What the Future holds for the Earth and the Sun - (And Possibly for Humanity)
In the year 4.9 billion AD, stepping outside on a summers day and looking up to see the Sun will present both a view and an experience quite unlike that of today. 4.9 billion years from today the Sun has progressed well up its evolutionary path. It has a luminosity of 1.4 times that of the present day Sun; that is, the Sun would be 40% brighter. With a surface temperature of 1000 Kelvin (1340oF) the oceans will have boiled away during a runaway greenhouse effect, creating an Earth not unlike the planet Venus in many ways. In the atmosphere of Earth there will probably be several layers of clouds many miles thick. If the Sun were visible through the constant overcast it would appear larger and brighter.
Mankind, if our species still exists, would probably be sufficiently advanced to compensate for the harsh conditions, but more likely we would have already found other locations to call home. However, even these grim conditions are paradise compared to what is coming. The Earth that gave birth to and nourished homo sapiens will have to be completely abandoned by its greatest progeny.
What's the Cause?
To understand what has happened to the Earth requires looking at the Sun's evolutionary track. When the Sun first arrived on the main sequence, it was fainter, only 70% as bright as it is now, had a diameter of 770,000 miles, compared to the present 860,000 mile diameter, and it would have been slightly cooler at 5586 K (9,950oF). While stars are on the main sequence they burn hydrogen in their cores at a fairly constant rate. For our Sun this is about 10 billion years, with about half of that time, 4.5 billion years, having already lapsed. When hydrogen is consumed in the fusion process in the Sun's core it becomes helium and occupies less space. The core shrinks and this shrinkage increases the pressure in the core making its temperature increase. The heat from this hotter core, called radiative pressure, increases the gas pressure in the outer envelope and the outer envelope expands to compensate. The Sun grows larger and more luminous even while the surface temperature cools as its gas envelope expands. So while the time spent by a star on the main sequence is considered a time of steady hydrogen burning it is actually a time of steady changes.
The Future of Earth
1.1 billion years from today the Sun's luminosity will increase by 10 %. This will cause a permanent moist greenhouse effect on Earth, evaporating most of the lakes, rivers and seas into the atmosphere. As the Sun continues to age it will continue to brighten, exacerbating the already grim climatic conditions.
By 4.9 billion years AD the Sun will be 40% brighter than today, double what it was when it first entered the main sequence and the Earth was new. All of the water will have long since vaporized into the atmosphere during a runaway greenhouse effect that was caused when the increased energy of the Sun quickened the evaporation of the oceans and the resulting increase in atmospheric water vapor retained even more heat leading to faster evaporation and ever-rising temperatures. Even the carbon dioxide in the rocks is baked out by temperatures around 1000 Kelvin (1340OF). This returns much of the carbon dioxide to the atmosphere that had previously been 'fixed" in the rocks during the first billion years or so of Earth's history. In many ways the Earth of the future is much like Venus today with a heavy carbon dioxide atmosphere with little carbon held in the rocks and no ocean to hold the rest. Additionally, as may have happened to Venus, the large amount of water vapor that boiled away would rise high into the atmosphere. If it rises above the ozone layer, or if the ozone layer is thinner or missing, the water vapor will be exposed to the increased intensity of the Sun's ultra-violet radiation and dissociate, that is, break down into oxygen and hydrogen atoms. With the higher temperatures, the increasing levels of carbon dioxide in the atmosphere, the disappearance of free water from the surface, and the disassociation of water vapor in the atmosphere, the Earth's surface experiences conditions unknown since primordial days.
But What About Mankind?
Initially, during the gradual warm up of the planet the population will probably relocate to cooler parts of the surface. But by 1.1 billion years AD there will be no place on Earth that is easy or practical to live on. These slowly changing conditions on Earth may make the previously cold planets and moons of the outer solar system look welcoming compared to Earth, for as the Sun heats up the Earth it warms them up as well.
It is estimated by some that within only 100,000 years or so mankind will have successfully traveled to the stars and explored much of the Galaxy. By then, perhaps much of the human race has already moved to such places as Mars as well as extrasolar planets which they have already successfully terraformed. Colonists will have been on various moons of the solar system, such as Europa, almost as long as they have been on Mars. Within the next 100,000 years colonists may have been extracting water, hydrogen and oxygen from the ice for crops, energy, and breathing air and have established a permanent and viable presence on Europa. They could supply the other colonies in the Jovian, Saturnian, Neptunian and Uranian systems with these commodities and trade for others. With the gradual warming of the Sun these colonies will increase in livability, as the Earth will be declining. Mars was previously cold but by 1.1 billion years AD, with the warmer Sun, Mars may experience surface temperatures not unlike the "old Earth". Even this will not last forever, however. Eventually, there will be no place safe in the solar system for most forms of life as the Sun continues to age.
If they have not already done so this gives whoever is left time to leave this solar system and hopefully find safe haven in other solar systems. These other systems will have been carefully picked since it is desirable to choose extrasolar planets for colonization that not only posses natural resources needed for human occupation but ones with suns that will remain friendly toward their dependent planets for billions of years to come. Stars located close to other stars that are about to nova and disrupt their neighboring solar systems would not be desirable either.
Sometime after 5.5 billion years AD, the Sun will leave the main sequence stage of its life when it ceases to burn hydrogen in its core. During this process the Sun's core will collapse and a shell of hydrogen outside of the core will begin burning. This will expand the Sun's outer envelope out to the orbit of Earth, a radius of about 1 AU (93 million miles), and the Sun becomes a red giant with a luminosity thousands of times brighter than at present. This will vaporize the inner planets and leave the outer planets without their thick gas envelopes which will evaporate away, leaving only the rocky cores of the once great gas giants.
Our Sun will never go nova, that is, blow itself up. During shell hydrogen burning it begins to burn helium in its core as well, making carbon and oxygen. When the helium begins burning in the core the core expands, reducing the shell hydrogen burning which in turn causes the envelope of the star to contract and heat up again. Once the helium is depleted in the core the Sun's core shrinks again and helium begins to burn in a shell. Thermonuclear processes in our Sun's core have now come to an end since our Sun is not massive enough to sustain nuclear carbon or oxygen burning. The Sun expands once more into a red-giant phase due to the helium burning in a shell around the core, this time not stopping its expansion until it sheds most of its outer envelope in a beautiful planetary nebula. No doubt one so beautiful that observers in other stars systems will admire it as we currently admire the planetary nebulas of other dying stars. Without the radiative pressure generated by a thermonuclear process there is nothing to hold up what little is left of the Sun's former envelope and its still hot core so it collapses inward until it reaches a diameter of around 10,000 miles, about the size of Earth. There our Sun will end its life as a white dwarf star, cooling slowly like a burned-out cinder over a period of many billions of years.
The increased luminosity of the Sun due to its normal evolution on the main sequence will create on Earth abominable climatic conditions including an oppressive greenhouse effect, a carbon dioxide atmosphere, the loss of all water, and extremely high temperatures long before the Sun leaves the main sequence. New extrasolar planets will need to have been discovered, explored, and prepared for human occupation. This extrasolar exodus will be fueled first by humanity's natural urge to explore, but also because of a growing population and the need for resources, and of course, due to the well established fact of the Sun's inevitable fate. It is reasonable to assume that long before 1.1 billion years AD that unimaginable technologies of that era would have lead to the terraforming of not only every worthwhile planet and moon in this solar system but travel and colonization throughout the Galaxy. Perhaps by 4.9 billion years AD there will already be no one left on Earth except for the occasional tourist with a sense of adventure. By 5.5 billion years AD even the most adventurous will have left.Published in the March 2003 issue of the NightTimes