When the Sun Becomes a Red Giant, Will Any Planet Be Safe?

Scientific American

2 days ago

As summer settles in and temperatures climb in the Northern Hemisphere, it's impossible to forget just how much the sun affects life on our planet. It's overwhelmingly the source of all 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, adapts 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 do change—a lot. And it's 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 is converted into energy (via everyone's favorite equation, E = mc 2). This is enough energy, it turns out, to power a star. If you like mind-boggling numbers, the sun produces 4 x 10 26 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 a billionth) of it is intercepted by Earth, heating our planet. Just how much heating takes place is a bit complicated, depending on the actual radiant flux from the sun, Earth's distance away and reflectivity, and more. Running 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 to see any change from the sun; Earth's current climate change is all us.
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, so it compresses. 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 therefore run the clock forward an eon or two, we find disaster. As the sun grows brighter, it will first raise Earth's temperature so much that we will lose all the water vapor in our atmosphere and then, eventually, all the surface water on the planet. Our oceans will evaporate. This global desiccation pretty firmly plants 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, so it becomes ruddier, even as it radiates energy 2,400 times stronger than it does now. This transforms 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 or not. As it stands, things don't look good.
Even 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 as well.
Is there any way for Earth to escape this fate? How do the other planets fare?
To answer both these questions honestly and in the order they were asked: ' not really ' and 'not well.' There is a slight reprieve because as the sun expands, its solar wind becomes much more powerful—so much, in fact, that the sun loses 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, currently with a temperature of 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, at that time, 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 can vaporize and possibly provide enough thermal retention to make the tiny world at least somewhat comfortable, if not exactly habitable.
What then? It gets worse, if you can imagine. The sun blows off its outer layers and the core is exposed to space, transforming into what astronomers call a white dwarf. While incredibly hot, the core is only about the size of Earth, so small that it provides very little heat to the planets. They 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 if 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 whole process starts up again.