What are the chances of man-habitable worlds? The guy quoted in the article says that 30% of the 200 billion star systems in the Galaxy may have earth-like planets. I think that that may be about right, but MHW are more than just Earth-sized... they have to be the right surface temperature, which means the right distance, and have air, water, life; it just goes on and on. I tend to suspect that life is the exception, rather than the rule, but I could be wrong and comets may regularly infect worlds with life, in which case most worlds are, were, and will be again, living, if conditions broadly so allow! We're talking extreme-life-as-we-know-it here on Earth versus what we 'normally' tend to think of, plants and animals and good weather, probably an order of magnitude more common than MHW.
So, boundary conditions? From one half to one and a half Earth Masses, from .9 to 1.1 times the insolation Earth gets, from a little less than one half to a little less than 100% water-covered, and about one half to one and one half atmospheres, with a partial pressure of O2 around one fifth of an atmo.
The likelihood of planets in that mass range is tied to the mass and the fraction of heavier elements (ahem, 'rocky stuff') in the star system. Smaller suns will tend to have smaller worlds, all other things being equal, but are more common than bigger suns... I have a quick and dirty method for roughing this out which will stop a statisticians' beating heart; I take the inverse of a number between 1 and 0 to the fourth root, subtract one from the result and roll with it. Nine thousand, nine hundred and ninety-nine times out of ten thousand, that returns a number less than nine... One sixteenth of all outcomes are greater than one, and fifteen sixteenths are less, sometimes much less. About 5% are between 1 and 2, and ~17% of the planets above would be between .5 and 1.5 Earth masses. I'm going to make another guess and say that there's about a 20% chance of a planet being in the Goldilocks zone, neither too close and hot nor too far out and cold. Water is a crap-shoot; call it even odds, whereas it's probably more likely than not (my Quick and Dirty Worlds method is to take the square root of a random number from 1 to 0, which averages ~71%! 8-). For the right mass, the chances of the partial pressure being right are pretty good; again, even odds, where it's likely I'm being overly pessimistic. Proper levels of O2 are a function of photosynthetic life. The level is probably self-regulating and 'seeks' one fifth of an atmosphere... Too low and animal life lags until O2 builds up. Too high and fires rage, lowering it. Call that even odds, too. That works out to about .4%, one in 240 star systems, but there are 200 billion in our galaxy; about 850 million MHW!
Incidentally, 1 out of 40 stars mass between 1.1 and .9 Sols by my method, so it's not very close to the real, but in my universe there would be ~21 million MHW around Sol-like stars; enough for me to spread Humanity out in 300-person packets! Which is just about twice the number that we humans are 'designed' for, but, yeah, we'd probably still be killing each other...
Let's look at two possible planets, cleverly calling them Morris and Leslie-
Planet Morris is 1.5 Earth masses and has a gravity of about 1.15 times that of Earth, 1.3 times the surface area, 663 million square kilometers. It has more water surface, having managed to get and hang onto more, 86% (~3/4^1/2), so the land area is only 93 million, but most of it is arable, nearly half again as much as on Earth. It's a little farther out and cooler, but has more atmosphere, ~2 bars, 1.8 N2 plus .2 O2, which makes it warmer all over.
Planet Leslie is .5 Earth masses and has a gravity of a little under .8 times that of Earth. 332 m km^2, 50% water, so the surface area is about 10% more than Earths' and about a quarter of that is arable, some 42 m km^2, a little bit more area than Africa or the Moon. Leslie is a little closer and hotter, but with less water vapor as a greenhouse gas, and less atmo over all (.2 inert plus .2 oxygen for .4 bar), the temperatures tend to swing more to extremes over the course of a day or a year.
People from Morris and Leslie wouldn't stand out much in a crowd of Earthlings, but over all, Leslies are taller and scrawnier than Morrisites, who are more heavily muscled, with less body fat, stronger and faster. They probably live a few years less, too, in a maritime environment, coastal farming, deep-sea mining perhaps, but warmer and milder temperatures. Probably no appreciable snowfalls, but monster storms race across the oceans, sometimes going around and around the planet, getting stronger and stronger until the they spend their fury upon the land...
Leslies live a little longer, in a more extreme climate with few big storms, but probably lots of desert, and lots of places where tornados stalk the land. The planet's core is more run-down than Earth and the surface more worn-down, too, so there are few mountains and more coastal plains and low, wet, flat marshy and swampy lands by the seas or ocean.
(I do believe I might be ready to write some stories about Earth, Morris and Leslie! 8-)