I don’t mean “appear to be in a line as seen from Earth” like what is kind of happening now. I mean, given any orbital resonances and whatever else, and ignoring the differences in their orbital planes, has it ever been / will it ever be the case that, looking down on the SS from above the Sun’s north pole, that the planets are in a straight line?
I’ve oft wondered this myself, and I imagine it would be a trivial thing to work out with any astronomy computer program. I also imagine that it would happen once every 4-5 billion years, and of course, the sun will have swallowed the inner terrestrial planets by then.
All the orreries I’ve seen initialise like that. I assume for packing purposes.
I should clarify: …looking down on the SS from above the Sun’s north pole, that the planets are in a straight line radiating out from the Sun in the order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Not in a straight line in the order, for example, Neptune, Jupiter, Earth, Mercury, Sun, Venus, Mars, Saturn, Uranus. Though maybe that’s an interesting question too.
If the orbits were all utterly stable and the sun never burned out, it would happen for the first time when the universe is a thousand times older than it is today.
So, I think what you ask is “Is there ever a time in which the planets are all in the same plane running through the Earth’s center of gravity and orthogonal to the ecliptic and on the same side of the Sun.” For simplicity in the purpose of performing a calculation, let’s assume that being “in that plane” consists of being within plus-or-minus a solar Earth day of being centered on that plane for the major eight planets in the Solar System. In that case, this alignment will be achieved no more frequently than once every 5,200,000,000,000,000,000,000 years, or about 358 billion times longer current existence of the universe. A more accurate alignment will be many orders of magnitude even less frequent, although given the exhaustion of the Sun, the oncoming merger of the Milky Way and Andromeda galaxies, and the metric expansion of space-time I don’t think greater precision is very useful.
Stranger
And I suspect that interactions between the planets make the odds even worse - since the interactions can lead to planets being expelled from the solar system in time periods much shorter than the alignment time, and an expelled planet would never align
Given the current organization of the Solar System I don’t think there is any potential for (major) planets to be ejected by internal interactions with each other or minor planets. An interaction with a very near passing star might knock the planets out of their alignment but the negative characteristic energy of the planets about the Sun is so great that there just is no way to achieve C3>0 for any major planet. It is possible that a minor planet like Pluto or Ceres could be ejected under the right circumstances but I think even that is unlikely without an external perturbation.
Stranger
Sone studies show instablities that give a small chance of expelling (or colliding planets in the few billion year time frame) so I figured the billion billion time frame would be even worse.
We’re discussing “view from above” but keep in mind that the orbits are not completely on the same plane…So never a “straight line”. Plus, none of the orbits are perfect circles, so discussing the probabilities using perfectly circulr orbits and constant orbital velocity will not be accurate either. (The orbit of Earth is about 92M x 94M miles) The closer to the sun the faster a planet travels. Not as extreme as comets, but certainly a factor.
Life is messy.
It’s my understanding that the Solar system’s orbits are chaotic, and thus there’s a unpredictable chance of random instabilities building up into something that could eject a planet. Although the likely time scale is long enough that the Sun will likely be a white dwarf by then anyway.
More to the point of the thread, that kind of instability doesn’t have anything to do with planetary alignment.
When the sun goes off main sequence and expands to larger than 1 AU radius, the inner planets, probably including Mars to some extent, will experience some degree of solar atmospheric drag (I think I figured the track of the Earth would be something like 3 times denser than it is now, which is a pretty small number). The sun gets dragged around significantly by Jupiter (they orbit an external barycenter), so the turbulence caused by Jupiter will be notable. This messes up the orbital math a little.
According to Wikipedia:
When the Sun enters its red-giant branch (RGB) phase, it will engulf (and very likely destroy) Mercury and Venus. According to a 2008 article, Earth’s orbit will have initially expanded to at most 1.5 AU (220 million km; 140 million mi) due to the Sun’s loss of mass. However, Earth’s orbit will then start shrinking due to tidal forces (and, eventually, drag from the lower chromosphere) so that it is engulfed by the Sun during the tip of the red-giant branch phase 7.59 billion years from now, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate.
Even if the inner planets aren’t outright destroyed, having a lot of their mass boil off will do unpredictable things to the orbit of the remnant.
But where did the planets form, originally?
My (rather vague) understanding is that a big cloud of interstellar ‘dust’ coalesced via gravity into objects, one of which was large enough to ‘ignite’ itself as our sun, and then smaller clumps of dust formed into planets, dwarf planets, asteroids, etc.
And it seems logical to me that this dust would have been fairly uniformly scattered, so that as the smaller clumps formed into planets, there would have been some formed on each side of the sun. And all rotating in the same direction (except one – where’d that come from?)
So getting them all together in a line on one side of the sun would take a really, really long time, if even possible. Am I understanding this correctly?
I think that is kind of a simplistic view. In fact, gravity is an incredibly weak force, which was probably not the initial driver for coalescence. Most likely, the somewhat uniform-ish dust cloud was affected by a combination of electrochemical attaction/repulsion forces, cosmic ray bombardment and the local gravitational and magnetic gradients from large bodies in the general area. Modeling system formation should include a lot of non-gravity variables.
The planets took a long time to form, hundreds of thousands or millions of years. You can’t pick a single instant where some object wasn’t a planet before and was a planet after. So saying a planet formed on one side of the Sun is meaningless. The material going into the planet was always rotating about the Sun during formation. Since that formation took so long, it was on all sides of the Sun during that formation.
The molecular cloud(s) which form a stellar nursery and the planetary system that forms around most if not all Population I stars are anything but uniformly scattered; they are initially collections of massive ionized collections of gas and heavier elements (‘dust’) which through their motion and interactions create large currents and turbulent fluid dynamic behavior which eventually start rotating about a central axis, and as more mass is accumulated the rotation becomes more regular and material falls into specific resonance bands, basically making rings (or at least crescents) of denser material. Eventually, material nearer the central body coalesces into planetary bodies and more diffuse material is flung outward to make an Öpik–Oort cloud within the main body’s Hills sphere. Binary and multibody stellar systems have additional dynamics but the basic formation steps are still the same.
Gravity may be ‘weak’ but it also acts over infinite distance and unlike electric charges it attracts itself (as there is no ‘anti-mass’), so while there are definitely electromagnetic phenomena that have to be simulated in the behavior of the initial gaseous stellar nursery, the actual condensation and organization of a protoplanetary disk into a mature star system is almost completely due to gravitational effects. Of course, once ‘heavy’ elements (basically anything heavier than hydrogen) condenses into solid material you have geotechnic phenomena that drive the formation and evolution or rocky worlds (including the interiors of ‘icy’ giant planets), and the weather of Jovian-type gas giant planets is driving by fluid mechanics while the core is condensed into hydrogen and helium in a superfluid and metallic states that have some very exotic properties and behaviors but from a standpoint of how mass ends up forming a planetary mass gravity is by far the dominant force. “Cosmic ray bombardment” and the galactic magnetic field have essentially no influence other than perhaps providing some very small amount of irregular heating.
Stranger
Apparently, the problem with trying to discover from the current position of the planets of our solar system if those planets can ever be aligned is an example of the Three-body problem - Wikipedia , which is, yes, what inspired The Three-Body Problem (novel) - Wikipedia .
According to some theories they mostly formed near the Sun and migrated outwards. The relatively stable modern solar system is an endpoint condition, not how it started.
I can’t see any options here; Mercury and Venus will certainly be destroyed, and Earth will be destroyed later, all long before any straight-line alignmennt is possible.
So the answer is never.