Alas, this version of Proxima b is just one possibility over a huge range of scenarios. So, yeah, from this study alone, Proxima b is probably not very Earth-like.
But wait, there's more.
Habitable Zones Not So Habitable?
Just because a planet orbits its star in the habitable zone, it doesn't mean it has the same life-giving qualities as Earth (keep in mind that both Mars and Venus also orbit the sun within our solar system's habitable zone).
Proxima b orbits very close to its star. It's the nature of the beast; red dwarf stars are small and therefore cooler than sun-like stars. Proxima Centauri's habitable zone is therefore one hell of a lot more compact than our sun's. The Proxima Centauri habitable zone is well within the orbit of Mercury. If a planet got that close to our hot sun, it would be burnt to a crisp; for a planet in orbit around Proxima Centauri, this location is an oasis.
But when you orbit so close to a red dwarf, a planet starts to succumb to some tidal difficulties. One face of an orbiting planet around a red dwarf will be constantly facing the star, meaning the planet's spin matches its orbital period. One hemisphere of the planet is in constant light while the other hemisphere is in constant darkness -- a situation called "tidal locking."
So, in this case, let's imagine the orbiting exoplanet really is a textbook "Earth-like" world with just the right composition. A world with an iron core, rocky mantle and enough water on the surface to create liquid water oceans that could support life. But this world is tidally locked with its star -- that's got to cause some problems, right?
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Let's assume that this planet somehow possesses an atmosphere (more on that later), to have one hemisphere being constantly heated while the other hemisphere is constantly frozen certainly doesn't sound like a good time. Many simulations have been run in an attempt to model the complexities of the atmospheric conditions in this situation and most outcomes aren't good. Some scenarios predict planet-wide hurricanes that act like a blast oven, other scenarios predict a dry wasteland on the star-facing hemisphere and a frozen solid dark hemisphere.
There are, however, some planetary models that could save the day for these unfortunate wannabe "second Earths". One fun prediction is the possible existence of "Eyeball Earths". These peculiar planets would still be tidally locked to their star, with one hemisphere a constantly baked desert and the other hemisphere in deep freeze, but there would be a region between day and night where the conditions are just right for a liquid water ocean to circle the world between the darkness and light. Oh, and it would look like an eyeball, seriously:
In other research around atmospheric dynamics of tidally locked exoplanets, there could be a situation where the world has efficient "air conditioning" -- hot air from one hemisphere is distributed about the planet in such a way to balance global temperatures. But this assumes a high degree of friction between the lower atmosphere and a craggy, rocky surface and efficient high-altitude air flow.
But the ultimate kicker when considering "Earth-like" exoplanets around red dwarf stars is that just because red dwarfs are small, it doesn't mean they are docile. In fact, red dwarf stars can be downright violent, frequently erupting with powerful flares, flooding any nearby planets with ionizing radiation. This radiation, plus inevitably powerful stellar winds, would likely blow any atmosphere away from our hypothetical burgeoning Earth 2.0. Without an atmosphere, the only vaguely habitable location on that planet would be under the surface, perhaps in a sub-surface ocean protected by an icy crust like Jupiter's moon Europa.
But, like Earth, if these planets have a powerful global magnetosphere, perhaps the worst of the stellar storm can be deflected and an atmosphere could form, who knows?
Just the Beginning
Though there are many challenges facing our search for "Earth 2.0", we are only just beginning our quest to seek out alien worlds orbiting other stars. Yes, it is an incredible stroke of luck to find a small world orbiting a neighboring star, but as red dwarfs are the most populous type of star in our galaxy, the odds are that a handful may well have just the right ingredients to support a habitable atmosphere. But is Proxima b one of those diamonds in the rough?
For now, with the tools at our disposal, we simply do not know. Perhaps with the launch of NASA's James Webb Space Telescope in 2018 we might be able to tease out the spectroscopic fingerprint of an atmosphere, but that would likely be beyond its capabilities. So we might just have to send an interstellar probe there to find out if Proxima b is really the habitable exoplanet everyone hopes it will be.
*In Proxima b years, I'm nearly 1,200 years old...
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