Context : Recently , the He i triplet at 10830 \AA has been rediscovered as an excellent probe of the extended and possibly evaporating atmospheres of close-in transiting planets .
This has already resulted in detections of this triplet in the atmospheres of a handful of planets , both from space and from the ground .
However , while a strong signal is expected for the hot Jupiter HD 209458 b , only upper limits have been obtained so far .

Aims : Our goal is to measure the helium excess absorption from HD 209458 b and assess the extended atmosphere of the planet and possible evaporation .

Methods : We obtained new high-resolution spectral transit time-series of HD 209458 b using CARMENES at the 3.5 m Calar Alto telescope , targeting the He i triplet at 10830 \AA at a spectral resolving power of 80 400 .
The observed spectra were corrected for stellar absorption lines using out of transit data , for telluric absorption using the molecfit software , and for the sky emission lines using simultaneous sky measurements through a second fibre .

Results : We detect He i absorption at a level of 0.91 \pm 0.10 % ( 9 \sigma ) at mid-transit .
The absorption follows the radial velocity change of the planet during transit , unambiguously identifying the planet as the source of the absorption .
The core of the absorption exhibits a net blueshift of 1.8 \pm 1.3 km s ^ { -1 } .
Possible low-level excess absorption is seen further blueward from the main absorption near the centre of the transit , which could be caused by an extended tail .
However , this needs to be confirmed .

Conclusions : Our results further support a close relation between the strength of planetary absorption in the helium triplet lines and the level of ionising , stellar X-ray and extreme-UV irradiation .