The Poynting-Robertson Effect forces things to fall into the sun

The sun's light feels pretty good, doesn't it? Fool! That warming light is the sun's method of destroying you. Using its light, it is slowly forcing you to fall inwards, where you will burn. Burn! This trap is the result of the Poynting-Robertson Effect, which causes all the objects in the solar system, especially the smaller ones, to swirl in a kind of very mild death spiral towards the sun.

The effect uses the same principle that scientists in laboratories use to cool tiny particles using lasers — the fact that particles will radiate the energy from light in every direction, even if they only take it in from one direction. Put a piece of iron over a fire, and it will get hottest on the side that faces the fire, but you will generally feel the heat even if you put your hand on the side that's away from the fire. Although the degree to which an object radiates heat in all directions depends on the thermal conductivity of the substance as well as its size and the heat source, almost every object will shed some of its heat away from the source of heat. So, as particles orbit around the sun, picture energy and motion coming in from only one direction — the sun — but energy radiating off in every direction. As that energy radiates away, the object is pushed a little in the opposite direction, the way throwing a ball forwards gives the thrower a little push backwards.

But why should the particle be falling into the sun at all? Shouldn't all those hits from the sun's photons be pushing it away from the sun? If it were standing still, yes. It would get pushed away from the sun slowly. But these particles are orbiting. And the photons always push against that orbit. Think about standing in vertical rain. You're not moving, and so the rain isn't resisting your motion. But as you begin to walk, the rain doesn't seem vertical anymore. Instead, it seems to be blowing at a slight angle so it hits you in the face. In fact, you're walking into it, but the effect is the same. It's resisting your motion. As the particles move around the sun, they're experiencing the same thing with the sun's energy. Instead of just coming from a neutral direction, photons are blowing into their "faces" and resisting their motion. If they could radiate heat in just one direction, picking up their speed again, it wouldn't matter. But since they're radiating energy in every direction, overall, they're slowing down. As they slow down their orbit, they fall towards the sun.

It's a clever trap that the sun has set up. Sure, get close and you can harvest energy and sustain life. But you are, inevitably, being slowed down by the very energy that you harvest. You'll slow, and you'll fall inwards. Sure, dust has it worse than planets in this respect, but we're all spiraling down.

Image: NASA/SDO/Steele Hill

Via Harvard College Observatory, NJU, and Icarus.