The online science magazine Nautilus recently published this piece, which is based on the work of Ken Caldeira and James Kasting (Caldeira, K. & Kasting, J.F. Nature 360, 721-723 (1992)), which predicted a biospheric doomsday some 500 million years from now, due to the decline in CO2 as the Sun inexorably brightens at 10% per aeon. The Nautilus author discusses some of the astroengineering options for moving the Earth outwards from the Sun – asteroid flybys, solar-sail gravity tractors. Alternatively there’s the (presently unproven) option of uploading into robotic/cyborg forms adapted to the heat.
In my view, the real problem is that the Earth isn’t reflective enough because it spins too quick. Based on advanced Global Circulation Models, the surprise result is that slow rotating “Water Worlds” can survive higher insolations (sunlight intensity) by reflecting more light/heat back into space. A thick, permanent cloud mass forms beneath the sub-solar point – the noon position – and this mass reflects so much light/heat that the planet retains its water up to more than twice the Earth’s present insolation. Could we slow the Earth sufficiently? A sol (the time from sunrise to sunrise) that’s more than 240 hours long seems to result in this cloud bank forming. Thus if the Earth were slower rotating, it might prove habitable for longer. At least in part. The equatorial zone could be too torrid for advanced plant life, but land plants aren’t the main source of oxygen for animal life, so this may not be as big an issue as imagined.
As our technology and biology become more interrelated we may find that uploading/cyborgization are quaint concepts from a bygone age. Techno-Adaptation, for all terrestrial life, may become the way the Biosphere adapts to the brightening Sun. Thus Life’s tenure is expanded all the way to the Red Giant stage, but what then?
Most popular discussions of the Sun’s Red Giant stage give the impression that it’s a sudden change in the Sun. Certainly all the TV depictions imply that (e.g. Star Trek & Doctor Who) but it’s actually a protracted process. When the Sun is about 10 billion years old it will leave the Main Sequence, when its core supply of hydrogen fuel is exhausted, and over the next ~2.2 billion years become a Red Giant. For the first billion years not much happens. The Sun is a bit brighter (rising slowly from 2.2 to 2.7 times the present day) and becomes a bit cooler and bigger (cooler stars are bigger for the same amount of light output.)
Two main sources of data inform my discussion of the Sun’s Red Giant phase. First is a classic paper by Boothroyd, Sackmann & Kraemer (1993) and a revision of that work by Schroeder & Smith (2008). There is some uncertainty as since both papers came out, there’s been some scholarly arguing over new data about the abundance of ‘metals’ in the Sun. Astrophysically speaking, metals are all the other elements other than hydrogen and helium. Just how much of those other elements is in a range between 1.5% and 2%, roughly speaking. What that difference means for the Red Giant Sun is, as yet, unclear.
The very pinnacle of the Red Giant process lasts about a million years and the Sun bloats to over 200 times its present size and is over 2,000 times brighter. While it’s bloating, the Sun is blowing itself away in an enhanced Solar-Wind, with ~1/3 of its mass blown into space by the end of the Red Giant phase. If nothing impeded them, the inner planets would expand in their orbits and escape the expanding Sun – except Mercury, though its orbit is sufficiently chaotic that it might no longer be there anyway. However there will be tidal drag – the tides raised in the Sun by the planets Venus and Earth will cause them to spiral into their fiery doom. All this happens in the last, crowded half million years of the 2.2 billion years of the Sun’s Red Giant “Life Change”.
And Life? Migration away from the Sun seems a sensible option, yet maybe there’s a way to tweak the Sun into behaving in a more Life Friendly way. We’ve discussed that here before.