In late 2015, in the Chilean desert, astronomers pointed a telescope at a faint, nearby star known as a red dwarf. Amid the star’s dim infrared glow, they spotted periodic dips, a telltale sign that something was passing in front of it, blocking its light every so often. Last summer, the astronomers concluded the mysterious dimming came from three Earth-sized planets—and that they were orbiting in the star’s temperate zone, where temperatures are not too hot, and not too cold, but just right for liquid water, and maybe even life.
This was an important find. Scientists for years had focused on stars like our sun in their search for potentially habitable planets outside our solar system. Red dwarfs, smaller and cooler than the sun, were thought to create inhospitable conditions. They’re also harder to see, detectable by infrared rather than visible light. But the astronomers aimed hundreds of hours worth of observations at this dwarf, known as TRAPPIST-1 anyway, using ground-based telescopes around the world and NASA’s Spitzer Space Telescope.
It paid off. There are not three, but seven rocky planets with Earth-like masses orbiting TRAPPIST-1, the astronomers reported Wednesday in the journal Nature. Even though it’s a star, TRAPPIST-1, located about 40 light-years from Earth, is only slightly bigger than Jupiter. If TRAPPIST-1 were the size of our sun, all seven planets would be well inside the orbit of Mercury. Despite the close quarters, the planets orbit in a part of the system where temperatures could be between 0 and 100 degrees Celsius, allowing liquid water to pool on their surfaces. Three of the planets are located firmly in the habitable zone, orbiting at a comfortable distance from the star, far enough to safely soak in the warmth without the heat vaporizing their atmospheres or boiling away water. The research, led by the University of Liège in Belgium, marks the largest bounty of Earth-sized planets ever discovered in a single star system.