carefully observed and measured then it is possible to calculate the movement of the star, and from that information the mass of the object that is tugging it out of position can be worked out. The evidence of the presence of three planets in orbit around Gliese 581 is hidden in the subtle movements of these absorption lines.
Other techniques are used to discover planets around distant stars. One of the more popular ones involves studying not the spectrum of light from the star but its brightness instead. There are a number of reasons why the apparent brightness of a star may vary, but occasionally it will be because a small amount of its light is being blocked by something such as another star or perhaps even a planet. The tiny variations in brightness as a result of a planetary transit are almost imperceptible but their signature is unmistakable. As the planet transits the star there is a tiny reduction in brightness of the overall system as the planet blocks a tiny amount of starlight from reaching us, and there will then be a secondary dip when the planet passes behind the star. This second dip is detectable because the planet usually reflects a small amount of starlight, thus increasing the brightness of the overall system, but when it passes behind the star that reflected light is not seen. If this were all plotted on a graph to show light against time then the curve would appear like a hilly landscape as the light level changes, with the valleys representing the time when the planet and star are in alignment. The existence of more than one planet around a star is revealed in repeating patterns in the light curve.
In the future, new technology may allow us to take direct images of the surfaces of these so-called exoplanets. Orbiting space telescopes that utilize a technique known as interferometry will be able to combine the light from telescopes separated by thousands of kilometres to offer views with unprecedented resolution, revealing fine surface detail on distant worlds. For now, though, all we can do is wonder what the conditions are like. Sadly it will be quite a few years before humans can even think about setting foot on a world around another star as there will have to be some significant improvements in rocket propulsion and supporting technologies for that to happen.
When we do finally make that journey, are we likely to find new civilizations thriving on other worlds? The chances seem quite high. After all, the Universe is a complex system of billions and billions of galaxies, and each one contains billions of stars. In fact it is often said that there are more stars in the Universe than there are grains of sand on Earth. Our current knowledge suggests that water is plentiful in our Solar System, planets around other stars are not a rarity, and even organic compounds seem to be abundant, so the chances for life to evolve seem high.
In case there is life on one of the planets around Gliese 581 and the Kaldi comes into contact with it, a plaque has been attached to the side of the ship to show where it has come from. A similar plaque was attached to the Pioneer spacecraft, which used a map of nearby pulsars (rapidly rotating dense stars) to identify the location of our Solar System. In addition to that there’s a map on the Kaldi of the Solar System and a depiction of the spacecraft coming from the third planet around the star, which is of course our home, planet Earth.
Even if the ship does not encounter alien civilization, the mission has been a great success. From the comfort and safety of home you will be able to look back on a fascinating voyage across the expanse of our Solar System. You have been closer to the Sun than any other human being, visited the hostile worlds of the inner planets, successfully negotiated the asteroid belt, saw the beautiful cloud tops of Jupiter and even flew through the raging hurricane that is the Great Red Spot. You have seen the awesome rings of Saturn and the subtle hues in the atmospheres of Uranus and Neptune, then explored the dark depths of the outer Solar System. After that you became the first human to leave the Solar System and enter interstellar space, before leaving the Kaldi to continue the voyage without you.
The journey required you to spend many years away from home, but the long-term space mission has been quite an adventure. Wanting to explore the unknown is an innate part of human nature, and just like our ancestors who set out to map every inch of the Earth’s surface, to cross seas, climb mountains and race to reach the poles, we have continued the quest to understand the natural world, peering into and continually wishing to explore the dark depths of space. Yours has been an amazing experience, and your trusty spacecraft will continue on through interstellar space for all eternity.