Small planets with a radius between 1-4 Earth radii are extremely common in the Milky Way but their planetary formation and evolution are not revealed completely. NASA's Kepler space telescope revealed that this population has a bimodal distribution to be consistent with the transition from rocky to non-rocky planets predicted by photoevaporation (e.g. Owen & Wu 2013), core-powered mass-loss model (e.g. Ginzburg et al. 2016), and gas-poor formation model (Cloutier & Menou 2020). In addition, small planets are likely possible to have various compositions and structures. Sub-Neptune-size planets (2-4 Earth radii) are particularly interesting because they are likely to have large oceans with habitable conditions under H2-rich atmospheres (Madhusudhan et al. 2021) in addition to a rocky core enveloped in a H2-He gaseous envelope and H2O-dominated ices/fluids (e.g. Zeng et al. 2019). To further explore the nature and origin of small planets, it is important not only to measure the planetary radii and mass with high precision but also to observe their atmospheres directly.

Helium 10830Å triplet lines are a good indicator to investigate the existence of a primary atmosphere. Previous studies have detected the He I triplet lines in the atmosphere of a half-dozen Jupiter and Neptune-sized planets (e.g. Nortmann et al. 2018) but only in two sub-Neptunes (Palle et al 2020, Orell-Miquel et al. 2022). Other studies on sub-Neptune planets have been able to place only upper limits on He I absorption, but they still can help to constrain the mass-loss rate (e.g. Krishnamurthy et al. 2021).

We have confirmed TOI-2136b, a new sub-Neptune-sized planet around a nearby M3 dwarf with a planetary radius of 2.2 ± 0.07 Earth Radii and a mass 4.7+3.1-2.6 of Earth Mass using TESS photometry, multi-color ground-based transit photometry, and high-precision RV measurements. We also conduct high-resolution transmission spectroscopy to search for helium in the atmosphere and place an upper limit on the equivalent width of < 7.8 mÅ (95% confidence) and on the absorption signal of < 1.44 % (95% confidence) around helium 10830Å absorption lines. In this poster, we will report on these results and discuss the physical property of TOI-2136b.