James Webb telescope discovers ancient 'water world' in nearby star system

Astronomers have finally peered past the clouds on the exoplanet GJ 1214b, a mini-Neptune planet around a star about 40 light-years away. Mini-Neptunes, like a shrunken down version of the familiar gas giant, are a common type of planet in our galaxy — but because there isn't one in our own solar system, these worlds have largely remained a curiosity for scientists.

Previous observations of the distant planet were foiled by thick cloud layers, but the powerful James Webb Space Telescope's (JWST) infrared heat vision allowed astronomers to find a new view through the haze. The results, published May 10 in the journal Nature, reveal that GJ 1214b has an atmosphere made of steam, hinting at its past as a possible "water world," according to NASA researchers. 

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"For the last almost decade, the only thing we really knew about this planet was that the atmosphere was cloudy or hazy," Rob Zellem, an exoplanet researcher at NASA's Jet Propulsion Lab, said in a statement. The team used JWST's Mid-Infrared Instrument (MIRI) to map the temperature of the planet as it moved through its orbit, capturing information on both its day and night sides and enabling astronomers to figure out what it's made of.

The temperature on GJ 1214b shifted dramatically from day to night, getting as hot as 535 degrees Fahrenheit (280 degrees Celsius) and then cooling down by more than 100 degrees F at night. Imagine a day on Earth with sweltering heat during the day, and then a blizzard overnight — that's what a 100-degree difference would look like here! On GJ 1214b, this huge temperature swing indicates that the planet's atmosphere can't be just light hydrogen molecules; instead, there has to be something else like water or methane. Researchers see this finding as an interesting clue into the planet's past, since the atmosphere doesn't match what the star is made of.

GJ 1214b "either lost a lot of hydrogen, if it started with a hydrogen-rich atmosphere, or it was formed from heavier elements to begin with — more icy, water-rich material," lead study author Eliza Kempton, a University of Maryland astronomer, said in the statement. "The simplest explanation, if you find a very water-rich planet, is that it formed farther away from the host star," she added.

Astronomers still have a lot left to figure out about GJ 1214b, but they hope to observe more mini-Neptunes with JWST in the near future. According to Kempton, they hope to figure out a "consistent story" for how mini-Neptunes are created, and how this particular one ended up with so much water.