James Webb Space Telescope studies mysterious exoplanet with a possible watery past

Link to article May 13, 2023 from space.com

NASA's $10 billion James Webb Space Telescope (JWST) has just detected a "mini-Neptune" planet GJ 1214 b about 48 light-years away. It's the most common type of planet we know. This one is not the first that has been seen, but it's the first showing an atmosphere, and the second mini-Neptune whose mass and radius were calculated.

Illustration of Earth, GJ 1214 b, and Neptune (Aldaron/Wikimedia Comons/CC BY-SA 3.0)

What do we mean by "mini-Neptune" anyway? The Neptune in our solar system is the eighth planet from our sun and is almost 58 times the Earth's volume. It has a rocky core surrounded by a mix of water, ammonia, and methane ice in a kind of slush. Mini-Neptunes are built of similar materials, but the have a radius about 1.7 and 3.9 times that of Earth's. Our Neptune is blue, but that's not because of water oceans; in fact, nobody knows why is is blue.

Neptune's core, slushy mantle, and thick atmosphere (image from space.com)

GJ 1214 b and Neptune are classified as ice giants, while Jupiter and Saturn are gas giants. But the term "ice" is not what you might think. The gases in Jupiter and Saturn (hydrogen, helium) are cold, but they have not frozen. Different gases (ammonia, water vapor, methane) make up the exterior of Neptune planets, and they do freeze, so scientists group the water ice, methane ice, and ammonia ice as just "ice". Ice giants are also sometimes known as "super Earths".

The JWST was launched on Christmas Day, 2021 and reached its final orbiting point in January 2022. The pictures below show it during construction (left, to provide scale with people) and an image of it after it opened its main reflector.

Pictures from Wikipedia

Not much was known if GJ 1214 b for ten years. It has a dense hazy atmosphere, and it was the only planet to circle its sun, which it does in about 1.6 days (Earth days, that is). That made it convenient for  the JWST. Better equipped than the Hubble to look at objects with infrared light, the JWST watched as GJ1214 b revolved around its start completely instead of just getting a glance as it passed in front of the star. The JWST used the infrared measurements to make a "heat map" of the entire planet.

heat map from Nature

This new mini-Neptune spins around its star like the Moon goes around the Earth, with only one side facing inward. So, on the sunlit side are as high as 279 degrees C, and on the dark side it goes down to 65 degrees C.

Because GJ 1214 b does not have much hydrogen or helium in its atmosphere, unlike Neptune, it is thought that maybe it lost those gases into space. The alternative is that it never had them when it formed, but nobody knows for certain. If it lost hydrogen or helium, then that explains why these types of planets are sometimes called "transition planets".

For more on the JWST, go to this link.

For more on Neptune, go to this link.