The relevance of all this to planets suitable for life is defined by celestial mechanics. When we have a star like our Sun, planetary orbits around it tend to be stable over long periods of time. The Earth has varied little in its distance from Sol, and hence in the amount of solar heating, during its whole lifetime. The mathematical description of the motion of the Earth around the Sun is provided by the "two-body problem," solved by Isaac Newton in the late seventeenth century. Perturbation effects of other planets, particularly Jupiter, were included by later workers such as Laplace, and confirmed the stability of the Earth's orbit.
When two or more stars are in one stellar system, however, the relevant mathematical problem for the motion of a planet is termed the "N-body problem." The formal exact solution has never been found, but approximate solutions can be obtained in any particular case, using computers. When this is done, the fate of a planet in an N-body system of multiple stars is found to be very different from the stable orbits of our own solar system. Orbits are far more chaotic. Close encounters of a planet with one or other of the primary stars will take place, distances vary wildly over time, and in extreme cases a combination of gravitational forces can eject the planet totally from the stellar system.
Even if the planet does not suffer such a fate, it moves through various extreme situations, now close to a star and baking in radiation, now far away in the freezing dark. This is, so far as we know, not a promising environment for the development of life.
There are two ways for a storyteller to avoid these problems. One is to be so blissfully ignorant of basic astronomy and astrophysics that you see no problem putting life and intelligence any place that you choose, and you hope for equal ignorance on the part of the reader. If you have come this far with me, you will know that I do not approve of such an approach.
The other way is to choose a star without companions, of a stellar type close to our own Sun. Suitable candidates that are also our stellar neighbors include Epsilon Eridani, at 11 light-years, and Tau Ceti, at 12 light-years. No one knows if either star has planets, though Epsilon Eridani has a ring of dust particles which is considered a promising sign. You are free to give either of these stars a world with the size and chemistry of Earth, and explain to the reader that this is the case.
You will then not have the chore of building a plausible world, and you will be safe from criticism. But as Hal Clement, justly famous for designing and explaining exotic worlds, says, "Where's the fun in that?"
TABLE 7.1. The Moons of Jupiter.
Physical properties
Name
Mass
Radius
Density
Albedo
(1020 kg)
(kms)
Galilean Satellites
Io
893
1,821
3.530
0.61
Europa
480
1,565
2.990
0.64
Ganymede
1,482
2,634
1.940
0.42
Callisto
1,076
2,403
1.851
0.20
Lesser Satellites
Metis
20610
0.05
Adrastea
10610
0.05
Amalthea
131x73x67
0.05
Thebe
50610
0.05
Leda
5
Himalia
85610
Lysithea
12
Elara
40610
Ananke
10
Carme
15
Pasiphae
18
Sinope
14
Orbital parameters
Name
Semimajor axis
Period*
Inclination
Eccentricity
(1000's kms)
(days)
(degrees)
Galilean satellites
Io
422
1.769
0.040
0.041
Europa
671
3.552
0.470
0.0101
Ganymede
1,070
7.155
0.195
0.0015
Callisto
1,883
16.689
0.281
0.007
Lesser Satellites
Metis
128
0.297
0
0.041
Adrastea
129
0.298
0
0
Amalthea
181
0.498
0.40
0.003
Thebe
222
0.675
0.8
0.0015
Leda
11,094
238.72
27
0.163
Himalia
11,480
250.56
28
0.163
Lysithea
11,720
259.22
29
0.107
Elara
11,737
259.65
28
0.207
Ananke
21,200
631R
147
0.169
Carme
22,600
692R
163
0.207
Pasiphae
23,500
735R
148
0.378
Sinope
23,700
758R
153
0.275
* The symbol R after the period indicates that the moon is in retrograde motion;
i.e., it orbits in the opposite direction to Jupiter's rotation on its axis.
TABLE 7.2 The Moons of Saturn
Physical properties
Name
Mass
Radius
Density
Albedo
(1020 kg)
(km)
Mimas
0.38
198.8
1.140
0.5
Enceladus
0.73
249.1
1.120
1.0
Tethys
6.22
529.9
1.000
0.9
Dione
10.52
560
1.440
0.7
Rhea
23.10
764
1.240
0.7
Titan
1,345.50
2,575
1.881
0.21
Hyperion
185x140x113
0.19-0.25
Iapetus
15.9
718
1.020
0.05-0.5
Phoebe 1
15x110x105
0.06
Lesser Satellites
Pan
10
0.5
Atlas
18.5x17.2x13.5
0.9
Prometheus
0.0014
74x50x34
0.270
0.6
Pandora
0.0013
55x44x31
0.420
0.9
Epimetheus
0.0055
69x55x55
0.630
0.8
Janus
0.0198
99.3x95.6x75.6
0.650
0.8
Calypso
15x8x8
0.6
Telesto
1
5x12.5x7.5
0.5
Helene
16
0.7
Orbital parameters
Name
Semimajor axis
Period*
Inclination
Eccentricity
(1000's kms)
(days)
(degrees)
Mimas
185.5
0.942
1.53
0.0202
Enceladus
238.0
1.370
0.02
0.0045
Tethys
294.7
1.888
1.09
0.0000
Dione
377.4
2.737
0.02
0.0022
Rhea
527.0
4.518
0.35
0.001
Titan
1,221.9
15.945
0.33
0.0292
Hyperion
1,481.1
21.277
0.43
0.1042
Iapetus
3,561.3
79.330
7.52
0.0283
Phoebe
12,952
550.48R
175.3
0.163
Lesser Satellites
Pan
133.6
0.575
Atlas
137.6
0.602
0
0
Prometheus
139.4
0.613
0.0
0.0024
Pandora
141.7
0.629
0.0
0.0042
Epimetheus
151.4
0.695
0.34
0.009
Janus
151.5
0.695
0.14
0.007
Calypso
294.7
1.888
0
0
Telesto
294.7
1.888
0
0
Helene
377.4
2.737
0.2
0.005
* The symbol R after the period indicates that the moon is in retrograde motion.
TABLE 7.3 The Moons of Uranus
Physical properties
Name
Mass
Radius
Density
Albedo
(1020 kg)
(km)
Miranda
0.659
240x234x233
1.200
0.27
Ariel
13.53
581x578x578
1.670
0.34
Umbriel
11.72
584.7
1.400
0.18
Titania
35.27
788.9
1.710
0.27
Oberon
30.14
761.4
1.630
0.24
Name
Mass
Radius
Density
Albedo
(1020 kg)
(km)
Lesser Satellites
Cordelia
1
0.07
Ophelia
16
0.07
Bianca
22
0.07
Cressida
33
0.07
Desdemona
29
0.07
Juliet
42
0.07
Portia
55
0.07
Rosalind
29
0.07
Belinda
34
0.07
Puck
77
0.07
Orbital parameters
Name
Semimajor axis
Period
Inclination
Eccentricity
(1000's kms)
(days)
(degrees)
Miranda
129.8
1.413
4.22
0.0027
Ariel
191.2
2.520
0.31
0.0034
Umbriel
266.0
0.36
0.0050
Titania
435.8
8.706
0.10
0.0022
Oberon
582.6
13.463
0.10
0.0008
Lesser Satellites
Cordelia
49.75
0.335
0.1
0.000
Ophelia
53.76
0.376
0.1
0.010
Bianca
59.17
0.435
0.2
0.001
Cressida
61.78
0.464
0.0
0.000
Desdemona
62.66
0.474
0.2
0.000
Juliet
64.36
0.493
0.1
0.001
Portia
66.10
0.513
0.1
0.000
Rosalind
69.93
0.558
0.3
0.000
Belinda
75.26
0.624
0.0
0.000
Puck
86.00
0.762
0.3
0.000
TABLE 7.4 The Moons of Neptune.
Physical properties
Name
Mass
Radius
Density
Albedo
(1020 kg)
(km)
Naiad
29
0.06
Thalassa
40
0.06
Despina
74
0.06
Galatea
79
0.06
Larissa
104x89
0.06
Proteus
218x208x201
0.06
Triton
214.7
1,352.6
2.054
0.7
Nereid
170
0.2
Orbital parameters
Name
Semimajor axis
Period*
Inclination
Eccentricity
(1000's kms)
(days)
(degrees)
Naiad
48.23
0.29
4.74
0.00
Thalassa
50.08
0.31
0.21
0.00
Despina
52.53
0.33
0.07
0.00
Galatea
61.95
0.43
0.05
0.00
Larissa
73.55
0.56
0.20
0.00
Proteus
117.65
1.12
0.55
0.00
Triton
354.76
5.88R
156.83
Nereid