Which brings us to an exceedingly interesting new result from the study of the formation rate of solar system planets. Mars in particular may have formed fast and early, in a mere 2 to 4 million years or so, as told by the hafnium-182 isotopic clock in martian meteorite samples. This places it into a very different category of object than the Earth. In effect it was and still is a large planetary embryo, even though we've all been calling it a small planet. Embryos have not undergone major collision with other similar bodies. By contrast, the Moon-forming impact that likely occurred on our homeworld (although modern reanalysis of lunar rock water content hints at possible complications) was part of the agglomeration process 4.5 billion years ago that puts Earth into a different section of the planetary zoo.
How fast different planets assemble is something we don't always remember to think about, but there are important implications. For example, if Mars did form this fast it would have coincided with a timespan in the young solar system where elements such as highly radioactive aluminum 26 would still be abundant (having been left behind from earlier supernova events that helped generate the very nebula out of which we formed). A young Mars could have been extra hot with internal radioactive decay, opening up the possibility of extensive interior melting and setting up a very different geophysical pathway compared to the Earth. This in turn could influence the nature of subsurface environments today, and their potential occupants.
Finally, on another related topic. An interesting paper appeared recently that employed a large ensemble of gravitational simulations to explore just how rare Earth-Moon type systems might be for planets in habitable zones around Sun-like stars. The Moon is a big satellite, and as such contributes significantly to the dynamical evolution of our planet - from its day-length to the gravitational tides that sweep across us every twelve hours. It also plays a role in the spin-axis stability of our planet (where our poles point) and therefore the long-term climate variation. It's a tricky problem to study for sure, but the results are intriguing. The kinds of giant impacts that can create Earth-Moon type systems may occur at rates of around 1 in 12 systems (with a range of possibly as many as 1 in 4, or as few as 1 in 45). This isn't hugely frequent, but in a galaxy with potentially millions of Earth-analogs that certainly adds up.
All three of these discoveries or studies are noteworthy because they help crack the door open a little further into our fast developing picture of not only our particular context as a habitable planet, but the opportunities for life elsewhere. Worms, isotopes, and collisions, it's a great mix.