Searching for exoplanets with HEPS:I.detection probability of Earth-like planets in multiple systems

The astrometry method has great advantages in searching for exoplanets in the habitable zone around solar-like stars. However, the presence of multiple planets may cause a problem with degeneracy when trying to compute accurate planet parameters from observation data and reduce detectability. The degeneracy problem is extremely critical, especially in a space mission which has limited observation time and cadence. In this series of papers, we study the detectability of habitable Earth-mass planets in different types of multi-planet systems, aiming to find the most favorable targets for the potential space mission–Habitable ExoPlanet Survey(HEPS). In the first paper, we present an algorithm to find planets in the habitable zone around solar-like stars using astrometry. We find the detectability can be well described by planets' signal-to-noise ratio(SNR) and a defined parameter S = M2/(T1-T2)2, where M2 and T2are the mass and period of the second planet, respectively. T1 is the period of the planet in the habitable zone. The parameter S represents the influence of planetary architectures. We fit the detectability as a function of both the SNR of the planet in the habitable zone and the parameter S. An Earth-like planet in a habitable zone is harder to detect(with detectability PHP< 80%) in a system with a hot Jupiter or warm Jupiter(within2 AU), in which the parameter S is large. These results can be used in target selections and to determine the priority of target stars for HEPS, especially when we select and rank nearby planet hosts with a single planet.

Searching for exoplanets by HEPS II. detecting earth-like planets in habitable zone around planet hosts within 30 pc

HEPS(Habitable ExoPlanet Survey) is a planning astrometry satellite that aims to find Earth-like planets in the solar neighbourhood. In this paper, we selected 140 planet harboring stars within 30 pc of the solar system to be potential targets for HEPS. We calculate the detection probability of the planet in habitable zone(HZ) for each system using the simulated data of astrometry measurements. For those host stars without planets in HZ, we inject an additional planet of 10 M⊕ in their HZs and check the stability of the systems. Considering five observation modes of different sampling cadence and total observation time, we obtain a table containing the total detection probability of the planets in HZs for all of the 140 selected systems. This paper provides a potential ranked list of target stars for HEPS, or other astrometric mission to detect Earth-like planets in the future. We also calculate an empirical fitted expression of the detection probability as a function of both sampling cadence and total observation time. We conclude a quantitative method to estimate the detection probability for certain planet hosts and observation modes via the empirical expression. We show the minimum requirements of both sampling cadence and observation time for Proxima Centauri, HD 189733 and HD 102365, if the detection probability of habitable-zone planets of these three systems needs to be 90%.