# Sunlight pressure on planets

We know that the pressure of sunlight can blow gas and dust away from a star and can even propel a spacecraft equiped with a sail.

Therefore sunlight must exert a outward force on the earth and other planets. Given enought time shouldn’t such a force alter the planets orbit? If so, how and by how much.

The earth weighs 5.972 sextillion (5,972,000,000,000,000,000,000) metric tons. That means the effect would be far, far less than 1/5,972,000,000,000,000,000,000 times as great as that on a solar sail.

We ain’t goin’ nowhere.

If I can remember correctly from my Astro class last year, the idea is now that some of the Jovian planets formed much closer in, but solar wind has blown them out to where they are now, over billions of years. The composition of Jupiters atmosphere indicates that if formed closer in to the sun than it currently is.

When you say “alter”, you imply a change between some before-time & some after-time. That’s not quite right. The solar pressure has always been there.

Right now our orbit is a result of gravity between Earth & Sun, AND solar pressure from the Sun. And also another hundred factors, such as gravity from the other planets, the Mooon, the drag from the galactic gas/dust cloud the Solar System is pushing through, the magnetic forces between our magnetosphere & the solar wind, etc., etc., etc.

Our orbit from moment to moment is the vector sum of all those influences. And as Exapno Mapcase points out, the solar wind ranks about number 54 in the list of influences. Computable in principal? Sure. Detectable in practice? Heck no.

Light pressure and the pressure of solar wind (charged particles emitted from the Sun) create a pressure which tends to push the planets away. However, the interplanety medium creates a drag on the planets that tends to slow them down, dropping them into a lower orbit (consequently, by the perverse laws of orbital mechanics, speeding them up). The effect of both, singly or in combination, is negligible on the orbit of the planets, although it has almost certain had a significant impact upon the atmospheres of the inner worlds.

Stranger

I don’t quite see how that works. Jupiter’s atmosphere is much richer in hydrogen and helium than the inner planets’, and those are the two elements that are going to get blown first away by the solar wind, on account of their small molecular mass.

I just don’t see how the entire planet could have been moved out from the sun to its current orbit without losing most of the H and He along the way. Plus, I doubt that the force of the solar wind is anything close to that required to move a planet the size of Jupiter out from close to the sun to where it currently is. Could you provide a cite?

The solar light pressure is an inverse square force pushing outwards. Gravity is an inverse square force that pulls inwards. To first order, the effect of solar light pressure is to reduce the force of gravity. By an incredibly tiny amount. So, no, it isn’t going to have a major effect on orbits.

An interesting effect, tyhough, is that for dusdt particles of the appropriate size (really tiny) the solar light pressure is large enough and the force of gravity small enough that the two cancel out, something predicted by Peter deBye about a century ago. Clouds of dust of this size will essentially pass through the solar system unaffected, since the forces are inverse-square-law and of opposite sign.

Of course, it’s only true to first order, and you’d expect impact with gases and solar wind partyicles and electrostatic interactions to still have an effect, and you;ll get gracvity effects when the particles pass through shadows.

the pressure of sunlight is known as Radiation pressure which is energy flux density divided by speed of light in the respective medium

As ever Wiki dscribes it much better than I can

So they reckon about 4.6 micro pascals or 0.0000046 N/m2

With a radius of 6,372 km and assuming our exposed profile is simple disc that gives us an exposed profile of 42 trillion m2, so an applied force of 194 million N
As mentioned above the earth mass at 5.94x10E24
Which gives an acceleration of 33 atto m/s2

Which is a long winded heavily rounded off with assumptions flying everywhere sort of way of saying what everyone else said, which was bugger all effect.#

No doubt a proper physics dude or dudette will be along shortly.

The aspect should actually be about 128x10[sup]12[/sup] m[sup]2[/sup], and thus an applied force of 587x10[sup]6[/sup] N. This still translates into several orders of magnitude less than a rounding error.

Also, any planet with an atmosphere has an elastic cushion around it; Newton’s laws and conservation of momentum still hold, of course, but this makes the calculation far more complex than a simple aspect pressure calculation; the planet can shed atmosphere to create a momentum balance.

Stranger

For the life of me I cannot work out how I got 42 trillion from pi r2
I blame Doug 10E9 times.

So, solar radiation pressure doesn’t count for Diddely. Still, we hear of the “sun beating down” on a summer day. The phrase surely comes from before any of us knew the concept of solar wind.

Actually, it does have an effect: there was a program on TV about the planets recently and it said that there was a gaseous trail stretching out behind Venus. Apparently, the Earth captures some of it.

But it would appear that the pressure of the solar wind is in equilibrium with the gravitational attraction of the sun, as LSLguy says.

And let’s acknowledge the distinction between solar radiation pressure and the solar wind. A photon of light from the sun stricking, say a molecule of O2, in the upper atmosphere will cause it to recoil slightly, but not accelerate much. It could break the molecular bond but it won’t drive it out of the atmosphere. A proton from the solar wind - that’s what the solar wind is mostly made of - going at 500 km/s striking the O2 molecule could accelerate it to 500/16 = about 30 km/s and could drive it out of the atmosphere if it is already far enough out.

So, how do solar sails work at night?

Eh, why do you think they carry torches on spaceships?

Paging Sunlight. Paging Sunlight.

Really, I think he should be given a chance to defend himself here.