Earth-like, Earth-sized, Earth-mass: habitable?




Maybe. Maybe not.

Dear readers, by now you must have heard or read news reports about the discovery of an “Earth-like” planet orbiting Proxima Centauri, the star that is closest to our solar system (4.5 light years away).

I put “Earth-like” in quotes because the term appears in many stories (especially in headlines) reporting the discovery. But what exactly does “Earth-like” mean? (See my blog post of July 23, 2015, about another announcement of a “near-twin” of Earth.) In this post I do not intend to criticize the research under discussion here or the media reporting on it. I am interested in exploring the optimistic and somewhat confusing framing of the discovery, the fuzzy terms used to describe it, the minimizing of considerable uncertainties.

The discovery of this “Earth-like” planet, Proxima b, apparently was first reported August 12 by the German magazine Der Spiegel. In the following week or so, a few science news outlets reported on Der Spiegel’s story. Some of the headlines: “Proxima Centauri may host Earth-like planet” (Spaceflight Insider), “Does an Earth-Like Alien Planet Orbit the Sun’s Closest Neighbor?” (, “Newly Discovered Earth-Like Planet Is Orbiting Proxima Centauri” (Nature World News), “Earth-like planet around Proxima Centauri discovered” (

The research paper reporting on this discovery – “A terrestrial planet in a temperate orbit around Proxima Centauri” – was published by Nature August 24 (Anglada-Escude et al, doi: 10.1038/nature19106): “we report…the presence of a small planet with a minimum mass of about 1.3 Earth masses… Its equilibrium temperature is within the range where water could be liquid on its surface.” In the last paragraph of their paper, the researchers note: “The habitability of planets like Proxima b – in the sense of sustaining an atmosphere and liquid water on its surface – is a matter of intense debate. The most common arguments against habitability are tidal locking, strong stellar magnetic fields, strong [stellar] flares and high ultraviolet and x-ray fluxes; but none of these have been proved definitive…. Proxima b suffers from X-ray fluxes that are approximately 400 times that experienced by Earth.”

A commentary in Nature on this paper (“Earth-like planet around Sun’s neighbor”) describes Proxima b as “Earth-like,” “Earth-mass,” “in the temperate zone” that “could theoretically support liquid water. ” Author Artie Hazes suggests that, “Until we understand what makes a planet habitable, it is better to say that Proxima…b lies in a temperate (the right temperature) rather than a habitable zone.” An accompanying news report in Nature (“Nearby star hosts planet”) describes Proxima b as “Earth-sized” and “potentially habitable,” though possibly “unlivable.”

Also on August 24, the European Southern Observatory (ESO) issued a public announcement (what we used to call a video news release) about this discovery: “the planet, Proxima b, falls within the habitable zone of its host star. The newly discovered Proxima b is by far the closest potential abode for alien life.” ESO defines “habitable zone” as a location in a planetary system where liquid water could (might?) exist. Toward the end of this announcement, uncertainties about habitability are mentioned.*

Media reports on the paper followed suit, emphasizing Proxima b’s alleged similarity to Earth and closing with a mention of uncertainties. Here are some headlines from August 24: “Potentially Habitable Planet Found Orbiting Star Closest to Sun “ (National Geographic), “Proxima b By the Numbers: Possibly Earth-Like World at the Next Star Over” (, “Proxima b: Alien life could exist on ‘second Earth’ found orbiting our nearest star in Alpha Centauri system” (The Telegraph).

You get the idea.

On August 26, Wired reported, “Y’all Need to Chill About Proxima Centauri b…. Astronomers have found other quote-unquote Earth-like planets in the habitable zone in recent years. According the Planetary Habitability Laboratory at the University of Puerto Rico, there are 15 “Earth-size” (in terms of mass or radius) potentially habitable exoplanets. And while, yes, Proxima Centauri b has the mass closest to Earth’s so far, its other characteristics may not be very earthy….” Also on August 26, my friend and colleague Sten Odenwald (who is an astronomer) blogged for the Huffington Post, “Proxima Centauri b: Earth-sized? Earth-like? Or Habitable?... The terms Earth-sized, Earth-like and habitable might sound very similar, but in fact they are not, and they are also not astronomically precise terms….”

Thank you, Sten.

On August 29, Popular Mechanics (predictably) asked about Proxima b, “How will we travel to that promising new planet?” On August 31, the Voice of America went way over the top with “Colonizing Proxima b, It’s Complicated.”

On September 4, Cosmos magazine addressed “The many potential lives of ‘Earth-twin’ planet Proxima b.” on September 6, Nature World News reported, “Co-Discoverer Says Proxima B is a Life-Friendly Planet; Life Outside Earth Possible?” And also on September 4 (I’m throwing this in just for fun), an alleged news website called Clapway claimed that the lead author of the Proxima b paper in Nature is “meeting with aliens from Proxima b.” (Sigh.)

So, Proxima b is described as habitable, Earth-like, Earth-mass, Earth-sized, terrestrial…have I forgotten anything?

I know that exoplanet scientists have thought about the imprecision of these terms – I’ve witnessed many a conversation among them on the subject (and thanks again, Sten, for your blog post). We all use fuzzy terms from time to time, knowing exactly what we mean in our own heads but not knowing what they might mean to others. In the case of the search for another Earth, I’m doubtful that we’ll find one. Over the 25-year course of the discovery of now 3,000-plus exoplanets, what amazes me most is not how many planets have been discovered but how different they all are. It appears to me that what scientists have discovered (so far) is that there’s no such thing as a typical planet or a typical planetary system.

We humans – and especially scientists – love to label and sort things into groups, in a never-ending effort to create order. In the case of exoplanets, we have “hot Jupiters” and “mini-Neptunes” “Earth twins” and “super-Earths” and so on and so on. I do hope that exoplanet scientists continue to work on more precise terms for characterizing their discoveries – especially when it comes to discussions of potential habitability.


* Habitability is complicated. As I noted in my blog post of July 1, 2015, among the many Big Questions yet to be answered by space science are: What is “habitable”? What is a “habitable zone”? How do we define the habitable zone of a planetary system?


2 Responses to “Earth-like, Earth-sized, Earth-mass: habitable?”

  1. Brett Says:

    I hate “super-Earth” in particular. We should replace it with “rocky super-terrestrial”, avoiding the loaded “Earth” descriptor entirely. Same goes for “ocean world” – the type of planets they’re calling these are planets with a thick water/liquid mantle, not “oceans”. “Ice dwarf” would probably be more accurate.

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