Helliconia

The SciFi novel series 'Helliconia' by Brian Aldiss describes a world similar to Earth that orbits a Sun-like yellow star, Batalix on a near-circular orbit - which in turn orbits, in the course of more than 2500 years (the 'longyear') a white supergiant, Freyr, on a highly eccentric orbit. This leads to the situation that for most of the long orbit, Helliconia is a cold, frozen world which thaws when coming closer to Freyr and actually changes to a warm, tropical world close to Freyr periapsis. The pattern allows human civilization to emerge and develop from 'spring' to 'autumn', but reliably it decays in the harsh, long winter before it can develop the science and tools (like nuclear energy) to continue evolving through winter.

In a situation where the planet is bound to Batalix which in turn orbits Freyr, there is no particular restriction where the two suns can be found in the sky like in the Janus case - for part of the short year, they are close to each other in the sky and Helliconia sees day and night, for the opposite orbit they illuminate both sides of the planet which never gets dark.

Axial tilt may introduce a powerful asymetry between northern and southern hemisphere here: When at periapsis the northern hemisphere is tilted such that it faces both stars, it gets maximal energy in this phase while the southern hemisphere gets very little. Half an orbit later, the planet is on the other side of Batalix, and so the southern hemisphere is now tilted towards Batalix whereas the northern hemisphere is still tilted towards Freyr, so the northern hemisphere receives more net heating.

Of course the situation reverses at apoapsis, but there Freyr is too far away to be relevant anyway.

Helliconia's orbit

The following file defines Helliconia and its two stars. Note the companion block, which works similar to the binary block but instructs the solver to let the star and planet orbit around the companion rather than the planet around the barycenter of the two stars.

config
orbital_sim_timestep 1000.0

input

star
mass 0.96
T_surf 5600.0
name Batalix

companion
mass 14.8
T_surf 11000.0
luminosity 60000.0
radius 65.0
name Freyr

planet
name Helliconia
mass 1.28
radius 1.21
semimajor_au 1.26
eccentricity 0.01
semimajor_longyear_au 473.0
eccentricity_longyear 0.50
axis_tilt 35
dec_offset 90.0
rot_period_h 24.0
mean_albedo 0.3
mean_heat_capacity 2.0
mean_depth 0.05
elements_lat 16
elements_lon 24

plot_orbit
body planet
file orbit_helliconia.dat
npoints 100000
timeframe_a 2592

end

After running this (which may take a while as over 2500 years need to be processed) this elliptic orbit around Freyr emerges:

Longyear orbit of Helliconia and Batalix around Freyr

Zooming in reveals the motion of Helliconia around Batalix superimposed:

Insert showing the shortyear orbit of Helliconia around Batalix

Thermal properties of Helliconia

Following the terminology Aldiss uses, 'winter' in the following refers to apoapsis and 'summer' to the periapsis of Batalix' orbit around Freyr. The appendix to the novels lists the axis tilt of Helliconia, but does not describe how the axis is oriented at the periapsis, so for the sake of illustration we assume that at periapsis the axis is tilted such that the northern hemisphere points towards Freyr (dec_offset 90 is set). Also, we call the situation in which the two suns are close to each other in the sky a conjunction and the situation where they illuminate opposite hemispheres of the planet an opposition.

Let us first take a look at summer in opposition:

Temperatures during Helliconia summer with the suns in opposition

At about 50 deg latitude is the strong peak of Freyr's radiation visible, at around 0 deg but shifted 180 degrees in longitude the weaker peak of Batalix. Generally the planet is heated fairly evenly, albeit somewhat more at northern latitudes.

When the suns are in conjunction however, the situation is much less symmetric and only the northern hemisphere receives lots of heat.

Temperatures during Helliconia summer, opposition vs. conjunction

(in this case, the heating peaks of the two suns are not separated in longitude, the planet sees day and night during this phase).

Looking at the situation in winter, the striking difference is that at apoapsis the energy flux from Freyr is no longer dominant and chiefly Batalix is heating the planet. For that reasons, temperatures are generally much lower. Also the relative axis tilt reverses - now the southern hemisphere receives more light and heat and during conjunction, the northern hemisphere cools dramatically.

Temperatures during Helliconia winter, opposition vs. conjunction

The combination of orbital motions and rotation of the planet here gives rise to a rich and complex pattern of seasons.

Now, when looking at the scale of the temperatures, what leaps into the eye that temperature contrasts are very harsh, and the world seems hardly inhabitable. This is because there are many mitigating factors, having to do with the atmosphere and the oceans (and Aldiss describes several of them in the novel). For instance the planet is somewhat heated by the Greenhouse effect, and atmosphere and oceans transport heat away from the warm parts of the planet towards the cool, unlit parts. Close to periapsis, cloud formation is much enhanced, bringing the overall albedo up and thus cooling the planet. All these effects are important to stabilize the temperature distribution, but we will cover their simulation at a later stage.

Continue with Moons.

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