Jocelyn Bell and the Breakthrough prize 2018

Pulsars were first detected in 1967 by a research student called Jocelyn Bell when she was taking observations for her PhD thesis. Her supervisor, Anthony Hewish, went on to win the Nobel prize in 1974 for the discovery, and her contribution was overlooked. Many at the time felt that Jocelyn Bell should have been given at least a share in the prize, since she was the person who had initially spotted the signal from the first pulsar.

Jocelyn Bell 

This was finally addressed this month when she was awarded the $2.3 million Physics Breakthrough Prize for the discovery. Previous recipients include Stephen Hawking, researchers at Cern who discovered the Higgs boson, and physicists on the Ligo experiment who detected gravitational waves.

According to the BBC website, Bell has decided to donate her entire winnings to set up a fund to help women, under-represented ethnic minority and refugee students to become physics researchers.

Bell who is now 75 years old told the BBC:

‘I don’t want or need the money myself and it seemed to me that this was perhaps the best use I could put it to.’

If you would like to know more, please see my post from last year on the discovery of pulsars below.


In 1967 Jocelyn Bell, a 24-year-old student from Cambridge University, was doing the research for her PhD. She was using a radio telescope to study radio waves emitted from compact astronomical objects known as quasars, and when she analysed the data, she noticed a signal which appeared to pulse on and off every 1.3 seconds. After doing this continuously for about an hour, the pulsing signal would stop altogether, but it would start again precisely 23 hours 56 minutes later after it had first started. As Jocelyn Bell – and indeed all astronomers – knew, the Earth rotates on its axis once every 23 hours 56 minutes, a period of time called a ‘sidereal day’, and the fact that the pulsating signal was detected at the same time in each sidereal day meant that it must almost certainly be coming from space rather than being man-made as shown below.


Not only was the time interval between each pulse very short at only 1.3  seconds, but this time interval was completely constant and didn’t vary to any significant degree.  The fact that the pulses were very short in duration and so regular meant that whatever was emitting the pulses must be extremely small in astronomical terms.  A larger object – something, for example, the size of the Sun – would not be able to generate such precise pulses. For a while Bell and her PhD supervisor Antony Hewish considered the possibility that the mysterious signals were generated by a signaling beacon left by an alien civilisation. For this reason, they briefly nicknamed  the unknown object ‘Little Green Men-1’. However, this explanation was rejected when it became clear that the pulses contained no information and when they also discovered additional pulsing sources in other parts of the sky.


The origin of the mysterious signals turned out to be a hitherto unknown class of astronomical objects, known as neutron stars. Neutron stars are very small, typically around 20 km in diameter, but have an enormous mass – between 1.1 and 3 times the mass of the Sun.  Having such a large mass squeezed into a small volume means that their density is incredibly high.  A cubic centimetre of neutron star material would weigh about 500 million tons. They are called neutron stars because they consist mainly of neutrons, which are subatomic particles found in the nucleus of ordinary atoms. In a neutron star the neutrons are so tightly squeezed together that they are touching each other. Neutron stars also have very strong magnetic fields – around 1 trillion times stronger than the Earth’s.

Neutron stars which rotate extremely rapidly, around once a second, are known as pulsars, and these are the objects which Bell and Hewish had detected. This rotation causes electrically charged particles around the neutron star to move rapidly in the intense magnetic fields. This causes electromagnetic radiation (such as radio waves) to be emitted in two cone-shaped beams along the magnetic North and South poles of the pulsar, shown as B in the diagram below.

In the diagram above A is the axis around which the pulsar rotates and B is the magnetic axis.

As you can see, the magnetic poles are at an angle to the line through the North and South poles of the pulsar about which it rotates. This means that the cone-shaped beams will rotate around the axis of the pulsar and only when either of the beams is pointing directly at the Earth will it be detected. A pulsar behaves like a lighthouse – the light is on all the time but appears to a viewer to switch on when it is pointing towards them and switch off when it is pointing away.

Neutron stars are formed when a large massive star explodes at the end of its life in a violent event known as a supernova. Most of the outer parts of the star are blown out into space by the force of the explosion. The remaining material in the star’s core collapses, forming a neutron star.  In fact, if the remaining material from the star’s core is more than three times the mass of the Sun, a neutron star won’t be formed at all.  Instead, an even more compact object called a black hole will result.

Large stars with a diameter of tens of millions of kilometres rotate relative slowly, taking around one year to complete one rotation. As the star collapses into a small massive object, millions of times smaller in diameter, a law of physics called the conservation of angular momentum causes its rotation to speed up massively.  A  more familiar example of this is from the world of ice skating: ice skaters spin more rapidly when they pull in their arms.

Why this knowledge has been useful to astronomy

In the last 50 years more than 2000 pulsars have been detected. Understanding the properties of pulsars has led to further discoveries in various areas.  For example, it has led to greater understanding of the diffuse gas between stars known as the interstellar medium, and it has also been used to test Einstein’s theory of general relativity. I will discuss these advances in more detail in future posts.

Knowledge of pulsars was also used in a very interesting way in the 1970s.  Four spacecraft were launched, destined to the leave the Solar system and head out into interstellar space. The missions are described in more detail in my previous post Artefacts from Earth.  On each spacecraft there was a diagram devised by the American astronomer, Frank Drake (1930-), consisting of a circle with 15 lines coming out of it.

The centre of the diagram, from which the lines radiate, represents the Sun.  The right-hand end of the longest line (at 3 o’clock) represents the centre of the galaxy. The end of each of the remaining line represents a pulsar and the length of the line between the Sun and the pulsar represents the distance to the pulsar.

The diagram relies on the fact that each pulsar has its own distinct period between pulses. So if, in the far distant future, an alien civilisation were to recover the spacecraft they would be able to identify the pulsars from their periods.  To enable this, each of the lines depicts the length of the pulsar’s period not in seconds (which as a man-made unit would be meaningless to an alien race) but in multiples of a ‘fundamental time unit’ that an alien might understand.  The alien civilisation could then have sufficient information to identify the fourteen pulsars and the distance of the Sun from each pulsar, and thus work out the location of our Solar system within the galaxy.

The Nobel prize controversy

Jocelyn Bell and Antony Hewish published their results in February 1968 (Hewish et al 1968). The discovery of this entirely new type of astronomical object was a major advance. Interestingly, it had been suggested as long ago as 1934 by the astronomers Baade and Zwicky that neutron stars would be the end result of supernova explosions. However, this prediction was ignored. Before 1967 most astronomers regarded neutron stars as hypothetical objects which might or might not actually exist in reality.

Such was the impact of the discovery that it led to the Nobel prize for physics in 1974. Two astronomers were awarded the prize – but Jocelyn Bell was not one of them. Antony Hewish and Martin Ryle were the recipients, the former for the discovery of pulsars and the latter for his work in radio astronomy.  Jocelyn Bell’s contribution was not recognised.

Antony Hewish (1924- ) 

Many at the time felt that Jocelyn Bell should have been given at least a share in the prize, since she was the person who had initially spotted the signal from the first pulsar. The British astronomer Fred Hoyle was particularly vocal on the issue and stated publicly  that Bell should have been given a major share of the prize to acknowledge her contribution. Bell herself said very little about the controversy in the years immediately afterwards. The few statements she made were, in general, supportive of the Nobel prize committee’s decision. In the 1960’s and 1970’s it was commonplace for the senior person leading a team of scientific researchers to get the credit for a major discovery on behalf of the entire team. This is largely still true today.

Bell went on to have a successful academic career and always has been a passionate advocate for getting women more involved in science. From 2002 to 2004 she served as the president of the Royal Astronomical Society, the organisation for British Astronomers. She was the first ever woman to hold that role.  She later served as the first ever female president of the Institute of Physics. In 2006, nearly 40 years after the discovery, she said in an  interview:

In those days, it was believed that science was done, driven by great men . . . And that these men had a fleet of minions under them who did their every bidding, and did not think. It also came at the stage where I had a small child and I was struggling with how to find proper childminding, combine a career, and before it was acceptable for women to work. And so I think at one level it said to me ‘Well men win prizes and young women look after babies.’

Postscript: a different kind of pulsar discovered in 2016

Stars such as the Sun do not end their lives in violent supernova explosions resulting in neutron stars. Instead, when they have used up all their nuclear fuel, the outer layers of the star are blown away into space and form a bright glowing shell of gas called a planetary nebula, shown below.

Planetary Nebula

The remnants of the star’s core collapse into a dense hot star called a white dwarf, an object which is roughly the same size as the Earth.  For the last 50 years astronomers have predicted that some white dwarfs might also form pulsars, although because white dwarfs are much larger and rotate more slowly than neutron stars, the radiation would be much weaker than neutrons star pulsars, making them harder to detect and the pulses would be much longer.  This prediction was finally confirmed in 2016 when a team led by Tom Marsh from Warwick University discovered that the white dwarf star AR Scorpii was a pulsar with a period of about 2 minutes.


Baade, W.and Zwicky, F. (1934) ‘Remarks on supernovae and cosmic rays’, Physical Review, 46(1), pp. 76-77.

Hewish, A., Bell. S. J., Pilkington, J. D. H, Scott P. F. and Collins, A (1968) ‘ Observation of a rapidly pulsating radio source’, Nature, 217(), pp. 709-713.


Lunar eclipse 27 July 2018

On 27 July 2018 there will be a total eclipse of the Moon, which will be viewable from many areas of the world. This will be the first total lunar eclipse able to be observed in the UK for nearly three years and it will be worth making the effort to see, especially since, for viewers in Europe, Africa and eastern Asia, it will occur at a sociable hour in the evening.

NASA Image Lunar Eclipse

The Moon during a recent total lunar eclipse – image from NASA


What happens during a lunar eclipse?

A lunar eclipse occurs when the Earth prevents some or all of the Sun’s light from hitting the Moon’s surface. This is shown in the diagram below:


Image from Wikimedia Commons

In this diagram in the region marked Umbra the Earth completely blocks the Sun. In the region marked Penumbra the Earth partially blocks the Sun.


The stages of the July 27 lunar eclipse

The next diagram below shows how, to someone on Earth, the Moon will move through the Earth’s shadow on 27 July. The six points labelled P1, U1, U2, U3, U4 and P4 are known as the eclipse contacts and are the times when the eclipse moves from one stage to the next.



Diagram from NASA


At point P1 the Earth will start to block some of the Sun’s light from reaching the Moon.  This will start at 5:15 pm GMT and is the start of the penumbral phase. The Moon’s brightness will dim a little, but this will be quite difficult to notice with the naked eye.


As the Moon continues in its orbit, more and more of the Sun’s light is obscured, until after about an hour some of the Moon will get no direct sunlight.  This is known as the partial phase. It will start at point U1 which will occur at 6:24 GMT. The part of the Moon which receives no direct sunlight will appear dark, as shown in the picture below.


Lunar_eclipse_ Partial

The partial phase of a lunar eclipse – Image from Wikimedia Commons


After a further hour the Earth will block all direct sunlight from reaching the entire Moon. This is shown as U2 is the diagram and this total phase will start at 7:30 PM. In the total phase, rather than disappearing completely, the Moon goes a dull red colour as shown in the picture at the top of this post. This is because, even though no direct sunlight can reach the Moon, some light from the Sun is bent round the Earth’s atmosphere towards the Moon. This light appears red because visible light from the Sun is a mixture of different wavelengths – red light has the longest wavelength and violet the shortest. Most of the light of the shorter wavelengths  (orange, yellow, green, blue, indigo and violet) is removed from this light bent by the Earth’s atmosphere by a process called scattering, which I discussed in an earlier post . The same effect causes the western sky to be red after sunset on a clear day.

Interestingly, if we could stand on the surface of the Moon and view the eclipse we would see a red ring around the Earth.

The Moon will emerge from the total phase (point U3) at 9:13 GMT, the partial phase (point U4) will end at 10:19 PM and the eclipse will finish (P4) at 11:29 PM.

Which areas of the world can see the eclipse?

The eclipse timings are summarised below

Data from NASA (2009)

Not all areas of the world will be able to see the eclipse. This is because the Moon will have already set after the eclipse starts or will not have risen before it finishes. Other places will only be able to see part of the eclipse.

  • In Manchester where Mrs Geek and I live, the Moon will rise at 9:06 PM local times which is 8:06 PM GMT, so when the Moon rises the total eclipse will already be underway.
  • In Manila, the Moon will set at 5:44 am on July 28, Philippine Standard Time (PST) which is 9:44 PM GMT, so viewers will miss part of the final partial phase because this will occur after the Moon has set.

I have adapted the diagram below from NASA (2009) and this shows where in the world the eclipse can be seen.

The regions labelled A to L are as follows




How often do lunar eclipses occur?

Even though the Moon takes roughly a month to orbit the Earth, lunar eclipses do not occur every month. The Moon’s orbit around the Earth is tilted at about five degrees with respect to the Earth’s orbit around the Sun, as shown below.


Moon Tilt

This means that during most lunar months, as seen from the Moon, the Earth passes just below or just above the Sun rather than obscuring it. There are only two time windows in a year when a lunar eclipse can occur.  These two points are known as the nodes (See note 2). Even then most lunar eclipses are partial eclipses where the Earth only partially covers the Moon.




  1. GMT versus UTC

Although the term Greenwich Mean Time (GMT) is often used in popular writing it is no longer used by astronomers.  Instead, they use two different times which agree with each other to within 1 second.

  • Universal Time, often abbreviated to UT1, is the mean solar time, the time determined by the rising and setting of the Sun at the Greenwich Meridian, zero degrees longitude.
  • Co-ordinated Universal Time, usually abbreviated to UTC, is the time measured by atomic clocks and is kept to within 1 second of UT1 by the addition of leap seconds.


In common use, GMT is often taken to be the same as UTC, which is the approach I have taken for this post. However, it can also be taken to mean UT1. Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, the term GMT is normally avoided in precise writing.


  1. Nodes when eclipses can occur

The two nodes when a lunar eclipse can occur aren’t the same dates every year but change from year to year due to an astronomical effect called precession of the line of nodes.


NASA (2009) Total lunar eclipse of 2018 July 2017, Available at: (Accessed: 8 July 2018).

Methane on Mars

I was very excited to read about the discovery published last week by NASA’s Curiosity rover of the seasonal variation in the amount of methane in Mars’ atmosphere. Curiosity found that the average methane concentration varied from 0.24 parts per billion (ppb) in the northern hemisphere winter to around 0.65 ppb in the summer.

This was widely reported in the world’s media e.g.the New York Times.The interesting fact is that, on Mars, methane should have a very short lifetime because it is destroyed by ultraviolet light from the Sun. The presence of any methane in Mars’ atmosphere means that there must be some process, as yet unknown, continually producing it.  On Earth nearly all methane is biological in origin and is generated by microorganisms as a waste product. The seasonal variation in methane in Mars’s atmosphere is certainly consistent with life, but there may be other non-biological processes making this methane.

A joint mission between the European Space Agency (ESA) and the Russian space agency, called the TGO has just started scientific operations and may shed more light on the origin of this methane. I’ve therefore attached an updated version of my earlier post about this spacecraft.


Two years ago on 14 March 2016 the ExoMars Trace Gas Orbiter (TGO) spacecraft was launched from Baikonur, Kazakhstan on a journey to Mars. The purpose of its mission is to study how the distribution of the gas methane varies with location on the planet’s surface and over the course of time.

Trace Gas orbiter

Image from ESA

What is the significance of methane on Mars?

Compared to the Earth, Mars has a very thin atmosphere. Its surface pressure is only 0.6% of that of the Earth. The atmosphere mainly consists of carbon dioxide. However, it also contains a small amount of methane, around 0.5 parts per billion. This is a puzzle to scientists because the ultraviolet light from the Sun should break up any methane within 600 years, and Mars is 4.5 billions old. Therefore there must be some process occurring which is constantly replenishing the methane.


Mars- Image from NASA

There are various possibilities for the origin of this methane. One is that it is released by geological processes such as volcanoes or a chemical process occurring within rocks called serpentinisation. This is not as exciting as it sounds (no snakes, I’m afraid) – it is simply a slow chemical reaction between olivine (a mineral found on Mars) carbon dioxide and water which can produce methane.  Another is that there is a large reservoir of methane locked away in the ice below the Martian surface and as the temperature varies some of the ice may melt, thus releasing the methane into the atmosphere.

A fascinating possibility is that the methane is created by microorganisms below the planet’s surface.  On Earth more than 90% of methane in the atmosphere is produced by living organisms (ESA 2014). There are over 50 species of microorganisms known as methanogens that live off organic matter and produce methane as a waste product.  These microorganisms are found not only in wetlands (producing what is known as marsh gas) and in the soil but also in the guts of many animals such as cows and humans.  At the risk of sounding somewhat vulgar, the methane gas escapes from both cattle and ourselves in the form of flatulence.



What will the TGO measure?

The TGO will measure how the methane content of Mars’s atmosphere varies with space and time. It will also be able to measure the concentration of other gases such as sulphur dioxide (which on Earth is normally associated with volcanic activity) and organic compounds such as ethane, methanol and formaldehyde (which on Earth are produced by living organisms).  Although the TGO won’t be able to say for definite what the sources of the methane found on Mars are, if the concentration of methane were found to vary with the seasons and also if methane were found in conjunction with other organic chemicals it would point towards a biological origin.

How long with the mission be?

The TGO is a joint mission between the European Space Agency (ESA) and the Russian Space Agency (Roscosmos).  The mission is described in more detail on the ESA website  (2018).  It arrived at Mars in October 2016 and was initially placed in a high elliptical orbit around the planet. It took until March 2018 to gradually manoeuvre into the intended orbit and, now it is in the correct position, will spend the next five years mapping the methane distribution. This is a low circular orbit only 400 km from the planet’s surface. The orbit is inclined at an angle of 74 degrees to Mars’s equator. This high inclination enables the spacecraft to see most of the planet’s surface.

The high inclination of the TGO’s orbit means that, as the planet rotates, all area of Mars between latitudes -74 degrees South and 74 degrees North will at some stage be directly below the TGO.

Footnote- Schiaparelli

When the TGO arrived at Mars, it it deployed a small lander called Schiaparelli. This was an ‘add on’ to the main mission and was only designed to operate for a week on the Martian surface. Schiaparelli was intended to measure the wind speed and direction, humidity, pressure and surface temperature, and determine the transparency of the atmosphere. Sadly, when Schiaparelli arrived in the upper regions of the Martian atmosphere, a parachute to slow it down failed to open properly and it crashed into the surface at thousands of km/h and will was destroyed by the impact.


ESA (2014) The enigma of methane on Mars, Available at: (Accessed: 20 March 2016).

ESA (2018) Robotic exploration of Mars, Available at: (Accessed: 12 June 2018)

June 21 2018 – the solstice

This year, the June solstice will fall on 21 June.  In the northern hemisphere, it is the day when there is the most daylight and when the Sun is at its highest in the midday sky.


Sunrise at the solstice at Stonehenge, England – image from Wikimedia commons

The origin of the word solstice is from two Latin words:  sol, which means Sun, and sistere, to stand still. This is because, at the time of the solstice, the Sun stops getting higher, appears to stand still at the same height for a few days, and then gets lower in the midday sky.


The graph below shows the maximum height, or elevation, of the Sun, measured in degrees above the horizon, during the month of June. The graph is for a location 50 degrees latitude North, which is the same latitude as the southern tip of the British Isles.  

The fact that the Sun’s elevation changes gradually means the amount of daylight also changes very little around the solstice. This is shown in the table below, which gives the sunrise and sunset times and the amount of daylight in hours, minutes and seconds for June in London.

Table of sunrise and sunset times for London (Time and Date 2018).


Precise definition of the solstice

The diagram above shows the Earth’s orbit around the Sun. For clarity the sizes of the Earth and Sun have been greatly exaggerated.




  • During June, marked as Ain the diagram, the Earth’s North Pole is tilted towards the Sun and the days are longer in the northern hemisphere.
  • During December, marked as Cin the diagram, the Earth’s South Pole is tilted towards the Sun and days are longer in the southern hemisphere.
  • At points Band D, known as the equinoxes, neither pole is tilted towards the Sun and the amounts of daylight in the northern and southern hemisphere are equal.

The precise astronomical definition of the June solstice (also called the summer solstice in the northern hemisphere) is the exact point in time when the North Pole is tilted furthest towards the Sun. The times for this event for the years 2016-2020 are given in the table below – in GMT, in Tokyo time (which is 9 hours ahead of GMT) and in Hawaiian time (which is 10 hours behind GMT).

June Solstice Times



As you can see, the time of the solstice varies from year to year. It can fall on 20, 21 or 22 June, depending on your longitude (and thus your time zone).

Importance of the solstice to early man

The solstice was of great importance to early man, and many prehistoric sites appear to have been built to celebrate it. The most famous of these is Stonehenge, which is located in Wiltshire, England. It is a set of concentric stone circles built between 4000 and 5000 years ago. It was an amazing feat of construction for stone age man. The stone circle is over 30 metres in diameter. The largest stones are more than 9 metres tall, weigh over 25 tonnes and were hauled over 30 km to the site. It is reckoned that the smaller stones were moved from western Wales, a distance of 225 km (Jarus 2014).


Image from Wikimedia commons 

At the centre of Stonehenge is a horseshoe arrangement of five sets of arches called triliths, each containing three stones.  The open side of the horseshoe points North East towards a large stone 80 metres away from the main circle. Today this large stone is given the name  ‘The Heel Stone’.



Image from Wikimedia commons

The monument is arranged in such a way that, for a few days either side of the June solstice and only at those dates, someone standing in the centre of the horse shoe and facing North East will see the Sun rise over the Heel stone.

Heel Stone Sunrise

How sunrise at the summer solstice at Stonehenge would have looked after the monument’s construction.

It is amazing that prehistoric man built such a large monument to line up with the June solstice. It clearly must have been a major event for a people living outdoors with only natural daylight, and in fact the solstice is still celebrated at Stonehenge today. Modern groups with ancient origins, such as Druids and Pagans, who revere the natural world more than many modern humans, join approximately 30,000 people who flock to Stonehenge to watch the Sun rise at the solstice each year.

Interestingly, to prevent damage to such an important ancient monument it is not normally possible to get right up to the stones. However, the charity which manages the site English Heritage open it up every year for the solstice, giving people a rare chance to get up close.

For the BBC report on the 2017 Stonehenge solstice celebrations click on the link below.

The southern hemisphere

To those of you who live in the southern hemisphere the June solstice is the winter solstice, when the midday Sun is at its lowest in the sky. After the solstice the days start getting gradually longer and the nights gradually shorter, although the change doesn’t really become noticeable until July.


Strictly speaking it isn’t true that for the whole northern hemisphere the midday Sun is at its highest in the sky on the solstice. At the Tropic of Cancer, which is 23.5 degrees north, and is shown as the upper red line in diagram below, the Sun is directly overhead at midday on the June solstice. At low latitudes between the equator and the Tropic of Cancer the Sun is directly overhead at midday on two dates either side of the solstice. For example, in San Juan, Puerto Rico, which lies 18.5 degrees North of the equator, the Sun is overhead at midday on May 13 and July 30.


Tropic of cancer


Jarus, O (2014) Stonehenge: Facts & Theories About Mysterious Monument, Available at: 10 June 2016).


Time and Date (2018) London, ENG, United Kingdom — sunrise, sunset, and daylength, June 2018, Available at: 4 June 2018).


The anthropic principle

In his 1988 book ‘A Brief History of Time’ the British theoretical physicist Stephen Hawkins (1942-2018) stated:

‘The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron. … The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life.’

In this post I will talk about the view that the laws of physics, and the properties of the Universe as a whole, are somehow finely tuned to allow our existence. The term ‘anthropic principle’ was coined in 1973 by the Australian physicist Brandon Carter (1942-) to describe this viewpoint. However, these ideas had been circulating for decades beforehand.

Brandon Carter – image from Wikimedia Commons

Since 1973 the ideas behind the anthropic principle have been reviewed in many books, both popular science and those aimed at the more specialised reader. They were developed in detail in a book written in 1986 by the theoretical physicists John Barrow and Frank Tipler called ‘The Anthropic Cosmological Principle’.


Background: atoms, the atomic nucleus and the four fundamental forces

Before we go into detail about the anthropic principle it is worth discussing, at a high level, the structure of matter and the fundamental forces which drive the way that everything in the Universe behaves.

All ordinary matter in objects like ourselves, planets and stars is made up of atoms. An atom consists of a central nucleus, which has a positive electric charge, surrounded by a cloud of negatively charged electrons. Atoms are very small, typically around 0.0001 microns in diameter (a micron is a millionth of a metre). However the nucleus, which contains nearly all the mass of the atom, is much, much smaller, typically around 0.000 000 001 microns in diameter.

The nucleus consists of a number of protons, which have a positive electric charge and neutrons which have no electric charge. Because the electrons have a negative charge, and the number of protons and electrons in an atom is always the same, atoms have a net charge of zero.

  • The number of protons in the nucleus is called the atomic number and determines the atom’s chemical properties. You may remember from high school chemistry that the atomic number gives the position in the periodic table.
  • The number of neutrons in the nucleus does not affect the chemical properties of the atoms. In fact, all elements have different versions of themselves called isotopes, which have a different numbers of neutrons but the same number of protons.

The simplest possible atomic nucleus is that of hydrogen, which consists of a single proton.  Ordinary hydrogen atoms have no neutrons, but a small fraction of naturally occurring hydrogen atoms are deuterium or heavy hydrogen which have one proton and one neutron. Atoms which have 2 protons (regardless of the number of neutrons) are helium atoms, those which have 3 protons are lithium atoms and so on.  The element with the highest atomic number which naturally occurs on Earth is uranium, which has 92 protons.




An atom of the most common isotope of carbon has 6 protons and 6 neutrons in the nucleus surrounded by 6 electrons. Other isotopes of carbon are found on Earth which have 7 and 8 neutrons in the nucleus.

The Universe is governed by four fundamental forces.  All other interactions, such as the combination of two hydrogen atoms and one oxygen atom to form a water molecule, are due to these fundamental forces.

  • Gravity – an attractive force which acts on all particles having mass.
  • Electromagnetic force – This only acts on electrically charged particles and can be an attractive or repulsive force. If two particles have the same charge, such as two protons, the force is repulsive. If two particles have different charges, such as a proton and an electron, the force is attractive.

  • Strong force – this force acts on fundamental particles called quarks.  There are six types of quark, which have the rather odd names of up, down, charm, strange, top, and bottom.
    • a proton consists of two up quarks and a down quark
    • a neutron consists of one up and two down quarks

Internal structure of a proton – image from Wikimedia Commons

Because protons and neutrons are made up of quarks they too are acted on by the strong force. The force between protons and neutrons which is sometimes called the ‘residual strong force’ or ‘nuclear force’ only works over extremely short ranges, of less than 2.5 femtometres, where 1 femtometre (fm) is one thousand trillionth of a metre. It is this nuclear force which binds protons and neutrons together into atomic nuclei. For more details on the strong force see the notes at the bottom of this post.

  • Weak force – this acts over a short distance, 0.01 fm to 0.1 fm. It is this force which is responsible for a particular type of radioactive decay called beta decay. Without the weak force there would be insufficient oxygen produced in stars to support life (Clavelli 2008).

In addition to these four forces there are other fundamental parameters such as the  mass of the proton and electron and the average densities of ordinary matter, dark matter and dark energy in the Universe. (See my previous posts for more information on dark matter and dark energy.)

Fine tuning

One thing that is apparent is that the relative strength of the four forces (and other  fundamental parameters such the density of matter in the Universe) appear to be finely tuned to enable the production of stars, planets and the eventual evolution of intelligent life. If they were only slightly different, the Universe would be a very different place and life would never emerge. To illustrate this I’ll give some examples below.

  • Hydrogen is the most common element in the Universe. In the early Universe 10 to 1000 seconds after the Big Bang, when it was at a temperature of billions of degrees, about 25% of the primordial hydrogen was converted to helium by nuclear fusion. If the strong interaction had been only 2% stronger than its current value then all its hydrogen would have been converted to helium in the first few minutes of the Universe’s existence. There would be no hydrogen compounds in the Universe, such as water which is, as far as we know, essential for life.
  • If the strong interaction were only a few percent weaker, then deuterium (heavy hydrogen) would not be stable. This would mean that certain elements essential for life such as nitrogen and phosphorous, which are made in the centre of stars by nuclear reactions which fuse deuterium with other nuclei, would not be formed to any appreciable degree.
  • If the electromagnetic interaction were three times stronger, and all other forces the same strength, then any element heavier than carbon (atomic number = 6) could not form. For these elements, the repulsive electromagnetic force between the protons in the nucleus would be stronger than the attractive nuclear force holding the nucleus together. Therefore elements such as nitrogen and oxygen on which life is based would not exist.

  • If gravity were a thousand times stronger, then stars would be much smaller and burn their nuclear fuel more quickly. Instead of living for ten billion years, a typical star would live for about 10 million years.  As readers of a previous post will recall, it took hundreds of millions of years from when the Earth was formed until the emergence of the first single-celled lifeforms. So, in a Universe with stronger gravity, these mini-suns and would have stopped shining before even the first steps in evolution had started.

  • If the strength of all the forces were the same, but there were much less matter in the Universe, then the way the Universe evolved would have been very different. In our Universe, initial unevenness in the distribution of matter in its early stages eventually became the structures such as stars and galaxies which we see today. If the density of ordinary matter were 10% of its current value, then structures such as stars and galaxies would not have formed.

The strong and weak anthropic principles

In his 1973 work Carter distinguished between the strong anthropic principle (SAP) and weak anthropic principle (WAP).  Since then there have been many slightly different definitions of the WAP. The one below is from Barrow and Tipler (1986):

‘the observed values of all physical  and cosmological constants are not equally probable but they take on values restricted by the requirement that carbon-based life can evolve..’

The WAP is generally accepted by most astronomers. In fact it has been criticised as a tautology – a statement which must be true. For if conditions were very different, so that life couldn’t evolve, humanity wouldn’t be around to observe them. Even so, it is remarkable how finely tuned the Universe is.

Many physicists believe in the multiverse –  a collection of possibly an infinite number of other universes.

In other universes different relative strengths of the fundamental forces might apply, so they would look very different from our Universe  In some of them there might be more or fewer than four fundamental forces, perhaps even more than three dimensions of space. It seems likely that in the vast majority of these other universes conditions are such that life can never evolve.

While the WAP is generally accepted, the SAP is more contentious. The definition given by Barrow and Tipler is as follows:

‘The Universe must have those properties which allow life to develop within it at some stage in its history.’

A variation to this definition was proposed by the physicist John Archibald Wheeler (1911-2008) in 1977, which he called the participatory anthropic principle.

‘Observers are necessary to bring the Universe into being’

What  the SAP is saying is that we cannot have a universe which doesn’t have, or have the potential to have, any observers. In some way the purpose of the universe it to give rise to intelligent observers. The SAP is not generally accepted by most astronomers. One particular criticism of it is that we cannot falsify it by observing a  ‘dead universe’ in which observers cannot exist. This is because other universes are, by definition, unobservable.

Final Anthropic Principle

Even more controversial is the Final Anthropic Principle (FAP). This idea was developed by Barrow and Tipler. They could see little point in having a Universe which has as its purpose giving rise to intelligent observers and these intelligent observers then become extinct. The FAP is defined as follows:

Intelligent information-processing must come into existence in the universe, and, once it comes into existence, it will never die out.

In the final chapter of The Anthropic Cosmological Principle they outline a future of the Universe in which the FAP is true. In this universe billions of years after the Big Bang intelligent carbon-based life (i.e ourselves ) eventually emerges. Over a long period of time as the civilisation develops it evolves into different forms of life, which are more robust and better able to survive the harsh conditions of interstellar travel and the long timescales needed. These new lifeforms will not be carbon based but could be for example intelligent self-replicating robots. The civilisation then spreads to neighbouring stars and eventually spreads throughout the galaxy.

Eventually it spreads to neighbouring galaxies and through the entire Universe. If in the far future, the Universe’s expansion slows down and stops and it starts contracting,  it may end in what is termed a singularity of infinite density where the Universe will come to an end and space and time will cease to exist. Barrow and Tipler called this point in time the Omega Point.

At the Omega Point life will have gained complete control of the Universe and will be able to process an infinite amount of information. The final two sentences of The Anthropic Cosmological Principle state:

‘At the instant the Omega Point is reached, life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know.  And this is the end’

In general the scientific community reacted with scepticism to the FAP.  To many it smacks of pseudo-science. In a review of The Anthropic Cosmological Principle the American science writer Martin Gardner (1914-2010) called it a ‘Completely Ridiculous Anthropic Principle’ which he suggested should be abbreviated to CRAP. In addition, although in the 1980s many astronomers believed in a Universe which wold would eventually collapse into singularity, as discussed in a previous post, this is not supported by current observations. The general consensus is that we live in a Universe which will expand forever and so there will be no Omega Point.



Although there are six types of quarks up, down, charm, strange, top, and bottom, which are sometime called ‘flavours of quark’, only the up and down quarks are found in nature. The other four flavours rapidly decay into either an up or down quark. Interestingly quarks are never found free but always in combination with other quarks. The combinations are:

  • mesons which consist of one quark and one antiquark
  • baryons which consists of three quarks or three antiquarks

The strong force, which acts on quarks because they have a property called colour charge, is carried by particles called gluons.  Gluons also have colour charge and so are subject to the strong force too. For a non-technical overview of the strong force see the following:


Clavelli, L. (2008) Problems in a weakless universe, Available at: 24 May 2018).

Jupiter at opposition 9 May 2018

On May 9 the planet Jupiter will be what is known as ‘at opposition’.  This event, which occurs every 399 days, happens when Jupiter is at its closest to the Earth and at its brightest.  To the naked eye it will be a brilliant white object, three times brighter than the brightest star. Features such as coloured bands and the famous great red spot can easily be seen with a small telescope.


Jupiter through a telescope – image from NASA

What is opposition? 

The series of diagrams below show Jupiter and Earth at different points in their orbits around the Sun. The Earth takes just over 365 days to complete an orbit. Jupiter, which is further away from the Sun and moves more slowly in its orbit, takes nearly 12 years.

In the first diagram, below, Jupiter is at its closest point to the Earth  As seen from Earth, Jupiter is in the opposite direction from the Sun. This is why it is called opposition. All night between sunset and sunrise, Jupiter is above the horizon and it reaches its highest point in the sky in the middle of the night. Jupiter is at its brightest at opposition because it is at its closest to Earth and the entire sunlit side is facing Earth.


In the diagram below, 133 days later, the Earth has completed more than one-third of its journey around the Sun, whereas Jupiter has done less than 3%. Jupiter appears less bright because it is further away from Earth and not all the sunlit side of Jupiter is facing us.  In addition, Jupiter appears fairly close to the Sun and for most of the time is only above the horizon during the daytime, when it is very difficult to see against the brightness of the daytime sky.

199.5 days later the Earth has completed more than half its journey around the Sun, whereas Jupiter has done just over 4.6%. At this point, which is known as a conjunction of Jupiter and the Sun, Jupiter is only above the horizon in the daytime and is impossible for an amateur astronomer to see without specialist equipment, because it is so close to the Sun.

399 days later, the Earth has caught up with Jupiter, so the two are level again in their orbits. Jupiter is once again at opposition.



How bright is Jupiter at opposition?

When discussing the brightness of objects in the sky, astronomers use a scale called magnitude, where the lower the magnitude the brighter the object.

The scale was invented by the ancient Greek astronomers who classified all the stars visible to the naked eye into six magnitudes. The brightest stars were said to be of magnitude 1, whereas the faintest were of magnitude 6, which is the limit of human visual perception (without the aid of a telescope). Today, the magnitude scale is applied to all objects in the sky, not just stars, and the magnitude of the very brightest objects is less than zero. The brightest objects in the sky are (obviously) the Sun, which has a magnitude of -26.7, followed by the Moon, which has a magnitude of -12.7, at a typical full Moon.

The scale is defined so that a decrease in magnitude by 1 means an increase in brightness by a factor of 2.512. Therefore, a decrease in magnitude by 2 would mean an increase in brightness by 6.31, because 2.512 x 2.512 = 6.31. So, for example:

  • a star of magnitude 1 is 15.9 times brighter than a star of magnitude 4. This is because 2.512 x 2.512 x 2.512 = 15.9.
  • a star of magnitude 1 is 100 times brighter than a star of magnitude 6. This is because 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100

Jupiter moves in an elliptical orbit around the Sun; this means that its closest distance to Earth is different at each opposition. Therefore, as shown below, the brightness at each opposition will vary as well.

The diagram above shows that if an opposition occurs where Jupiter is closest to the Sun (point A,) Jupiter will also be closest to the Earth and thus brighter than an opposition which occurs where Jupiter is furthest from the Sun (point B) . In the diagram the elongation of Jupiter’s orbit has been exaggerated. 

The closest Jupiter can get to Earth is 589 million km; when this happens it shines with a magnitude of -2.9. On the May 9 opposition, it will be 658 million km from Earth and will shine with a magnitude of -2.5, making it three  times more luminous than the brightest star Sirius, which has a magnitude of -1.4.  For comparison, the table below shows the average magnitude at opposition of the planets which lie outside the Earth’s orbit.


Data from

How Does the brightness of Jupiter compare to Venus? 

For the two inner planets Venus and Mercury, the situation is a little different. They can never be at opposition because they lie inside the Earth’s orbit. This is shown in the diagram below for Venus.

Venus Phases

As Venus orbits the Earth it goes through phases, similar to those of the Moon. However, because Venus is so small, they are only visible through a telescope. When Venus is closest to the Earth, the point known as its inferior conjunction – labelled A in the diagram – it is almost impossible to see because it is almost in a direct line of sight with the Sun and its sunlit side is facing away from Earth. Venus is actually at its brightest just before and just after inferior conjunction,  points B and F, when it has a magnitude of around -4.5.

Although Venus appears brighter than Jupiter, unlike Jupiter it is never above the horizon all night. At point B it is only clearly visible for few hours before sunrise where it is known as the Morning Star. At point F it is only visible for a few hours after sunset where it is known as the Evening Star.

Properties of Jupiter

As nearly all my readers will know, Jupiter is the largest planet in the Solar System. Its diameter is on average 140,000 km which is roughly 11 times that of the Earth, making its volume 1320 times larger (Williams 2017). It is its large size which causes Jupiter to appear brighter than Mars despite it being more than three times further from the Sun.

Unlike the smaller inner planets (Mercury, Venus, Earth and Mars) which have large iron cores surrounded by rocky materials, Jupiter is mainly composed of gas. It is not known if it has a solid core, but if one exists it will only make up a small proportion of the planet. Being made up of largely of gas means that its density is only 25% that of the Earth. Even so, its mass is still 320 times greater, making it more massive than all the other planets, moons, asteroids and comets in the Solar System put together.

Exploration of Jupiter

So far nine unmanned spacecraft have visited Jupiter. The first was Pioneer 10 in 1973 shown below.


Image from NASA

This was followed by Pioneer 11 (1974), Voyager 1 and Voyager 2 (both 1979), Ulysses (1992), Galileo – which went into orbit around Jupiter between 1995 and 2003, Cassini (2000) and New Horizons (2007). The latest mission Juno (shown below) arrived in 2016 and is currently orbiting the planet studying its atmosphere, magnetic and gravitational fields. For more details on the Juno mission see my post Mission Juno.

Juno at Jupiter

Image from NASA

Jupiter’s moons

Jupiter has its own mini “solar system” of over 60 moons in orbit around it. The four largest moons were discovered by Galileo in 1610 and one of them, Ganymede, the largest moon in the solar system, is bigger than the planet Mercury. The innermost moon Io is the most volcanically active object known to exist anywhere. Europa, the second innermost, is of particular interest because its surface is composed of ice underneath which are thought to lie oceans of liquid water, warmed by a process called tidal friction. Many scientists think that Europa is one of the most promising places in the Solar System to find extraterrestrial life.

NASA  and the European Space Agency (ESA) are developing missions to Jupiter’s moons, will tell us a lot more about them . The ESA mission is called  (JUICE)  which stands for JUpiter ICy moon Explorer and the NASA mission is called Europa Clipper. Both missions are scheduled for launch in 2022.

Jupiter Moons

The four large moons of Jupiter: Io, Europa, Ganymede and Callisto -Image from NASA

I hope you’ve enjoyed this post and that you will have a clear night on the daysaround 9 May to see this sight.


In the discussion of magnitudes all the values quoted are visual magnitudes. This is how bright the objects are in light of wavelength of 540 nanometres, which is in the green part of the spectrum. The values at other wavelengths will be different for different objects. For details on light and the way the eye sees different wavelength as different colours see


Williams, D. R. (2017) Jupiter Fact Sheet, Available at: (Accessed: 28 April 2018).

The Rare Earth hypothesis

Ever since the pioneering work of Frank Drake (1930-) in 1960, astronomers have been looking for radio signals from extraterrestrial civilisations and have failed to find anything. This could be because Earth-like planets containing complex life forms (such as ourselves) are rare in the Universe and only a series of highly improbable events led to the evolution of intelligent life on Earth. In 2000 geologist and palaeontologist Peter Ward and astronomer Donald Brownlee published a book in which they explained the term ‘Rare Earth Hypothesis’ which they had coined to describe this viewpoint.

However, they were not the first people to arrive at this conclusion. These ideas had been circulating for decades before the publication of their book. For example, the astronomers John Barrow and Frank Tipler discussed them in detail in their 1987 book ‘The Anthropic Cosmological Principle’.

A little background – the Drake Equation

Back in 1961, Frank Drake invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  His equation, known as the Drake equation, consists of seven numbers multiplied together:

N=  R* x FP x NE x FL x FI x FC x L

in which

  • R* is the average number of stars formed per year in our galaxy. Current estimates are that this has a value of around 10.
  • FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Current estimates are that FP is very close to 1.
  • NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. For this to happen liquid water must exist somewhere on the planet.  A reasonable value which many astronomers would agree with is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.
  • FL is the fraction of bodies with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL.
  • FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, again expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value is.
  • FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is.
  • L is the average lifetime of a civilisation in years.  Again, there is no consensus on this point.

As the values of many of the terms in the Drake Equation are not known to any degree of accuracy, it cannot be used to provide a meaningful estimate of N.  However, it is still very useful to illustrate the factors involved

Could we be alone? Could FI could be very very low?

Perhaps the term with the biggest degree of uncertainty in the Drake equation is FI, the fraction of bodies having life on which life has evolved into intelligent civilisations. I will outline the arguments for believing that this number is very low indeed.

The Earth is roughly 4.6 billion years old. Simple single-celled lifeforms emerged 300 million years later, a relatively short time after the Earth had cooled enough for liquid water to exist. These simple cells, called prokaryotes, cannot form complex organisms where different types of cells perform different functions. However, individual prokaryotes  can group together in colonies, forming a kind of slime.

Colonies of prokaryotes

All complex life on Earth is based upon cells called eukaryotes. These cells have a nucleus (containing the genetic material of the cell), structures called mitochondria, which regulate the cell’s energy and other specialised units known as organelles.

The first eukaryotes didn’t emerge until 2 billion years after prokaryotes, indicating that it was a much bigger step in evolution than the emergence of the first simple cells. It could well be the case that the large number of sub-steps needed in the evolution from prokaryotes to eukaryotes means that even where simple lifeforms have emerged,  in the vast majority of cases there has been no further evolution beyond this stage.

Even after the emergence of these complex eukaryotes, it would take over one billion years before multi-cellular life forms such as the first plants and animals appeared. In these organisms cells are specialised, so different types of cells perform different functions within the organism. Given the large amount of time taken to move from eukaryotes to complex organisms, it might well be the case that even if something akin to eukaryotes emerge on a planet, which have the potential to eventually evolve into multi-celled organisms, evolution proceeds no further. The average time taken to evolve from complex cells to multi-celled organisms given favourable conditions might be 2, 5 or even 10 billions years.

The role of mass extinction events

It has taken 600 million years from the appearance of the first animals to the emergence of Homo sapiens in Africa, around 200,000 years ago. Over this vast amount of time the vast majority of species have disappeared and have been replaced by other species which are better suited to the changing environment. However, the disappearance of species and emergence of new ones doesn’t occur at an even rate. Every 50-100 million years there have been catastrophic events which have caused periodic mass extinctions when a large number of species failed to survive. The most dramatic of these was at the end of the Permian period, around 230 million years ago, when 95% of land and 70% of sea species became extinct.

Fossil of a Trilobite – one of many species to disappear in the mass extinction at the end of the Permian Period

Perhaps the best known mass extinction occurred 65 million years ago when a massive comet or asteroid 10 km in diameter hit the Earth at a speed of up 50,000 km per hour. Its high speed coupled with its huge mass meant that it smashed into the Earth in the area now known as the Yucatan peninsula with an energy 6 billion times greater than the atomic bomb dropped on Hiroshima at the end of the Second World War.

KT extinction asteroid

The impact melted much of the local crust and blasted molten material outwards. Any object near to the impact site would have been instantly vapourised. Such was the energy of the impact that some of the Earth’s crust was thrown upwards with so much velocity that it went out into space. Over the next few hours molten rock, dust and ash rained down on an area millions of square kilometres in area. This hot material would have ignited fires, destroying plant and animal life within a large area.

Kt ejecta map

The impact 65 million years ago occurred in what is now the Yucatan peninsula in Mexico.  The outer ring shows the area which become covered in debris.

A fine cloud of dust from the impact circled the entire Earth. This blocked out sunlight, causing the Earth’s temperature to fall by about 15 degrees Celsius. The sudden drop in temperatures and lack of sunlight getting through meant that plant growth stopped for several months.  As a result, many species of herbivorous animals which fed on those plants became extinct and the carnivores which fed on the herbivores became extinct too. Those species which did survive had their numbers drastically reduced.  Nearly 75% per cent of the species of animals alive before the impact became extinct, including every species of dinosaur. After the disappearance of the large reptiles, mammals became the dominant land animals.

Although we as a species are now probably sufficiently advanced as to be able to survive a mass extinction event, even if only in small numbers, this would not have been the case if one had occurred earlier in our development. If one had happened 150,000 years ago, when Homo sapiens were few in number and less equipped to withstand a major famine – those hunter gatherers would have had nothing to hunt and nothing to gather for many months – we would have become extinct long before we could develop civilisation.

Indeed,  if mass extinctions were to occur every million years, rather than every 50 million years, it is difficult to see how any intelligent species could ever evolve. It would be wiped out before it became fully established. The reason why mass extinctions occur infrequently is due to the unusual layout of the Solar System, and I’ll talk about this next.

Our special Solar System

The Sun is a single star in an uncrowded region of space, in the outer regions of the Milky Way galaxy. The nearest star to the Sun is over 4 light years (40 trillion km) away, more than ten thousand times further than the outermost planet Neptune. If the Sun were in a more crowded region of the galaxy, a passing star’s gravity could easily disrupt the Earth into a different orbit, which might be closer to Sun, making it too hot to support life, or further away, making it too cold to support life.

The giant planet Jupiter is more than 300 times the mass of the Earth. As comets enter the inner Solar System from its outer reaches, Jupiter’s gravity slings most of these fast-moving ice balls out of the Solar System before they can get close to Earth.  Without Jupiter, comets like the one which hit the Earth 65 million years ago would collide with our planet much more frequently.

Image from NASA

Observations of planets detected around other stars have shown that arrangements of planets similar to our Solar System, with small inner rocky planets surrounded by massive giants in the outer reaches, are relatively rare.

Another factor which may be essential to the emergence of complex lifeforms is the presence of a large moon close to the planet. The Moon is 25% of the Earth’s diameter and is only 400 000 km away, a very short distance in astronomical terms.

Relative sizes of the Earth and the Moon

The Moon  is much larger in comparison to its parent planet than any other moon in the Solar System. This means that the Moon’s gravity stabilises the Earth’s tilt so that it doesn’t vary too much from its current value. Without the Moon there would be massive swings in the tilt between 0 degrees, when there would be no seasons, and 50 degrees, where there would be extreme seasons where much of the planet would be in darkness or full daylight for months at a time.

The Earth’s magnetic field

The Earth’s magnetic field is generated by convection currents stirred up by rotation in its liquid outer core, which is made out of iron. In fact for any planet to have a strong magnetic field, part of its interior must consist of a liquid which conducts electricity and it must be rotating rapidly enough to generate convection currents.

Earth Interior

Without a magnetic field the planet would not have a protective ozone layer shielding its surface from deadly ultraviolet radiation. It would be very difficult for advanced lifeforms to exist on the surface.  It is not known how many planets around other stars satisfy the criteria to have a magnetic field. However, the Earth is the only one of the inner planets in our Solar System (Mercury, Venus, Earth and Mars) to have a strong magnetic field.

Planetary size

In order for life to evolve, the planet must be large enough to retain a significant  atmosphere, without which liquid water cannot exist. Mars is roughly one tenth the mass of the Earth and its gravity is so low that its atmospheric pressure is only 0.6% of that of Earth – too low for liquid water to exist.

Man has been lucky to survive

In addition to periodic mass extinctions which wipe out most species, all populations come under pressure due to sudden changes in climate, such as ice ages, major volcanic eruptions, loss of habitat, disease and competition from other species for food and shelter. The first Homo sapiens were confined to a single region of Africa and were few in number, and thus vulnerable to becoming extinct. 70,000 years ago a super-volcano erupted in Indonesia causing the Sun to be partially blocked out and resulting in a fall in global temperatures. Only an estimated 15,000 humans are thought to have survived (Edwards 2010).


If we take all the factors I’ve talked about into account, for single-cell organisms to evolve into intelligent lifeforms capable of building civilisations, all of the following must happen.

  • Single cell creatures must have evolved in into complex cells, something akin to eukaryotes.
  • To reduce the frequency of mass extinctions, the planetary system must have a large giant planet in its outer reaches.
  • To ensure the planet remains at a habitable temperature, the parent star must be a single star in a less crowded region of the galaxy.
  • To ensure the planet remains at a habitable temperature over a long period of time, the parent star’s energy output must not vary too much.  This constraint rules out advanced life around the most common type of star in the galaxy, red dwarfs .
  • To reduce the frequency of dramatic changes in climate leading to mass extinctions, the planet must have a large satellite.
  • The planet must rotate rapidly enough to generate a magnetic field, as without this complex life would not be possible on its surface.
  • The planet must be large enough to hold an atmosphere.
  • Multi cellular organisms must have evolved.
  • The planet must have a significant fraction of its surface as dry land. The assumption is that although aquatic creature such as dolphins are undoubtedly intelligent, only land based creatures could build civilisations.
  • A land-based intelligent species has emerged during a survival of the fittest.
  • The intelligent species has managed to survive major environmental changes such as ice ages and has built a civilisation.

Putting all these together, followers of the Rare Earth hypothesis believe that the probability of all the above happening on a planet on which simple life has evolved could be as low as a billion to one.

If it were this low, then the expected number of intelligent communicating civilisations in a galaxy such as ours would be very low.  If the disputed figures within the Drake equation were guessed to be on the low side, N could be as low as 0.000016.  This would mean that not only are we the only intelligent communicating civilisation in our own Milky Way galaxy, but also that there are none in any galaxies within tens of millions of light years of us.

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed!

I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


Edwards, L. (2010) Human were once an endangered species, Available at: (Accessed: 7 April 2018).