FUSION Makes Amazing Discovery!

The results of the recent Sizing the Earth from Your Garden experiment have shown that the tilt of the Earth has shifted considerably from its former angle and the North Pole was pointing almost directly at the Sun in June, which may explain the recent heat-wave in the northern hemisphere. We received only two measurements of the sun's elevation, 60.95 degrees at Bar Hill in Cambridgeshire and 61.5 degrees at Lincoln, some 100km further north.

At the summer solstice in the northern hemisphere (which is when the experiment was carried out) the only region where a more northern latitude experiences a higher angle of elevation for the midday sun is between the equator and the Tropic of Cancer. This implies that the Tropic of Cancer (the locus of points where the sun is overhead at the summer solstice) has moved to a latitude higher than that of Lincoln - to about 81 degrees North, in fact, if you do the maths. At the same time, the Arctic Circle will have moved to a latitude of about 8 degrees north; so that while Greenland will be experiencing tropical climates in summer, everywhere with a latitude higher than that of Lagos will get six months of darkness in winter.

Alternatively, it could just be experimental error ...

Web Site News

New physics Q&A link
Visit http://scienceworld.wolfram. com/physics for answers to a wide variety of physics and maths questions. Also check out other sites listed on our Links page.

Event and Photo Reports
This April's visit to UKAEA Winfrith (a former nuclear power research site).
CAT (The Centre for Alternative Technology) at Machynlleth, awash with OU FUSION students on the 12th of July.
Plus pictures from the NEXUS trip to DESY, Hamburg in June.

Ask-a-Boffin

Accelerating Inertia!

FUSION member Zied Nehdi would like someone to explain the following statement in Richard Feynman's Lectures On Physics (volume 1, chapter 19, page 3), about an object which "is held in some manner inside a box, and ... the box, and everything contained in it, is accelerating".

Zied asks "What does it mean, especially when it is said: 'to make the object go along with the box, we have to push on it to accelerate it, and this force is balanced by the force of Inertia', because the object is already inside the box! And why there will be an effective force due to inertia for someone at rest with respect to the box?"

Confusion Fusion

In Ask-a-Boffin in our Winter 2002/3 Newsletter we asked: "What is the fundamental difference between what happens (or will happen) in a fusion reactor and what happens in an H-bomb - thereby guaranteeing that fusion power is safe?" We got the following answer from Chris Warrick, Educational Outreach Manager at UKAEA Culham:

"It is worth stressing straight away that the magnetically confined fusion research carried out at Culham (and other laboratories around Europe and the world) is no way related to development of nuclear weapons.

In a device like JET (Joint European Torus) or a future fusion reactor, the plasma is created in a vacuum chamber (by heating up a D-T gas mixture), heated to the type of temperatures required for fusion of D (deuterium or ²H) and T (tritium or ³H) to start occurring (150-200 million degrees C) by various means and confined inside the chamber using powerful magnetic field structures. In this way, the plasma can be sustained (by feeding in D and T as necessary ) and fusion power extracted (through the energetic neutrons that are produced from the fusion reaction). There is very little plasma present in the vessel at any one time (D and T are fed in to refuel as the process goes on) and if too much gas is fed in, the plasma temperature will drop as radiation increases. For example, less than 1g of Deuterium is used in a typical JET plasma. This use of a small amount of fuel at any one time is one of the reasons why exploitation of the fusion process in this way is intrinsically safe.

In a nuclear fusion bomb, an uncontrolled chain fusion reaction is initiated - but this requires extreme conditions to be maintained for the nuclear explosion to occur. Typically, a surrounding fission reaction forces the fusion core to implode increasing its temperature and pressure significantly until fusion starts to occur. The short reaction time, large amount of fuel and high plasma pressure all contribute to the uncontrolled rapid release of fusion energy.

There are no similarities between a nuclear bomb a fusion power plant. The density of fuel and the pressures are much lower in the power plant and most crucial of all we don't try to heat the fuel with a fission bomb".

Chris has promised to write a longer article about nuclear fusion research for a future newsletter; current estimates are that the article will be available by 2050 ... (sorry, Chris, only joking!)

Circle Time...

In the Spring 2002 Fusion Newsletter we launched the Ask-A-Boffin column with a query from David Parker, who works for a laser cutting company, and asked us why it is that he can cut 15mm mild steel but only 3-4mm aluminium.

We asked Science Line, the free public information service which provides answers to questions on all aspects of science, medicine, technology and engineering; and a few weeks later they came up with an explanation. (See Fusion Newsletter Winter 2002/3 for full details). Rather worryingly, however, the wavelength of the laser, quoted by David as 10.64 nanometres, had been multiplied by a factor of 1000 in the process, and Science Line quoted it as 10.64 micrometres. We decided, on balance, to use their version in the resulting newsletter article. But... who was right?

We contacted Science Line and asked them to check their information. Several more months down the line, in early June 2003, they were able to report to us that they had indeed found an article on laser cutting which corroborated their version of the wavelength. The publication concerned was Focus, the Open University Physics Society's magazine.

Now, apart from the fact that a complete loop had been traversed in cyberspace, and the implication that this may not be such a rare event, raising the possibility that whole rafts of scientific theory might turn out to be supported by nothing but their own vanity, and might (as Douglas Adams might have said) be in danger of disappearing in a puff of logic... WHERE did they get that name from? It turns out that MANY years ago (so long ago in fact that even Ray Mackintosh can only just remember it) there was a short-lived OU physics society called... yes, you guessed... Focus.

Spooky, or what?

BOOK REVIEW

by Lorna Pain

Some Time with Feynman
Leonard Mlodinow
(Allen Lane - The Pengiun Press)

This book is a compelling read and easy to get into, even for a non-scientist like me. Mlodinow has a chatty yet intelligent style that describes, with frank and often amusing insight, his time spent as a postdoc at the prestigious California Institute of Technology. Faced with his own mental block after joining Caltech, he turns to others on campus for inspiration and eventually summons up the courage to talk to Nobel Prize-winner Richard Feynman. Through conversations with the enigmatic Feynman over the next few years, he learns more about his own views of physics and life, and gradually is able to find a new perspective for himself.

Into this rich backdrop Mlodinow weaves a description of the then current issues in theoretical physics - quantum chromodynamics and string theory in particular - and what others like Murray Gell-Mann thought of them. The account is funny, sad, enlightening and thought provoking. Some Time with Feynman is more about Mlodinow's own quest to find a place in life, than it is about Richard Feynman, but is worth reading to find out what makes scientists tick.

Where is Everybody?

On the way to lunch at Los Alamos Scientific Laboratory one day in 1950, Enrico Fermi and three other physicists - Emil Konopinski, Edward Teller and Herbert York - chatted about flying saucers. At lunch, when the talk had turned to other matters, Fermi suddenly said, "Where is everybody?" His companions realized that the talk of flying saucers had turned his mind to the possibility that there is intelligent life elsewhere in the universe and that he was asking why, if there is, we have seen no sign of it. The question encapsulates what is now known as the Fermi paradox. In his recent book If the Universe is Teeming with Aliens ... Where is Everybody? (Springer, Heidelberg) Steven Webb, a physicist at the OU, presents 49 solutions that have been proposed for the paradox, grouping them according to whether they hold that intelligent extraterrestrials are here, exist but have not communicated, or do not exist. He concludes with his own solution, the 50th: "We are alone."

Tangling with Entanglement

by Ray Mackintosh of the Department of Physics and Astronomy

Not long ago, it was pointed out in the UK parliament that 'something like 25% of the GDP of the USA is based upon physics which is totally derived from quantum mechanics research.' So the practical importance of quantum mechanics is officially recognized, then! But that's not the reason that there is such a plethora of popular books on quantum mechanics in your local bookstore, some with strange feline-invoking titles harking back to Schrödinger. Quantum mechanics, more than any other discovery in physics, even relativity, has fundamentally changed our understanding of the world, calling into question even our most cherished beliefs about the nature of 'reality'. In view of my oblique reference to Schrödinger's cat just now, it might surprise you to know what Schrödinger thought was the most deeply counter-intuitive aspect of quantum mechanics. Writing in 1935, Erwin Schrödinger said "I would call entanglement not one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought."

Before I try to explain what entanglement is, I should say that it is not only a matter of quantum weirdness; it could soon increase that figure of 25% of the GDP derived from quantum theory! There is currently intense activity around the world, with reports reaching the popular press, involving quantum cryptography, quantum teleportation and possibly quantum computing. What these subjects have in common is their exploitation, to varying degrees, of entanglement. As well as giving rise so some of the most deeply mystifying consequences of quantum mechanics, entanglement affords the real possibility of the science fiction writer's dream, teleportation, as well as secure cryptography, something of profound interest to bankers, on-line businesses and their customers and governments alike.

But there is another side to entanglement: it leads directly to tests of quantum mechanics against the trenchant critiques of Einstein and others in the 1930s. For many years, quantum physicists got on with the business of applying quantum theory to understanding solids and other forms of matter, atoms, nuclei, fundamental particles and their interactions with light and so on. Questions of what it all meant seemed less important. However, the deep and unsettling question raised by Einstein, Podolsky and Rosen (EPR) in the thirties never completely went away. It is not for nothing that the only other aspect of science that attracts the attention of philosophers to a comparable degree is Darwinian evolution.

Soon after the EPR paper, Schrödinger published the article in which he identified the key element in EPR's argument as entanglement; his paper contained the quote given above. Perhaps not everyone would go so far as to agree that the essence of what is most weird in quantum mechanics arises from entanglement, but it has certainly proved true in recent years that all the tests of quantum mechanics concerning a key issue, that of the existence of hidden variables, have involved entanglement in a crucial way. So Schrödinger's use of the word 'enforce' in the quotation above now seems deeply prophetic!

What are hidden variables? Most physicists believe that quantum theory says that, for example, an electron in an atom does not have a position until it is measured to have one. In other words, our ignorance of the electron's position is not like our ignorance of the position of individual atoms when we describe the pressure of gas using the gas laws. In the case of the atoms in the gas, we have an excellent statistical description, but we believe that a description in terms of the position of all the atoms would be possible in principle except for the overwhelming number to be kept track of. But quantum mechanics appears to imply that the wavefunction gives a complete description of an atomic system: it says all we can know, even in principle. EPR, however, argued that quantum theory must be incomplete, leading to various proposals for hidden variables (interestingly, not by Einstein, though). Roughly speaking, this means that there is an invisible description according to which electrons do, after all, have a position (or other observable) prior to measurement, much as the atoms in a classical gas do. At this point we shall simply say that (i) many people see this as a deep issue concerning how we understand the world, and, (ii) all tests of whether there might be a more complete theory of the world than QM involve entangled states. Before saying what the modern resolution is, I must first explain 'entanglement.'

What is entanglement? As usual, quantum concepts are most easily discussed using equations, but here I shall try to explain without. (The equations will be found on Video Band 7 of SM355, although the word entanglement is not used! But that video band does explain the physics behind the EPR paper and modern tests of quantum mechanics.) Two particles are said to be in an entangled state if neither particle on its own has a well defined state although the two particles as a pair do. For example, two electrons can be in a so-called 'singlet state' in which we know what the total 'spin angular momentum' of the pair is. In particular, we know that the component of spin angular momentum in any direction is zero. But in such a state neither electron individually has a determined spin component in, say, the z-direction. (Just as an electron in a hydrogen atom is not in a definite position until its position is measured.) But if the component of spin of one of the pair is measured to be +h^-/2 say, in some direction, we know for sure that the spin of the other will be -h^-/2 in that direction (h^- is Planck's constant h divided by 2π). This is true whatever direction is chosen for the first measurement! And as if that is not strange enough, suppose the electrons are billions of light-years apart, maybe having been created in a singlet state and then sent flying apart for billions of years. Now, suppose they encountered nothing in their travels, until one of them encountered a device that measured its spin, somewhere on the surface of a planet. Then the measurement of the spin of that one electron would instantaneously 'collapse' the wavefunction of the other so that its spin is the opposite of whatever the spin of the first-measured electron was. If you are not amazed by that sentence, then you've either read about this before and become blasé, or you haven't taken it in, in which case you should read it again!

No wonder Schrödinger thought it was weird. No wonder EPR made an example equivalent to this the centre-piece of their argument that quantum theory is incomplete. No wonder, finally, that when Bell proved a famous theorem that (to simplify a bit) quantum mechanics and hidden variables theories could not both be right and that experiments could tell which was right, another physicist (Stapp) proclaimed that Bell's theorem was 'the greatest discovery of science.' (Bell explains these things in his own words on Video Band 7* of SM355.)

So what is the answer: is quantum mechanics true, or are there hidden variables? Many experiments have now been performed that make it pretty clear that quantum mechanics is correct and that there are no hidden variables. (Strictly speaking, there are no so-called 'local' hidden variables... everything in quantum mechanics seems to needs qualification of one kind or another!)

One question always comes up when entanglement is discussed: 'But surely this instantaneous collapse of the two-particle wavefunction over indefinitely large distances contradicts Einstein's special relativity that nothing travels faster than light. Needless to say, this question has been much discussed, and the key point is that this collapse mechanism cannot be used to send information or any material object faster than light. Nevertheless, the fact that it somehow breaks the spirit of special relativity, although not the legal requirements, has been the subject of discussion.

There has been a remarkable outgrowth from the discussions over the last twenty years concerning these issues of interpretation: the realization that entanglement and other characteristic quantum properties provide means for some extraordinary new technologies mentioned above. Recently, there has been a tendency to link all these topics under the heading of 'quantum information'.

In short: quantum entanglement and other fundamental quantum properties are connected with at least four topics of intense current interest:

1. What does quantum mechanics tell us about the world? This relates to what the study of entangled states has to say about Einstein's trenchant 1935 critique of how quantum mechanics is generally understood. Another relevant question is that of just where the boundary lies between the microscopic world (that requires quantal description) and the classical world of macroscopic objects.

2. Quantum teleportation (teleporting the whole of Captain Kirk is not possible just yet).

3. Quantum cryptography (security issues relating to the WWW make this a very live issue).

4. Quantum computing (a huge subject, with entanglement just one of many issues related to quantum theory).

The list is growing. The last three topics, at least, are part of the more general topic, quantum information.

Where does entanglement enter into OU courses? Well, there is video-band 7* (non-examinable) of SM355, and there is something in SMXR355. In fact, although SM355 contains nothing examinable on entanglement as such, it does contain a great deal that is very relevant to a thorough discussion of it, including singlet states. Students who have passed SM355 are in a good position to go more deeply into the subject since one of the topics for the new project course, SXP390, is entanglement! The SXP390 course CD-ROM will provide material that will fill the gaps between what is taught in SM355 and the papers and articles you are likely to read as you undertake a project on one of the variety of subjects based on an old subject that is now at the frontiers of knowledge: entanglement.

*Available to loan from FUSION.

Physics and/or Balls

by Ian Saunders

In a world which includes something so improbably good as the Open University, anything at all may be possible, but it seems unlikely that any other FUSION member has been both a professional snooker referee and a professional physicist. Hence I guess that I am uniquely qualified to comment on the mechanics of striking a billiard ball, prompting as a contribution to this journal a revised version of a note previously published by the M500 society.

Striking the ball in billiards, snooker and pool is more subtle than at first appears. Clearly you must give it the right speed and direction, but there is also rotary motion to consider. To impart pure rolling motion, the height above the table at which a ball should be struck horizontally with the cue is exactly 0.7 of the diameter d, as discussed below, i.e. ⁷/₁₀d (and not d /√2 or anything else suggested by mathematicians). Striking below that level, e.g. hitting at the centre of the ball in the usual manner of novices, causes the ball to slide across the cloth initially, until friction slows it down and eventually produces smooth rolling. A stroke higher than the 0.7d level gives the ball too much forward spin for rolling. This causes a skidding rotation on the table surface which generates friction to accelerate the ball until it is rolling smoothly. In a collision between two balls the impact is always at the height of a ball centre, and there is a slight exchange of rotational motion between the balls via friction at their surfaces. Such a collision involves complex effects and prediction of subsequent behaviour is best made on an empirical basis from experience and much practice. Physicists tend to apply over-simplified theories and in general do not make good snooker players (as I amply demonstrated in my own playing career!).

A little physics at about S207 level yields the required height above the table. Suppose a horizontal force of magnitude F is applied at height x to a ball with mass M and moment of inertia I about an axis through the ball centre C. The impact causes C to move at speed v and the ball to spin at angular speed ω about C. For pure rolling motion the bottom of the ball in contact with the table at P is stationary, not rubbing and skidding on the cloth. As the centre C is above point P by a radius r, rolling motion requires v = rω. To get the ball to this speed the cue supplies accelerations in both linear and angular motion, and a sufficient relation for instant rolling is obtained by differentiation of the speed equation above, giving dv/dt = r dω/dt. (It is not necessary that the two accelerations are uniform. So the force need not be constant, and strictly it is only required that the average accelerations have this relation, but see below.)

The principles of dynamics give us for linear motion F = M dv/dt (Newton's 2nd law) and for rotational motion Γ = I dω/dt, where Γ = F (x - r) and is the magnitude of the torque about the centre C (also known as a couple or moment). Substituting F and Γ into the relation between the accelerations gives F / M = r Γ / I = r F (x - r) / I, and F cancels here to yield (x - r) = I / M r as the sole condition for a blow on the ball to give pure rolling motion. For a sphere we have I = ²/₅Mr² and using this we must have x - r = ²/₅r, and so x = ⁷/₅r = ⁷/₁₀d.

The condition thus derived for rolling motion from the start has no dependence on the size of the force, only on where it is applied to the ball. Any variation in the magnitude of the force and consequent non-linearity of acceleration during the cue action is not relevant. Hitting at the right height automatically ensures the correct ratio between the linear and rotational accelerations and hence between the speeds v and ω for pure rolling.

All this is closely related to the concept of the "centre of percussion", e.g. for the bats and racquets used in sports like cricket and tennis. Here the idea is to strike the ball so that there is no unpleasant jarring or twisting force on the hand of the player, which involves a similar condition to requiring no skidding of the ball on the billiard table cloth. The same analysis applies, and we find that the best place to hit the ball is at distance x on the bat from the hand where again (x - r) = I / Mr. Here r is the distance from the handle to the centre of mass for the bat, and the bat's moment of inertia I should be measured around an axis at the centre of mass, not at the handle.

Most real sports bats and racquets have rather complicated shapes, and to be honest, for players the physics theory is not very helpful in practice, but may be for designers of course. The players know from experience where the "sweet spot" of their own bat is located, and try to use it. For the simple case of a uniform rod (or plank) of length 2r with the same thickness all along I = ¹/₃Mr², and if it is held at one end the centre of percussion is ²/₃ of the way along. This was well known to cavalry soldiers, who would try to strike their opponent about ²/₃ down the sweeping sabre blade, and not with the tip or near the hilt. Not all that easy, however, on a galloping horse! The ²/₃ down from the handle is also roughly correct for a cricket bat. For a tennis racquet the centre of percussion is near the middle of the strings. For a large mass on a weightless handle, it is right at the end where the mass is. Anybody who has used a hammer knows that! Golf clubs are similar.

Here is an easy way to find the exact centre of percussion for your own bat (or sabre, etc.). Suspend it hanging down at the angle of use, swinging freely from the point where it is usually gripped. (Unless you are willing to drill a hole in the handle you'll have to attach it by a loop of string.) Make a pendulum by tying a small heavy object to a piece of thread, and adjust the length of this thread until the pendulum swings to and fro at exactly the same rate as the bat. Let the bat come to rest and then place the upper end of the pendulum thread at the suspension point of the bat. The middle of the pendulum weight will hang at the centre of percussion.

If anybody wants to explore more of this (or to hear how trying and nasty Alex Higgins could be and how skilful and nice Stephen Hendry was) I'll be happy to chat over a pint at a summer school or revision weekend. You'll find a different proof for the billiard cueing and a lot more about balls, particularly the golf variety, in an excellent book The Physics of Ball Games, by C.B. Daish, but I don't know if it is still in print. My own copy is a Hodder and Stoughton paperback published in 1981.

When not playing with spherical objects or tutoring OU students, Ian Saunders works in the Department of Physics at Lancaster University.

S280 Matters!

by Jim Grozier

One of the reasons OU students tend to talk in numbers is the rather embarrassing titles that are given to some courses. S280 (Science Matters) is no exception. No doubt someone at Milton Keynes was jolly pleased with him or herself for thinking up the title. But what is wrong with "Science Communication"? Or "Science, the Media and the Public"? These would give a more accurate picture of the course content.

The communication and interpretation of science is examined using six topical issues; first the actual science is taught, to a fairly basic level, enabling the student to then critically analyse scientific articles from the media, or write his or her own. (Just how much work there is to do in this field is illustrated by this extract reproduced from the Nuclear Power book.

It shows the tabloid press's idea of what might happen after a nuclear meltdown - the core sinks to the centre of the earth and then carries on, defying gravity, until it emerges on the other side!)

S280 is a popular course, with over 700 students registering each year. Most of the criticisms that have been levelled at it are completely unjustified, amounting as they do to accusations that the material is "dated". This shows a lack of understanding of the main aim of the course, which is to teach, not the science itself, but the skills of communication, analysis and criticism. True, S280 deliberately addresses some very cutting-edge issues, such as genetic engineering and climate change - fields which are changing so quickly that any OU text will be out of date once it has been running a few years. But so what? The point is to look at how the science was being interpreted and communicated at the time, and that time could just as easily be 100 years ago as now - a fact reinforced by the inclusion of a historical account of the exploration of the deep oceans as one of two optional modules. And in any case, the Genetic Engineering module has a supplement, which updates the original text.

Nevertheless, the Course Team does address this issue by including more recent material in TMA questions and the exam. Indeed, in last October's exam, I had to write an article for the editor of a local newspaper about the likely effect of a hijacked airliner crashing into a nuclear power station - and you can't get much more up-to-date than that! Also, the final TMA required us to produce a briefing paper for a Government committee on climate change, based on a report issued the previous year. The Climate Change block certainly left me realising how much disinformation and misunderstanding there is about this topic, and how vital it is to combat them.

If I had any criticism of the course at all, it would be that the techniques for data handling and analysis are not well taught. Because everything is done in terms of one of the six topics, data skills are picked up almost accidentally along the way; then one is confronted with a compulsory exam question worth 25% of the marks, which is entirely about data analysis and interpretation, and for which no revision is possible! There should be a short "data" module right at the beginning, and it should include some elementary statistics.

It is perhaps in response to the charges of "datedness" that the OU decided some time ago to withdraw and/or rewrite the course. I was astonished to learn that these activities would not necessarily be concurrent, and that the rewrite might not happen at all. The future is still uncertain, although a little brighter - according to Course Team member Audrey Brown, "We are definitely going to run S280 next year - and possibly, even the year after. It is an important component of the new Natural Sciences Degree - and it seems unlikely that the replacement will be ready before February 2006".

So sign up for it now - just in case!

Extrasolar Planets

by Keith Tritton

Are there any planets in orbit around the stars? This age-old question was finally answered only as recently as 1995 - but the first extrasolar planets discovered have unearthed a host of new riddles for astronomers to puzzle over.

If our understanding of the formation of our own Sun and Solar System is correct, then we might expect the same process to have occurred throughout the Universe. Since it is estimated that there are 100,000 million stars in our galaxy alone, then there could be many Earth-like planets orbiting other stars, and perhaps capable of supporting life.

How could this idea be confirmed or disproved? The problem is that the direct observation of extrasolar planets is impossible. We cannot simply turn our telescopes towards other stars and look for planets in orbit around them. Any planets would be lost in the glare of the star itself.

The evidence for the existence of extrasolar planets comes indirectly, from the gravitational effects that the planets have on their parent stars. As the planet orbits, it perturbs the star's position a little so that the star also executes a miniature orbit. In fact the planet and the star orbit around their common centre of mass, since each pulls gravitationally on the other. For most planets the effect on the star is minute. Even in the case of Jupiter, the largest planet of the Solar System, the size of the "orbit" of the Sun is only a little greater than the radius of the Sun itself, so both Jupiter and the Sun orbit around a point that lies just outside the solar surface.

In October 1995, Michel Mayor and Didier Queloz from theGeneva Observatory announced that they had detected small periodic fluctuations in the Doppler shift in the light coming from a star about 50 light-years away called 51 Pegasi, revealing the presence of an invisible planetary companion. They had analysed their data extremely carefully, only releasing their conclusions when they were very confident of their interpretation. Their caution was understandable, for the planet they had discovered was quite unlike anything in our Solar System. It is a giant, with a mass lying between that of Jupiter and Saturn, yet it orbits 51 Pegasi once about every four days at a distance of only 7.5 million kilometres. This is eight times closer than our innermost planet Mercury is to the Sun.

Since then, further discoveries followed rapidly one after the other and the number of confirmed planetary companions has now passed the 100 mark. A few systems contain two or even three planets.

The planets discovered so far are all giants, and well over half of them are significantly more massive than Jupiter. In addition, their orbits are generally very much smaller than those of the giants of the Solar System - most are smaller than the orbit of Mercury. Of course, the more massive planets and the ones that orbit most closely to their stars are precisely the ones that cause the greatest gravitational effects and are therefore the easiest to detect. Because of this selection effect it is still too early to say whether high-mass close-orbit planets are the commonest kind or whether arrangements like our Solar System are more typical.

Many of the extrasolar planets have been found to travel in highly elliptical paths, in sharp contrast to the Solar System, where the planetary orbits differ only slightly from circles. If the planet that orbits the star ε Eridani were in our Solar System, it would in turn sweep out beyond Jupiter at the furthest extent of its orbit then loop back in to pass Mars at its closest. A massive planet in an orbit like this would be highly disruptive, ousting any other planets from its neighbourhood and possibly eliminating Earth-like planets altogether.

Smaller worlds may well exist in these or other systems, but the detection of Earth-sized planets, where we might expect conditions to be more favourable for the development of life, is beyond the capability of current instruments. It is impossible at the moment to say whether such planets are common or rare and their discovery will have to await the new generations of telescopes now under development.

Keith Tritton tutors S282, S283 and S357, and is an astronomer by background, with a special interest in astrobiology. This article is based on an extract from his new book Earth, Life and the Universe... Exploring our Cosmic Ancestry. The book is available to FUSION members at a discounted price of £14.95 (£5 off) if they order direct from Curved Air Publications, Premier House, Boldero Road, Bury St Edmunds IP32 7BS (fax 01284 750482, email info@premier-printers.co.uk), mentioning "Fusion offer". They will post the book with an invoice so there is no need for cash with order.