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arXiv 2103.07753astro-ph.EP
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热岩系外行星上的水

Water on Hot Rocky Exoplanets

Edwin S. Kite, Laura Schaefer

摘要:

数据显示,大多数轨道周期为$p$ $<$ 100 d的岩石系外行星(“热”岩石系外行星)形成于富含气体的次海王星,随后失去了大部分的外壳,但这些岩石系外行星是否仍然有大气层是未知的。我们确定了一条路径,通过该路径,轨道周期为10-100天的1-1.7 $R_{Earth}$(1-10 $M_{Earth}$)岩石系外行星可以获得长寿命的10-2000巴的大气,这些大气是H $_2$ O主导的,平均分子量为$>$ 10。这些大气层是在行星从亚海王星演变成岩石系外行星的过程中形成的。由硅酸盐岩浆中的铁氧化物还原而成的H $_2$ O在岩浆中高度可溶,形成溶解的储层,只要H $_2$占主导地位的大气持续存在,就不会损失。大尺寸的溶解储库缓冲H $_2$ O气氛,防止H $_2$分散后的损失。在我们的模型中,一个长寿的,以水为主的大气是一个共同的结果,有效的相互作用之间的星云衍生的大气(峰值大气质量分数0.1-0.6重量%)和氧化岩浆($>$ 5重量% FeO),其次是大气损失。这一观点预测,大多数轨道周期为10-100天,半径在半径谷下边缘0.1-0.2 $R_{Earth}$范围内的岩石行星仍然保留着H $_2$ O大气层。这一预测即将通过JWST进行测试,并对凌日超级地球的数据解释产生影响。

英文摘要:

Data suggest that most rocky exoplanets with orbital period $p$ $<$ 100 d ("hot" rocky exoplanets) formed as gas-rich sub-Neptunes that subsequently lost most of their envelopes, but whether these rocky exoplanets still have atmospheres is unknown. We identify a pathway by which 1-1.7 $R_{Earth}$ (1-10 $M_{Earth}$) rocky exoplanets with orbital periods of 10-100 days can acquire long-lived 10-2000 bar atmospheres that are H$_2$O-dominated, with mean molecular weight $>$10. These atmospheres form during the planets' evolution from sub-Neptunes into rocky exoplanets. H$_2$O that is made by reduction of iron oxides in the silicate magma is highly soluble in the magma, forming a dissolved reservoir that is protected from loss so long as the H$_2$-dominated atmosphere persists. The large size of the dissolved reservoir buffers the H$_2$O atmosphere against loss after the H$_2$ has dispersed. Within our model, a long-lived, water-dominated atmosphere is a common outcome for efficient interaction between a nebula-derived atmosphere (peak atmosphere mass fraction 0.1-0.6 wt%) and oxidized magma ($>$5 wt% FeO), followed by atmospheric loss. This idea predicts that most rocky planets that have orbital periods of 10-100 days and that have radii within 0.1-0.2 $R_{Earth}$ of the lower edge of the radius valley still retain H$_2$O atmospheres. This prediction is imminently testable with JWST and has implications for the interpretation of data for transiting super-Earths.

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