[论文解读] Predictions for the Period Dependence of the Transition Between Rocky Super-Earths and Gaseous Sub-Neptunes and Implications for $η_{\mathrm{\oplus}}$

Eric Lopez, Ken Rice|arXiv (Cornell University)|Oct 28, 2016

Astro and Planetary Science被引用 2

一句话总结

本文研究了短周期类地超级地球是否为海王星型行星的剥离核心，还是原本即为无气体包层的类地行星。通过光致蒸发模型与N体模拟，研究预测：若行星为蒸发残留核心，则岩石超级地球与气态海王星型行星之间的过渡半径应随轨道周期延长而减小；若行星无初始气体包层则应随周期增加而增大——这一差异为利用TESS径向速度后续观测提供了可检验的区分依据。

ABSTRACT

One of the most significant advances by NASA's ${\mathit Kepler}$ Mission was the discovery of an abundant new population of highly irradiated planets with sizes between the Earth and Neptune. Subsequent analysis showed that at ~1.5 Earth radii there is a transition from a population of predominantly rocky super-Earths to non-rocky sub-Neptunes, which must have substantial volatile envelopes. Determining the origin of these highly irradiated rocky planets will be critical to our understanding of low-mass planet formation and the frequency of potentially habitable Earth-like planets. These short-period rocky super-Earths could simply be the stripped cores of sub-Neptunes, which have lost their envelopes due to atmospheric photo-evaporation or other processes, or they might instead be a separate population of inherently rocky planets, which never had significant envelopes. Using models of atmospheric photo-evaporation, we show that if most bare rocky planets are the evaporated cores of sub-Neptunes then the transition radius should decrease as surveys push to longer orbital periods, since on wider orbits only planets with smaller less massive cores can be stripped. On the other hand, if most rocky planets formed after their disks dissipate then these planets will have formed without initial gaseous envelopes. In this case, we use N-body simulations of planet formation to show that the transition radius should increase with orbital period, due to the increasing solid mass available in their disks. Moreover, we show that distinguishing between these two scenarios should be possible in coming years with radial velocity follow-up of planets found by TESS. Finally, we discuss the broader implications of this work for current efforts to measure $\eta_{\mathrm{\oplus}}$, which may yield significant overestimates if most rocky planets form as evaporated cores.

研究动机与目标

• 确定短周期类地超级地球的起源——是海王星型行星的剥离核心，还是原本即为类地行星。
• 评估在不同形成情景下，岩石超级地球与气态海王星型行星之间的过渡半径如何随轨道周期变化。
• 评估这些形成路径对η⊕（类地行星频率）估计的影响。
• 识别可通过即将发布的TESS数据与径向速度后续观测区分两种形成情景的观测检验方法。

提出的方法

• 建模大气光致蒸发，模拟不同轨道周期下海王星型行星气体包层被剥离的过程。
• 利用行星形成的N体模拟，评估在不同距离处原行星盘中固态质量的可得性。
• 比较两种形成情景下（核心残留 vs. 原初类地形成）岩石超级地球与海王星型行星之间预测的过渡半径。
• 分析过渡半径对轨道周期的依赖性，以推导可检验的观测特征。
• 将理论预测与TESS及径向速度巡天的预期观测能力相结合，评估两种情景的可区分性。

实验结果

研究问题

• RQ1 若类地超级地球主要为海王星型行星的蒸发残留核心，其与气态海王星型行星之间的过渡半径如何随轨道周期变化？
• RQ2 若类地行星在无初始气体包层的情况下形成，其过渡半径如何随轨道周期变化？
• RQ3 TESS发现的系外行星的径向速度后续观测能否区分蒸发残留核心与原初类地行星形成情景？
• RQ4 类地行星的形成路径对η⊕估计有何影响？
• RQ5 不同轨道距离处的盘中固态质量分布如何影响观测到的过渡半径？

主要发现

• 若类地超级地球为海王星型行星的蒸发残留核心，则由于外轨道光致蒸发效率降低，过渡半径应随轨道周期增加而减小。
• 若类地行星在无气体包层情况下形成，则由于外盘区域固态质量更丰富，过渡半径应随轨道周期增加而增大。
• 预测的过渡半径周期依赖性为区分两种形成路径提供了清晰的观测检验手段。
• 未来几年内，TESS系外行星的径向速度后续观测有望实现此区分。
• 若大多数类地行星实为蒸发残留核心而非原初类地天体，则当前η⊕的估计值可能被显著高估。
• 两种形成路径的区分对理解行星形成机制及宜居行星频率具有关键意义。