The James Webb telescope’s launch has been delayed till at least December 22
after a clamp fastening it to its ride malfunctioned. But when the
spacecraft finally gets to orbit, exoplanet astronomers look forward to a
game-changing view of exoplanets.
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.”
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