Do planets move toward or away from their Star?

[farmerben]

The closest thing to a perpetual motion machine is a planet.  But there are tiny forces on it.  First is random gas and debris swept up from its orbital path during the early formation of a solar system.  This brings the planet inward, I think.  

Lets take a planet without a moon like Venus as an example.  There is very little impacting it except from the sun.  There is a small degree of pressure outward from the solar wind.  And, I think also, a small retrograde pressure due to encountering more solar wind on the prograde side of the planet.  The net result, I think, is the planet slowly getting closer to the star.

What does the  other literature suggest? 

[cubinator]

Whether an object is pushed toward or away from the star by radiation depends on its size and the way it re-emits heat, I don't remember exactly how that goes though. There is also a lot of influence from other planets, especially Earth and Jupiter. My guess is Venus will get pushed and pulled by gravity from the other planets more than radiation pressure. The orbit will also precess due to relativity over millions of years.

[tomf]

Good question. My guess is that both drag effects and solar wind are significantly smaller on something planet sized than the outwards spiralling caused by mass loss of the star. I hope someone who actually knows the answer comments though because I am curious.

[mikegarrison]

I am pretty sure the Earth is projected to slowly move away from the Sun, although since the Sun is expected to become a red giant, there is uncertainty about whether the Sun will ultimately swallow the Earth up or not. People will be long gone by that point in time anyway. And the Earth will be inhospitable to any known form of life. So it's kind of a moot point for us in many ways.

[Gargamel]

The answer to the title is yes.  
 

Some mechanics will force a planet farther away, and some will drag it down.  

[StrandedonEarth]

30 minutes ago, Gargamel said:

The answer to the title is yes.  
 

Some mechanics will force a planet farther away, and some will drag it down.  

It can be argued that since no planetary orbit is a perfect circle, at some points the planet is moving away from the primary, and at other points it is moving towards the primary…

[sevenperforce]

22 hours ago, StrandedonEarth said:

It can be argued that since no planetary orbit is a perfect circle, at some points the planet is moving away from the primary, and at other points it is moving towards the primary…

That's what I was assuming from the title of the thread TBH

[darthgently]

I figured the OP was about semi major axis increasing or decreasing with time.  Solar wind effects etc

[Terwin]

On 6/3/2023 at 12:08 PM, farmerben said:

The closest thing to a perpetual motion machine is a planet. 

The entire point of a perpetual motion machine is that it generates more energy than it consumes.

Planetary orbits are in a state of dynamic equilibrium, which is something entirely different.

From certain points of view a planetary body can seem to gain or lose energy, but that is usually an energy exchange with some other body in the system(usually Jupiter, if only because it has > 70% of the planetary mass in our solar system)

[Hannu2]

I think the most important effect is mutual interaction of planets. Solar systems are unstable many body systems during extreme long periods. They may converge to quite stable orbits but for example nearby stars can disturb systems. Also beginning may be chaotic. Some simulations show that outer planets have moved significantly, maybe even changed their order, and  one gas or ice giant may have been kicked out of the system. It seems to be quite common that gas giants drift near their stars (and destroy smaller planets in migration) and as far as I know it is not known why it has not been happened in our solar system. Such many body motion can shrink or expand orbits or even cause collisions or kick planets out from their system.

Second most important is slow drifting outwards because stars loose their mass during life. Stable stars like Sun lose negligible amounts during main sequence period but after that they begin to loose. Larger stars can loose significant part of teheir mass during main sequence period (which is very short due the extreme radiation power).

I think drag or radiation pressure are not noticeable for planet sized objects, except if they are completely swallowed in red giants. However, such effects drop small particles to star in astronomically short time and affect orbits of small asteroids.

In thermodynamics perpetuum mobile is a mechanism which does not obey conservation of energy. For example rogue planet can rotate in space whole lifetime of the Universum but it obeys conservation laws and is not considered to be perpetuum mobile.

Edited June 7, 2023 by Hannu2

[The1Paxman]

Planets, moons, dwarf planets, asteroids, comets, everything and anything in a solar system (just pick any ol' name because it's all just stuff, right?) is always moving towards or away from its star (if it only has one; if it has more than one, that's even more fun to think about, but if there's only one, then really we are talking about the barycenter, right?) all of the time.  If there's anything that we've all learned from playing KSP, it's that everything is always falling.  Falling is the name of the game.  It's just picking a place to determine where you choose to stop falling relative to.
Remember to think in 4D:  time.  It takes time for a planet to form.  If you're really asking about whether a planet is slowly working it's orbit spirally down closer and closer to it's star/barycenter, remember that it took millions of years for said planet to form.  And, as others have posted, the planet has probably wandered in it's orbit, not originating with it's current averaged distanced that we find so easily listed in textbooks.  These orbits are dynamic, chaotic, interactive, always changing.    What I don't like about KSP is the whole SOI concept: that only one object is exerting a gravitational influence at a time.  That's a simplistic way of looking at things and makes for a fine game, but makes for a completely inaccurate picture of what's really going on out there.
Over the extremely long periods of time that it takes for the solar system to form, the star(s) to form, the proto-planets to form, the planets to form, the star to go through it's various phases of life (pick a star type, because that'll impact the conditions/variables), etc etc There's just way too many variables and too much time consider.  The planet will get closer to its star if variables/circumstances line up a certain way and further if they line up a different way.

In the simple view, if a proto-planet can hang around in an elliptical orbit of any kind long enough to form into an actual planet, it's an orbit.  Everything else in and nearby the star/planet/solar system will nudge the orbit around moving the planet toward or away from the star.

But remember, the star isn't fixed.  The star is also moving around the galaxy and the galaxy itself is moving through the universe.  Everything is falling.

[kerbiloid]

They randomly oscillate near their quantized orbits, predicted by the Titius-Bode law (i.e. a discrete exponential distribution), under various perturbations.

(The Neptune is a particular exception, which probably should be taken into account together with the Kuiper Belt, with Pluto as a placeholder.)

They are as stable, as the electron orbits are. I.e. sometimes are, sometimes not are.

Edited June 7, 2023 by kerbiloid

[lajoswinkler]

On 6/7/2023 at 7:42 PM, Terwin said:

The entire point of a perpetual motion machine is that it generates more energy than it consumes.

That is only the first kind of perpetual motion machine. There are three kinds. OP mentioned the third kind - perpetual motion due to the lack of energy dissipative forces (mechanical friction, etc.) and planetary bodies far from stellar bodies are almost such things.

Edited June 10, 2023 by lajoswinkler

[darthgently]

On 6/7/2023 at 1:42 PM, Terwin said:

The entire point of a perpetual motion machine is that it generates more energy than it consumes

Not so sure about this definition.  But it is a unicorn anyway, so no matter

[mikegarrison]

A "perpetual motion machine" in the sense usually meant is a closed system with a decrease in entropy.  A violation of the second law of thermodynamics. Something simply flying through a dragless void would not be a perpetual motion machine in the usual sense, because it violates no rules of physics. It has no change in entropy at all, nor would it be expected to have any. It also can do no work, so describing it as a "machine" is quite questionable, even if it is perpetually in motion.

Edited June 10, 2023 by mikegarrison

[lajoswinkler]

15 hours ago, mikegarrison said:

A "perpetual motion machine" in the sense usually meant is a closed system with a decrease in entropy.  A violation of the second law of thermodynamics. Something simply flying through a dragless void would not be a perpetual motion machine in the usual sense, because it violates no rules of physics. It has no change in entropy at all, nor would it be expected to have any. It also can do no work, so describing it as a "machine" is quite questionable, even if it is perpetually in motion.

But it is defined as one type of a perpetual motion, the third kind.

[mikegarrison]

5 hours ago, lajoswinkler said:

But it is defined as one type of a perpetual motion, the third kind.

It's a definition that I disagree with.

[Terwin]

On 6/11/2023 at 7:51 AM, lajoswinkler said:

But it is defined as one type of a perpetual motion, the third kind.

Would not the solar wind cause a trivial amount of drag on planetary bodies casing them to be continuously slowing down by amounts too small to measure?

That sounds like a long duration, but not perpetual scenario.

Not sure how that balances with the loss of mass from the star due to both fusion and expelled mass(solar wind), but even if that keeps the system balanced, it is still not a 'perpetual' state, as it depends on the star converting mass to energy.

[lajoswinkler]

12 minutes ago, Terwin said:

Would not the solar wind cause a trivial amount of drag on planetary bodies casing them to be continuously slowing down by amounts too small to measure?

That sounds like a long duration, but not perpetual scenario.

Not sure how that balances with the loss of mass from the star due to both fusion and expelled mass(solar wind), but even if that keeps the system balanced, it is still not a 'perpetual' state, as it depends on the star converting mass to energy.

Sure, there is no such thing as perpetual machine of the third kind, but some things are close to it, considering our ephemeral experience with the universe.

Even a rotating body in intergalactic space isn't perfectly spherical and gives off gravitational waves. Unmeasurably tiny energy is given off, but still, it exists.