What will happen to the Solar System after the Sun's death?

About 4.5 billion years ago, a giant cloud of gas and dust collapsed onto its structure and eventually gave rise to the Sun and planets in the Solar System. Since then, our star has continued to emit energy and light and should continue to do so for a few more billion years. However, there will come a day when our star will no longer have the fuel to sustain the reactions that sustain it and will end its “life” expanding – which is not very good news for some planets in our space backyard, especially the rocky ones.

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About % of the stars in the universe — including the Sun — are considered “main sequence” stars, which means that they perform nuclear fusion of hydrogen atoms to, thus, form helium. The life of these stars depends on their mass: the most massive, for example, have more fuel available, but consume it more quickly due to the high temperatures in the core; years old.

Detail with the size of the Sun today, as a main sequence star, with its future version as a red giant (Image: Reproduction/Oona Räisänen)

In our star, the fusion of hydrogen into helium produces an external pressure that balances the internal one, caused by gravity that “try” collapse it on its own structure and thus makes it stable. The bad news is that hydrogen will run out in about 6 billion years and, in parallel, there will be a large amount of helium produced, a compound that requires much higher pressure and temperature for nuclear fusion. With this “waste” left over, it will be more difficult to keep the fusion going and, consequently, to avoid the collapse.

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So, to keep the balance and its structure, the Sun will have to increase the temperature of the fusion reactions, so that its core will be hotter. Thus, the helium-filled core will be surrounded by a layer of molten hydrogen, and then gravitational forces will compress it. As a result, the Sun will become a red giant star, whose outer layers will expand throughout the Solar System — and this is where the situation will get complicated for the planets that are closest to the Sun.

Mercury, for example, is very close to the star, and even so it managed to retain some blocks of ice in its structure, but hardly these volatiles — and the planet itself — will resist this expansion. Venus, in turn, will likely have its dense atmosphere burned away and will be consumed in a fate similar to the one that awaits Earth. Our planet’s atmosphere will be destroyed and our oceans will evaporate — and depending on how much the Sun expands, not even the Earth will be spared and could be consumed in less than a day.

When the Sun becomes a red giant, its atmosphere must expand by 1 astronomical unit, which is the distance between us and our star (Image: Reproduction/Wikimedia commons / fsgregs)

Even if the expansion stops happening before it reaches Earth, the extreme energies released by the Sun will be intense. enough to vaporize the rocks here, leaving behind only our planet’s metallic core. Perhaps Mars is a little lucky and is right on the edge of the Sun’s range, which would allow it to narrowly escape destruction; besides, the water that exists there today under the surface would probably have been evaporated until our star began the end of its life.

And the rest of the Solar System?

With the increase in radiation, the outer planets will lose their atmospheres , which are as fragile as Earth’s, and their ice structures won’t hold up either. But on the other hand, there is the possibility that this expansion of the Sun will reach them and “feed” them, making them bigger than ever. Astronomers have already found gas giant exoplanets orbiting red giant stars up close, and even so they managed to maintain their atmospheres, which apparently ended up “bloated” by the star’s intense radiation. Therefore, the fate of Jupiter and Saturn is still somewhat uncertain.

During these final stages, our star will be unstable and will pulse for millions of years, increasing and contracting, and this will push the outermost planets in directions other than those they are in today. As the red giant loses mass, the gravitational pull the star exerts on the planets gets weaker, so their orbits expand. As the Sun must lose nearly half its mass, the planets’ orbits can be twice as wide as they are today.

Comparison of the current habitable zone of the System Solar with the Sun as a red giant, which will “push” it (Image: Reproduction/Cornell University)

In addition, the red giant will also cause some changes in the habitable zone of our system, which is the region where water can exist in a liquid state. As the Sun is expected to remain in this state for about a billion years, the habitable zone of our system will be migrated further away, which could provide friendlier conditions for life in the outer Solar System, which will be closer to what it was. the Sun — the dwarf planet Pluto, for example, could have surface temperatures close to those on Earth today.

This warm period for distant objects won’t last long: how is not massive enough to explode in a supernova, the next step in the end of the Sun’s life will be to expel its outer layers of atmosphere in a sequence of emissions, which will leave behind only a carbon and oxygen nucleus called the “white dwarf”. This is a dense, extremely high-temperature object that will emit X-ray radiation for some time and, after another billion years, the white dwarf will become more stable and will have a new habitable zone.

Since the temperature would be lower, this region would be closer to the white dwarf than Mercury’s orbit is with respect to the Sun, so any planet or planetary core would be disrupted by the force of tides. Furthermore, if the Earth survives the red giant phase and stays there until it encounters this white dwarf, our planet’s orbit would end up decaying due to gravitational waves. Then, after a few trillion years, the Earth would spiral towards the Sun as what was the Solar System comes to an end.

Source: Space.com (1, 2), Discover Magazine, Astronomy.com, NASA, Bad Astronomy

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