First, the bad news: in just under 8 billion years, the Sun’s expanding red giant will engulf Earth.
Things get worse from there, as you can imagine. Not that it was great to begin with; the process of the Sun starting to die and growing huge actually starts a few billion years before that. As the available hydrogen fuel runs out in the Sun’s core, it will begin to expand and become what we call a subgiant, blasting enough light to cook the Earth. And honestly, billions of years ago This one it gets bad because as hydrogen is converted to helium in the Sun’s core, the helium builds up and gets hotter, so the Sun is slowly getting hotter now; in about a billion years it will be hot enough for the Earth to lose all its water.
Oh, and that’s okay: to be brutally honest, even a hundred million years from now, the Sun’s warming will cause a runaway greenhouse effect on Earth that will burn it to life. So yes.
And yet, and yet, none of this is as bad as tThe Earth being physically inside the Sun, which is where we’ll be when our star expands into a proper red giant. The Sun will remain that way for hundreds of millions of years, so the Earth will likely spiral toward the Sun’s core over time and be incinerated.
The Earth will also affect the Sun, but probably only minimally. However, this is because the Earth is small. We know that some stars have giant planets like Jupiter orbiting very close – we call these planets hot Jupiters – and they will be swallowed early in the red giant process. Studies have shown that they can have a profound effect on the star. As they orbit, they experience a lot of drag, passing through the star’s gas, and this can spin the star, causing it to spin faster, causing it to eject its outer layers. This could be why so many planetary nebulae have such fantastic shapes.
A newly published new study carefully analyzes this process, using the physics of a planet inside a Sun-like star as it dies to see what the effects are given to different masses of planets. [link to paper].
They use sophisticated three-dimensional models of hydrodynamics, the physics of how gases flow, to observe the interaction of a planet within a star. They measure the ram pressure as the planet passes through the star’s gas and the drag on it that will cause its orbit to shrink and, in reflex, the star’s gas to expand.
What they found is that if the planet is big enough, it will greatly affect the shine of the star. This has to do with conservation of energy, the idea that energy cannot be destroyed; it just changes form from one type to another. In this case, there is a lot of energy in the movement of a planet around the star. Think of it this way: it only takes a little fuel to accelerate a car at highway speeds, but a lot more for a semi-loaded one. Think about how much energy a planet’s mass has, given that it could also be orbiting at many hundreds of thousands of kilometers per hour!
That’s a lot of energy, and as the planet passes through the outer layers of the star, the energy is transferred to this gas. The star responds to this addition energy by rotating faster, but also by getting brighter. Astronomers have found that a planet like Jupiter can increase the star’s luminosity by a factor of several hundred times over several years as it spirals. A much larger object, such as a brown dwarf with 80 times the mass of Jupiter, can make a star. tens of thousands of times brighter for a short period of time as it inspires! The peak of this effect only lasts for a year or so, which is very short on a cosmic scale, but lower levels of brightness can last for centuries.
In many cases, so much energy is deposited in the star’s outer shell that it will completely explode, flying off into space. They found that this can’t happen until the star expands to about ten times the diameter of the Sun, because when it’s smaller than that, its surface gravity is strong enough to hold on to the gas. As it expands, gravity weakens, allowing material to be ejected. Since the star is a fully bloated red giant, however, a planet just ten times the mass of Jupiter – which is not uncommon – can cause the star to eject its material.
There are a lot of subtleties in these calculations, but in the end they show that what happens to a star depends on its mass, how big it is when it swallows a planet, and how massive the planet is. As these calculations are refined, they can help astronomers really look for these events occurring in the galaxy. The Gaia space observatory, for example, monitors billion of stars, so while this engulfed planet-induced glow is ephemeral, it’s possible it could be caught red-handed. That would be awesome.
If this is all bad news, then there is a small amount of good news:
We are starting to understand how this process works. I know this is cold comfort given the, ah, rather bleak nature of it all, but science is all about understanding things; whether they are good or bad is a value judgment. But if I can be one little optimistic, understanding a problem is the first step to solving it. Hopefully, if humans – or anything like us – are still around in this future, we can head out into the galaxy and find a planet around a younger star to live on. Or, if we’re nostalgic, there are ways to move the Earth slowly away from the Sun to counteract its effects.
I’ll leave that to our distant descendants to worry about, though. We still have hundreds of millions of years to figure out the details.