The solar core is now fusing helium from hydrogen, with the result that with each nuclear reaction, a small amount of mass is converted into pure energy, according to Einstein's E = mc squared.
But this cannot last forever, because the amount of fuel in the core is limited. The sun has already lost in this process a mass equivalent to the mass of Saturn, and in 5-7 billion years it will completely use up all the fuel in the core. Swelling to a red giant, it will eventually shed its outer layers, creating a planetary nebula, and its core will shrink and turn into a white dwarf. For an outside observer, it will be a beautiful and colorful sight. In the first photo, the Cat's Eye Nebula is a gorgeous and colorful example of this possible fate.
But inside the solar system, it will lead to disaster.
The first thing to know about red giants is that they are huge. It seems to us that our Sun is large: 1.4 million km across, with a mass 300,000 times that of the Earth, but compared to the red giant, this is nothing. With this mass, our Sun will grow 100 times its previous size, absorbing Mercury and Venus. The Earth is likely to be pushed further as the Sun grows and loses mass, and although it could be swallowed up by a star, scientists are still debating whether it will survive or not.
If the calculations are correct, the Sun will not have to swallow the Earth when it swells to a red giant.
In this case, Earth and Mars will turn into charred, barren worlds. The oceans and atmospheres of these planets will boil and disappear from the surface, and these worlds will become airless and hot, like today's Mercury. These effects will extend far beyond the orbits of the inner rocky worlds of the solar system.
You see, the red giants are not just huge, they are also still heated to many thousands of degrees, and shine thousands of times brighter than today's sun. Most of the ejected material - about a third to half the sun's mass - will remain heated to extreme temperatures and travel to the outer edges of our solar system. Asteroids will melt, losing all volatile components, and only rocky cores will remain of them.
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Asteroids have a number of volatiles, and they often show tails as they approach the Sun. Over time, as the Sun grows to a red giant, these asteroids will melt, lose all volatile materials, and become either piles of cobblestones or molten rocks - in any case, becoming much smaller than their current size.
But the gas giants will be massive enough to hold onto their gas blankets, which may even grow when the Sun enters this phase. For example, today we find only gas giants in orbit around the red giants, much larger than even Jupiter. Perhaps this is the result of selection - and we see them because they are the easiest to see - but perhaps it is the result of an inevitable process.
Vast quantities of material leaving the Sun will collide with giant worlds with powerful gravitational fields. Much of the material that encounters these atmospheres will emit a splash of cosmic proportions and increase the size and mass of these worlds. As a result, Jupiter, Saturn, Uranus and Neptune may be larger and more massive than they are today.
Visually, the large gap between the sizes of terrestrial worlds and planets like Neptune is immediately striking - and the transformation of the Sun into a red giant will only increase this difference. Earth and Mars will lose their atmospheres, and possibly some of the surface, while the gas giants will grow, absorbing more and more matter as the Sun sheds its outer shell.
However, the Sun will become so bright and hot that much of the outer solar system will be completely destroyed. Each of the gas giants has their own rings; the most famous are the rings of Saturn, but all of our four giants have them. Basically they consist of various ices - water, methane and frozen carbon dioxide. Thanks to the extreme energy emitted by the Sun, these ices will not only melt - their individual molecules will acquire such energy that they will be thrown out of the solar system.
The rings of Neptune, captured with the wide-angle camera of the Voyager 2 spacecraft with a long exposure. You can see how continuous they are. The rings of Neptune, like the rings of all gas giants, are composed of volatile ice components, and will melt, boil and sublimate when the Sun turns into a red giant.
The same will be true for water-rich moons orbiting these worlds. The frozen surface of Europa, under which there is water ice, will completely boil away. The same will happen with Enceladus, which will evaporate almost everything except a rocky core with an admixture of metals. Virtually all the moons of Jupiter, Saturn, Uranus and Neptune will shrink significantly in size, their atmospheres will boil away, their outer layers will melt and disappear; only the cores of these satellites, consisting of stone and metal, will remain. Some moons that are completely volatile can disappear completely.
Even the largest and well-known Kuiper Belt objects are not immune to this disaster. Even worlds at such vast distances as Triton, Eris or Pluto will receive four times more energy per unit surface than the Earth receives today. Their atmospheres and surfaces, now covered with various types of ice, and possibly containing subsurface oceans, will also completely evaporate. When the Sun becomes a red giant, and the inner worlds turn into charred remains or are swallowed up by the Sun, worlds like Pluto will not become potentially habitable planets: they will burn up. They will turn into bare cores of stone and metal, and will look like today's Mercury.
Geological structure beneath the surface of the Sputnik plain. It is possible that Pluto has an ocean of liquid water underneath its thin crust. When the Sun becomes a red giant, all the outer layers will sublimate and boil away, leaving behind only a core of stone and metal.
For several tens or hundreds of millions of years, there will be hope for more acceptable conditions in the Kuiper belt, at a distance 80-100 times greater than the distance from the Sun to the Earth. For this small, by cosmic standards, period of time, objects at this distance will receive about the same amount of sunlight as the Earth receives today. However, the world needs more than sunlight for habitability; you need to have enough weight, the right size and the right ingredients. The Moon and Earth are very different in habitability, despite receiving almost identical amounts of solar energy per unit area.
The orbits of the known Sednoids, along with the putative Ninth Planet. Even when the Sun becomes a red giant, Planet Nine - whose existence is controversial as it is - will not reach a temperature high enough to become potentially habitable. Other worlds in the Kuiper belt, even those that are at the right distance, will be too shallow from this point of view.
However, even the hypothetical Ninth Planet will be too far away to become potentially habitable, and anything at the right distance will be too shallow for life to exist there. The solar system will become a molten disaster, leaving only the bare cores of planets, moons and other objects. The gas giants may swell and grow, lose their rings and many satellites, but everything else will become nothing more than pieces of metal-rich garbage. If you're hoping that the frozen outer worlds of the solar system finally get their chance to shine, you will be disappointed. When the Sun reaches the end of its life, these worlds, like our hopes for survival, will face the fact that all the most important will melt and disappear.
