All About Space - - Fu­ture Of The So­lar Sys­tem -

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.

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