What Does “Earth-like” Mean for a Planet?

Classifying Exoplanets

In the wake of the long-teased and barely embargoed story about the discovery of Earth-like planet Proxima Centauri b last week, the press really ran with the idea of it being “just like Earth”. That’s really a misleading way to report on an astonishing finding. Judging by some reports (and comments) I’ve seen in social media and in the mainstream media, you’d think that little green Proxima Centaureans were already panicking at the thought of Earthlings finding them and hogging up all the spaces in the drive-up at the local Starbucks.

What Makes a Planet Earth-like?

For the record, “Earth-like” is a term that says a planet has some characteristic like Earth. It means that world is similar in some way to Earth. It might be a rocky world, it might be about the same size as Earth, it might be in the habitable zone of its star. But, it does NOT mean that it’s exactly like Earth in every way. That’s just incorrect to assume. Why?

Earth formed and evolved in an environment that supplied what it needed to become the Earth we know. There were planetesimals rich in water and minerals that slammed together to make early Earth (and supply some or most of its water). The life that exists on our planet reflects the chemical mix that existed in the system.

The planet around any other star is also a creature of its birth environment. The mix of chemical elements and the dynamics of its formation all affect what it turns out to be. They also affect the ultimate forms of life that may evolve on other worlds. Of course, in any case of a planet around a star, the energy output of the star (and its activity) also affect the chances for the origin and evolution of life. Each situation has a complex set of factors that govern its ultimate form.

“Earth-like” and Planetary Science

Planetary scientists use the term “Earth-like” to describe a world that is more like Earth than it is any other type of planet. It may not be Earth 2.0 hosting people with three eyes and giant brains. But, it can still be a rocky world with oceans or other surface water features. It might be a little smaller than Earth or a little larger, or it might be exactly the same size. The point is, it’s more Earth-like” than, say, a super-Jupiter. In the case of Proxima Cen b, it means a rocky planet in the habitable zone of the star it orbits. It’s too soon yet to know if it has water vapor or oxygen in its atmosphere, or water on its surface. That understanding will come with more and better observations.

Proxima Centaur b is “Earth-like”

So, for the record, “Earth-like” does not mean Earth 2.0. You have actually read and/or listen to the scientists when they describe the planet they’ve found. The answer is NOT in soundbites such as “Earthlike”, but in the fuller, more nuanced descriptions they give about where the planet orbits, its composition, and so on. It pays big dividends to actually pay attention to what the scientists say, to look beyond the clickbait headlines by Websites that are harvesting your “likes”, and to totally ignore the conspiracy theorists who are clamoring for your attention by using misleading characterizations of what the scientists DID say in their ignorant social media postings.

Maybe “Earthlike” isn’t the best term. I’ve also seen “Earth analog” and “rocky world”, but neither of those has the same punch as “Earth-like”. It conveys a meaning that gives you a general idea about a planet. It’s not talking about a gas giant or a tiny frozen world. It’s the scientists’ job to decipher the meaning for us, and their press offices need to learn to write better headlines. However, it’s up to us as consumers of science knowledge to actually read what the scientists are telling us.

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