Exploring the Diversity of Exoplanets in the Mass-Density Diagram

质量密度图是理解系外行星体积密度差异的重要诊断工具。在这张图中，质量低于~0.3 MJup的低质量行星的密度很大。例如，质量约为10 MEarth的超级地球可以具有与岩石行星（密度= 5-8 gm/cm 3）或海王星状结构（密度约为2 gm/cm 3）一致的体积密度。了解这种多样性可能会为行星的形成提供线索。另一方面，质量范围在0.3 - 62 MJup的巨行星和褐矮星在质量密度图中显示出紧密的线性关系，这两类物体之间没有区别特征。因此，质量密度图可以提供线索的性质，巨行星和棕矮星轨道恒星。它们真的是不同的物体吗？从CoRoT和开普勒等太空任务中获得的凌日行星的光变曲线为我们提供了行星半径的最佳测量。然而，CoRoT和开普勒的目标是相对暗淡的恒星。因此，从地面通过多普勒测量确定行星质量是困难的。重新改造的开普勒使命，K2，正在观测比开普勒和CoRoT目标亮3个星等的恒星。到我们提出的项目结束时，K2将获得近30万颗恒星的精确光变曲线。TESS的光曲线将在2017年成功推出后纳入该项目。K2和最终TESS的发现将大大增加太阳系外行星的数量，我们可以精确测量其质量，半径和体积密度。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。我们有专家处理来自太空仪器的原始光变曲线，光变曲线的检测，以及获得行星半径所需的凌日光变曲线的建模。 我们还有恒星光谱学专家，他们将确定宿主恒星的性质。我们希望这个项目能产生许多十几个凌日系外行星，在整个质量密度图中具有准确的体积密度。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.