Why do planets, just like our moon, have their sidereal paths almost the same (with only slight deviation) as that of the ecliptic? Is it mere coincidence? Or is there a better solution?

This question arose when I was going through the following lines in the book- "Astronomy - Principles and Practice 4th ed. - A. Roy, D. Clarke" (for some context):

More information, too, would be acquired about the star-like objects that do not twinkle and which have been found in the course of a month to have a slow movement with respect to the stellar background. These planets, like the Moon, would never be seen more than a few degrees from the plane of the ecliptic, yet month after month they would journey through constellation after constellation. In the case of one or two, their paths would include narrow loops, though only one loop would be observed for each of these planets in the course of the year.

I just got reading this book, so I am new to astronomy. Could anyone please provide an easy-to-understand solution to this?

Here's a table showing the approximate deviations of the sidereal paths of the planets and the Moon from the ecliptic plane in the solar system:

Celestial Body Approximate Deviation from Ecliptic (degrees)
Mercury 7
Venus 3.4
Earth 0
Mars 1.8
Jupiter 1.3
Saturn 2.5
Uranus 0.8
Neptune 1.8
Moon 5.1

As we can see, the deviation is almost always low, except Mercury—$7°$! If so, why was it only Mercury?

  • 4
    $\begingroup$ As regards Mercury see astronomy.stackexchange.com/questions/54933/… $\endgroup$
    – Leos Ondra
    Mar 4 at 14:56
  • 20
    $\begingroup$ If I were Jupiter, I'd be inclined to say that my orbit was the ecliptic and everybody else was at an angle to it. $\endgroup$
    – EvilSnack
    Mar 4 at 16:55
  • 4
    $\begingroup$ As the invariable plane is related to the orbital angular momentum of the planets, it is obvious that the orbit of Jupiter comes closest to it, because of its mass. Still, the ecliptic is by definition given by Earth's orbit. It would be highly misleading to use this word for Jupiter or any other planet, $\endgroup$
    – Thomas
    Mar 5 at 8:21
  • 3
    $\begingroup$ By the 1990s, astrophysicists thought they had planet system formation all figured out. Nice and simple: terrestrial planets form inside the ice line, gas giants form just outside the ice line, and smaller (but still massive compared to the Earth) form a bit further out, and beyond that, there are some oddballs. But then astronomers began discovering exoplanets that break all the rules: planets with highly eccentric and/or highly inclined orbits, massive planets well inside the ice line. Our solar system apparently got lucky and escaped the chaos that appears to be the rule. $\endgroup$ Mar 5 at 16:27
  • 3
    $\begingroup$ Or the chaos is less common than something more like us, but more visible, because it's easier to detect exoplanets that are massive and swing close to the star. $\endgroup$ Mar 6 at 17:24

2 Answers 2


The ecliptic is the path through the sky along which the sun seems to travel during the year. If you flip your perspective around, that means the ecliptic is basically the path of Earth's orbit around the sun, projected onto our view of the sky.

The question "Why do all the planets lie near the ecliptic?" is therefore the same as asking "Why are the orbits of all the planets more or less in the same plane?" Apart from very distant objects like comets, the Kuiper belt, and the Oort cloud, everything in the solar system is orbiting in pretty much the same orientation and going the same direction around the sun. How did that happen?

Keeping a complex topic down to its most basic level, the answer to that is that all the planets formed from the same protoplanetary disc of gas and dust, so there wasn't a bunch of "stuff" out orbiting in random directions that could form planets with wildly different orbital characteristics. The disc was all moving in pretty much the same direction in pretty much the same plane, so when it all conglomerated into planets, they had to keep moving the same way.

The two notable exceptions are Pluto and Mercury. (I'm excluding the moon, because the moon was formed from a planetary collision in the early solar system, and its orbit is dominated by Earth's gravity, so its angle is a little high but for reasons only vaguely related to the formation of the solar system.)

Pluto has a very high tilt at over 17 degrees, which is probably due to being disrupted by Neptune's gravity until the two bodies reached the stable orbital resonance they have today.

Mercury, meanwhile, probably had a similar relationship with the sun. The sun is notably oblate (that is to say, it's slightly flattened by its own spin, so the equator bulges outward a little bit). From the Earth or Venus, that oblateness isn't a significant factor, but as close as Mercury is, the sun's gravity affects it a little differently when it's above or below the plane of the sun's rotation versus right along the plane. That means Mercury's orbit was slightly unstable early on.

We think that over millions of years, the tiny off-center forces from that difference slowly wobbled Mercury's orbit, pushing it into a higher inclination and altering its rotation and orbital eccentricity until it reached a stable resonance that it has maintained since then. In short, Mercury is so close to the sun that its orbit has to be a little weird to stay stable.

  • 11
    $\begingroup$ I'll just add as a comment: ScienceSnake had a very good point that all this is our best guess at what happened, based on what we can observe, but a lot of the details are still under ongoing investigation and debate. How the solar system formed is not an easy thing to study since we can't actually look back and see how it happened. $\endgroup$ Mar 4 at 15:54
  • 2
    $\begingroup$ Even most asteroids have orbital inclinations under 10°, but there are still significant numbers out to 20° or so. space.stackexchange.com/a/49027/38535 $\endgroup$
    – PM 2Ring
    Mar 5 at 1:05
  • 3
    $\begingroup$ youtube.com/watch?v=tmNXKqeUtJM $\endgroup$
    – Leos Ondra
    Mar 5 at 9:17
  • $\begingroup$ Unlike the Oort Cloud, objects in the Kuiper Belt are mostly orbiting in the same direction and relatively close to the pane of the ecliptic (with some moderate inclinations, like Pluto). $\endgroup$ Mar 5 at 12:07
  • 1
    $\begingroup$ "The disc was all moving in pretty much the same direction in pretty much the same plane, so when it all conglomerated into planets, they had to keep moving the same way." Not exactly; at some early point there was plenty of "random" motion, but inelastic (sticky) collisions between dust particles (and later, larger agglomerations) averaged out the randomness. The consistent angular rotation is what's left over after the averaging. $\endgroup$ Mar 5 at 18:15

As a single principle:

Conservation of Angular Momentum

Which means:

The material that a star and solar system forms from inevitably has some spin to it. This spin is about some main axis and is quantified by angular momentum: to calculate it, sum up the mass * distance_from_axis * tangential_velocity of every particle in the system. Like regular momentum, this quantity is conserved by the laws of physics. This means that motion perpendicular to this axis is special, and it cannot be "reduced". Motion parallel to this axis, however, is subject to no such constraint.

Now in a proto-stellar system things are moving around in a hopelessly complicated way. As a general trend, however, when small particles bounce (in-elastically) against each other, or crash into each other, or interact with things like tidal forces to stress and compress each other, some of the kinetic energy of the system is dissipated as heat. But losing kinetic energy means slowing down. But, because of the conservation of angular momentum, motion in the plane of rotation is protected. Hence, it must be the motion perpendicular to this plane which is gradually lost, which is the same thing as saying that orbits tend to move towards the same plane - the one that we end up calling the plane of the ecliptic. This is why accretion disks are disks rather than spheres: it's the lowest energy configuration that preserves the initial angular momentum of the system.

Note that this is a very rough and purely qualitative description of what's happening. The rate at which this convergence to the ecliptic happens depends on many factors. It's going to be much faster in the densest and most active parts of the system. A proto-planet in a stable system far away from anything that it could interact with is going to keep its angle to the ecliptic for much longer than something in a busy cloud of debris. This, however, is all talking about averages and general process, anything can happen to a specific planet (orbital resonances, a collision at a strange angle, etc...) to make it buck the trend.


Planetary formation is an active area of research and exactly how often different kinds of planets form in different types of orbit is not settled science. But the principles above are the physicists intuition for why we should expect everything in the a stellar system to eventually be spinning in more or less the same direction unless some specific event happens to make something else happen.

  • $\begingroup$ Neptune's moon Triton is the notable exception of a large body with a retrograde orbit (157° tilt) $\endgroup$
    – Karl
    Mar 7 at 21:49
  • $\begingroup$ Yes, I believe some retrograde exo-planets have been found as well $\endgroup$ Mar 9 at 19:29
  • $\begingroup$ Hm, it's relatively easy to determine the rotation rate of stars, but how on earth ;) can you determine the absolute direction of movement of anything as small and far out? The number of exoplanets that have been imaged directly can be counted on one hand, I think. $\endgroup$
    – Karl
    Mar 10 at 20:07
  • 1
    $\begingroup$ @Karl I haven't read it, but knock yourself out: arxiv.org/abs/0908.1553 or more casually newscientist.com/article/… $\endgroup$ Mar 10 at 21:01
  • 1
    $\begingroup$ Ah, OK! A fairly simple trick, en.wikipedia.org/wiki/Rossiter%E2%80%93McLaughlin_effect. You get no idea about the absolute direction, but the relative direction of star surface and planet movement is directly obvious if the planet passes in front of the star and you can take a time-resolved spectrum. $\endgroup$
    – Karl
    Mar 10 at 21:22

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .