Ask Ethan No. 27: Will the Earth and the Moon survive?

And they will not see what the darkness hides if the Sun has dimmed and the Moon has died.
- J.R.R. Tolkin

The reader asks:

In the distant future of our Solar system, according to your words, when the Sun begins to expand, "the Earth / Moon system will be pushed out, and will avoid the fate of our neighbors from internal orbits." Could you explain why?

Let's start from today and talk about what awaits us in the future.



This is our sun. A huge plasma ball of 1.4 million km. in diameter. At this diameter, you could place 109 planets of the Earth close to each other. With every second in the Sun, 4 × 10 ^ 38 protons turn into helium, that is, more than 4 million tons of mass is converted into energy (E = mc ^ 2).

And although it is so large and hot in the core region, it has enough fuel to maintain combustion for 10-12 billion years. 4.5 billion years have passed by now. But even during this time, small changes will occur with the Sun.



The newly formed Sun was a little fainter than today, and a little more massive. The sun is not very different from any other star visible in the night sky, and having studied millions of stars, we more or less imagine how they work. We know that with age, stars undergo two important changes: the

temperature of the nucleus rises, and they begin to burn fuel faster and shine brighter
stars constantly emit atomic nuclei (mainly protons).

Over the course of days, these changes are not noticeable, as well as over time years, or even millions of years. But over billions of years, something can be noticed.



Since the birth of the Solar System, the Sun has become brighter by 20%, and after a couple of billion years it will become hot enough to boil the Earth's oceans, which is likely to lead to the disappearance of life. But the most interesting will begin in 5-7 billion years, when protons begin to end in the core of the Sun.



The sun will still burn hydrogen in a shell around an inert core, and this phase will last several hundred million years. At this time, the Sun will expand approximately two times, become 5-10 times brighter, and begin to emit particles faster.



Why? Remember how gravity works - the farther you are from the center of the object, the weaker it is. Therefore, if a star increases, then particles with a small kinetic energy (which most) will be able to leave the star. Very slowly, the sun will begin to lose quite a lot of mass. And this will affect the orbits of all bodies.



When a star loses mass, planets move away from it in a spiral due to a decrease in gravitational attraction. Everything from Mercury to Kuiper Belt objects will move away from the Sun.

But only you decided that this will continue neatly - our star suddenly inflates to the red giant, and begins to synthesize carbon from helium.



This process with high energy efficiency, leading to dramatic changes in the star. It swells up hundreds of times larger than its original size and glows hundreds of times brighter. The sun will swell to approximately the size of the Earth’s current orbit. As a result, it will absorb Mercury and Venus. But what about the earth?

And this, oddly enough, is still an open question. But we think we already know the answer.



Matter on the outskirts of the red giant will hold on to it weakly, so in a short time a rather large fraction of the mass of the Sun will be lost. Consequently, the orbits of the planets will continue to increase. Due to rapid mass loss, the Earth / Moon system is likely to be outside the photosphere of a giant star.



If, in the phases of the subgiant and the red giant, the star did not lose mass, we would certainly be consumed by it. But apparently, the losses of the Sun will turn into luck of the Earth and the Moon - we will remain intact even when the Sun goes through the last phases of its development.

People, and indeed life in general, most likely will not catch this event, but you may be comforted by the fact that our planet, the only known world where there is life, is likely to exist even after the Sun disappears.