James Webb Space Telescope discovers its first exoplanet

Posted on: 25 June 2025

Exoplanets are key targets in observational astronomy, as they help better understand how planetary systems form, including our own. While thousands have been detected indirectly, obtaining images of exoplanets represents a genuine challenge. They are less bright, and seen from the Earth are located very near their star; their signal, which is drowned out by that of the star, does not stand out enough to be visible.  

To overcome this problem, the research team developed, in collaboration with the CEA, a telescopic attachment for the JWST’s MIRI instrument – a coronagraph, which can reproduce the effect observed during an eclipse: masking the star makes it easier to observe the objects surrounding it, without them being hidden by its light.  

It is this technique that allowed the team, led by Centre national de la recherche scientifique (CNRS) researcher Prof. Anne-Marie Lagrange, at the Observatoire de Paris-PSL associated with the Université Grenoble Alpes, to discover the new exoplanet. It is located within a disk of rocky debris and dusts.

The finding is reported today in leading international journal Nature and can be read on the publisher's website. 

Rings in debris disks 

Scientists have focused on the most promising targets of observation: systems that are a few million years old and that can be seen “pole-on”, which allows for seeing the disks “from above”. The recently formed planets in these disks are still hot, which makes them brighter than their older counterparts.  

Low-mass planets are in principle easier to detect in the mid-infrared thermal range, for which the JWST has provided a unique window of observation. Among the disks seen from the front, two drew special attention from researchers, with previous observations revealing concentric ring-like structures within them. 

The scientists had until now suspected that these structures resulted from gravitational interaction between unidentified planets and planetesimals. One of the two systems, named TWA 7, has three distinct rings, one of which is especially narrow, and surrounded by two empty areas with almost no matter.

Image of the disk around the star TWA 7 recorded using ESO’s Very Large Telescope’s SPHERE instrument. The image captured with JWST’s MIRI instrument is overlayed. We can clearly see the empty area around TWA 7 B in the R2 ring, CC #1. © A.-M. Lagrange et al. Evidence for a sub-jovian planet in the young TWA7 disk.  

The image obtained by the JWST revealed a source within the heart of this narrow ring. After eliminating the possibility of a potential observation bias, the scientists concluded that it was most probably an exoplanet. Detailed simulations have indeed confirmed the formation of a thin ring and a “hole” at the exact planet’s position, which perfectly corresponds to the observations made with the JWST. 

The research team included Luca Matrà, Associate Professor in Trinity’s School of Physics, who used observations of the TWA 7 system with the ALMA telescope (at longer, millimetre wavelengths) to confirm at high likelihood that the detected TWA 7b was indeed a planet and not a background object.  

Prof. Matrà said: “This first imaging discovery of a Saturn-mass planet showcases the full extent of JWST’s planet-finding potential; particularly as these sub-Jupiter mass planets are the ones we expected to find within these structured rings of exocomets.”  

What prospects for the future discovery of other exoplanets? 

The discovery of TWA 7 b, ten times lighter than those previously captured in images, is likely just the beginning, marking a new step in the research and direct imaging of increasingly light exoplanets. The JWST has the potential to go even further in the future, with the scientists hoping to capture images of planets with just 10% of Jupiter’s mass (around three times lighter than TWA 7b).  

Excitingly, this discovery paves the way to direct imagery of terrestrial exoplanets. They will be prime targets for the future generations of space-based and ground-based telescopes, some of which will use more advanced coronagraphs. The most promising systems are already being identified for these future observations.

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