Due to the Sun’s weakening gravity, the orbits of the planets will spiral outwards. The effect won’t be enough to save Mercury or Venus if they are still near their current orbits, but it might be enough to save Earth – if not for another twist in the tale.
“As the Sun expands and loses mass, its rotation rate slows down,” says Smith. “By the time it is 200-times larger than its current radius, its rotation period will be 2,500 years instead of one month. For Earth this has a drastic consequence: as it orbits past a Sun whose surface is now rather close, it raises a tidal bulge on the Sun’s surface.
As it moves ahead of the bulge, this exerts a tidal drag on Earth, causing it to lose energy and spiral in towards the Sun. Interaction with the Sun’s outermost atmosphere introduces a further drag force – the combined effect of these two forces is enough to cause a fatal spiral into the Sun, and the vaporisation of Earth.”
Earth, it seems, is doomed by a very similar effect to that which is currently pulling Phobos to its fate. But what would be the effect of these dramatic changes further out in the Solar System? “The increased luminosity of the Sun means that all the planets also heat up, with some of them becoming potentially habitable,” explains Smith.
“There is a habitable zone in the Solar System in the form of a ring, which currently only includes Earth near its inner edge. As the Sun brightens the habitable zone will expand, leaving Earth behind in about 1 billion years. Unfortunately the habitable zone moves rather slowly for most of the Sun’s red giant evolution, speeding up only in the last halfbillion years before helium ignition. This means that Mars, for example, will only enter the habitable zone 6 billion years after Earth leaves it, and will stay in it for 100 million years.”
Such periods are far too short for any life to evolve and take advantage of them, and before the habitable zone can reach the outer edge of the Solar System, the Sun will undergo another dramatic change. An event called the helium flash will see the core reignite, fusing helium into heavier elements. As its internal structure changes, our star will become much less luminous and shrink back rapidly, taking the habitable zone with it. Helium burning may last for about 100 million years before the core’s supply of that fuel is also exhausted.
After swelling to a red giant once again, the Sun will become unstable, discarding its outer layers in a small planetary nebula and ultimately leaving behind just the burnt-out, incandescently hot planet-sized core – a white dwarf.
Is that the end for our Solar System? Remarkably, recent work by astronomers at the University of Hawaii suggests it might not be. Ben J. Fulton and his colleagues investigated the possibility that new planets might form in the aftermath of the Sun’s burnout, and asked whether they might be able to support life. “The idea is that the remains of these destroyed first-generation planets might be able to re-coalesce into a second generation of planets once the star runs out of hydrogen and helium and contracts into a white dwarf,” explains Fulton. The small size of the Sun’s white dwarf remnant would make it far fainter than the present-day Sun and would mean that any habitable planets would need to orbit much closer in.
“White dwarfs are like hot embers left after a fire goes out,” continues Fulton. “They start out bright and hot and slowly cool for the rest of eternity. This means that the orbital radius of the habitable zone around a white dwarf evolves with time. Just after the formation of the white dwarf the habitable zone is relatively far away and evolving very rapidly, but as the white dwarf ages the distance to the habitable zone moves inward and the evolution slows down. A planet orbiting at a distance of 0.01 AU will be within the habitable zone for 8 billion years from about 2 billion years after the formation of the white dwarf.”
Compared to ‘normal’ exoplanet systems, such white dwarf planets might be rarer. Fulton and his colleagues carried out a survey and found that ‘super-Earths’ are expected in the habitable zones of seven per cent of white dwarfs. That is in contrast to estimates of 15 to 20 per cent for super-Earth planets around Sun-like stars, but there are billions of white dwarfs out there. “Our survey was not sensitive to smaller planets or asteroidal debris, which have recently been discovered around a white dwarf,” admits Fulton. “It’s possible that these smaller planets commonly orbit white dwarfs throughout the galaxy.” Even if the Solar System as we know it is doomed to die when the Sun expands into a red giant in 7 billion years’ time, it’s comforting to imagine a new system of strange planets orbiting our burnt-out star in the future.
As our star uses up the hydrogen fuel in its core, it brightens slowly over the next 5 billion years. At its greatest extent the Sun will probably have a diameter equivalent to Earth’s current orbit. As hydrogen fusion moves into shells around the core the Sun brightens and swells in size, with its surface growing cooler. Eventually the Sun’s core is exposed as an incandescent, slowly cooling white dwarf star. Over billions of years this fades away to become a cold black dwarf. After a brief respite as the core reignites and fuses helium, the Sun swells again, entering a period of pulsations in brightness and size. Due to the time it takes for them to evolve, a black dwarf has never been observed. The Sun becomes ever more unstable and its outer layers are blown off to form a spectacular but short-lived planetary nebula.
2050 -180°C to 120°C (-292°F to 248°F) Pressurised environment No atmosphere – however, this means that environmental conditions on the Moon will change less dramatically than they will back here on Earth. 2100 -55°C (-67°F) Pressurised environment or terraformed planet Carbon dioxide – but planetary engineering could trigger a greenhouse effect and release trapped ice, warming the planet to produce breathable air. 2200 -140°C (-220°F) Pressurised environment None. Callisto would remain too cold to terraform until the Sun enters its red giant phase, but is the safest of all Jupiter’s moons in terms of radiation risk. 2300 -180°C (-292°F) Pressurised environment Nitrogen and methane – although it will remain deep frozen for billions of years, Titan has a thick atmosphere and many resources to support a human colony.