The Real Science Behind TRAPPIST-1e

Forty light-years from Earth, in the constellation Aquarius, a small and dim red dwarf star is orbited by seven rocky planets — the largest family of Earth-sized worlds ever discovered around a single star. The system is called TRAPPIST-1. Its fourth planet, TRAPPIST-1e, has spent nearly a decade at the top of every scientific shortlist for "most likely to harbor life outside Earth."

When I was writing What the Stars Encoded, I needed a world that felt real enough to be terrifying. Not a generic "alien planet." Something that could plausibly broadcast — or receive — a signal that mattered. TRAPPIST-1e wasn't a creative choice. It was the obvious one.

Here's what the actual science says.

A dwarf star, seven rocky worlds

TRAPPIST-1 is what astronomers call an ultra-cool red dwarf. It's smaller than Jupiter in radius, burns at roughly 2,500 Kelvin (compared to our Sun's 5,778 K), and emits most of its light in the infrared. It is also absurdly old — estimates place its age at somewhere between 5.4 and 9.8 billion years, which means it has been quietly doing its thing for longer than Earth has existed.

This matters for two reasons.

First, red dwarfs like TRAPPIST-1 are the most common type of star in the galaxy. If life tends to emerge around any kind of star, it is statistically most likely to emerge around one of these.

Second, red dwarfs burn for an extraordinarily long time — potentially trillions of years. Our Sun has a main-sequence lifespan of about 10 billion years. TRAPPIST-1 will outlive it by orders of magnitude. Whatever life might evolve around such a star has more time to do so than we can reasonably imagine.

Why TRAPPIST-1e, specifically

Of the seven planets, TRAPPIST-1e sits in what scientists call the habitable zone — the orbital band where a planet's surface temperature could, theoretically, support liquid water. It is almost exactly Earth-sized, with a similar density and (probably) a rocky composition. Climate models suggest a moderate greenhouse effect could maintain surface temperatures suitable for oceans.

What makes TRAPPIST-1e remarkable isn't that it might be habitable. It's that so many of our requirements for habitability are satisfied at the same time.

That said, habitability models for TRAPPIST-1e also come with a hard caveat. Because the star is so small, the habitable zone is extremely close in — all seven planets orbit within a radius smaller than Mercury's orbit around the Sun. This proximity creates a phenomenon called tidal locking, where one face of the planet always points toward the star. One side in perpetual day; one side in perpetual night.

"The question has never truly been whether life exists elsewhere. It has been whether we would recognize it when we found it — and whether, when we did, it would recognize us."

The flares

Red dwarfs have a dark side, in every sense. Unlike our relatively placid Sun, they are prone to violent stellar flares — bursts of high-energy radiation that can strip a nearby planet's atmosphere over cosmological timescales. For any life on TRAPPIST-1e, this would be a genuine problem.

But life, on Earth at least, has a remarkable capacity for adaptation. Extremophiles thrive in conditions that would kill virtually everything we consider "normal." And evolution has more than enough time on a trillion-year star system to work with worst-case scenarios.

Why it matters for fiction

When I set out to write What the Stars Encoded, I didn't want a story about aliens who look like us, speak like us, or want what we want. I wanted a story grounded in what the universe actually appears to be — strange, distant, older than we can comfortably imagine, and organized on principles that are not obviously aligned with ours.

TRAPPIST-1e gave me that. A real place, at a real distance, with a real set of conditions that make it simultaneously the most promising and most foreign candidate for extraterrestrial life we have discovered so far.

The novel asks what happens if something does come from there. Not a message of peace. Not a message at all. An instruction set — encoded in a form we understand only because we share enough biology with the sender for the instructions to read.

The question the book asks is not "are we alone." The question is: if we are not alone, what if what comes first isn't greeting, but intention?

Further reading

For the actual science of TRAPPIST-1:

• NASA — TRAPPIST-1 Explained
• James Webb Space Telescope observations of TRAPPIST-1
• Nature: The original TRAPPIST-1 discovery paper (2017)