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What Does “Earthlike” Mean?

I make no secret about my love for exoplanet research and discovery. When you consider that a mere 25 years ago, our Solar System hosted the only known planets, and even 10 years ago the number of confirmed discoveries of planets orbiting other stars was small. In the midst of all the wrangling over defining what a planet is in the context of our own little star system, astronomers have found over 500 exoplanets with over 1,000 more from the Kepler mission slated for follow-up observation.

In the early days, discovery of a single exoplanet was enough to create headlines; now the rate of discovery is high enough that something unusual about the planet has to be known to get people excited. Read that last sentence in Grandpa Simpson’s voice if you like, but I think that’s a good thing: enough exoplanets are known that we can do statistics now (and I have to keep changing the answer key to my introductory astronomy exoplanet exercise). We are slowly getting an idea of how many planets are in our galaxy, and what other star systems are like — in other words, how typical our Solar System is in the scheme of things. In my non-specialist’s view (and I invite commentary from people who study the topic), there are still a lot of unanswered questions about how planetary systems form, but with enough data, I’m confident we’ll get a coherent picture.

Another way the stakes have changed: it’s no longer sufficient to announce exoplanets. Now the mission is to identify Earthlike planets, a laudable, if difficult, goal. Earth is pretty small, and while observations are getting closer and closer to finding Earth-mass objects, it’s tricky business. (The lowest-mass planet yet found is 1.4 times Earth’s mass — nearly half again as large.) It’s a lot easier to find planets that are massive in comparison to their host stars, or planets that orbit very close in, since these produce the largest observable effects.

A more difficult question is what we even mean by “Earthlike”. Case in point is the planet described in this National Geographic post: the planet orbits within its host star’s “habitable zone”, which means the amount of heat it receives from the star is sufficient to keep water in its liquid state, but not so hot as to boil it away. Water is a pretty common molecule, so its presence on another world is a reasonable assertion, although not a certainty — no water has actually been found on this exoplanet.

On the other hand, the exoplanet is 3.6 times Earth’s mass. While this puts it into the terrestrial planet category — rocky composition, as opposed to the hydrogen-based Jovian type — there’s no guarantee it will look like Earth at all. As I wrote in an earlier post about another possibly Earthlike planet, merely falling into the same general category is no guarantee greater of further similarity. To phrase it another way: is Venus Earthlike? Its mass is close to Earth’s, but its atmosphere is thick carbon dioxide with an admixture of sulfur compounds, including sulfuric acid (pleasant stuff, that). Even though Venus is technically in the habitable zone, the composition of its atmosphere keeps the surface at a lovely constant temperature of 460° C (hot enough to melt lead) — not very Earthlike in the aspects living things care most about. The newly-discovered exoplanet is positioned more like Venus with respect to its star, but that doesn’t say it will be Venus-like either: the conditions on Venus, as with Earth, are dependent on its detailed properties. (In all fairness, the National Geographic article itself provides many of the same caveats I do, while the lede and the headline are designed to grab attention through exaggeration.)

Earth has changed a lot in it’s 4.5-billion-year history. When it formed, its atmosphere was very different in composition: little oxygen, a lot more carbon dioxide, and other gases that are poisonous to humans. Life in the form of cyanobacteria (what my biology class in high school called blue-green algae) helped change the composition of the atmosphere by metabolic processes, using sunlight and carbon dioxide and producing oxygen as a waste product. This obviously would have climatic consequences too, as the ability of our atmosphere to retain heat depends on the details of its chemical composition (as we are learning the hard way in this era of climate change). The role of the Moon in churning up the oceans, the fact that Earth’s axis is tilted so that the planet experiences seasons, and a host of smaller influences add up to make Earth what it is.

Paleontology writer extraordinaire Brian Switek has mocked the tendency of science writers to call every carnivorous dinosaur a “distant relative to Tyrannosaurus rex”, whether they are close in evolutionary terms or not. (He even calls himself a “distant cousin of T. rex” in his Twitter profile: it’s true, after all.) I fear astronomy writers run a similar risk of diluting interest in exoplanet research by calling any world “Earthlike” that has a passing resemblance to Earth. What makes each world unique may tell us as much about the history and evolution of planetary systems as general characteristics (as much as it pains this theoretical physicist to write it). Let’s not exaggerate resemblances to Earth too often, not least since astronomers are likely to find planets even more like Earth than anything yet discovered, changing our tale from “Goldilocks” to “The Boy Who Cried Wolf”.

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