Exploring the Mysteriously Watery Atmosphere of GJ 1214b with the James Webb Space Telescope

Friday - May 12 2023, 04:59 UTC - 1 year ago

NASA's James Webb Space Telescope has observed GJ 1214 b, a distant mini-Neptune exoplanet outside our solar system, to reveal what is likely a highly reflective world with a steamy atmosphere. Data collected suggests that the atmosphere of GJ 1214 b is not composed mainly of lighter hydrogen molecules, but a water-rich one. Its shiny atmosphere reflects a large fraction of the light from its parenthost star which results in cooler than predicted temperatures.

NASA’s James Webb Space Telescope has observed a steamy, highly reflective mini-Neptune exoplanet, GJ 1214 b, using its Mid-Infrared Instrument (MIRI) to reveal details about its atmosphere. The watery atmosphere suggests that the planet formed farther from its star before spiraling inward, but more observations are needed to understand mini-Neptune formation.

NASA’s James Webb Space Telescope has observed a distant planet outside our solar system – and unlike anything in it – to reveal what is likely a highly reflective world with a steamy atmosphere. It’s the closest look yet at the mysterious world, a "mini-Neptune" that was largely impenetrable to previous observations.

And while the planet, called GJ 1214 b, is too hot to harbor liquid-water oceans, water in vaporized form still could be a major part of its atmosphere.

"The planet is totally blanketed by some sort of haze or cloud layer," said Eliza Kempton, a researcher at the University of Maryland and lead author of a new paper, published in Nature, on the planet. "The atmosphere just remained totally hidden from us until this observation." She noted that, if indeed water-rich, the planet could have been a "water world," with large amounts of watery and icy material at the time of its formation.

To penetrate such a thick barrier, the research team took a chance on a novel approach: In addition to making the standard observation – capturing the host star’s light that has filtered through the planet’s atmosphere – they tracked GJ 1214 b through nearly its entire orbit around the star.

The observation demonstrates the power of Webb’s Mid-Infrared Instrument (MIRI), which views wavelengths of light outside the part of the electromagnetic spectrum that human eyes can see. Using MIRI, the research team was able to create a kind of "heat map" of the planet as it orbited the star. The heat map revealed – just before the planet’s orbit carried it behind the star, and as it emerged on the other side – both its day and night sides, unveiling details of the atmosphere’s composition.

"The ability to get a full orbit was really critical to understand how the planet distributes heat from the day side to the night side," Kempton said. "There’s a lot of contrast between day and night. The night side is colder than the day side." In fact, the temperatures shifted from 535 to 326 degrees Fahrenheit (from 279 to 165 degrees Celsius).

Such a big shift is only possible in an atmosphere made up of heavier molecules, such as water or methane, which appear similar when observed by MIRI. That means the atmosphere of GJ 1214 b is not composed mainly of lighter hydrogen molecules, Kempton said, which is a potentially important clue to the planet’s history and formation – and perhaps its watery start.

"This is not a primordial atmosphere," she said. "It does not reflect the composition of the host star it formed around. Instead, it 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." .

And while the planet is hot by human standards, it is much cooler than expected, Kempton noted. That’s because its unusually shiny atmosphere, which came as a surprise to the researchers, reflects a large fraction of the light from its parenthost star, resulting in temperatures that are cooler than predicted.