One in two sun-like stars in our galaxy has a sub-Neptune exoplanet, a world between the size of Earth and Neptune, says Jacob Bean, an exoplanet astronomer at the University of Chicago and co-leader of two planned exoplanet observations outlined in a recent NASA announcement. Astronomers still know little about the formation and composition of these plentiful worlds. Are they rocky Earth-like planets that grew a little larger and acquired a thick atmosphere? Or are they composed of ice like Neptune? Most exoplanet experts hypothesize that these worlds are rocky, Bean says. And if that’s the case, the planets are also important for understanding how Earth-like planets form. “If you want to understand anything about planets, you have to at least understand the most common type of planet,” meaning sub-Neptunes, says Björn Benneke, an astrophysicist specializing in exoplanets at the University of Montreal. Benneke isn’t involved with Bean’s team but will work on other Webb exoplanet observations. He previously found evidence there may be water on at least one sub-Neptune exoplanet. But even the composition of sub-Neptunes’ atmospheres is tricky to observe because they’re often obscured by haze or clouds. Such features are thought to be common in many planets’ skies. In fact, all the atmospheres in our solar system contain clouds or haze. Those are two different ways that aerosols—tiny specks of liquid or solid matter—manifest in the atmosphere, Bean says. A cloud forms when something that’s normally a gas in the atmosphere “condenses out”—a phenomenon when a gas such as water, which is typically in the air as vapor, condenses into clouds if the air is saturated. A haze is similar to clouds, Bean says, but occurs when ultraviolet radiation from a star splits up molecules in the atmosphere which makes them condense out when they wouldn’t otherwise. This planetary haze is different from a pollution-caused haze on Earth. A go-to example of a nearby hazy atmosphere is the one covering Saturn’s moon Titan, where nitrogen and methane in the atmosphere are broken up by sunlight and form aerosols. James Webb can see into “mid-infrared” wavelengths that are longer than what other space telescopes see. This will allow astronomers to see deeper into planetary atmospheres because when viewed at longer wavelengths, clouds and haze are generally more transparent, Benneke says. The Spitzer Space Telescope specialized in infrared and was also a powerful tool for exoplanet astronomy, he says. James Webb is the successor to the Hubble Space Telescope, the first and most iconic large space telescope that has taken a vast trove of images over three decades in service. The Webb will be larger and more precise than both of them. Webb’s infrared view will also be able to precisely measure the temperature of exoplanets such as GJ 1214 b—one of the oldest known, most thoroughly studied exoplanets, Bean says. GJ 1214 b is easier to see because of its large size relative to its parent star, which is a red dwarf and is only 40 light-years away. The team hopes to be able to directly measure what kind of molecules are present in its atmosphere, Bean says. But even if they can’t, because the planet is tidally locked, with one side always facing its star, the team should be able to learn about its composition with a different approach: They can take the planet’s temperature and study how the atmosphere transfers heat from the day to the night side through the super rotating jet—an equatorial jet stream that flows around the planet. Most of the sub-Neptunes that are easy to explore are close to their stars, which makes their existence even harder to explain. They retain thick gaseous atmospheres, which should be very difficult for a small planet to hold onto with the intense solar winds and heat that close to a star. “How in the heck could these planets pull in so much [gas]?” Bean says. Because sub-Neptunes are “at the borderline” of what Hubble could detect, James Webb could be a “complete game-changer” for studying them, Benneke says. Webb’s large size, gold-plated mirrors, and the fact that it’s kept very cold, mean it “was really built to take the temperature of the universe.”
