What secrets do you hold, planet GJ 1214 b?

NASA turned the powerful James Webb Space Telescope to a world 48 light-years away, a type of planet called a "mini-Neptune" that's bigger than Earth but smaller than Neptune. Astronomers found the world well over a decade ago, in 2009, but didn't have an instrument capable of peering into its atmosphere.

But with Webb, which can identify the make-up of distant planets well beyond our solar system (called exoplanets), they now can.

"The planet is totally blanketed by some sort of haze or cloud layer," Eliza Kempton, an exoplanet astronomer at the University of Maryland who led the new research, said in a statement. "The atmosphere just remained totally hidden from us until this observation." The research was published in the science journal Nature.

A past water world

What astronomers found raises many questions about the planet's past. Might it have once been a water world blanketed in oceans?

Today, the atmosphere is likely "steamy," meaning there could be lots of vaporized water in the hot atmosphere. Yet where did all this water come from? GJ 1214 b whips closely around its star every 1.6 days, and a star's front doorstep is a torrid place that's not outwardly friendly to sustaining a watery world. But, mused Kempton, it may have started off as a world teeming with water and other icy materials.

In fact, this curious world may have formed deeper in its solar system, where it's colder. Then, the planet could have migrated closer to its star, a red dwarf that's smaller, less bright, but more long-lived than the sun.

"The simplest explanation, if you find a very water-rich planet, is that it formed farther away from the host star,” Kempton said.

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The planet is quite hot today — but not nearly as hot as it could be. It's a whopping 326 degrees Fahrenheit at night, and some 535 F during the day. But its thick atmosphere reflects bounties of light and heat from its nearby star.

Exoplanet scientists used the Webb telescope to learn things about GJ 1214 b — such as concluding the atmosphere likely contains "heavier" molecules like water and methane — by watching the planet as it transited in front of the red dwarf. As light passes through the planet's skies, certain molecules block this light from filtering through, ultimately providing astronomers unprecedented details about what's present in such a distant, alien atmosphere. An instrument on Webb called a spectrometer makes these specialized light observations. What's more, Webb also followed the planet as it orbited the star, and day turned to night, giving insight into how dramatically, and quickly, this world changes.

Astronomers will keep peering at this mini-Neptune, and other mini-Neptunes, with Webb. Looking at exoplanets is a major part of the instrument's mission, which was formed in close collaboration between NASA, the European Space Agency, and the Canadian Space Agency. These mini-Neptunes are the most commonly found exoplanets in the galaxy, yet they remain largely mysterious to us: Curiously, our solar system doesn't have one.