|models of solar systems can and ought look more reasonable|
In considering the orders of the planets, Bode-Titius discovered their rule of planetary orbit placements, .4+.3*2^(n-2) AU for each planet, n, except that: only asteroids are found at 2.8 AU, n=5; Mercury is too close to the sun at .39 AU, as though n=-infinity, far below n=1; giant Saturn at 9.5 AU is 5% inside of 10 AU, n=7, closer to gianter Jupiter at 5.2 AU, n=6; Neptune at 30.1 AU instead of 38.8 AU, n=9, encroaches on Uranus at 19.2 AU, n=8; leaving Pluto at 39.5 AU near Bode-Titius' assignment for Neptune.
We supply reason by noting the relations of resonances: a simple 3:1 orbit-period resonance between adjacent planets, equates to 2.08:1 orbit-radial resonance; and expand on that: allow offsets for solar tidal forward pull; allow that massive giant planet Jupiter dominates the outer planets gravitationally; thence planets arrange accordingly: Jupiter:Saturn is neatly at 2:5 orbit-period (3-sided) resonance more accurately than Bode-Titius' 3:8 (5-sided); Jupiter:Uranus is at 1:7 (6-sided) dominating Saturn:Uranus at 7:20 (13-sided); Jupiter:Neptune is at 1:14 (13-sided) dominating the 1:2 (1-lopsided) orbit-period resonance of Uranus:Neptune; And Neptune is pumped by Planet X.
But something more is needed, and we look to the planets themselves: formed in the early solar nebula-blanket enshrouding the sun before it ignited on the first day-(type) of GENESIS, to bring daily not yet clear light to the Earth, (On the fourth day-type the sun's nebula cleared to such optical thinness, the disks of the sun and moon and the points of the stars could then be distinguished).
Earth and Venus are but ±10% different at 1.0 and 0.82 Earth-mass; (Note that Earth's moon possibly made the Earth:Venus base-difference 1% closer: if Earth swallowed its own moon and captured Venus' moon in the early jostle for orbit rank); And the Earth+Venus pair are many-times larger than inner-system moons and asteroids, (Possibly than outer-system too but not those 'few' catalogued so far).
Neptune and Uranus at 17.1 and 14.5 Earth-masses are ±8% different; But the difference between the pairs, Neptune+Uranus and Earth+Venus, is a huge factor of 17×; Clearly distinct: not a happenstance probability....
Mars and the dis-asteroid planet Astrus can't compare but might have been twins, (but their rocky cores smaller than Earth, formed farther from the sun).
And, if an upset quadruplet can become unbalanced twins, we find Jupiter:Saturn, at 315:95 Earth-masses, +2%/-8% different from 3:1 = 2+1:2-1; And, a big factor of 13× differentiating the pairs Jupiter+Saturn and Uranus+Neptune... accounting for our significant planets twinned adjacently, each having developed with its near match in the same orbital position in the pre-solar nebula disk: and as individual masses congealed; and identified their exact orbital ephemerides, these twins vied for resonant orbit-period positions: each throwing the other inward or outward until they settled in more stable orbit-period resonances among the mass-significant planets - a non-wavial exclusion principle.
Neptune on the outskirts may have lost some of its orbit-angular momentum to further [yet untelescoped] Pluton and smaller asteroids in retrograde orbit, throwing them further outward, and Neptune further inward, as the larger mass tends to overwhelm the smaller, and impart some velocity [increased energy] - and, leaving Pluto-Charon tossed aside at its original more resonant orbit distance in the process: thus imputing a significant mass for the retrograde Pluton.
So we may expect to find many Earthlike planet twins around other stars, too: Those closer-in will have lost much of their gas-planet atmospheric bulk, and one or two or three will have the right balance for habitation: A 'Venus' might survive a little further from the sun, without freezing-out an 'Earth' - certainly its poles would be balmy, were its atmosphere not thermally switched to greenhouse mode - and a 'Mars', were it Earth-sized, might remain an ice-planet permanently wintered at its equator - maybe fish and penguin could habitate - and more importantly, it might be interesting to visitors from Earth.
[cf Sampling-generations run by astronomers interested in Drake-Green-Bank-equation planetary developments and Earthlike probabilities found about one twinning among dozens of program-simulated planets, whereas we'd expect 3-4 pairs per system]
Henceforth we presume that a paucity less than 2 habitable planets is the exceptional case among near-sunlike systems; And, solar systems with 2 or more habitable planets may have planet-hopping society much in advance of Earth space sciences: They may telescope each-others' constructions, canals, fires, radio-signals: Earth may be the gardenlike exception among the local suns systems' planets (and Cain and Abel may be spacer analogies for adjacent twin planet societies).
(A secondary note about the extant solar system twins: Each of the twin sets has an unusual member, typically the inward twin: Venus (inward) is rotation locked synchronized to the Earth; planet Astrus (outward) is shattered; Jupiter (inward) has its pole almost vertical, like the sun; Uranus (inward) is flipped on its side: It is also noteworthy that the inward twin is in earlier ring quadrature resonance formation pushed toward equi-distant among its own mass-doubles and both of the next outward twin's two mass-doubles.)
(A third note is that the planets mostly have significant tilt from the ecliptic: possibly from an early warp in the pre-solar nebula, or a significant "other" in the system, eg. Pluton, likely the central pre-nova mass, and another star long gone may be twin to our sun.)
The recent discoveries, in the space-telescopic search for distant solar systems, of large gas planets, in multiples, orbiting close-in about their stars, closer than the orbit of Mercury, leads to a reassurance that inner planets are guaranteed: The 'old' theory of the early nebular Frostline aka Iceline and Snowline wherein large gas planets could grow rapidly outside the inward sublimation zone, gives way to a better theory, that such gas planets started out there, but in the balance of mass-versus-orbital-energy cumulation they all moved deep inward as typically "mini-neptunes" where they'd then shed their overburden of atmosphere meanwhile vying for orbital position... This may even explain the near-stop rotation of Venus: as the innermost realm of the protosolar nebula would not supply enough prograde gas-and-dust, nor to draw Venus further-in, and solar-spin tides and interplanet-throwing gradually pushed the surviving planets out: even as our Mercury is now at the Bode-Titius 'infinite-bottom'. (And may even explain the existence of Mercury as the lost moon of Venus: where-if the Venus-Earth-pair had actually begun as a sub-paired resonance-quad Venus-Mercury-Earth-moon (1)+(.16)+(1)+(.16), so close to the sun that it never formed a full quad but so loaded the Venus-Mercury sub-pair that it parted them and drew Mecury futher down and heavier... even the mass-pairing shows that Mercury has the difference between Venus and Earth.)
A premise discovery under the title,