Exploring the Diversity of Exoplanets in the Mass-Density Diagram

质量-密度图是理解系外行星显示的体积密度的巨大多样性的重要诊断。在这张图中，质量在~0.3兆焦耳以下的低质量行星在密度上表现出很大的散射。例如，质量约为10兆瓦的超级地球的体积密度可能与岩石行星一致(密度=5-8克/厘米~3)，或类似海王星的结构(密度~2克/厘米~3)。了解这种多样性可能会为行星的形成提供线索。另一方面，质量在0.3-62MJup范围内的巨行星和棕矮星在质量密度图上表现出紧密的线性关系，两类天体之间没有区别的特征。因此，质量密度图可以提供关于围绕恒星运行的巨行星和褐矮星的性质的线索。这些天体真的不同吗？从CoRoT和开普勒等太空任务中获得的凌日行星的光曲线为我们提供了对行星半径的最佳可能测量。然而，CoRoT和开普勒的目标是相对较暗的恒星。因此，通过地面的多普勒测量来确定行星质量是困难的。重新调整的开普勒任务K2正在观测比开普勒和CoRoT目标亮3等的恒星。到我们提议的项目结束时，K2将获得近300,000颗恒星的精确光曲线。来自TESS的光曲线将在2017年成功启动后纳入本项目。K2和最终TESS的发现将极大地增加太阳系外行星的数量，我们对这些行星的质量、半径和体积密度有准确的测量。为了这个项目，我们组建了一个来自三个研究所的团队来开发K2和最终TESS任务的光曲线。我们的团队以其作为CoRoT系外行星科学团队成员的丰富经验为基础。我们有来自空间仪器的原始光曲线的处理专家，光曲线中的检测，以及获得行星半径所需的过境光曲线的建模。我们也有恒星光谱方面的专家，这将决定主恒星的性质。我们预计这个项目将产生十几颗凌日太阳系外行星，在整个质量-密度图中都有准确的体积密度。这将有助于我们了解低质量行星的体积密度的多样性，以及巨行星/棕矮星的性质。

The mass-density diagram is an important diagnostic for understanding the large diversity in bulk densities that exoplanets exhibit. In this diagram low-mass planets with masses below ~0.3 MJup show a large scatter in density. For example, super-Earths with masses ~10 MEarth can have a bulk density consistent with a rocky planet (density = 5-8 gm/cm3) or a Neptune-like structure (density ~ 2 gm/cm3). Understanding this diversity may yield clues as to the planet formation. On the other hand, giant planets and brown dwarfs in the mass range 0.3 - 62 MJup show a tight linear relationship in the mass-density diagram with no distinguishing features between the two classes of objects. The mass-density diagram can thus provide clues as to the nature of giant planets and brown dwarfs orbiting stars. Are these really different objects?The light curves of transiting planets obtained from space-based missions like CoRoT and Kepler provide us with the best possible measurements of the planetary radius. However, CoRoT and Kepler targeted relatively faint stars. The determination of the planetary mass via Doppler measurements from the ground was thus difficult. The re-vamped Kepler mission, K2, is observing stars that are 3 magnitudes brighter than Kepler and CoRoT targets. By the end of our proposed project K2 will have obtained precise light curves for almost 300,000 stars. Light curves from TESS shall be included in this project after its successful launch in 2017. The discoveries from K2 and eventually TESS will greatly increase the number of extrasolar planets for which we have accurate measurements of the mass, radius, and bulk density.For this project we have assembled a team from three institutes to exploit the light curves from the K2 and eventually TESS missions. Our team builds on its vast experience as members of the CoRoT Exoplanet Science Team. We have experts in the processing of raw light curves from space instruments, the detections in the light curves, and the modeling of the transit light curves needed to obtain the planetary radius. We also have experts on stellar spectroscopy which will determine the properties of the host stars. We expect this project to yield manyl dozen transiting extrasolar planets with accurate bulk densities throughout the mass-density diagram. This will help us understand the diversity in bulk densities for low-mass planets and the nature of giant planets/brown dwarfs.