Sciency Words: Superhabitable

Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we talk about the meaning and origin of scientific terms.  Today’s sciency word is:


The word “habitable” traces all the way back to ancient Latin.  Think of a residence or dwelling.  Think of tenants and landlords and the act of paying rent.  That’s the sort of thing words like habitabilis, habitator, or habitatio referred to.

Of course when we talk about planets, the meaning of “habitable” and “habitability” is a bit different.  Unless…

In our ongoing search for extraterrestrial life, it’s generally assumed that Earth is typical of habitable planets.  But why should we assume that?

In this 2014 paper, physicists René Heller and John Armstrong claim that Earth is not as perfectly suited for life as it seems.  In some ways, Earth is kind of a dangerous place to live, and there have been several instances where life on Earth nearly got snuffed out.  Heller and Armstrong then go on to argue that other worlds may “offer more benign environments to life than Earth does.”

If we insist on calling Earth “habitable,” then Heller and Armstrong propose calling those other worlds “superhabitable.”  Though really, if we’d stop being so geocentric and anthropocentric in our terminology, it is the “superhabitable” planets that should set the standard for habitability, and Earth would be better described as “marginally habitable.”

So what sort of planet would offer a more benign environment for life than Earth does?  Well, according to Heller and Armstrong, planets that are two to three times as massive as Earth would do nicely.  More massive planets will remain geologically active for longer, and they’ll have stronger magnetic fields to protect life from solar and cosmic radiation.  Shallower oceans and a thicker atmosphere would help too.

A smaller and cooler star would also be preferable.  A K-type “orange dwarf” would spew out less harmful radiation than our own G-type Sun, and K-type stars last longer.  A whole lot longer.  No need to worry about the day the sun dies if your planet orbits a K-type star!

Personally, I feel like Heller and Armstrong are making a lot of big assumptions in describing their superhabitable planets.  There may be some wishful thinking at work here.  But then again, it’s also a pretty big assumption to assume that Earth is a typical example of a habitable world.  There’s probably some wishful thinking at work there too.

Next time on Planet Pailly, the nearest superhabitable planet could be a lot closer than you think (unless you clicked that link above, in which case you probably know where Heller and Armstrong said the nearest superhabitable planet might be).

15 thoughts on “Sciency Words: Superhabitable

  1. I think I read somewhere that planets couldn’t get much more massive than Earth without essentially becoming mini-Neptunes rather than super Earths. As the gravity climbs, the atmosphere becomes deeper, thicker, with higher pressure, with the surface just becoming the edge of the rocky core. Of course, that could be as speculative as anything else until we get a chance to actually look at one.

    But if it is possible, can you imagine how difficult of a gravity well it would be to climb out of?

    Liked by 1 person

    1. In that paper, Heller and Armstrong are looking at a range ofplanets between about 1 to 5 Earth masses. They say 2 to 3 Earth masses is optimal for superhabitable planets. But yes, anything about 5 Earth masses would start to get into mini-Neptune territory.

      At some point I was looking at a chart of surface gravities for known super Earths. Some of them were surprisingly close to 1G, but some were much higher. A lot depends on a planet’s density.

      Liked by 1 person

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