How can three planets in our sky form a triangle?

[atkinsonr]

Looking at the sky this week we were fortunate to have some amazing views of Mercury, Venus and Jupiter in conjunction.

Given that the planets formed from the protoplanetary disc, and given the conservation of angular momentum, why did Mercury, Venus and Jupiter form a triangle from the perspective of Earth?

Was it due to an early interaction or collision between two planets? If so what are the details?

I'd love to know the answer to this in as much detail as possible :)

[astronobob]

Hi Rich,

Here are 2 links that may get you some further insight to your question.

1st shows that each planets obit is not 'exactly' on the celestial plane, explains why you do see triangle positioning when 3 planets are near each other in our line of sight
Orbital Inclination :
https://en.wikipedia.org/wiki/Orbital_inclination

2nd has a brief understanding on how they got that way
https://van.physics.illinois.edu/qa/listing.php?id=16884

Hope this helps

[iasuka]

Hi Rich, this is a very good question. :thumbsup:

While planet formation from a protoplanetary disc can be considered confirmed by recent discoveries of protoplanetary discs around other stars, there are no complete and detailed theories that would be proven accurate at this moment. This is as far as I know. I can easily be wrong of course. To get more details one would probably need to dive deep into the actual science papers.

Personally, I'm convinced that planets orbit inclination was caused by an early interaction between numerous planetesimals and protoplanets. Actual planet collisions in later times are unlikely but still possible.

Another interesting detail is solar system planets have very different rotational axis inclination. For instance, Venus have 177 degree axis inclination meaning it actually spins in the opposite direction to the Earth! Can you imagine how fierce and terrifying those early days of the solar system could be?

BTW, few years ago I had a good time playing games like Super Planet Crash (http://www.stefanom.org/spc/). That particular one probably isn't very precise, but quite educating nonetheless.

[OICURMT]

Had to try this... loads of fun!...

Got a stable system with one planet (Planet 5) crossing paths with another (Planet 4)... consequently, for Pluto lovers...

Are the two planets that cross-over each other technically not planets, as they haven't clear their respective orbits?

It was interesting to see the elongated orbit (Planet 5) slowly go more circular, only to be pulled back into high elongated by the Brown Dwarf (Planet 2) and Planet 4. Planet 5 actually ended up outside of Planet 4's orbit several times, slowly shifting into the crossing mode again.

OIC!

[atkinsonr]

Thanks Dmitry and Bob. I'm going to read those links tonight

[NTgazer]

Super Planet Crash... well there goes the rest of my afternoon. Great fun!

[Eratosthenes]

....can you describe how 3 celestial objects, or any 3 objects CANNOT form a triangle? (Euclidianly speaking of course)

[Hodur]

Perspective

[julianh72]

It's called an Eclipse!

[bkm2304]

Maybe even 4 - if you add the viewer's planet! And in this case it would be a straight line - or is it a ray? And depending on the relative sizes and arrangement in the line it may be a mix of eclipses and occultations! Oh vey!!!:shrug::shrug::shrug::shrug:

Richard

[iasuka]

Maybe I took it wrong, but my impression of the original question was that the TS wanted to know how come planets form triangle instead of a straight line. On other words, why planets in Solar system are not in the same orbit plane?

This problem is not as trivial as it might seem. Look at the rings of Saturn, for instance. It consists of billions of ice pieces all going in virtually the same plane. On the other hand, if you look at the planets' orbit planes - those are noticeably inclined, for up to 7 degrees for Mercury.

[sil]

I think it would be a line segment as it the two outer planets would ben endpoints in this example. But yes a line or ray could be drawn through them in an illustration.


The original question is a good one and we have even more simple but complex questions in our own solar system. Why some bodies (planets/moons) in our solar system orbit or spin in a different direction to everything else (well, the consensus direction). Its questions like these that get us closer to understanding how the solar system formed and whether ours is typical of other solar systems or different. It wasnt that long ago that it was presumed all moons in the solar system were boring, dead worlds like our own moon so NASA questioned the benefit of taking any photos when it sent probes out to jupiter and saturn. If we didnt actually look we wouldnt have discovered some of the most amazing locations and possibilities for extraterrestrial life.

[xelasnave]

If the Sun is moving thru space, which on all accounts it does, and if the message of gravity travels at a finite speed, I think that be the speed of light, one would think the planets each orbit a point where the Sun was such that you would expect the orbits mark out a cone shape.
I don't know if the observations would show that but seems reasonable to me.
I think the solar system should move thru space in a "pie in the face" orientation but I think observations show the orbits are inclined.
Does anyone understand the point I am trying to make?
Alex

[julianh72]

I believe that gravity does indeed propagate at the speed of light, so that at this precise instant, the Earth is being pulled towards the Sun where we see it, which is where it was about 8 minutes ago. (Or more correctly, it's probably that distortion and curvature in the fabric of Space-Time propagate at the speed of light? :question: )

The Earth is orbiting around the Sun at about 30 km/s. The Sun is orbiting around the centre of the Milky Way at about 200 km/s, which is much faster than the Earth's orbital speed, so by the time the Earth has completed one orbit of the Sun in one year (about one billion km), the Sun has moved about 6.3 billion km.

If the Solar System was travelling "edge on" around the Milky Way, the Earth's trajectory would be a relatively small side-to-side wobble about the Sun's trajectory. If the Solar System was travelling "face on", the Earth's path would be a long slender helix, where the pitch is much longer than the diameter.

In fact, the plane of the Solar System is tilted about 63 degrees relative to the galactic plane - and it makes my brain hurt trying to work out what the Earth's trajectory looks like! :confused2:

And then we need to think about the movement of the Milky Way through Space-Time ..... the Milky Way orbits around the centre of mass of the Local Group, and the Local Group is moving at about 600 kilometres per second in the direction of the constellation Hydra. (And since we're about 25,000 light years from the centre of the Milky Way, I guess that the Sun is really orbiting around the point where the heart of the Milky Way was 25,000 years ago?)

I think I'm sticking with the Earth-centric Universe - it's a lot easier to get your head around! :screwy:

[Eratosthenes]

Can you cite an instance where the earth is NOT eclipsing the sun?

....to see it depends on whether you are located in the shadow cast.

[Eratosthenes]

I would be more surprised if the planets in any solar System lined up precisely on a common orbital plane.

[xelasnave]

So Julian is there any hint that the solar system is cone shaped?
Does Earth orbit where the Sun was 8 minutes ago?
If the Sun travels at 200 klms per second the center of our orbit would be 200 x 60 x 8 klms.
If we find no hint of a cone shape would that indicate gravity propagates faster than light?
I don't know what GR says.
We need a time exposure of the solar system passing thru the field of view.
There must be a computer simulation showing same.
I wonder why we have the incline.
Alex

[sil]

The solar system is really a pebble skipping across the lake of reality? :) So we have an incline to provide lift and not sink into the depths where cthulu awaits?

MIB 4 anyone?

Seriously the incline is easy for me to understand, what I find tricky is why there aren't tight conical columns of particles/energy radiating outwards from the top/bottom of the suns spin axis? Thats how we see black holes directly isn't it? and pulsars? the spin of a cloud of matter creates a disc but there is a pair of funnels perpendicular too, like stirring a drink a funnel forms, is it just gravity causing the funnels or is it high energy collisions in the middle of the spinnig region where anything ejected back out into the disc just get added to it and the perpendicular column is essentially in balance like a spining top where the disc doesnt effect it enough.

Its too early to think cosmologically.

[xelasnave]

Hi Sil,
Would you care to share your understanding re the incline or are you suggesting the stone skipping holds the understanding?
I think we can observe the conical columns you suggest, at least I think I saw something showing same.
I am not sure about that so it may be I imagined seeing something and without an authority to refer to I treat vague recollections as suspect.
But something like you suggest seems reasonable.
Alex

[sil]

pebble was a bit of fun nonsense.

Incline though: consider a cloud of matter and energy in space. There's almost zero chance it could be perfectly spherical with perfectly uniform distribution. As gravity (or whatever force) draws it all inwards it will do so at different rates and maybe not even vectors that take it through the ideal center of mass. So the unbalance can start the clumping that leads to planets (presumably the same mechanism as the whole but on a smaller localised scale). Not sure why a spin is induced or needed, but may be the result of a clump moving on a straight vector towards the center but not exactly through a center point so it goes past and falls back again in a tight ellipse. This motion taken over enough time smooths out towards a circular path. Now the ideal plan of the spin is a disc averaged by the masses and distribution but because nothing is ideal most of the actual elements won't be precisely on this plane, they may be close and their individual plane is on an incline.

Thing to remember is our solar system is NOT in a static fixed state. I feel its tending towards circular orbits on the same plane but its still in a dynamic state making its way towards that eventuality. It all happens on a time scale much larger than a human lifespan so the changes happen too slowly for us to notice and possibly measure yet with confidence.

Experiment: get a childrens spining top and spin it. The forces want it to spin upright on a vertical axis, but the tiny variations in density of the tops material, tiny changes in air currents, tiny imperfections in the table surface, the direction of energy you gave the top to start it spinning, etc all contribute enough to give it a noticable wobble from its ideal state.

re the conical columns, my understanding is we see them in the xray spectrum directly from black holes. I believe its our only direct way of photographing them and the movements of other objects is how we infer the presence of a black hole and where it should be. But pulsars tend to be described thein a similar way but with the conical jets out along the spin disc so as it spins it flashes like a lighthouse, but I dont understand how this could come about or why all stars don't exhibit columns as we presume they are all spinning or why they dont just tend to being football shaped. Or maybe they do but to a degree thats too small for us to measure yet (we know the earth is not a perfect sphere but bulging towards a disc direction so why these xray columns from the axis? thats making less sense as i think about it).

btw I am an enthusiastic amateur, am relearning all maths i can but not yet up to tackling general relativity or quantum physics yet (I want to be able to do them all for myself and explore datasets and confirm findings one day, not sure its possible or if i'm capable. I had a deep understanding of Newton in high school that I've since lost and want to regain. I keep a journal of things to one day explore and of course my ideas will be wrong but I do want to under WHY they are wrong for myself and not just take the word of textbooks. The exploration is fun regardless the outcome.

sorry if this went on and on and on :)

[julianh72]

Star systems are formed by the gravitational collapse of a cloud of gas and dust; as the material collapses in on itself, it retains its initial angular momentum as it forms a disc, and then the star and planets coalesce from that disk, so all tend to lie in a plane. However, the initial angular momentum of the gas / dust cloud is pretty random, so the inclination of star systems within a galaxy is pretty random.

Our Solar System happens to be aligned at an angle of about 63 degrees to the plane of the Milky Way, but other star systems are at arbitrary angles to our own - which is why we see some of them edge-on (which means we can see transiting planets or eclipsing binary stars), while others are more face-on to us.

[xelasnave]

Hi Sil,
Thank you for taking the time to present your understanding on the matters you addressed.
Alex

[xelasnave]

Hi Julian,
Thank you for your explanation.
Alex

[Eratosthenes]

How do you know so much about Astrophysics?

[julianh72]

I dunno... Any description of the workings of the universe which doesn't mention the four elephants which are borne through space and time on the back of the Great A'tuin is obviously incomplete!

[Eratosthenes]

Yes I have heard about that theoretical hypothesis. Surprised CERN or LIGO arent involved in some way.

Personally, I think 4 is not enough in Non-Euclidian space-time geometries.

[julianh72]

I believe Ponder Stibbons (Head of Inadvisably Applied Magic at Unseen University in Ankh Morpork) heads up the leading research team in this field.

[AussieTrooper]

Interesting point. You could argue that such a high concentration of small objects would have many collisions, averaging out the orbits. Then again, the asteroid belt has a huge number of highly inclined objects.

[markbakovic]

sorry to gravedig. the answer is "the Nice II model".

i deadset love the cosmology pondering with an almost preset countdown timer to elephants and wizardry though. may i suggest space donkeys, as proposed by Noble (2007).

and in that Q & A link: follow up question 2 is a fantastic example of how to out "science" common sense and leave a kid thinking "well, that sounds like bollocks I'm going to believe in pixies now"... -"everything rotates anticlockwise when viewed from 'above' LOCALLY which is defined to be the direction from which rotation appears anticlockwise by convention" was the answer LeeH should have been looking for... [/rant]

sil: vortex shedding of rotating spheres in fluid?. My guess (so not my field but I'll guess anyway whynot) is difference between star and BH is energy level of infalling particle: relativistic effects probably apply for BH, notsomuch for mainsequence yellow dwarf. also someone mentioned x-rays: remember that the soft x-rays generated by curving electrons are tightly focussed because relativity, so you get a tangential beam from an electron curving around near constant radius, but the emission of the x-ray obviously drops the electron's energy so it doesn't just emit another beam straight away: sound applicable?