<p>The mass of a planet does not affect its orbit significantly unless it is a very large planet.  On the opposite end of the spectrum would be a very small mass planet that was evaporating, and that is essentially what a comet is.  It's all Keplerian, though comets can have their orbits affected slightly by the tiny push they get from the evaporating gases.  So it's not the change in mass, but how the evaporation actually happens, but this is a small effect.  Even a large planet like Jupiter does not affect the Sun much (the Sun orbits around a point that is not even external to the Sun), so losing mass would not affect the orbit of Jupiter in any significant way either.</p>

CC BY-SA 3.0
2016-10-15T13:02:49.563Z
So how does the evaporation happen and how is this measured and analysed? Even if the effect is small, how is it analysed?
Spoilt Milk
CC BY-SA 3.0
2016-10-15T22:52:28.777Z
Evaporation of comets is what leads to their tails, which we have observed up close using space missions.  We also track their trajectories, and note small deviations from Keplerian orbit, both from the gravity of other planets, and from impulses from the evaporation.  The latter produce small unpredictable kicks, but again it is not the change in mass that matters, it is how evaporation can act like a very tiny jet engine.
Ken G
CC BY-SA 3.0
2016-10-16T05:21:46.690Z
so would the comet follow a hyperbolic trajectory as it comes closer to the sun?
Spoilt Milk
CC BY-SA 3.0
2016-10-17T02:21:44.587Z
That does indeed seem possible, if it is a long period comet and gets enough of a push from its evaporation.
Ken G
CC BY-SA 3.0
2016-11-14T17:49:44.233Z
To nitpick a point you made, the point the sun orbits [(the barycenter of the solar system)](https://en.wikipedia.org/wiki/Barycenter#Inside_or_outside_the_Sun.3F) is occasionally outside of the sun, when [Jupiter and Saturn and some of the ice giants line up](https://en.wikipedia.org/wiki/Barycenter#/media/File:Solar_system_barycenter.svg).
Cody
CC BY-SA 3.0
2016-11-14T23:58:20.533Z
@Cody is correct. The solar system barycentre is frequently external to the Sun.
ProfRob
CC BY-SA 3.0
2016-11-15T06:58:05.780Z
Yes, by "not external to the Sun," I mean that the point is not permanently outside the Sun.  One would not say it *is* external to the Sun!  But yes, it's ambiguous.  The point is simply, negligible feedback onto the orbit from mass loss,
Ken G
CC BY-SA 3.0
2016-10-15T12:27:44.230Z
2016-10-15T12:27:44.230Z
Ken G
<p>A planet orbits around its sun on an elliptic orbit, and loses mass slowly due to evaporation. How will the parameters of the orbital ellipse change as a function of time?
Could we do a generalisation and mathematically model it?</p>

CC BY-SA 3.0
2016-10-15T11:39:34.507Z
As a curiosity, is this a homework question, or for "worldbuilding" consideration, or??
Fattie
CC BY-SA 3.0
2016-10-15T11:49:43.200Z
@JoeBlow Nope this is not a HW question. If it were, I would have a used a HW tag.
Spoilt Milk
CC BY-SA 3.0
2016-10-15T12:05:25.740Z
If you just mean in a Keplerian sense, as Earth gets lighter the orbital path would be identical but, let me think, slower, the year would be longer.  Mass does not come in to Kepler's laws.
Fattie
CC BY-SA 3.0
2016-10-15T12:14:49.937Z
@JoeBlow Not in a keplerian sense.
Spoilt Milk
<planet><astrophysics><celestial-mechanics><orbital-mechanics>
On orbital mechanics of evaporating planets
Spoilt Milk
# On orbital mechanics of evaporating planets

(Asked by: username_0 on 2016-10-15)

A planet orbits around its sun on an elliptic orbit, and loses mass slowly due to evaporation. How will the parameters of the orbital ellipse change as a function of time?
Could we do a generalisation and mathematically model it?

**Tags:** planet, astrophysics, celestial-mechanics, orbital-mechanics
> **Comments:**
> **username_1**: As a curiosity, is this a homework question, or for "worldbuilding" consideration, or??  
> **username_0**: @JoeBlow Nope this is not a HW question. If it were, I would have a used a HW tag.  
> **username_1**: If you just mean in a Keplerian sense, as Earth gets lighter the orbital path would be identical but, let me think, slower, the year would be longer.  Mass does not come in to Kepler's laws.  
> **username_0**: @JoeBlow Not in a keplerian sense.  

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## 1 answers

### Answer by username_2 on 2016-10-15. Score: 2

The mass of a planet does not affect its orbit significantly unless it is a very large planet. On the opposite end of the spectrum would be a very small mass planet that was evaporating, and that is essentially what a comet is. It's all Keplerian, though comets can have their orbits affected slightly by the tiny push they get from the evaporating gases. So it's not the change in mass, but how the evaporation actually happens, but this is a small effect. Even a large planet like Jupiter does not affect the Sun much (the Sun orbits around a point that is not even external to the Sun), so losing mass would not affect the orbit of Jupiter in any significant way either.

> **Comments:**
> **username_0**: So how does the evaporation happen and how is this measured and analysed? Even if the effect is small, how is it analysed?  
> **username_2**: Evaporation of comets is what leads to their tails, which we have observed up close using space missions.  We also track their trajectories, and note small deviations from Keplerian orbit, both from the gravity of other planets, and from impulses from the evaporation.  The latter produce small unpredictable kicks, but again it is not the change in mass that matters, it is how evaporation can act like a very tiny jet engine.  
> **username_0**: so would the comet follow a hyperbolic trajectory as it comes closer to the sun?  
> **username_2**: That does indeed seem possible, if it is a long period comet and gets enough of a push from its evaporation.  
> **username_3**: To nitpick a point you made, the point the sun orbits [(the barycenter of the solar system)](https://en.wikipedia.org/wiki/Barycenter#Inside_or_outside_the_Sun.3F) is occasionally outside of the sun, when [Jupiter and Saturn and some of the ice giants line up](https://en.wikipedia.org/wiki/Barycenter#/media/File:Solar_system_barycenter.svg).  
> **username_4**: @Cody is correct. The solar system barycentre is frequently external to the Sun.  
> **username_2**: Yes, by "not external to the Sun," I mean that the point is not permanently outside the Sun.  One would not say it *is* external to the Sun!  But yes, it's ambiguous.  The point is simply, negligible feedback onto the orbit from mass loss,  

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