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首页> 外文期刊>Astronomy and astrophysics >Characterization of the K2-18 multi-planetary system with HARPS - A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
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Characterization of the K2-18 multi-planetary system with HARPS - A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit

机译：带有HARPS的K2-18多行星系统的特征-一个可居住的区域超地球，并在非共面轨道上发现了第二个温暖的超地球

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Aims. The bright M2.5 dwarf K2-18 ( M _(s)= 0.36 M _(⊙) , R _(s)= 0.41 R _(⊙) ) at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star’s habitable zone; K2-18b. Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet’s mass which is required to interpret atmospheric properties and infer the planet’s bulk composition. Methods. We have obtained precision radial velocity measurements with the HARPS spectrograph. We then coupled those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a correlated stellar activity model based on Gaussian process regression. Results. We measured the mass of K2-18b to be 8.0 ± 1.9 M _(⊕) with a bulk density of 3.3 ± 1.2 g/cm ~(3) which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction ? 50%. We also find strong evidence for a second, warm super-Earth K2-18c ( m _(p,c)sin i _(c)= 7.5 ± 1.3 M _(⊕) ) at approximately nine days with a semi-major axis ~ 2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with the orbit of K2-18b although only a small mutual inclination is required for K2-18c to miss a transiting configuration; | Δ i | ~ 1?2° . A suite of dynamical integrations are performed to numerically confirm the system’s dynamical stability. By varying the simulated orbital eccentricities of the two planets, dynamical stability constraints are used as an additional prior on each planet’s eccentricity posterior from which we constrain e _(b)< 0.43 and e _(c)< 0.47 at the level of 99% confidence. Conclusions. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which, along with its proximity to the solar system, makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.
机译：目的已知34 pc的明亮M2.5矮K2-18（M _（s）= 0.36 M _（⊙），R _（s）= 0.41 R _（⊙））拥有一个超地球大小的过渡地球在恒星宜居区域内运行的行星; K2-18b。考虑到该系统用于研究与太阳相似的日照水平的系外行星大气的最高级特性，我们旨在表征行星的质量，这是解释大气性质和推断行星的总体组成所必需的。方法。我们已经通过HARPS光谱仪获得了精确的径向速度测量值。然后，我们将这些测量值与K2光度法结合使用，共同对观测到的径向速度变化与行星信号和基于高斯过程回归的相关恒星活动模型进行建模。结果。我们测得的K2-18b的质量为8.0±1.9 M _（⊕），堆积密度为3.3±1.2 g / cm〜（3），这可能对应于一个主要的岩石行星，该行星具有明显的气态包壳或海洋行星与水的质量分数？ 50％。我们还找到了有力的证据，表明在大约九天的半长轴上第二次温暖的超地球K2-18c（m _（p，c）sin i _（c）= 7.5±1.3 M _（⊕））比过渡的K2-18b小2.4倍。在重新分析了K2-18的可用光曲线后，我们得出结论，尽管在传输过程中未检测到K2-18c，所以它的轨道可能与K2-18b的轨道不共面，尽管对于K2-18b而言只需要很小的相互倾角即可。 K2-18c错过了过渡配置; | Δi | 〜1？2°。执行了一套动态集成，以数字方式确认系统的动态稳定性。通过改变两个行星的模拟轨道偏心率，动态稳定性约束被用作每个行星偏心率后验的附加先验，由此我们将e _（b）<0.43和e _（c）<0.47约束在99％的水平上置信度。结论。内行星K2-18c的发现进一步强调了M矮星周围多行星系统的普遍性。对K2-18b密度的表征表明，该行星可能具有较厚的气态包膜，这使其与太阳系相距较近，这使得K2-18行星系成为研究接收地球的系外行星进行大气研究的有趣目标。喜欢日晒。

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