<p>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.</p>
<p>The question &quot;Why do all the planets lie near the ecliptic?&quot; is therefore the same as asking &quot;Why are the orbits of all the planets more or less in the same plane?&quot; 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?</p>
<p>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 &quot;stuff&quot; 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.</p>
<p>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.)</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2024-03-04T15:54:08.927Z
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.
Darth Pseudonym
CC BY-SA 4.0
2024-03-05T01:05:13.027Z
Even most asteroids have orbital inclinations under 10°, but there are still significant numbers out to 20° or so. https://space.stackexchange.com/a/49027/38535
PM 2Ring
CC BY-SA 4.0
2024-03-05T09:17:00.540Z
https://www.youtube.com/watch?v=tmNXKqeUtJM
Leos Ondra
CC BY-SA 4.0
2024-03-05T12:07:32.547Z
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).
Peter Erwin
CC BY-SA 4.0
2024-03-05T15:33:52.067Z
Hot-group KBOs have a much wider range of inclinations than the planets do (and indeed, that's what defines the hot group), and even the cold group is now thought to be much more widely inclined than previously (the cold group was defined as within 5 deg and now it's more like 12). It's not as wild as the Oort is theorized to be, but still significantly more varied than the planets, and that's why I included KBOs as an exception.
Darth Pseudonym
CC BY-SA 4.0
2024-03-05T18:15:46.723Z
"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.
Russell Borogove
CC BY-SA 4.0
2024-03-05T18:52:44.677Z
I said, "Keeping a complex topic down to its most basic level..." I understand there are vast complexities that I'm skipping right over because they're not really relevant to understanding why the planets all share a common plane of revolution. I'd argue that before the random motion got averaged out, you didn't really have a disc yet, and anyway the details of the evolution of the protoplanetary disc is not relevant to this answer.
Darth Pseudonym
CC BY-SA 4.0
2024-03-06T16:20:01.467Z
I'd say Mercury's orbit is "different" rather than "weird". Its not at all weird once you understand why. (No, I'm not worried about the feelings of a planet, or demanding it be called "differently orbited"!)
T.E.D.
CC BY-SA 4.0
2024-03-06T17:25:04.093Z
Eh, potato potato.
Darth Pseudonym
CC BY-SA 4.0
2024-03-04T14:47:48.213Z
2024-03-05T22:19:44.280Z
2024-03-05T22:19:44.280Z
Darth Pseudonym
<p>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?</p>
<p>This question arose when I was going through the following lines in the book- &quot;Astronomy - Principles and Practice 4th ed. - A. Roy, D. Clarke&quot; (for some context):</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>I just got reading this book, so I am new to astronomy. Could anyone please provide an easy-to-understand solution to this?</p>
<p>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:</p>
<div class="s-table-container"><table class="s-table">
<thead>
<tr>
<th>Celestial Body</th>
<th>Approximate Deviation from Ecliptic (degrees)</th>
</tr>
</thead>
<tbody>
<tr>
<td>Mercury</td>
<td>7</td>
</tr>
<tr>
<td>Venus</td>
<td>3.4</td>
</tr>
<tr>
<td>Earth</td>
<td>0</td>
</tr>
<tr>
<td>Mars</td>
<td>1.8</td>
</tr>
<tr>
<td>Jupiter</td>
<td>1.3</td>
</tr>
<tr>
<td>Saturn</td>
<td>2.5</td>
</tr>
<tr>
<td>Uranus</td>
<td>0.8</td>
</tr>
<tr>
<td>Neptune</td>
<td>1.8</td>
</tr>
<tr>
<td>Moon</td>
<td>5.1</td>
</tr>
</tbody>
</table></div>
<p>As we can see, the deviation is almost always low, except Mercury—<span class="math-container">$7°$</span>! If so, why was it only Mercury?</p>

CC BY-SA 4.0
2024-03-04T14:56:18.263Z
As regards Mercury see https://astronomy.stackexchange.com/questions/54933/why-is-mercurys-orbit-so-unusual
Leos Ondra
CC BY-SA 4.0
2024-03-04T16:55:14.330Z
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.
EvilSnack
CC BY-SA 4.0
2024-03-04T17:04:42.047Z
True in our own Solar System, but not in at least some others...
ProfRob
CC BY-SA 4.0
2024-03-04T21:04:33.810Z
@EvilSnack Indeed! Jupiter's inclination to the [invariable plane](https://en.wikipedia.org/wiki/Invariable_plane) of the Solar System is only 0.32°.
PM 2Ring
CC BY-SA 4.0
2024-03-05T08:21:47.847Z
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,
Thomas
CC BY-SA 4.0
2024-03-05T16:27:49.010Z
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.
David Hammen
CC BY-SA 4.0
2024-03-06T17:24:42.497Z
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.
Darth Pseudonym
<ecliptic><inclination>
Why do most planets remain within a few degrees from the ecliptic?
Celestia Novalis
# Why do most planets remain within a few degrees from the ecliptic?

(Asked by: username_0 on 2024-03-04)

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?

**Tags:** ecliptic, inclination
> **Comments:**
> **username_1**: As regards Mercury see https://astronomy.stackexchange.com/questions/54933/why-is-mercurys-orbit-so-unusual  
> **username_2**: 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.  
> **username_3**: True in our own Solar System, but not in at least some others...  
> **username_4**: @EvilSnack Indeed! Jupiter's inclination to the [invariable plane](https://en.wikipedia.org/wiki/Invariable_plane) of the Solar System is only 0.32°.  
> **username_5**: 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,  
> **username_6**: 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.  
> **username_7**: 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.  

---

## 1 answers

### Answer by username_7 on 2024-03-04. Score: 22 | Accepted answer

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.

> **Comments:**
> **username_7**: 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.  
> **username_4**: Even most asteroids have orbital inclinations under 10°, but there are still significant numbers out to 20° or so. https://space.stackexchange.com/a/49027/38535  
> **username_1**: https://www.youtube.com/watch?v=tmNXKqeUtJM  
> **username_8**: 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).  
> **username_7**: Hot-group KBOs have a much wider range of inclinations than the planets do (and indeed, that's what defines the hot group), and even the cold group is now thought to be much more widely inclined than previously (the cold group was defined as within 5 deg and now it's more like 12). It's not as wild as the Oort is theorized to be, but still significantly more varied than the planets, and that's why I included KBOs as an exception.  
> **username_9**: "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.  
> **username_7**: I said, "Keeping a complex topic down to its most basic level..." I understand there are vast complexities that I'm skipping right over because they're not really relevant to understanding why the planets all share a common plane of revolution. I'd argue that before the random motion got averaged out, you didn't really have a disc yet, and anyway the details of the evolution of the protoplanetary disc is not relevant to this answer.  
> **username_10**: I'd say Mercury's orbit is "different" rather than "weird". Its not at all weird once you understand why. (No, I'm not worried about the feelings of a planet, or demanding it be called "differently orbited"!)  
> **username_7**: Eh, potato potato.  

---

