What Makes A Planet Earthlike?

Three Earth-size planets orbit a small, dim star. Are they Earthlike? It’s not clear yet.

Earlier this week, a string of somewhat breathless news stories reported that three Earthlike exoplanets could be the most likely hosts for life outside the solar system.

But that’s not exactly true. There’s a lot we don’t know about these newly revealed planets, and a vast ocean of data that needs gathering before an Earth-size world can make the leap to truly Earthlike. Already, a new study has suggested that two of these planets could be desiccated, parched, and decidedly un-Earthy.

Yet even if the trio aren’t as Earthlike as advertised, they’re still interesting and notable—even without trumped up reports of habitability.

“All of the stuff about habitability, surface environments, etc., is merely idle speculation and conjecture,” says Greg Laughlin of the University of California, Santa Cruz. “Even with our own solar system, there’s been zero success in predicting what surface environments for a large moon or planet look like until the body is viewed up close.”

The journey from Earth-size to Earthlike depends on which definition is at the destination. How much of an Earth twin does a planet need to be to fit the description? Does a similar size and temperature qualify? Or does it also need to orbit a star like the sun and have an Earthlike composition, atmosphere, and ability to host life?

My guess is, that definition varies depending on who’s using it, and it’s worth clarifying what it means each time.

Right now, most of those latter characteristics are complete unknowns in the system. At this point, all scientists know for sure are the sizes, orbits, and potential temperatures of the two innermost planets, which are so close to the star they keep the same face pointed inward all the time. The team knows the third planet’s size, but its orbital period is mostly a mystery. And they know the planets are basking in infrared light, which is what most of the TRAPPIST-1 star’s light is—which is very different from the sun.

But while studies of other exoplanets suggest it’s likely the planets are a mixture of ice and rock, it’s not clear at all what they’re actually made of, whether they have atmospheres that affect surface temperatures (and if so, how), and if liquid water could pool on their surfaces, at least in narrow temperate bands bordering the regions of perpetual day and night.

And what about the worlds’ ability to support life—either as we know it or in a form that has evolved to thrive on infrared photons? That’s a completely different set of questions, and one that isn’t even close to being answered.

Still, “it looks like the middle planet, with the 2.42-day orbital period, could potentially be quite Earthlike in its properties,” Laughlin notes. “I don’t know of any other exoplanet that is potentially as Earthlike.”

How is Laughlin defining “Earthlike” here?

“I mean a planet that is close to Earth’s radius and which receives a similar energy flux from its parent body. I definitely don’t mean oceans, plate tectonics, dolphins, stock markets …,” he says. “As the authors argue, however, it’s likely that the planet is spin-synchronized so that it presents one hemisphere to the star, which is very different from Earth.”

OK, so the planet is definitely not Earth’s twin, but it’s not exactly unrelated, either.

Regardless, the worlds are still noteworthy, and for a slew of reasons. They live around a tiny and cold star known as an ultracool dwarf; until now, scientists weren’t sure these stars, which comprise 15 percent of the stars nearest the sun, could host relatively large worlds. And it turns out that answer is yes, which means—to put it simply—more planets!

Second, at roughly 40 light-years away, the system is so nearby that closely scrutinizing the planets is not only possible, but it’s possible now. Those observations will be even easier because the star is so dim, meaning that disentangling which information is coming from the planets and which is coming from the starlight is much simpler. And lastly (for now), studying the system will help scientists learn more about how planets evolve around stars very different than our own—which is the subject of the follow-up paper taking a look at water loss on worlds around ultracool dwarfs.

In short, it’s worth remembering that responsibly covering science means being faithful to the discoveries and the data—shortcomings, vagaries, and all. Sure, it’s fun to speculate about what we might eventually learn, but it’s not worth overstating findings and misleading readers for the sake of clicks. The beauty of science is in its complexity, in its messiness, in its rigorous examination of the unknown, in amassing enough data to reasonably interpret—and then using that as a foundation to start the whole process over again, with a different set of questions.

Three Earth-size alien worlds circling a nearby stellar underdog? That’s fantastic.

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