Why do the planets have roughly the same inclination with respect to Earth?

Looking at the inclinations, which I got from http://www.astronomynotes.com/tables/tablesb.htm, we see that Mercury has the highest inclination at about 7 degrees with respect to Earth. Why are there no inclinations at approximately 90 degrees, or even slightly less, like 45 or 30? My guess is after some long time, gravitational influence causes all the planets to line up.

Question two is then: Do we know of any planetary systems where the planets have high degrees of inclination with respect to each other?


While the other answer here and the link provided by Jeremy give excellent explanations, I believe a bit more nuanced reasoning is required.

Although the theory of planet formation is currently still incomplete, it is generally accepted that planets form in a so-called proto-planetary disk as a part of stellar formation process. This is backed up by several observation of such disk and even by directly observed planets in systems where remnants of the disk are still present (most notably Fomalhaut-b).

However, exactly how these planets form is still unclear. A popular theory is that these planets form in place and therefore keep the orientation and inclination of the disk. This beautifully explains the Solar System and specifically why all planets have roughly the same orbital inclination. But that's not surprising as the model was specifically designed to do just that! Yet the Solar System does not have to be the only possible type of planetary system, or even the most common for that matter. Indeed the study of exoplanets has confirmed that very different planetary systems are possible.

An alternative theory is that planets are created far away from the star and migrate inwards. During this migration multiple planets can interact with one-another and be pushed out of the disk plane to higher inclinations. They could even flip over completely and become retrograde.

Now to come back to the actual question; the Solar planets probably have about the same inclination simply because they were created that way.

You are also right in saying that gravity will tend to align the planets. How this works physically is that the star can exerts a torque on the planet, which produces a tidal effect (like how the Moon creates tides on Earth). Over time this aligns the angular momentum of the planet to that of the star by pushing the planet towards the equatorial plane of the star.

As for your second question, yes, there are probably planetary systems which are misaligned with respect to the star. At least there are certainly planets that are not aligned with the equatorial plane of the star. The well named HAT-P-11b is a known example, but there are a lot more.

Note that for exoplanets the important parameter is not the inclination (which for exoplanets is the angle between the normal of the orbit and the line of sight), but rather the obliquity or tilt of the planet rotation with respect to the rotation of the star.

As for a third twist to this story, very recently there was a discovery of an entire proto-planetary disk that is misaligned with respect to the star, which opens the door to even stranger configurations.

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    $\begingroup$ Nice overview of the subject of solar system formation. For those who want a bit of detail without mathematical rigor, the recent book "From Dust to Life: The Origin and Evolution of Our Solar System" is worth considering. $\endgroup$ – Ben Aug 2 '14 at 17:17
  • $\begingroup$ The question was whether the planets' orbital planes were misaligned with each other, not with the star. Roughly one third of "hot Jupiters" have strong orbital misalignments with the star's spin. $\endgroup$ – Rob Jeffries Dec 11 '14 at 14:02

To answer your second question:

Upsilon Andromedae c and d have mutual inclination of 30 degrees http://en.wikipedia.org/wiki/Upsilon_Andromedae

Note that you cannot calculate mutual inclination by subtracting one inclination from another from exoplanet data catalogs because the "inclination" used in exoplanet studies is a 2D line-of-sight-to Earth inclination. Mutual inclination must be got from full 3D measurements of a system using astrometry.

The GAIA telescope will probably find more such non-coplanar systems with large mutual inclinations.

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It starts with the formation of the solar system. The solar system started off in a large cloud of gas, under gravity, this cloud started to collapse. Most of the mass was concentrated at the centre which eventually formed into the sun. Due to the conservation of angular momentum, as the cloud collapse it started to spin and form into a disk with a bulge at the centre where the sun was forming. The resulting planets were formed in this disk and that's why most of the planets have roughly the same inclination and why they all orbit in the same direction. We do have some bodies that we have captured that came from outside of the solar system that has an extreme inclination with respect to the earth.

As for your second question, As far as I know we have not found any other systems that have extreme inclinations as our methods of detection aren't that advance to tell much detail about other planetary system other than probably number of planets, size of planets and so forth. But it could be possible if a rouge planet that has no star is captured by a star.

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  • $\begingroup$ Incorrect - extreme inclinations (and by that I mean the angle the orbital angular momentum makes with the stellar angular momentum) are commonly found using the Rossiter-McLaughlin effect. $\endgroup$ – Rob Jeffries Dec 11 '14 at 14:04

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