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When the sun eventually expires and swells to engulf Earth, might the remnants of landfill and deeply buried nuclear waste be recycled to form the star dust that will become new planets or life forms?

Ron Dippold
San Diego, California, US

Quite possibly! Though you certainly won’t be able to say, “Oh hey, that was my old shirt”.

In the first stage, after the sun runs out of hydrogen in its core in about 5 billion years, it will initially collapse a bit from loss of outward pressure – but all of that gravitational energy from in-falling mass will be a huge energy injection. It will start burning some of the outer hydrogen in a shell around the core, and begin expanding into a red giant. It will get over 1000 times brighter. The intense radiation will blast Earth’s atmosphere into outer space. Earth’s surface temperature will reach over 1500°C. At that temperature, its crust and most other things will just melt into a sea of magma. A few things may survive intact – metals like tungsten and molybdenum can survive over 2500°C.

Some synthetic materials, like aluminium oxide and zirconia, are similarly hardy, and many gems, like diamonds, sapphires and rubies, would be fine.

Earth will drift outwards a bit because the sun is losing mass, but in the second stage – about 7.5 billion years from now, according to most theories – the sun will still swallow Earth.

The entire planet will disintegrate before too long from the heat, the buffeting by dense plasma, and the tidal forces. At this point, everything will become ionised plasma like the rest of the sun – even those things that avoided melting in the first stage.

The sun is about eight times too small for a core-collapse supernova. However, it still has several coughing fits to go through as it runs out of various elements and starts fusing heavier ones. Eventually, it will blow off its outer layers in an expanding cloud of gas and radiation called a planetary nebula. The remaining sun will shrink to a white dwarf, which, in theory, can glow for trillions of years – if the universe lasts that long. Anything stuck in that is never getting out, unless a black hole rips it to pieces. But most of Earth would still be in those outer layers blown off as a planetary nebula. As that cools over millennia, the atoms in the nebula will arrange into tiny grains of cosmic dust and interstellar gas. In billions of years, these may become part of a large molecular cloud like the one that became our solar system, and might one day collapse again into a brand new solar system. My Iron Maiden concert shirt will live on!

Hillary Shaw
Newport, Shropshire, UK

The first question here is will the sun engulf Earth as it becomes a  red giant? Probably it will, despite the reduction in the sun’s mass – resulting in Earth orbiting further out – as the sun’s extended atmosphere may cause drag on Earth, slowing its orbit and causing it to fall back further in.

Consuming the remains of Earth, Venus and Mercury, the red-giant sun expands, throwing off material like dust and gas that may well end up as part of another solar system, possibly hosting life. That life probably won’t use our nuclear-waste atoms – which by then will have decayed to stable isotopes anyway – directly, but if it is intelligent, it may find uses for these heavy elements as we have.

However, if the red-giant sun doesn’t consume Earth, its alternative is far more dismal, dark and lonely. The red giant loses fuel and transitions into a white dwarf. The husk of Earth orbits the white-dwarf sun for quadrillions of years as it cools to a black dwarf. A passing star may strip Earth away, or it may fall into another star, or a neutron star, or a black hole. At some point, this entity may collide with another black hole or neutron star, and some matter containing heavy atoms is ejected, possibly to seed another life-bearing planet.

Or this collision never happens, and Earth and the black-dwarf sun linger on, and in 1020 years, Earth’s orbit decays so it falls into the black-dwarf sun. Or Earth survives, perhaps ejected and wandering space, and in 1033 years, proton decay may convert it to radiation.

Or proton decay does not occur,and in 10100 years, the last black hole has evaporated and the universe is now a thinning, cooling void of sparse radio-wave radiation and matter. Time becomes meaningless and a dark infinity of nothingness –containing odd lumps of matter, like Earth, that never fell into the now-extinct black holes – lingers on forever. Very occasionally, and ever less frequently, distant faint flashes of light occur as objects like black dwarfs and stray planets collide. Extremely unlikely, but possibly, a single, small, dim, red main-sequence star forms, with a life-bearing planet. It is doomed to a solitary, albeit long, existence in the black, empty void. Then nothing. Forever.

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