It’s impossible to forget just how much the sun affects life on our planet. It’s overwhelmingly the source of our light and heat, providing just enough to maintain the delicate climatic balance we enjoy. That’s not a coincidence; life on Earth evolved under the sun’s influence and, given time, adjusts to any changes.
Adapting to the whims of a star is no small task, however. The sun may appear to be constant from day to day, but let time stretch out for millions or even billions of years, and things will change—a lot. And not always for the better.
For example, in its thermonuclear-driven core, the sun fuses about 700 million tons of hydrogen into 695 million tons of helium every second. The missing five million tons are converted into energy (via everyone’s favorite equation, E = mc2). This energy is enough, it turns out, to power a star. If you like mind-boggling numbers, the sun produces 4 × 1026 watts of power—400 trillion trillion watts. In other words, the energy our star emits in a single second is sufficient to satisfy humanity’s total consumption for about 650,000 years.
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It’s also enough to warm our planet to its current comfy clime. In fact, by using some basic physics principles, it’s possible to mathematically calculate how warm Earth should be given the sun’s energy-emission rate. That solar energy flows into space in all directions around the sun, and a tiny fraction (about half of one billionth) of it is intercepted by Earth, heating our planet. Just how much heating takes place is a bit complicated and depends on the actual radiant flux from the sun, Earth’s distance from it and reflectivity, and more. When we run the numbers, Earth’s average calculated temperature today is approximately –15 degrees Celsius, colder than the freezing point of water.
Actual measurements of Earth’s temperature, however, give an average that’s much warmer: about 15 degrees C. The difference exists because greenhouse gases in the air essentially trap heat from the sun, warming Earth above the calculated temperature. This warming is mostly from natural greenhouse gases, mind you, but we’re adding approximately 40 billion tons of carbon dioxide into the atmosphere every year, significantly increasing the warming effect. Note that this increase has occurred over the past century or so, a timescale far too short for there to have been any change in the sun; Earth’s current climate change is all us.
Astronomers argue about whether the expanding sun will reach Earth. As it stands, things don’t look good.
But the sun’s production of energy does change noticeably—over hundreds of millions of years. That helium created in the core is inert; think of it as ash from the nuclear fusion. It settles in the center of the sun, building up over time at the rate of 695 million tons per second! As it gains mass, it also gets squeezed by the tremendous weight of the sun’s layers above it and becomes compressed. A basic law of physics is that compressing a gas heats it, so even though the fusion rate is mostly the same, the core of the sun is still slowly heating up over time—which means the sun itself is getting more luminous.
If we run the clock forward an eon or two, we find disaster. As the sun grows brighter, it will raise Earth’s temperature so much that the planet will lose all the water vapor in its atmosphere and then, eventually, all its surface water. The oceans will evaporate. This global desiccation will pretty firmly plant a stop sign for all life on Earth. Still, if it’s any comfort, that won’t happen until three billion years from now.
The reactions going on in the sun’s core get very complicated after this point, but the biggest effect is that our star’s energy output will eventually increase prodigiously. All that energy will get dumped into the sun’s outer layers. When you heat a gas, it expands, so the sun will swell up to huge proportions—100 to 150 times as wide as it is now. At the same time, its surface temperature will drop, making it ruddier, even as it radiates energy 2,400 times stronger than what it puts out now. This shift will transform the sun into a red giant star.
The sun will be so big, in fact, that it will consume Mercury and Venus. Earth may escape this fate; astronomers argue about whether the expanding sun will reach Earth. As it stands, things don’t look good.
If Earth does survive, it won’t be pretty. The temperature of our planet will be about 1,300 degrees C, hot enough to melt lead. During the day rocks on the surface will melt, and Earth will be a lava world. On top of that, our planet will lose its atmosphere to space when it gets this hot. Is there any way for Earth to avoid this destruction? How will the other planets fare?
To answer both these questions honestly and in the order they were asked: not really and not well. There will be a slight reprieve because as the sun expands, its solar wind will become much more powerful—so much so, in fact, that the sun will lose a substantial amount of mass. This means our star’s gravity will weaken, and the planets will migrate outward, away from the solar system’s central blast furnace.
But it’s not enough. Jupiter, which is currently a chilly –110 degrees C, will heat up to more than 300 degrees C. Its icy moons will melt and start to boil away. We’ll find no sanctuary there.
If you want to find even a marginally clement climate anywhere in that far-future solar system, you might have to look toward Pluto, which, by then, will be about 50 times more distant from the sun than Earth is now. Its surface temperature will be roughly –10 degrees C. That’s still chilly, but remember the greenhouse effect: there is a lot of frozen methane and carbon dioxide on Pluto, so these ices could vaporize and possibly provide enough thermal retention to make the tiny world at least somewhat habitable, if not exactly comfortable.
What then? It gets worse, if you can imagine. The sun will blow off its outer layers, and the core will be exposed to space, transforming into what astronomers call a white dwarf. The incredibly hot core will be only about the size of Earth, so small that it will provide very little heat to the planets. They will cool once again, eventually dropping well below the freezing point of any biologically useful molecule.
If there’s any modicum of good news here, it’s that all of this won’t happen for many billions of years. Who knows what humanity will look like by then or whether we’ll still be around? If we are, well, more stars are born all the time, and they’ll have planets, too. Packing up and moving is never fun, but if your house is on fire, there’s not much choice. Perhaps we can find other Earths out there where we can settle down for an eon or three before this entire process starts up again.
