Rare Earth

4.75-4.49 billion years ago

A big day on Logarithmic History, the biggest since the beginning of the year: the origin of our Solar System including planet Earth. First a note on what’s odd about our planetary system.

Two preceding posts wrestled with the Fermi Paradox: If the universe is full of advanced civilizations, why haven’t we seen any sign of them so far? One answer to the paradox might be that our solar system is wildly unusual, so that abodes for the evolution of complex life are rare. We can finally start to address this matter with some real evidence. According to the NASA exoplanet archive, we’ve now discovered 3885 exoplanets (planets outside our solar system; up from 3587 last year at this date), with many more unconfirmed candidates. This is enough to do some statistics, and indications are that our solar system might indeed be out of the ordinary.


Exoplanets smaller than Jupiter are overwhelmingly closer, mostly a lot closer, to their primary stars than Earth is to the Sun. And the same models of planet formation that have done a pretty good job predicting some of the wild variation we see in other systems – “Hot Jupiters” orbiting closer to their primaries than Mercury, “Super Earths” in between Earth and gas giants in size – don’t readily generate systems that look much like ours. One model that does seem to do a good job with our solar system involves something special, a Grand Tack, where Jupiter and Saturn are caught in an orbital resonance that carries them into the inner solar system and back out, shaking up inner-system planet formation in the process. Wild stuff, but another latest model is even wilder: at the beginning of planet formation, there may have been a generation of Super Earths in the inner solar system. The Grand Tack of Jupiter and Saturn would have sent these planets colliding into one another. The Super-Earths and most of the debris of these collisions would have fallen into the sun, but what the debris left would then have condensed into the unusual inner planets we know, Mercury, Venus, Earth, Mars. And Theia. (Theia? you ask. See the next post).

If this model holds up, the formation of our solar system, like other major events in our history, takes on some of the flavor of mythology. This isn’t quite the old story about Chronos slaying Ouranos, and Zeus slaying Chronos. Instead, in the new story, two giants, Jupiter and Saturn, travel closer to the sun and set a generation of Titans – their like will not be there again – to fighting and destroying one another. Jupiter and Saturn depart, and a new generation is spawned from the wreckage. An unlikely sequence of events, but then our planet could be a very unlikely place. And all the more special for that.

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