Since its launch in December 2021, the James Webb Space Telescope (JWST) has been breaking records.

Now the instrument has spotted its first planet around a star other than our own, and with an estimated diameter equaling around 99 percent of Earth's it looks somewhat familiar.

Observations from NASA's Transiting Exoplanet Survey Satellite (TESS) hinted that the planet was there, but now the high-resolution imagery offered by the Near-Infrared Spectrograph (NIRSpec) on board the JWST has confirmed it.

Despite the size similarity, the planet is thought to be much hotter than our own home world, circling a red dwarf star close enough to complete an orbit in just two days.

"There is no question that the planet is there," says astronomer Jacob Lustig-Yaeger, from the Johns Hopkins University Applied Physics Laboratory in Maryland. "Webb's pristine data validated it."

This newly found object sits 41 light-years away in the constellation Octans, and has been given the designation LHS 475 b. As with other exoplanets, it was spotted by looking at the shadow created as it passes in front of its star.

What makes the JWST special is that it can look at transmission spectra; the assortment of light wavelengths being filtered around the planet that can reveal qualities of its atmosphere.

Transmission spectrum readings
Readings suggest LHS 475 b doesn't have an atmosphere dominated by methane. (NASA/ESA/CSA/L. Hustak (STScI))

For now, we don't have enough data to tell us what kind of atmosphere LHS 475 b has, if it has one at all. Astronomers are so far confident it lacks a thick, methane-rich atmosphere, like the one enveloping Saturn's moon Titan.

"The telescope is so sensitive and the data is so precise that we could have easily detected several different molecules, but we don't see much yet," says astrophysicist Ortiz Ceballos, from the Harvard–Smithsonian Center for Astrophysics in Massachusetts.

But they can't rule out a shallow atmosphere consisting of pure carbon dioxide.

The extra precision offered by the JWST means that researchers can look for stars and planets that are much smaller. Ordinarily, telescopes are looking for exoplanets larger than Jupiter, some 11 times wider than Earth.

Information is also gathered at a speedy pace: it took just two transits (or passes in front of its star) for the JWST to identify LHS 475 b and some of its characteristics. Further readings should tell us more about what we're dealing with here.

We're also seeing the JWST produce some absolutely stunning imagery from deep space, thanks to the sensitivity of its on-board instruments – and it's only been going for a little over a year. There's plenty more to come.

"These first observational results from an Earth-size, rocky planet open the door to many future possibilities for studying rocky planet atmospheres with Webb," says Mark Clampin, the Astrophysics Division director at the NASA Headquarters in Washington, DC.

"Webb is bringing us closer and closer to a new understanding of Earth-like worlds outside our Solar System, and the mission is only just getting started."

The findings were presented at a gathering of the American Astronomical Society on Wednesday, January 11, 2023.