Ask Ethan #27: Will the Earth and Moon survive?

Someday, the Sun will expand, engulfing Mercury and Venus, and then some. Will we survive?

“They do not see what lies ahead, when Sun has faded and Moon is dead.”
-J.R.R. Tolkien

It’s the end of the week on Starts With A Bang, and so I’m proud to debut at Medium our long-running Ask Ethan series, where you can send me your questions and suggestions for topics. If you get lucky, the next question we answer could be yours! For this week’s, our question comes from Malcolm Schongalla, who wants to know:

In The Far Future of our Solar System, you briefly mention that “the Earth/Moon system will probably be pushed outwards, and be spared the fiery fate of our inner neighbors,” when the Sun expands. Can you please explain why this could happen?

Let’s start at the present day, and talk about what’s coming in the future.

This is our Sun. A giant ball of plasma some 1.4 million kilometers in diameter, large enough that you’d have to line up 109 Earths just to go from one end to the other. With every second that goes by, the Sun fuses an incredible 4 × 10^38 protons-per-second into helium, which converts more than four million tonnes of mass into energy by Einstein’s famous E=mc^2.

As large as the Sun is, and as energetic and hot as its core is, it’s got enough fuel inside to keep it burning for a total of about 10-to-12 billion years. (We’re about 4.5 billion years into that as we speak.) But even those 10-t0-12 billion years hold some slight changes for our parent star.

When our Sun first formed, it was slightly dimmer than it is now, and just a tiny fraction-of-a-percent more massive than it is now. The Sun is not much different from any of the other stars in the night sky, and — having studied millions of them — we have a good idea how they work. And we’ve discovered that as stars age, they undergo two important changes:

• their core temperature goes up, meaning that they burn through their fuel a little more quickly and shine more brightly, and
• due to sustained stellar winds, stars radiate atomic nuclei (mostly protons) away over time.

These changes are insignificant on a day-by-day, year-by-year or even a millennium-by-millennium basis. But over billions of years, we’d start to notice.

Since the birth of our Solar System, the Sun has brightened by an estimated 20%, and by time another billion-or-two years goes by, it will be hot enough to boil the Earth’s oceans, likely ending life-as-we-know-it on our home world. But things really begin to get exciting about 5-to-7 billion years from now, when the very core of our star starts to run out of protons to fuse.

Initially, the Sun will be able to keep burning hydrogen in a shell around the inert core, a phase which will last a few hundred million years. During this time, the Sun will expand to about twice its original size, become five-to-ten times as bright, and star to emit matter particles at a greater rate.

Why is this? Think about how gravity works: the farther you are away from the center of an object, the weaker its gravitational pull is on you. Remember also that when you have a lot of particles at a certain temperature, their energies follow a distribution. So if the star is significantly greater in size, then particles with smaller kinetic energies (and hence, more particles total) can escape from the star. Very slowly, our Sun will begin to lose an appreciable amount of mass. And this affects the gravitational orbits of… well, everything!

As the star begins to slowly lose mass, the planets begin to spiral outwards, as the central gravitational pull is gradually being lessened over time. Every world — from the innermost Mercury to the gas giants to the Kuiper Belt Objects and beyond — will experience this spiraling-outward motion.

Only, just when you thought this might gently continue (and for a long time), our parent star swells into a red giant, and starts fusing helium into carbon in its core.

This is an incredibly energetic process, and it causes some intense and remarkable changes in the Sun very quickly. Swelling to hundreds of times its initial size and thousands of times its original brightness, the Sun will be approximately the physical size of Earth’s orbit around it. As a result, Mercury and Venus will definitely be engulfed by our star. But what about the Earth?

Perhaps surprisingly, this is still an open question, although we think we know the answer.

The material on the outskirts of this red giant star will be held onto only loosely, and a significant percentage of our Sun’s original mass will be lost in this phase. As this happens, all orbits will move appreciably outwards, including the Earth. Due to the relatively rapid and significant mass loss that the Sun will undergo, the Earth/Moon system will likely, by this point, be located outside the photosphere of this giant star.

If it weren’t for stellar mass loss during the subgiant and (especially) during the red giant phase, we very likely would be engulfed by our parent star. But to the best of our understanding, the Sun’s loss is the Earth’s (and Moon’s) great gain: we get to remain intact, even as our Sun goes through the final phases of its aeons as a true star!

Humans (and life in general) likely won’t be around to see it, but perhaps it provides you with a little solace and comfort to know that our planet — home to the only life in the Universe we’ve ever yet known — will still likely exist, even once our Sun is no more.

And that will bring us to the end of today’s Ask Ethan! Have a question or suggestion for the next column? Let us know, and you could be the star of next week’s feature!

Have a comment? Head over to the Starts With A Bang forum on Scienceblogs!