Prevailing theory is that the planets were all created from leftover “stuff” writ large from the formation of the sun.
Is the type of planet and rough distance from Sol predicated upon which materials coalesced into which planets?
If more “stuff” had been present in the area where the Earth was forming, would there have been a second Earth on the other side of our orbital ellipse from where we spin? Could it have developed as ours has?
Is the formation of planets around other suns sort of the same distribution of type and distance?
Two planets in one orbit is essentially impossible. On timescales much shorter than the evolution of a star & planets, any two proto-planets in similar enough orbits will either collide or throw one another into radically different orbits. For certain. Peaceful coexistence is the one true impossibility.
So more “stuff” in Earth’s orbit would have resulted in a bigger single Earth, not two smaller ones.
I’ll let somebody more skilled than I tackle the rest of your questions / ideas.
Planets formed closer into the Sun and migrated outward (or not) as they interacted with one another or extrasolar influences, potentially driven by transitory resonances. Our solar system has been pretty stable as far as the major planets are concerned for billions of years but we see a diverse array of different configurations in exoplanetary systems, some of which are unlikely by conventional theories of planetary system development and are probably in transition. In our solar system, all of the dense-core rocky worlds are closer to the Sun and the large and icy ‘gas giants’ are in the outsystem, divided by the Asteroid Belt, and this has influenced our conventional models for planetary development.
As @LSLGuy has already noted, having two comparable-sized bodies at opposing points in the orbit is not stable. It is possible to have smaller bodies in the leading and trailing (L4 and L5) libration points, or stable couplets of two similar mass co-orbiting bodies which are tidally locked to each other, but because orbits aren’t perfect circles and are subject to perturbations by other planets, bodies opposite each other in orbit will advance or retard their progress, eventually getting completely out of phase and ‘rubber-banding’ each other into separate orbits that will overlap and eventually make close approaches to one another.
So far not a single solar system discovered is similar to our own. They are surely out there, but small, distant planets are much harder to detect than large close ones (or small close ones).
Filling that excellent explanation in just a bit more …
Our present ability to detect exo-planets is very limited and results in us seeing only a small subset of what’s (probably) out there. If there was a twin of our system nearby we couldn’t detect it at all even if it was conveniently aligned. Which most systems won’t be.
Punchline: the bias built into our discovery methods overwhelms the actual distribution of exo-planet systems and we can conclude basically nothing about exo-planet systems from our sample set, beyond the brute fact exo-planets exist.
Explanets discovered via their gravitational effect on their star are only large, close ones, because those are the only ones that method can detect. But they can also be detected by planets passing in front of their star and hence briefly decreasing the light we see. This method usually doesn’t work (only around 1% of the time), because most solar systems aren’t lined up right for this to happen from our point of view… but it’s easy to calculate exactly what percentage of the time it would work, for any given sort of system, and so it’s possible to extrapolate reliably from the systems we do see this way.
They could, given a collision or not, end up in orbit around each other. Note our moon is by far the largest of any moon vis a vis the size of the primary in our entire solar system.
It would be nice to think so. But as others have said, there is zero evidence.
There do seem to be a lot of Hot Jupiters, and I think it can be safely assumed that if there’s one of those, the system is unlikely to contain an earthlike planet.
I wonder if anyone has compiled a breakdown of observation of stars so far? What percentage of those near enough so observation of planets is theoretically possible have yet been examined?
And of those, how many fall into the categories:
Hot Jupiter or other features ruling out earthlike planet detected.
Hot Jupiter would have probably been detected if present (which of course doesn’t imply a sol-like system.
Not examined yet, or insufficient data so far to draw any conclusion.
Alas, it does start to look as if the SF universe with lots of habitable or terraformable planets is not the one we live in…
The problem is that with two comparable Earth-sized bodies in relatively close orbit (and especially with active, liquid cores and a plastic mantle) the tidal interactions will end up siphoning off gravitational energy via internal friction and they’ll ultimately end up colliding after spiraling down. The Earth-Luna system is actually a special balance of a largish moon that is nearly at the limit of a being within stability such that the loses to tidal friction are small enough that the system will be consumed by expansion of the Sun long before they collide. Why the system is this way and whether we can attach any significance (i.e. would any similar combination of bodies naturally tend to evolve into this configuration or is Earth-Luna a really unlikely configuration that is just-so and perfect for life as we know it) is an area of active research.
There have been a number of SF stories involving co-rotating planets; Robert Forward’s ‘Rocheworld’ being a classic. Also Charles Sheffield’s “Summertide”.
But as @Stranger points out, this is probably not a long-term stable configuration, not to mention being a pretty improbable situation in the first place.
The universe is both big in extent and long in time. Any configuration that is not long term stable will be very unlikely to be found now even if it’s reasonably likely to have occurred at some time in the life of any given system.
Yes, due to the size of the universe and the speed of light, many things we can observe are being observed as they used to be, not as they are now. But by the same token, what we can see over one single night is one single-night’s worth of radiation emissions from everywhere. They may not all have started their journey to us at the same time (whatever “same” means under relativity), but they all represent the same time interval of emission; some particular 24 hour period long ago.
Oh, come on: I want a spindizzy so I can go out and look around the universe. Surely there will be some improbable things now and then, here and there?
Unfortunately the spindizzy breaks every known conservation law, as well as all of relativity. I still want one though.
Apart from Charon/Pluto. Admittedly Pluto has been demoted to a dwarf planet, but it is still large enough to be spherical, and so is Charon. That makes two in one solar system; I suspect binary paired planets are relatively commonplace, even if most of them are smaller in scale than the Earth/Moon system.
Also a strong hypothesis to answer to the so-called “Fermi Paradox” of why we don’t see technologically advanced extrasolar civilizations communicating with each other or omnidirectional radiating in observable electromagnetic bands; even if such civilizations are relatively common and survive for thousands of years, the likelihood that the period they exist would overlap with our ability to detect them, and that they would be close enough and not otherwise obscured for a detection to be favorable is extremely unfavorable. Unless such civilizations build enduring cosmic-scale structures (which, despite how often it is portrayed in science fiction and speculation by enthusiasts, there are reasons to consider this unlikely and unnecessary) or manage to subsist at a relatively unchanged level of industrial development for millennia our expectation that we should see a brightly lit up galaxy of overlapping civilizations (or at least their energy signatures) in regular communication is greatly exaggerated as compared to the more realistic prospect that interstellar ‘empires’ are unlikely and ability to see clear signs with our current and very limited tools over more than hundreds of light years is very constrained.
There could literally be thousands of contemporary civilizations spread throughout the galaxy with sufficient mean distance that none would detect others or be able to distinguish their residual emissions or signs of activity against the background. Which is not to say that we shouldn’t look or explore; just to stop making absurdly optimistic predictions about when such a discovery will happen, or abruptly declaring 'Aliens!" every time an extrasolar object is detected passing through our solar system.
Pluto and Charon are definitely a doublet (and have a series of four other, smaller bodies that orbit Pluto) but they are tidally locked and so lose almost no energy in their interactions. Pluto is almost certainly a trans-Neptunian object (TNO), likely originating in the Kuiper Belt and captured into a resonance by Neptune where it acquired Charon and perhaps its smaller moons (or they may have been formed by a separate impact). It’s a very compact system even for as small as Pluto is. Such doublets are likely common in exoplanetary systems, especially in systems with multiple gas giant or ‘super-Earth’ planets, or systems with multiple stellar objects where complex interactions and resonances are frequent. The diversity of possible planetary system configurations is essentially infinite, limited only by the geophysics and geochemistry of planetary formation and the laws of celestial mechanics, and even with our understanding of those fields there are often surprises about what is possible and even relatively common, though as @LSLGuy said, we are constrained in terms of the different type of celestial objects and configurations that we can see by the limitations of our current observational systems.
That may be correct, but I wonder about Jupiter and Io? As far as I know Io is orbitally locked but has a lot of tidal energy leading to the famous sulfur volcanos?
Io is in dynamic tension between Jupiter and the other Galilean moons (Europa, Ganymede, and Callisto), and Jupiter has angular momentum to spare. I can’t find any estimate of the expected lifetime of stable orbits offhand but these moons were all very likely developed from Jupiter’s originally circumplanetary disk, so they’ve been around since the beginning, with very round profiles and low eccentricity orbits that have fallen into natural resonance so I suspect it is at least in the many billions of years.
Actually there’s one other important thing we’ve been able to confirm: that our particular arrangement of planets (inner rocky planets, outer gas giants) is not in fact the only way star systems form. Which some people used to claim was likely, before exoplanets actually being detected proved them wrong. Something like a “Hot Jupiter” being possible would have been considered wild speculation before they were actually discovered.
Not true at all. We have an example of two objects of similar size sharing an orbit and not where they’re in orbit around each other nor in an L4/L5 configuration. That example is Janus and Epimetheus, two satellites of Saturn. They’re in a co-orbital arrangement that involves them coming close to each other every few years, at which point they “swap” orbits – the inner one moving to be the outer one and vice versa. There’s no reason two planets couldn’t fall into this kind of co-orbital situation.
This is not the arrangement (always on the opposite sides of the sun) as suggested by the OP. That is unstable in even fairly short runs.
It’s possible that two planets were in such a co-orbital arrangement in the early days of Solar System. There’s a widely accepted hypothesis that the Moon was formed by another planet (roughly 10% the mass of the Earth which they’ve named Theia) colliding with the Earth some 30 to 50 million years after they formed. Well, where was Theia for those 30M years? Could it have been in co-orbit with the Earth? Which orbit was then destabilized by a passing star or rogue planet or something dramatic like that?
