Five years ago, newspapers ran front-page headlines announcing that the first seven Earth-sized planets had been discovered outside our solar system. The discovery revealed that complex life may exist elsewhere in the universe. A discovery has raised my curiosity even more. Astronomers say that a star called TRAPPIST-1 hosts a close-knit system of seven sweltering, Earth-size planets. It’s the largest planetary haul ever found beyond our sun. And this time, astronomers have made a giant leap in their understanding of these alien worlds, revealing for the first time that they all lie within what’s known as the habitable zone, where surface temperatures could permit liquid water to form.
This planetary menagerie circles a star that flew under the radar of exoplanet hunters, who typically look for signs of planets by watching stars wobble as the gravitational pull of giant planets tugs them. The researchers could spot the planetary menagerie by searching through Kepler’s last week of high-quality data, efficiently scouring for the few stars that showed tiny dips as planets passed before their host star and blocked a bit of its light.
The astronomers also used telescopes on the ground and in space to measure the exact timing of these transits, revealing a wealth of information about the planets’ sizes, compositions, and orbits. They confirmed that the inner six planets are rocky, like Earth, and that the outermost planet, dubbed Planet H, is likely tidally locked with its star. This means that one half of the planet always faces its host star, much as our moon keeps one face toward us while the other is perpetually dark and cold.
What’s more, astronomers have confirmed that at least the three inner planets receive more radiant heat from their host star than any planet in our solar system does, suggesting they could have surface temperatures between 0 and 100 degrees Celsius (32 to 212 degrees Fahrenheit). They also say that the fourth and fifth planets orbit in the “habitable zone” of their star and, therefore, would have enough sunlight to generate abundant oxygen.
All of this adds up to a stunning planetary haul, which is not only the biggest that’s been discovered so far but also the most promising for hosting life. The fact that all the planets are rocky makes them even more intriguing, as it’s thought that only rocky worlds can sustain liquid water.
The astronomers’ next step is to use the James Webb Space Telescope, which will be launched in 2021, to start getting very detailed atmospheric properties for these worlds. They’ll be looking for “biosignatures,” such as high oxygen levels, that would indicate the presence of living organisms. They’ll also try to determine whether the planets can support photosynthesis, allowing for life production. But if no life is found, the discovery will still help to reinforce a theory that has been around for over a decade: The so-called Rare Earth hypothesis. It’s the idea that, despite billions of other stars and trillions of planets, complex life is unlikely to be widespread in the universe.