The earth is dominated by the sun, whose formation lead in turn to the formation of the earth, and whose supply of electromagnetic energy (and gravitational attraction) has driven nearly all of the processes affecting earth’s development[i].  The emergence and development of life is key amongst these influences.  Therefore, the future of life on earth is linked to the future of the sun.

The energy of a main-sequence star (of which the sun is an example) is primarily derived from the fusion of hydrogen into helium.  This ‘burning’ of hydrogen takes place in the core of the sun, where the temperature and pressure are at the levels required for hydrogen fusion to occur.  This process, which releases energy in the form of gamma-ray photons, alters the relative proportions of these elements within the sun.  Over time, the hydrogen fuel within the sun’s core will be expended.  At this point, a star is said to evolved through its main sequence.  The ‘life expectancy’ of a star is therefore based on its mass, as this directly determines the quantity of fuel available to the star. Based on measurements of the sun, it has been burning hydrogen for approximately 4.6 billion years, and has a life span of 10 billion years[ii].  Thus, in the year 4.9 billion AD, the sun will have been burning hydrogen for 9.5 billion years[iii], and will be approaching the end of its main sequence life cycle.  This process has significant implications for planet earth, and life on it. As the sun consumes hydrogen, and creates helium, the helium percentage in the core rises.  Because a helium atom’s nucleus contains 4 particles, it has a greater density than individual hydrogen atoms, which have just one proton[iv].  The increased density increases the core sun’s temperature.  This rise in temperature results in an increase in outward pressure, which causes the sun to expand.  The increase in energy output increases the temperature of the sun’s surface (as per the Stefan-Boltzman law for blackbody radiation emissions - energy flux is proportional to the fourth power of temperature), and also increases the luminosity of the sun (which will be effected by both the increased radius and temperature). When the sun has completed hydrogen burning, it is estimated that it will have a radius of one astronomical unit – that it’s surface will be at the distance of the earth’s current orbit.  However, this radius is the result of expansion caused by the processes that follow the exhaustion of hydrogen fuel, rather than  a steady increase in radius over the suns lifetime. Since the sun entered the main sequence, its radius has increased about 6%[v].  Estimates of the increase in size over the next 5 billion years are similar to the rate of growth that has been experienced so far[vi].  Therefore, the sun will increase in size, but only by about 10-15%.  Thus to a space-borne observer at 1 astronomical unit from the sun, it will appear larger and significantly brighter than it currently is. However, to an earth based observer, this may not be the case.  As the sun’s temperature rises, the earth will receive a greater amount of electro-magnetic radiation.  A rise in temperature will heat the earth’s surface, and increase the concentration of carbon-dioxide and water vapour in the atmosphere (mostly from the ocean).  Whilst this temperature increase will only be of a modest amount, the increase of these gases in the atmosphere will trigger a greenhouse effect, whereby visible light will penetrate the atmosphere, but reradiated infrared radiation will be trapped by the atmosphere.  This heat capture will raise the temperature more dramatically, and cause further release of greenhouse gases, creating a runaway greenhouse effect, that will eventually lead to the oceans boiling, and the earth’s atmosphere resembling that of Venus – much higher temperature (480° C) and atmospheric density (90 earth atmospheres). Estimates for the timeframe for this occurrence range from 500 – 800 million years[vii], although the icecaps may melt in tens of millions of years (this assumes no other factors altering the levels of greenhouse gases in the intervening time frame).  As a result, the hypothetical earth based observer would most likely be unable to see the sun through the dense atmosphere of the earth. As water is a key component for the existence of life, this heating of the earth would make the planet very inhospitable to life, and humans would need to take steps to ensure mankind’s continued existence.  Fortunately, the lengthy time frames involved allow a number of options to be considered. Remaining on earth would require moving to an underground existence (such extreme temperatures would melt most building materials).  However, underground living would be limited to a small section of the earth’s crust (the lithosphere is heated underneath by the mantle, increasing temperatures as depth increases), and population densities would need to be drastically reduced.  Also, retaining water within this environment (i.e. preventing escape into the atmosphere where it would be very difficult to retrieve) would be a great challenge. A more viable option would be to move to an environment further from the sun, which would potentially be made more hospitable by the sun’s evolution.  A key candidate would be Mars, which would be warmed by the sun’s increase in energy output, making it more ‘earth-like’.  This could require the creation of a much denser atmosphere, with a higher oxygen content, or simply the hosting of an enclosed civilisation.  Because of their large distances from the sun, the Jovian planets (or more likely their satellites) would remain too cold and receive too little solar energy to by viable habitats in this phase of the sun’s evolution. However, any settlement on a planet would be temporary, as further solar evolution would eventually render all planets uninhabitable.  Therefore, mankind’s greatest chance for ongoing survival is to build inhabitable worlds separate from any planet (space stations), that can be moved into orbits that take greatest advantage of prevailing solar conditions.  Looking beyond the life time of the sun, the capability to travel to other stars, and either populate suitable planets orbiting them, or build extra-planetary worlds would need to be undertaken.  Whilst the technical challenges are formidable (compared to current technology levels), and the distances involved enormous (considering the physical limitations of travelling at relativistic speeds[viii]), 5 billion years is a lot of time to develop solutions to these problems.

[i] The decay of radioactive material in the earth’s core and crust, and the gravitational influence of the moon being the other key factors. [ii] Kaufmann, W.J., Freedman, R.A., “Universe”, p518, WH Freedman & Company, 1999 [iii] The life expectancy of the sun is an estimate, and figures do vary.  For the purposes of this report, the figure of 10 billion years will be used. [iv] Other isotopes exist, such as deuterium, with 1 proton and one neutron, however this is quite rare in the sun’s core [v]Kaufmann, W.J., Freedman, R.A., “Universe”, p520, WH Freedman & Company, 1999 [vi] http://image.gsfc.nasa.gov/poetry/ask/a11467.html [vii] http://image.gsfc.nasa.gov/poetry/ask/a10603.html [viii] Speed that represent a significant percentage of the speed of light – say 1-10%