In part one, I examined the question of the "habitable zone" -- the range of distances around a star in which a planet has a climate that allows the existence of liquid water on its surface -- and noted that the evidence accumulated over the past few years is that the zone is comfortably wide after all.

But orbiting in the Habitable Zone is by no means a guarantee by itself that a planet can allow the development of life - and in particular complex life - on its surface.

One other serious possible problem is the planet's "obliquity" -- which is simply another name for the tilt of its spin axis. Right now, most of the Solar System's planets have nicely moderate axial tilts. Three of them -- Mercury, Venus and Jupiter -- have tilts of only a few degrees; they are almost perfectly side-on toward the Sun and have no seasons.

Earth, Mars, Saturn and Neptune all have reasonable tilts of 23-29 degrees, so that they have moderate seasons as one pole or the other tilts modestly toward the Sun and receives somewhat more sunlight. Only two planets -- Uranus and tiny Pluto -- "lie on their sides", so that during part of their orbits around the Sun one of their poles or the other faces almost directly toward the Sun.

Keep in mind that a rotating planet is like a gyroscope -- it continues to tilt in the same direction as it revolves around the Sun during one of its orbits. But this situation may not always have been the case.

Smashing Planets
It is now completely accepted that, during the formation of the Solar System, there was a long period during which large "proto planets" or "planetesimals" several hundred to several thousand kilometers across were sailing all over the Solar System and crashing into each other and into the gradually forming planets.

It seems very likely that both our own Moon and possibly Pluto's moon Charon were created when such objects struck Earth and Pluto with a major blow, splashing a plume of debris into orbit around each planet that later coalesced into a moon. There is a good chance that Uranus' strange tilt was also caused when a protoplanet that may have been ten times as massive as Earth crashed into one of its poles, tipping the planet on its side.

Furthermore, recent research in the past few years indicates that the initial tilts of the four small inner metal rich planets may have had completely different tilts from their current angle of tilt.

Any planet that rotates has a very slight bulge around its equator because of centrifugal force; and if its axis is tilted, the tidal tuggings of the Sun at this bulge cause the planet's spin axis to very slowly "precess" -- that is, its spin axis slowly slews around like a wobbling top.

Earth precesses every 26,000 years -- a process which means, for instance, that thousands of years ago the Pole Star was Vega rather than Polaris. And, mostly as a result of the gravitational tuggings of the giant planets Jupiter and Saturn, the tilt of the inner planets' actual orbits' around the Sun also wobble slightly, in a complex set of rhythms.

These shifts in the tilt of the planets' orbits around the Sun are very tiny -- but for over 25 years, it has been recognized that they can interact with a planet's precession wobble to produce unexpectedly dramatic results.

A Resonance of Wobbles
What happens is that, if a planet's precessive wobble is slow enough, it can get into rhythm with some of the periodic rocking motions of its orbit around the Sun -- and it turns out that, when this happens, the Sun's rhythmic tidal tuggings at the planet's bulge can slowly cause its spin axis to tilt over more and more extremely, and then tilt slowly back again to lesser levels.

This is a so-called "resonance" effect, like pushing rhythmically at a child on a swing to increase his swing to greater and greater levels. Such amplifying resonance effects are common in the Solar System.

Mars' precessive period is 157,000 years long, and periodically gets into rhythm with the slow rhythmic tilts of its solar orbit. Since 1973, it's been known that this causes its axial tilt to slowly rock back and forth between 15 and 35 degrees over a period of several hundred thousand years.

But in 1993, astronomer Jacques Laskar carried out more detailed computer analyses which showed that the effect was more extreme than had been thought, and that -- over cycles of several tens of millions of years -- Mars' axial tilt swings back and forth between 0 and 60 degrees!

It is now accepted that, much of the time, Mars joins Uranus and Pluto as the third planet in the Solar System that lies on its side. It's only by chance that its tilt right now is at a reasonable 25 degrees.

Because Mercury and Venus are closer to the Sun, long ago the Sun's larger tidal tuggings greatly slowed their rotation down until their days became months long, and then pulled their equatorial bulges "into line" with the Sun so that they have almost no axial tilt -- but before that happened, their spin axes rocked back and forth even more dramatically. In fact, Venus may have turned completely topsy-turvy at one point, which could explain why today it rotates slowly backwards.

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