超短周期系外行星研究进展

超短周期(ultra-short-period, USP)行星是指轨道周期小于1 d的系外行星,是近年来系外行星研究领域中一个新的前沿目标。USP行星的搜寻与确认需要借助傅里叶变换(Fourier transform, FT)和盒最小二乘法(the box least, BLS)等光变曲线分析算法,以筛选和确认精准的周期信号。利用统计方法可得到目前USP行星的轨道周期、行星半径、宿主恒星类型等分布特征。大部分USP行星半径小于2R⊕,受行星质量限制,大多数USP行星无法通过视向速度信号测得精确的行星质量。根据已有的观测结果可算出,部分USP行星的质量小于10M⊕,由此推测这些USP的组成更接近金属与岩石混合的类地行星。由于密近轨道可能发生光致蒸发等物质损失过程,USP行星大气的存在情况尚不明确。目前,USP行星被认为起源于热木星(hot-Jupiters)或亚海王星(sub-Neptunes),但USP行星与热木星的主星金属丰度的分布存在较大差异,亚海王星的光致蒸发起源理论可能性更高。USP行星轨道演化机制包括低偏心率轨道迁移和潮汐耗散的原位起源模型等。

The Recent Research Progress of Ultra-Short Periods Exoplanets

Ultra-short-period planets(USPs)with orbital periods of less than 1 day are emerging scientific goals in the field of exoplanets.Search and identification of USPs require light-curve analysis algorithms such as the Box Least Squares(BLS)and the Fourier transform(FT)to screen and confirm accurate periodic signals.By using statistical method,distribution of USPs’orbital period,planetary radius and spectral type of host stars are available.Majority of USPs move around“main sequence stars”in extremely close-in orbits.Among USPs,there are Jupiter-size giants as well as smaller terrestrial planets,even though most researches focus on USPs with R<2 R.However,precise mass measurements for most USPs through radial velocity are difficult due to their low(expected)masses.Observations showed that,some USPs have masses of less than 10 M,which indicates that they are more likely to be rocky planets.Besides,whether USPs have atmosphere remains uncertain since they have a great possibility to be bare under the photo-evaporation of sub-Neptunes or roche flow of hot Jupiters in close-in orbits.In addition,USPs are thought to originate from hot Jupiters or sub-Neptunes,while differences in metallicity distribution of host stars between USPs and hot Jupiters make the sub-Neptunes hypothesis more likely.The formation and orbital evolution mechanisms of USPs include in-situ origin model,low eccentricity orbital migration and tidal dissipation,when taking the classical planetary formation and evolution theories into account.