In Astrobiology as presently discussed, the word “habitable” is different to ‘Earth-like’. A habitable planet is defined as one on which liquid water is possible on the surface and thus some kind of Life-As-We-Know-It (LAWKI) is possible. In popular imagination the word ‘habitable’ means ‘Earth-like enough for humans’ – and that’s a much tighter set of constraints. Life can survive in a much broader environmental range than ‘unprotected’ humans. Even most of planet Earth is inimical to human life without technological adaptations, but that’s another discussion.
Clearly there’s some preconditions to Earth-like. Similar size, similar gravity, similar water/land ratio, similar insolation levels, and similar atmosphere mix. For example, we can probably live comfortably in a pressure range between half and twice current levels, if the gas mix remains the same. More oxygen, and the pressure minimum goes down, or more of some other mixing gas, and the pressure range we can live in goes up. Presently there’s no hard data on how a planet’s atmospheric pressure varies with its other features, so I’ll leave it for future discoveries to inform us. On Earth the pressure has varied between maybe half to maybe ~1.5 times current levels. Even in early Dinosaur times, oxygen was once so low we’d’ve found it hard to breathe. Yet animals did survive so maybe we could tweak our own biology and survive too.
Two possibly essential features have had some interesting recent results.
(1) Water delivery. Earth-like means significant amounts of water. At least oceans of some depth and water in the mantle to keep the geophysical wheels lubricated. Sean Raymond & Andre Izidoro posted this preprint:
…which simulated the delivery of water rich planetesimals to the Asteroid Belt and the Inner Planets, during the formation of Jupiter and Saturn. They concluded that such water delivery was a pretty generic process of Giant Planet formation. But just how frequent are Jupiter-analogs? The latest work indicates that about 6% of Sun-like stars (i.e. FGK stars, about 20% of stars) have Gas Giants between 3-7 AU. Red dwarfs, the most abundant type of stars, have about half that frequency of Gas Giants.
Sean Raymond gives a popular account of his paper here: Where did Earth’s (and the asteroid belt’s) water come from?
(2) Plates Tectonics powers geochemical cycles on Earth, keeping elements from being buried in the oceans by erosion. A new study suggesting it’s possible for planets around 1/3 of stars appeared this week: Stellar Chemical Clues As To The Rarity of Exoplanetary Tectonics
The basic idea is that tectonics is driven by the ability of certain crustal mineral mixes to increase in density as they’re buried and transformed in the mantle. This pushes old crust down, allowing new crust to erupt. It’s a balance between the tendency to float and the tendency to sink. Tectonics needs both. Thus the right mix of minerals is required, though it’s a pretty broad range. In about 1/3 of the stellar chemical composition range, a planetary crust wouldn’t float, while for another 1/3 the crust won’t sink. And the middling range combines sinking and floating in the right way.
If we’re looking at *just* Sun-like stars, then we get a frequency of ‘Earth-like’ mixes of ~3/50 x 1/3 = 1/50 Sun-like stars probably has more ‘Earth-like’ planets. And thus 1/250 stars in general. About ~400 million planets in our Galaxy of 100 billion stars. Of course all the data suggests that *every* star has at least a planet and a significant fraction of those sit in the “Goldilock’s Zone” of just the right insolation. But they’ll be *different* even if they’re warm enough to be ‘habitable’.