How did the James Webb Space Telescope discover its first exoplanet?

It’s a first and a record: in June 2025, Anne-Mar­ie Lag­range’s team at the Par­is Obser­vat­ory announced that the James Webb Space Tele­scope (JWST) had just dis­covered its first exo­plan­et, TWA 7 b. The size of Sat­urn, the new­comer becomes the light­est extra­sol­ar plan­et ever observed by dir­ect ima­ging. This dis­cov­ery illus­trates the tele­scope’s excep­tion­al per­form­ance and marks an import­ant mile­stone in the search for new worlds.

To under­stand what new per­spect­ives this res­ult opens up, let’s go back to the mid-1980s, ten years before the dis­cov­ery of the first exo­plan­et. At that time, debris disks—crowns of dust and particles sur­round­ing cer­tain stars—fascinated some astro­phys­i­cists. “We were only begin­ning to be able to see them. The first image of such a sys­tem, the disk sur­round­ing Beta Pic­tor­is, dates from 1984,” recalls Anne-Mar­ie Lag­range. “Rap­idly, oth­ers were observed—about a hun­dred are cata­logued to date—revealing a fas­cin­at­ing diversity of structures.”

At the turn of the 21st cen­tury, research­ers pos­tu­lated that cer­tain “anom­alies” could, in the­ory, be due to the pres­ence of planets

At the turn of the 2000s, research­ers pos­tu­lated that cer­tain “anom­alies,” such as the fine gaps carved into cer­tain disks, could the­or­et­ic­ally be due to the pres­ence of plan­ets. But these plan­ets, mil­lions or even bil­lions of times less bright than their stars, remained invis­ible. To try to work around this dif­fi­culty, research­ers decided to apply to the search for dis­tant plan­ets an old tech­nique that appeared in the 1930s for observing the sur­round­ings of our sun: coro­na­graphy. It con­sists of cre­at­ing an arti­fi­cial eclipse of the star—in the past through occulta­tion, today through destruct­ive inter­fer­ence. Thus, we now have dir­ect images for a few dozen exoplanets—out of the approx­im­ately 6,000 iden­ti­fied to date. But this meth­od has its lim­its, and until 2025, it had only allowed us to see super-Jupiters [Editor’s note: plan­ets with a mass great­er than Jupiter’s], much more massive and bright­er than the objects sup­posed to cre­ate the fine gaps in debris disks.

Hence, we had to wait for the JWST—and its record-break­ing instruments—to hope to go fur­ther. Because one of the telescope’s innov­a­tions lies in the coro­na­graphs integ­rated into the MIRI (Mid-Infrared Instru­ment). Optim­ised to enhance the con­trast between the star and the plan­et in the mid-infrared, and there­fore much more sens­it­ive than their pre­de­cessors, they were not­ably developed by teams from CNRS and CEA. “There is a remark­able expert­ise pipeline in optic­al instru­ment­a­tion in France. The first coro­na­graph was inven­ted in 1931 by a French­man, Bern­ard Lyot. It was also a French­man, Pierre Léna, who played a cent­ral role in the devel­op­ment of adapt­ive optics, now used in all ter­restri­al tele­scopes. These remark­able skills are explained not­ably by the pres­ence in France of an excel­lent engin­eer­ing school in this field, the Insti­tut d’Op­tique, cre­ated in 1917,” explains Anne-Mar­ie Lagrange.

But it was still neces­sary to find a can­did­ate worthy of the telescope’s per­form­ance. TWA 7, a small star loc­ated 111 light-years from Earth, was thus care­fully selec­ted from among the hun­dreds of thou­sands of stars access­ible to JWST. Not only did its debris disk, highly vis­ible, unam­bigu­ously present the sought-after gaps, but these gaps were far enough from the star for the tele­scope to dis­tin­guish a plan­et hid­den inside, without being blinded by the star’s light. Even bet­ter: TWA 7 is no more than 6.5 mil­lion years old, and this great youth guar­an­teed that its poten­tial plan­ets, still hot, would emit massively in the infrared, JWST’s domain of choice.

Mis­sion accom­plished: at 50 astro­nom­ic­al units from TWA 7, inside a nar­row gap carved into the debris disk, MIRI cap­tured TWA 7 b, ten times light­er than the least massive exo­plan­et dir­ectly observed until then. “It was loc­ated exactly where our sim­u­la­tions pre­dicted,” emphas­ises Anne-Mar­ie Lagrange.

Bey­ond the tech­nic­al feat, what does the dis­cov­ery of TWA 7 b teach us? “First of all, it con­firms our the­or­ies about the struc­tur­ing of debris disks, and opens the way to many more advanced mod­els, to bet­ter account for the form­a­tion and evol­u­tion of these disks,” com­ments the researcher.

“But this obser­va­tion also rep­res­ents an import­ant con­tri­bu­tion to under­stand­ing the form­a­tion and vari­ety of plan­et­ary sys­tems.” Indeed, the rich­er a plan­et is in gas, the more its form­a­tion is sup­posed to occur rap­idly after the birth of its star, because the hydro­gen and heli­um not used to form the star escape very quickly into inter­stel­lar space. “TWA 7 b proves that a very young sys­tem can already har­bour a sub-Jovi­an mass plan­et, and this is pre­cious inform­a­tion for refin­ing our models.”

“Finally, the dis­cov­ery of the exo­plan­et also provides an oppor­tun­ity to study, per­haps even with JWST, the prim­it­ive atmo­sphere of a Sat­urn-like plan­et.” And this is valu­able, because it proves very dif­fi­cult to “go back” in time from obser­va­tions alone of the sol­ar system’s plan­ets, which are more than 4.5 bil­lion years old.

Will JWST go fur­ther? Will it flush out even light­er exo­plan­ets? Will it be cap­able, above all, of see­ing Earth analogues—those ter­restri­al plan­ets loc­ated in the hab­it­able zone of their star, on which research­ers hope to find traces of life? “We would still need to gain a factor of 10 to 30 on the planet’s mass,” reminds Anne-Mar­ie Lag­range. “JWST won’t be able to make that leap.”

But the future gen­er­a­tion of instru­ments is already in pre­par­a­tion: the Extremely Large Tele­scope (ELT), with a dia­met­er of 39 meters (nearly 4 times wider than the largest cur­rently in ser­vice), under con­struc­tion by Europe in Chile should enter ser­vice by 2030. Sim­il­arly, the Amer­ic­an space tele­scope Hab­it­able Worlds Obser­vat­ory, equipped with coro­na­graphs even more sens­it­ive than James Webb, could be launched around 2040. It will fall to them to take the step toward these hab­it­able worlds.

Anne Orliac