Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía S/N, 18008-Granada, Spain.
LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Universités Paris 06, Universités Paris Diderot, Sorbonne Paris Cité, France.
Nature. 2017 Oct 11;550(7675):219-223. doi: 10.1038/nature24051.
Haumea-one of the four known trans-Neptunian dwarf planets-is a very elongated and rapidly rotating body. In contrast to other dwarf planets, its size, shape, albedo and density are not well constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system, and the Centaur Chiron was later found to possess something similar to Chariklo's rings. Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multi-chord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea's equator and the orbit of its satellite Hi'iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea's spin period-that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea's largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates. In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected.
海王星外天体矮行星家族中的四颗已知矮行星之一——海王星是一个非常细长且自转迅速的天体。与其他矮行星不同，它的大小、形状、反照率和密度都没有得到很好的约束。半人马座小行星卡里克罗是除巨行星之外第一个已知拥有环系的天体，而半人马座小行星恰伊隆后来被发现具有类似于卡里克罗环的结构。在这里，我们报告了来自多个地面观测站的观测结果，这些观测结果表明海王星在一颗遥远恒星前经过（多弦恒星掩星）。在海王星主体周围观测到的次要事件与一个具有 0.5 不透明度、70 公里宽度和半径约 2287 公里的环一致。该环与海王星的赤道和卫星 Hi'iaka 的轨道共面。该环的半径使其接近与海王星自转周期的 3:1 平均运动共振，即海王星在一个环粒子完成一次公转的时间内绕其轴旋转三次。主体的掩星提供了一个瞬时的椭圆投影形状，其轴长约为 1704 公里和 1138 公里。结合旋转光曲线，掩星约束了海王星的三维取向及其三轴形状，这与处于静力平衡的均匀体不一致。海王星的最长轴至少为 2322 公里，比之前认为的要大，这意味着其密度的上限为每立方厘米 1885 公斤，几何反照率为 0.51，均小于之前的估计值。此外，与之前的估计值相比，这一海王星密度的估计值更接近冥王星，这与预期相符。没有检测到全球性的氮或甲烷主导的大气。
