Bad astronomy | New research into what happens when a star eats a planet

First, the bad news: In just under 8 billion years, the expanding red-giant Sun will engulf Earth.

Things get worse from there, as you can imagine. Not that it’s great to begin with; the process by which the Sun begins to die and become huge actually begins a few billion years earlier than that. As the available hydrogen fuel runs out in the core of the Sun, it will begin to expand and become what we call a sub-giant, emitting enough light to bake the Earth. And honestly, billions of years before this it gets bad because hydrogen is converted to helium in the core of the Sun, the helium builds up and gets hotter, so the Sun slowly gets hotter now; in about a billion years it will be warm enough for the Earth to lose all its water.

Oh, well: to be brutally honest, even in a hundred million years or so, the warming of the Sun will cause a runaway greenhouse effect on Earth that will burn it lifeless. So here is.

And yet, and yet, none of this is as bad as tthe Earth being physically inside the Sun, where we will be when our star transforms into a true red giant. The Sun will stay that way for hundreds of millions of years, so the Earth will likely spiral down towards the Sun’s core over time and be incinerated.

The Earth will also affect the Sun, but probably 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 up very 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. Perhaps that is why so many planetary nebulae have such fantastic shapes.

A new study that has just been published takes a close look at this process, using the physics of a planet inside a Sun-like star as it dies to see what effects are given to different planetary masses. [link to paper].

They use sophisticated three-dimensional hydrodynamic models, the physics of gas circulation, to observe the interaction of a planet inside a star. They measure the dynamic pressure as the planet passes through the star’s gas, and the drag on it that will cause its orbit to shrink and, reflexively, the star’s gas to expand.

What they discovered is that if the planet is large enough, it will greatly affect the brightness of the star. It has to do with the conservation of energy, the idea that energy cannot be destroyed; it simply changes form from one gender 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 to highway speeds, but a lot more for a fully loaded tractor-trailer. Think of the amount of energy that the mass of a planet possesses, given also that it can be in orbit at several hundred thousand kilometers per hour!

That’s a lot of energy, and when the planet passes through the outer layers of the star, the energy is transferred to that gas. The star reacts to this extra energy by itself spinning faster, but also by becoming brighter. Astronomers have found that a planet like Jupiter can increase the star’s brightness by a factor of several hundred for several years as it spirals. A much larger object, like a brown dwarf with around 80 times the mass of Jupiter, can make a star tens of thousands of times brighter for a short time as he inhales! The peak of this effect only lasts about a year, which is very short on a cosmic scale, but lower levels of brightening can last for centuries.

In many cases, so much energy is deposited in the outer layer of the star that it entirely blows away and flies off into space. They found that this cannot happen until the star has grown to about ten times the diameter of the Sun, because when it is smaller than that its surface gravity is strong enough to hold the gas. As it expands, gravity weakens, allowing material to be ejected. Once the star is a fully inflated red giant, a planet with only ten times the mass of Jupiter – which is not uncommon – can cause the star to eject its material.

There are a lot of intricacies in these calculations, but in the end they show that what happens to a star depends on its mass, its size when it engulfs a planet, and the mass of the planet. As these calculations are refined, they could help astronomers actually search for these events happening in the galaxy. The Gaia space observatory, for example, monitors Billions of stars, so while this sunken planet-induced brightening is fleeting, it’s possible it could be caught red-handed. That would be great.

If this is all bad news, there is some good news:

We are beginning to understand how this process works. I know that’s cold comfort given the, ah, rather dark nature of it all, but science is about figuring things out; whether they are good or bad is a value judgment. But, if I can be a little optimistic, understanding a problem is the first step to solving it. Hopefully if humans – or something like us – are still around in this future, then we can move forward in the galaxy and find a planet around a younger star to live on. Or, if we’re nostalgic, there are ways to slowly move the Earth away from the Sun to counter its effects.

I will leave that to our distant descendants to worry about. We still have hundreds of millions of years to work out the details.

Leave a Comment