With the discovery of a planetary system around TRAPPIST-1, there has been a surge of interest in ultracool dwarfs as potential planet hosts. Planetary systems around ultracool dwarfs represent our best chance of characterising temperate rocky-planet atmospheres with JWST. However, TRAPPIST-1 remains the only known system of its kind, and the occurrence rate of planets around ultracool dwarfs is still poorly constrained. We seek to perform a complete transit search on the ultracool dwarfs observed by NASA's K2 mission to constrain the occurrence rate of planets around these stars. We filter and characterise the sample of ultracool dwarfs observed by K2, by fitting their spectral energy distributions and using parallaxes from Gaia. We build an automatic pipeline to perform photometry, detrend the lightcurves, and search for transits. Using extensive injection-recovery tests of our pipeline, we compute the detection sensitivity of our search, and thus the completeness of our sample. We infer the planetary occurrence rates within a hierarchical Bayesian model (HBM) to treat uncertain planetary parameters. With the occurrence rate parametrised by a step-wise function, we show a convenient way to directly marginalise over the second level of our HBM (the planet parameters). Our method is applicable generally and can greatly speed up inference with larger catalogues of detected planets. We detect one planet in our sample of 702 ultracool dwarfs: a previously-validated mini-Neptune. We thus infer a mini-Neptune (2 - 4Re) occurrence rate of η = 0.20+0.16-0.11 within orbital periods of 1 - 20 days. For super-Earths (1 - 2Re) and ice/gas giants (4 - 6Re) within 1 - 20 days, we place 95% credible intervals of η < 1.14 and η < 0.29, respectively. If TRAPPIST-1-like systems were ubiquitous, we would have had a 96% chance of finding at least one.
Upper Limits on Planet Occurrence around Ultracool Dwarfs with K2
围绕超冷矮星运行的行星的出现没有受到很好的限制。我们目前的结果，从一个客人观察员计划在美国宇航局的K2航天器上寻找过境行星轨道的样本827超冷矮星。在我们的样本中没有发现凌日行星，我们确定了行星出现的上限。我们模拟了围绕我们样本运行的行星的轨道周期和大小。对于凌日的模拟行星，我们将凌日光变曲线注入到真实的K2光变曲线中，然后尝试恢复注入的行星。对于给定的出现率，我们计算了没有看到行星的概率，并使用结果将行星出现的上限作为行星半径和轨道周期的函数。我们发现，短周期，迷你海王星和海王星大小的行星是罕见的超冷矮星周围，与早期和中期M型矮星的结果一致。我们约束行星的发生率$η$之间的$0.5$和$10$ R $_{\oplus}$与轨道周期在1和26.3天。
Astrometric planet search around southern ultracool dwarfs: V. Discovery of the exoplanet candidate PALTA-1 b
European Space Agency（欧洲空间局）；Main Astronomical Observatory, National Academy of Sciences of the Ukraine（乌克兰国家科学院主天文台）；Instituto de Astrofísica de Canarias（加那利群岛天体物理研究所）；Université de Genève（日内瓦大学）
Ultraviolet-Driven Atmospheric Degeneracies Challenge Conventional Biosignature Frameworks for Terrestrial Planets with Ultracool M Dwarf Hosts: An Archean-Analog TRAPPIST-1 e Case Study
University of California, Riverside（加州大学河滨分校）；Blue Marble Space Institute of Science（蓝大理石太空科学研究所）；University of Maryland, Baltimore County（马里兰大学巴尔的摩县分校）；NASA Goddard Space Flight Center（NASA戈达德太空飞行中心）；SETI Institute（搜寻地外文明研究所）；NASA Ames Research Center（NASA艾姆斯研究中心）
Discovering and Characterising Exoplanets and Ultracool Dwarfs with the Square Kilometre Array
University of Amsterdam（阿姆斯特丹大学）；ASTRON, The Netherlands Institute for Radio Astronomy（荷兰射电天文研究所）；Observatoire de Paris（巴黎天文台）；National Astronomical Observatory of Japan（日本国立天文台）；University of Tokyo（东京大学）；California Institute of Technology（加州理工学院）；University of Colorado Boulder（科罗拉多大学博尔德分校）；University of Groningen（格罗宁根大学）
