In 7.8 billion years, by current solar models, the unadulterated Sun will be a white-dwarf corpse. No longer fusing hydrogen or helium, the Sun will become a cooling mass of carbon/oxygen. In 2011 Eric Agol posited that white-dwarf stars might have Earth-like planets in their habitable zone for longer than the Sun will be hospitable to the Earth – up to 8 billion years or so. Since then several searches have been underway, or in preparation, and one such was discussed in the a recent “Centauri Dreams” blog, here: Life Around Dying Stars.
In the comments I noted:
The astro-engineering possibilities are worth exploring. For example, in Olaf Stapledon’s “Last and First Men”, there was talk of moving the planets inwards as the Sun was expected to cool after the catastropic brightening that forced the migration to Neptune. If a very efficient neutrino reaction and rocket could be developed, using some kind of inverse baryogenesis, then Earth (and other planets) could be sent in-spiralling towards the Sun after it begins its white dwarf phase.
According to Martin Beech’s astro-engineering work the Sun could have its useful lifespan extended many-fold by siphoning off “excess” mass. A necessity of the “easiest” scenario involves shifting the planets outwards as its luminosity increases slowly. Then, once a helium core develops, the Sun can be allowed to become a helium white-dwarf and the planets can spiral back inwards. The excess mass could be used to make low-mass companion stars, eventually creating a quintet of red-dwarfs. The various terraformed planets can be shared out between the new low-mass stars. Depending on the exact parameters chosen, the Earth could end up orbiting a quasi-terraformed gas-giant around one of the new stars.
How does one terraform Jupiter or Saturn, you might wonder. That’s a whole other story…
Martin Beech’s planet moving machinery was via asteroid-flyby or solar-sail, as my Bob Forward inspired neutrino rockets or Stapledon’s “sub-atomic energy beams” are based on speculative physics. Beech discusses engineering the Sun in his astrophysical work on “Blue Stragglers” and in his book “Rejuvenating the Sun and Avoiding Other Global Catastrophes” (Springer, 2008). To keep the Sun going while avoiding the runaway bloat of the Red Giant phase, the Sun needs to go on a diet – it has to lose mass. In one mass-loss scenario the Sun is shrunk to 11% of its current mass and extending its life to more than 12 times its normal Main Sequence lifespan (~10 Gyr, so 122 Gyr with mass-loss at the right rate.) Eventually the Sun, despite our best efforts, will form an unfusible Core, a White-Dwarf, and an inexorable decline, as its stock of heat trickles away, will begin.
Red-dwarf stars, made from the liberated mass from the Sun, might undergo a somewhat different fate. In the late 1990s, Fred Adams, Greg Laughlin and Peter Bodenheimer ventured into Deep Time – to compute the lives of the smallest stars. What they found was a multi-trillion year life-span quite different to that of the heavier stars, as related in their paper “The End of the Main Sequence”.
The diagram needs some exposition. The y-axis is the Luminosity in Log10(solar) units – so -2 is 1/100 and -3 is 1/1000 etc. The x-axis is the temperature in kelvin. The star’s development begins on the right, descending in luminosity along the Hayashi Track until fusion ignites in its core and it settles zags upwards on the Main Sequence. The initial point of the Main Sequence is the ZAMS, or Zero-Age Main Sequence, and forms a distinct change in the star. For most of its life the star is fully convective – the whole of its mass cycles through the fusion core, unlike the Sun which will only cycle 10% of its mass through the core on the Main Sequence. Eventually the core becomes radiative, like the Sun’s, and it rises in brightness at higher pace. As you can see the star gets as hot as the Sun (5800 K) but never as bright as 1/100th of the Sun. Thus why its Main Sequence lifespan is over 6 trillion years of hydrogen burning. Eventually all the hydrogen is fused and the star becomes a Helium Dwarf, which is a star only seen in the present day Universe due to extreme mass-loss processes. Such a star never becomes hot enough to initiate the Helium Main Sequence, which in the case of the Sun runs 50 times brighter than the Hydrogen Main Sequence and lasts 1% as long.
Slightly bigger stars live significantly brighter, shorter lives – the luminosity is roughly proportional to the 3rd power of the mass, thus a 0.2 solar mass star is about 8 times brighter than a 0.1 solar mass star, but with twice the fuel its life-span is 1/4 of the 0.1 solar mass star, about 1.5 trillion years or so. A 0.16 solar mass star approaches the brightness and hotness of the Sun for several billion years near the end of its Main Sequence – heavier stars show more of a trend towards forming Red Giants. Above ~0.25 solar masses and a proper Red Giant phase occurs.
Stars of the same mass and metallicity (fraction of elements heavier than helium) as the Sun have ~10 Gyr Main Sequence life-spans. Lower metallicity levels mean significantly shorter lifespans, with such a star brighter than the Sun at its ZAMS entry to the Main Sequence. Increase the mass slightly and the stars also have significantly shorter life-spans, living brighter and hotter lives than the Sun. Yet such stars might be ideal candidates for life-bearing planets. With these two factors in mind this suggests there have been significant numbers of habitable planets that have been faced with the prospect of a Red Giant Sun.
How might a civilization adapt? Astro-engineering via mass-loss requires engineering over billions of years, something that inhabitants of a shorter-lived star don’t have. What are the options then?
Greg Matloff, interstellar solar-sailing Guru, has examined this question in several papers. His NIDS Essay, “The Re-enchantment of the Solar System” is a provocative classic, as it posits the ETIs escaping such stars might be in our outer Solar System, with observable consequences. In “Red Giants and Solar Sails” he more formally looks at the boost in final speed that a Red Giant Sun can give a Star-Sail – up to 2-3 times what would be expected from the Sun. In “Giant/Red-Dwarf Binaries: New SETI Targets and Implications for Interstellar Migration”, he discusses wide binaries of a higher-mass star with a red-dwarf. An F star might last ~5 billion years, while a 0.2 solar mass red-dwarf will last over 1 trillion years. In a wide-binary separated by ~100s of AU, the red-dwarf would be a very attractive target for refugees from the Red Giant. Even in a post-Red Giant binary, where a red-dwarf circles a White-Dwarf, there might still be Star-Sail activity and observable radio-traffic. Interestingly, for “Star Trek” fans, the star Keid (40 Eridani), host to the planet Vulcan, is a trinary featuring a K-dwarf and a white-dwarf/red-dwarf pair. Perhaps added reason for SETI surveys of this already interesting near-by star-system.