Nakajima Miki, Genda Hidenori, Asphaug Erik, Ida Shigeru
Department of Earth and Environmental Sciences, University of Rochester, P.O. Box 270221, Rochester, NY, 14627, USA.
Department of Physics and Astronomy, University of Rochester, P.O. Box 270171, Rochester, NY, 14627, USA.
Nat Commun. 2022 Feb 1;13(1):568. doi: 10.1038/s41467-022-28063-8.
One of the unique aspects of Earth is that it has a fractionally large Moon, which is thought to have formed from a Moon-forming disk generated by a giant impact. The Moon stabilizes the Earth's spin axis at least by several degrees and contributes to Earth's stable climate. Given that impacts are common during planet formation, exomoons, which are moons around planets in extrasolar systems, should be common as well, but no exomoon has been confirmed. Here we propose that an initially vapor-rich moon-forming disk is not capable of forming a moon that is large with respect to the size of the planet because growing moonlets, which are building blocks of a moon, experience strong gas drag and quickly fall toward the planet. Our impact simulations show that terrestrial and icy planets that are larger than ~1.3-1.6R produce entirely vapor disks, which fail to form a fractionally large moon. This indicates that (1) our model supports the Moon-formation models that produce vapor-poor disks and (2) rocky and icy exoplanets whose radii are smaller than ~1.6R are ideal candidates for hosting fractionally large exomoons.
地球的独特之处之一在于它拥有一颗相对较大的卫星，据信这颗卫星是由一次巨大撞击产生的月球形成盘形成的。月球至少将地球的自转轴稳定了几度，并有助于地球气候的稳定。鉴于在行星形成过程中撞击很常见，那么系外卫星，即太阳系外行星的卫星，应该也很常见，但尚未有系外卫星得到证实。在此我们提出，一个初始富含蒸汽的月球形成盘无法形成相对于行星大小而言较大的卫星，因为正在生长的卫星小块（卫星的构成单元）会受到强烈的气体阻力，并迅速向行星坠落。我们的撞击模拟表明，大于约1.3 - 1.6R的类地行星和冰行星会产生完全由蒸汽构成的盘，这些盘无法形成相对较大的卫星。这表明：（1）我们的模型支持那些产生贫蒸汽盘（蒸汽含量少的盘）的月球形成模型；（2）半径小于约1.6R的岩石质和冰质系外行星是拥有相对较大系外卫星的理想候选者。
