A Rock in a Lot of Space
JWST’s First Exoplanet
January 31, 2023 | Written by Jackie Appel
The James Webb Space Telescope has done something amazing again.
On January 11, the telescope confirmed its first-ever new exoplanet. Carrying the poetic name LHS 475 b, this little rocky planet that’s almost exactly the same size as our Earth is proof that not only will JWST live up to its expectations as a camera, but that it will be an outstanding exoplanet analysis machine.
Even though exoplanets are high on the list of priorities for JWST, this discovery was not a foregone conclusion.
“Before launch, Vegas’ betting odds that the first exoplanet JWST confirmed would be a rocky one were probably astronomical,” said Kevin Stevenson, a staff astronomer at Johns Hopkins Applied Physics Laboratory and one of the leaders of this project. “JWST wasn’t designed to search for transiting exoplanets.”
According to Stevenson, space telescope missions come in two types — detection and characterization. Missions like the Transiting Exoplanet Survey Satellite (TESS) and Kepler are focused on detection and meant to scan the sky in order to identify as many objects as possible. Basically, they want to create maps of everything we can see.
On the other hand, missions like Hubble and JWST are characterization missions. They aim to take the candidates spotted by detection missions, confirm what they are, and tease out the details that tell us what’s really out there. They’re meant to fill in the metaphorical roads and towns on the maps made by the detection missions, gathering the kind of data that would tell you what it would look like to stand on the surface of an exoplanet and look up.
JWST is the most highly advanced characterization mission we’ve ever sent into space. “It was designed to search for and directly image young, hot, massive planets that are roughly the size of Jupiter,” said Stevenson. Which is no easy feat.
Direct imaging is exactly what it sounds like: pointing a camera at an exoplanet and observing it directly. It’s a very difficult way to spot new exoplanets — most exoplanets are detected through the transit method — but it’s also the best way to obtain a lot of detail about a far-away body, and it’s great for confirming planets that have already been spotted by other methods.
Already, it has taught us a lot about LHS 475 b. It’s shown us that the planet fully orbits its star in about two days, it’s incredibly hot, and it either has no atmosphere or a thin one with a very specific chemical makeup.
And that’s what makes this announcement such a big deal – the atmosphere. The ultimate goal for many of these observations — and one of the biggest goals for JWST in particular — is to truly understand exoplanet atmospheres. It’s a very difficult data set to gather, as it requires both incredibly precise measurements and for the planet to be observed passing in front of its star without getting fully blotted out by the star’s light.
We’ve done it before, but very infrequently, and mostly on large gas giants that orbit very close to their stars. In order to analyze the atmosphere of an exoplanet, researchers collect a spectral reading of what the light from an exoplanet’s host star looks like after it has filtered through the planet’s atmosphere. Then, they can compare that to pure light from the star, and the differences tell researchers what is in the planet’s sky.
The biggest reason it’s so tricky to do is that even the thickest atmospheres are relatively thin on the scale of stars. That’s why we’ve analyzed mostly puffy gas giants so far. Any extra thickness in an atmosphere is a huge help in making these measurements.
JWST won’t have any of that help with its newly-spotted rocky planet. It hasn’t told us exactly what LHS 475 b’s atmosphere looks like yet, but it has definitively shown that there isn’t any kind of puffy Jupiter-like gas surrounding the surface. For now, theories on atmospheric type for this planet range from a very thin, pure carbon dioxide atmosphere to no atmosphere at all.
We’ll know a lot more once follow-up observations are taken. “The signal sizes produced by a transiting rocky planet require repeated measurements using extremely precise instruments,” said Stevenson. “In this case, we still need more observations of LHS 475b to determine whether or not it has an atmosphere.”
Those observations are scheduled for the near future, so we should know more soon. And even with what we have on a first pass, the results are incredibly promising for what JWST will be able to do over the rest of its mission.
Analyses of planetary atmospheres are key to teaching us how planets, and whole planetary systems, form. Researchers are getting better and better at reading the history of a planet in the chemical composition of its atmosphere the more data sets they get their hands on. And on top of that, being able to understand exoplanet atmospheres is critical to being able to identify which planets might be able to host life.
LHS 475 b is probably too close to its host star to support life, but its atmosphere will still be interesting. And until JWST sends back the next round of observations, the possibilities for what the planet really looks like are wildly varied.
“LHS 475 b could look very much like planets in our own solar system — planets like Mercury, Venus, or Mars,” said Stevenson. “It could also look like several of our moons, maybe Io or Titan. At the moment, the possibilities are nearly endless.”
Meet the Writer
Jackie Appel is a freelance writer for Continuum with a degree in astrophysics from Ohio State University and a master’s degree in Science, Health and Environmental Reporting from New York University. She has previously written for Scienceline and loves sharing the weird wonders of the universe with anyone who wants to listen. Jackie thinks “spaghettification” is the best science word ever (and a very cool phenomenon!).