Probing planet atmospheres and finding small planets nearby: paving the way for detecting life elsewhere

在过去的二十年里，数以千计的系外行星被发现，使我们能够极大地扩展我们对它们的出现速度和体积性质的了解。到目前为止，这些发现已经引发了改进行星形成和进化模型的重大工作，并推动了我们在其他地方寻找生命的探索。在围绕明亮恒星运行时捕捉到的系外行星使我们能够窥探它们的大气层，我们已经探测到了一些分子，并对系外行星大气的物理条件施加了宽松的限制。然而，我们用现有设施进行的观测面临着重要的限制。詹姆斯·韦伯太空望远镜(JWST)将于2019年底开始运行，它将彻底改变这一领域。 我是JWST的四台仪器之一NIRISS仪器团队的成员，利用JWST保证的200小时观测时间，我将领导一个项目，使用NIRISS获取16颗系外行星的凌日和日食光谱观测，这些样本跨越适合大气特征的行星的全部可用平衡温度(300-3000 K)和质量(1 Mearth-10 MJup)范围。我们的观测将测量系外行星大气中存在的分子和气溶胶的丰度，并以前所未有的精度确定它们的垂直和水平温度结构。反过来，这将使我们能够解决基本问题，如行星的形成过程及其随后的演化，行星大气中颗粒云的存在和特征，以及可能在行星大气中发挥作用的非平衡化学效应。我们的目标中有四个是岩石目标，对于这些目标，我们打算对其大气的平均分子量和体积组成施加一些初步限制。 我也是SPIRou和NIRPS科学团队的成员，这是两台新型的高分辨率、光纤馈送的近红外échelle光谱仪，能够精确地测量径向速度。我将使用这些新的强大仪器来探测系外行星大气，使用相对较新的高色散光谱技术。这些观测将是对较低分辨率JWST观测的高度补充，改进了我们对大气成分的确定，并提供了有关行星大气中气体流动的新信息。利用SPIRou和NIRPS，我还将参与大型径向速度测量，以在太阳附近的低质量恒星周围发现新的岩石行星。鉴于它们明亮的宿主和较短的轨道周期，这些调查将揭示在不久的将来寻找生物特征的最佳类地行星，并将有力地确定拥有一颗宜居类地行星的低质量恒星的比例。

Thousands of exoplanets have been discovered over the past two decades, enabling us to considerably expand our knowledge of their occurrence rate and bulk properties. The findings to date have triggered significant work to improve models of planet formation and evolution, and fuel our quest to find life elsewhere. The exoplanets caught in transit around bright stars allow us to peek through their atmosphere, and already we have detected a few molecules and placed loose constraints on the physical conditions in exoplanet atmospheres. However, the observations within our reach with current facilities face important limitations. Starting operation in late 2019, the James Webb Space Telescope (JWST) will completely revolutionize this field. I am a member of the instrument team of NIRISS, one of the four instruments of JWST, and using 200 hours of our guaranteed JWST observing time, I will lead a program to use NIRISS to acquire transit and eclipse spectroscopy observations of a sample of 16 exoplanets that span the full available range of equilibrium temperatures (300-3000 K) and masses (1 MEarth-10 MJup) for planets amenable to atmospheric characterization. Our observations will measure the abundance of the molecules and aerosols present in the exoplanets' atmosphere and determine their vertical and horizontal temperature structure with unprecedented precision. In turn this will allow us to address fundamental issues such as the formation process of planets and their subsequent evolution, the presence and characteristics of particulate clouds in planets' atmosphere, and non-equilibrium chemistry effects that might be at play in their atmosphere. Four of our targets are rocky and for these we intend to place some of the first constraints on the mean molecular weight and hence bulk composition of their atmosphere. I am also a member of the science teams of SPIRou and NIRPS, two new high-resolution, fiber-fed, near-infrared échelle spectrographs capable of precision radial velocity measurements. I will use these new powerful instruments to probe exoplanet atmospheres using the relatively new technique of high dispersion spectroscopy. These observations will be highly complementary to the lower resolution JWST observations, improving our determination of atmospheric composition and providing new information about the flow of gas in the planets' atmosphere. Using SPIRou and NIRPS, I will also participate in large radial velocity surveys to find new rocky planets around low-mass stars in the immediate neighborhood of the Sun. These surveys will uncover the best earth-like planets to search for biosignatures in the near-future, given their bright hosts and short orbital periods, and will provide a robust determination of the fraction of low-mass stars that host a habitable earth-like planet.