What’s so special about the Earth?

Image adapted from NASA’s public-access library (www.nasa.gov)

I set myself a challenge earlier in the year: to put together a talk for our local branch of the U3A (see here for details) highlighting some of the combination of factors which foster the life that abounds on our planet. What is it, from the perspective of a physical scientist, which helps to make this ‘third rock from the Sun’ into a jewel? Meeting this goal turned out to require significantly more time and thought than I had bargained for. It’s a topic that has intrigued me since taking an optional course in geophysics whilst I was an undergraduate Physics student in the early ‘70s. However, getting stuck back into some reading – actually, quite a lot of reading – and trying to craft an equation-free talk which would encapsulate some key areas for a group of intelligent non-specialists needed all my creative ‘muscles’. Hindsight assures me that this was no bad thing as I learned a lot in the process. What’s that old saying? ‘If you want to understand something better, teach it’, or words to that effect. So true.

Rather than consume paper (and toner) generating hand-outs for my lovely U3A participants, I decided to post the core of the material on this blog so that they can access it at their leisure. Given its motivation, the post is almost necessarily on the long side, and it’s information/fact-heavy, so you may want make yourself a nice brew before you sit down to read it. The act of publishing this synopsis may of course mean that, were I to offer the talk again next year, no-one would register because it would be easier simply to read this post. However, my experience of making notes available to students, or even of audio/video-recording lectures during the latter decade of my career (see here), tells me that a good (!) ‘live performance’ will always draw people in. Numbers didn’t drop off at all back then, and I’ve no reason to think that a mere blog post would do anything similar now; so, here it is …

After introducing myself, and making it clear that this was not an area of particular expertise – not itself an issue within the U3A framework since it’s designed to foster a form of community learning – we took a look at where we are. Starting at the scale of the Milky Way, our home galaxy amongst the billions of others to have formed since the Big Bang, we zoomed in to the Solar system. Not that ‘zooming in’ seems entirely sensible in this context, but it does help to set the scale of things. Having established where we are and the approximate size of things, the next obvious question relates to how the Earth and other planets came into being. I’ve touched on this in an earlier post (click here) so I won’t needlessly take up space by repeating it. There is, however, a piece of news hot-off-the-press which does need to be added to this earlier account. On 2nd July – so a few days before this post was drafted in support of my U3A talk, the European Southern Observatory issued a press release (available here) outlining the first confirmed direct observation of a planet in formation around a dwarf star. 

Our Solar System: Note the distance scale: our ‘measuring stick’ is the distance between the Earth and the Sun (150 million km, called an astronomical unit – AU). On this scale, the dominance of the Sun’s magnetic field and the Solar wind extends beyond the planets to about 100 AU. Travelling at prodigious speeds since its launch in 1977 (currently in excess of 17 km per second) Voyager 1 only reached this region in 2012. There is a diffuse orbiting collection of cometary material called the Oort Cloud, left over from the birth of the system, a further factor of 100 beyond. (Image: photojournal.jpl.nasa.gov/catalog/PIA17046)

The rest of the talk served to explore the beneficial consequences to life of a few key facts:

• Our Solar System is only 4½ Gy (billion years) old, which makes it quite young in the context of the time since the Big Bang (13.8 Gy). The lyric from the 1960s musical ‘Hair’ becomes apt at this point: “we are stardust, we are golden”. The point being that there have been multiple generations of stars before the Sun, many of which exploded towards the end of their lives as supernova and in the process created all the heavier elements of the periodic table. These new types of atoms were blown out across space, eventually to be incorporated into planetary systems around later generations of stars. New stars are still being formed within the Milky Way. 

• The Sun, which represents 99.9% of all the mass in the Solar system, is a ‘middle-aged main sequence’ star which means that it’s been stable for about 4 Gy – lots of time for life to develop. It resides in what we might term a ‘quiet suburb’ of our galaxy; we have no black holes or analogous threats in our neighbourhood, which is good. Our nearest neighbour galaxy, Andromeda (M31 in the formal catalogues) is actually heading towards the Milky Way at over 400,000 km/h – but because it’s 2½ million lightyears away we still have several billion years before it arrives. 

• The Earth is a rocky planet with a molten core, travelling in a near-circular orbit around the Sun with an average radius of 150 million km; one complete orbit takes 365¼ days. All of which tells us that, given the energy output of the Sun, we’re at just the right distance for there to be liquid water at the planet’s surface – if there’s any water present that is. This relatively narrow band of distances from a star is often referred to as ‘The Goldilocks Zone’; Venus and Mars, our nearest neighbour planets exist right at the inner and outer fringes of the zone respectively. Moreover, whilst a highly elliptical orbit might take us repeatedly in and out of the zone, our near-circular orbit keeps the Earth within it all the year round. If the Sun were to be cooler than it is – and there are plenty such stars out there – the Goldilocks (or ‘Habitable’) Zone would have a smaller radius. Planets close to their star tend to have very short ‘years’ and are often locked into having a single face pointing towards the star (- much like the Moon with respect to the Earth: we only ever see one face). This means that half the planet’s surface would be warm and the other half cold – even if there was an atmosphere, the weather patterns would be quite unlike our own and one might even see any water present condense on the cold side. 

• The Earth has a radius of 6,378 km at the equator and mass 6 x 10²¹ metric tonnes (6000 billion billion). This tells us immediately that it has the sort of density that allows such phenomena as tectonic plate movement to occur. Venus, for example, also has tectonic plates, but their density is such that subduction apparently does not take place; this is the process whereby one plate ‘dives’ down below another as they drift towards each other, generating life-giving volcanic activity for instance. Tectonic plates form a crust on the Earth’s surface and move because we they float on a molten core beneath. This fact implies that the Earth’s temperature, beneath its surface layers, must be high enough to create and maintain the molten magma. Some of this heat energy derives from when the Earth was formed out of the violent impacts between dust, asteroids and comets – there hasn’t been time for it to have radiated away into space yet – but at least half of the heat energy comes from continuing radioactive decays. Which fact takes us right back to the benefits of being formed relatively late in the life of the universe such that all these usually heavy radioactive elements, like uranium, had already been created and spread by earlier generations of stars exploding as supernova. 

The Earth’s mass is high enough for us to hang on to an atmosphere, unlike Mars which has lost much of its atmosphere. Our atmosphere is very thin, and it’s fragile, but it’s there – and tectonic plate movements help to regulate its makeup of gases as well as helping to regulate the planet’s near-surface temperature. This stunning image of the sunrise viewed from the International Space Station was taken by Canadian astronaut Chris Hadfield – it reveals the thin shell of our precious atmosphere covering the curve of the Earth.

• The Earth possesses a magnetic field, which is generated because we have an inner core kept solid by the immense pressures at those depths, that rotates within a fluid outer core. The relative motion generates the magnetic field. This turns out to be far more important to life than simply providing a means of navigation. The central point here is that any charged particle moving through a magnetic field will experience a force. This is the same school-level physics that explains why an electrical current in a wire (which is another way of talking about the movement of electrons) can be used to create the motion of an electric motor if suitable magnets are place appropriately nearby. Thus, the Sun’s solar wind, which is largely made up of fast-moving charged particles, is mostly deflected around the Earth by this invisible shield and never reaches the surface. This is a good thing since energetic solar wind particles could have similar detrimental effects on living tissue as exposure to radiation. Furthermore, as Mercury, Venus and Mars tell us, the Solar wind is well-able to strip away the gas molecules that make up a planet’s atmosphere if they’re close enough to the Sun. (Venus retains a fairly dense atmosphere: it’s mass is high enough that its gravitational pull can, by and large, hang on to it – but its lack of a magnetic field means that measurable amounts of it are continually being stripped away from the planet.) 

Shields up! The two regions of the Earth’s atmosphere which do experience the effect of the solar wind are the poles. Here, the magnetic field lines dip down towards the north and the south poles, allowing charged particles to travel into the atmosphere. Collisions with oxygen and nitrogen molecules in the atmosphere give rise to the aurora. (The images above are from https://scijinks.gov/aurora/ and https://ase.tufts.edu/cosmos/view_picture.asp?id=356, left and right respectively. For a stunning view of the aurora, extending as though a coronet around the Earth, watch this video – shot from the International Space Station, or a slightly longer compilation of ISS videos here.)

• The Moon is the fifth largest moon in the Solar System, and by far the most massive moon in proportion to its planet. Indeed, the Moon’s mass is a full 1.2% of the Earth’s. This might not sound a lot, but consider the solar system’s larger moons: they orbit planets that have far, far higher masses than the Earth. For example, Titan has more than three times the mass of the Moon but that still represents only 0.04% of the mass of its planet, Saturn. Thus, we have a moon that exerts a strong effect on our oceans, creating the tides. There is a more subtle element to our relationship with the Moon however: it is massive enough to stabilise the angle of tilt of our rotation. In other words, it stops us from ‘wobbling’ too much, thereby granting us long-term stability in terms for our climate’s seasons. The Earth spins at an angle of 23.5º. That’s what gives us our solstices and our beneficial seasons as any given region of the Earth’s surface will tilt towards or away from the Sun as its orbit (the year) progresses. This is a ‘middling’ value – Mercury’s is 0.03º whilst Uranus’ tilt is at 82.2º. However, more important for the emergence and sustainability of life is the fact that it doesn’t vary much. Compare this to Mars’ tilt, which shifts between 10º and 60º in timescales of a mere million years or so and thereby alters its climate fairly rapidly. It takes a proportionately big moon to be able to stabilise the tilt of its host planet in this way. 
• There’s another consequence of the Moon’s large mass relative to the Earth which may well have an impact on tectonic plate movement. We naively think of the Moon orbiting the Earth in the same way as the Earth orbits the Sun, and in a sense it does. However, the physics of the situation tells us that whenever two bodies are tethered together – in this case via a gravitational force – they will rotate around their mutual centre-of-mass. In other words, both the Moon and the Earth rotate around this centre-of-mass, and it’s the centre-of-mass that orbits the Sun. I have tried to illustrate this in the simple diagram shown below. The fact that the Moon is massive enough to pull the Earth to and fro during each of the 28 days of the lunar month, albeit by a small amount, may well be important in terms of plate tectonics. 

An equal mass at either end of our beam means that the centre-of-mass – the balance point – must be in the middle of the rod connecting them. However, if one of the masses is only half the other then the balance point shifts towards the more massive end; the position of the centre-of-mass shifts along the rod in proportion to the masses. Now, the Earth is 81 times as massive as the Moon so, for the Earth-Moon system the centre-of-mass is actually within the Earth: about 1700 km beneath the surface in fact. This animation may help you to visualise what’s going on.

Thus, whilst there are undoubtedly many millions of planets even in our own galaxy, there are several important things that need to be in place before any of them could truly be called ‘Earth 2’. We inhabit an amazing planet that has nurtured life. It behoves us to treat it accordingly.

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Postscripts

1. Although drafted in the week prior to delivering my talk, I delayed publishing this post until afterwards ... just in case. It is for others to tell me whether I succeeded in conveying my passion as a non-expert scientist for this topic, but I must record my appreciation for the participants. There were some cogent and challenging questions posed throughout - the answers to some of which lay outside the bounds of my amateur understanding - and several genuinely helpful contributions. I got to 'talk science' and I came away having learnt something - I think that's called 'win-win'!
2. This opinion piece in the Guardian newspaper (here, published several days after my talk and after this post was uploaded) perhaps adds fuel to the debate on whether we are 'alone' in the universe or not. You may have heard of the Drake Equation, which set out to quantify estimates for intelligent life existing other than on the Earth and concluded that there is likely to be many examples, even in our Milky Way. Enrico Fermi, a hugely important person in the annals of 20th century physics, articulated a paradox (e.g. see here): if there are so many civilisations out there why is it that we've seen precisely none?  The Guardian's opinion piece reflects on this.