Understanding the Architecture of TRAPPIST-1

Top row: artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity as compared to those of Earth.
[Image credit: NASA/JPL-CALTECH]

Talkshop analysis of some of the data follows this brief report from Astrobiology at NASA.

A team of researchers has provided new information about putative planets in the outer regions of the TRAPPIST-1 system. Currently, seven transiting planets have been identified in orbit around the ultra cool red dwarf star. The scientists determined the lower bounds on the orbital distance and inclination (within a range of masses) of planets that could be beyond the seven inner planets.

The research could inform future radial velocity studies, which will be used to determine the architecture of the TRAPPIST-1 system. This system is of particular interest to astrobiologists because it supports Earth-sized planets; and some studies suggest that these planets could be rocky and a few might even be capable of supporting liquid water at their surfaces.

Source here.
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Talkshop analysis

Graphic: b,c,d sizes are a good match to f,g,h with e (the middle planet) in between the two groups.
Planets b,c,d radius = 1.12,1.10,0.78 (sum = 3.0)
Planets f,g,h radius = 1.05,1.15,0.77 (sum = 2.97)
Planet e in the middle obliges by having a radius nearly halfway between the lower and higher values of the others: 0.91

A similar pattern is shown for the planetary masses.
For the density it’s mostly a similar pattern again, except that the central planet e has the highest density (possibly due to compression?).

The graphic then turns to the solar system’s rocky planets, but here we’ll look at the Jovian and Saturnian moons for comparison of radius and mass distribution, to see what if any parallels turn up.

The four main moons of Jupiter are its Galilean moons, in order from the planet: Io, Europa, Ganymede, Callisto.
I + C radius = 8470 km.
E + G radius = 8384 km.

I + C mass = 19,690,900 kg.
E + G mass = 19,619,000 kg.

The pairings seem fairly clear here, so nothing to add.

Turning to Saturn, its first four spherical moons in order from the planet are: Mimas, Enceladus, Tethys and Dione.
M + D radius = 1519.2 km.
E + T radius = 1566.2 km.

M + D mass = 1,132,945 kg.
E + T mass = 725,471 kg.

Radius is a good match but mass is only about 3:2, mainly due to Dione being heavier than the other three combined. However we find that the sums of their distances from Saturn (semi-major axis) are of interest:
M + D sma = 562,800 km.
E + T sma = 532,569 km.

Also the sma ratio of M:E is 1:1.283 and that of T:D is 1:1.281.

So it appears there must be some factor(s) causing these bodies to form pairs or groups in terms of at least some of their physical parameters, and as shown at least one similar example can be found beyond the solar system.

Note that all the bodies mentioned here have orbit periods (whether of a star or a planet) of less than 20 days, whereas the shortest planetary orbit in our solar system belongs to Mercury at nearly 88 days. That may mean the close proximity of the bodies to each other is an important element in the observed data.

Related: Why Phi? – the resonance of Jupiter’s Galilean moons