The influence of stellar activity on detecting Earth-like planets via nulling interferometry

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
² Key Laboratory of Modern Astronomy and Astrophysics, Ministry of Education, Nanjing 210023, PR China

^★ Corresponding author; huigen@nju.edu.cn

Received: 23 October 2024
Accepted: 10 February 2025

Abstract

Context. To investigate the influence of stellar activities on the detection of Earth-like planets via the nulling interferometer, we aim to estimate the signal-to-noise ratio (S/N) values of Earth-like planets around typical dwarf stars G and M, with different stellar activities. We also study the fitted accuracy of the planetary locations with different S/N values, which is crucial to evaluating the possibility of a planet in the habitable zone (HZ).

Aims. The direct imaging of Earth-like planets in solar neighbors is challenging. Both transit and radial velocity (RV) methods suffer from noise due to stellar activity. Here, we focus on the differential nulling interferometer with an X configuration and examine whether stellar activities are likely to influence the detection of planets in the HZ and the location accuracy. We aim to provide a basis for selecting target stars according to their specific activity levels for future interferometer observatories in space.

Methods. By choosing a typical configuration of an X array interferometer, we used theoretical formulas to calculate the intrinsic Poisson noise and the noise of stellar activities. Assuming a fixed array with no rotation and ignoring other systematic and astrophysical noises, we considered a single active region on a stellar disk, including both spots and flares with different parameters, for instance, the position, size, and temperature of the active regions. Then we simulated the S/N of Earth-like planets in HZ around G dwarf stars (solar-like) and M dwarfs (Proxima-like), with different stellar activities in the mid-infrared (MIR) band (7–12 µm). Furthermore, we used analytic and numerical ways to investigate the influence on the determination of the planet’s location, based on the deduced S/N caused by the stellar activity.

Results. The noise attributed to stellar activity has much less of an influence than the TV and RV when attempting to detect Earthlike planets around both G and M dwarfs. Stellar activity may hardly influence the detection of Earth-like planets around G dwarf stars, with a S/N exceeding 10 for both flares and spots even when the active region reaches three times Jupiter’s radius. However, detecting Earth-like planets around M dwarfs, which are usually more active, can be significantly hindered. If the stellar activity is violent enough, the S/N can drop below 5 for both flares and spots when the active region is as large as three times Jupiter’s radius. We also analyzed the uncertainty of the planet’s location due to the deduced S/N. Consequently, we have determined the possibility of mistaking a planet in the HZ as being outside the HZ based on an erroneous S/N measurement.

Conclusions. Since the nulling interferometer largely suppresses the stellar signal, the impact of stellar activities, such as spots and flares, on the detection of Earth-like planets is significantly reduced. However, for some extremely active M stars, such activities can make the detection of planets obscure or lead to greater uncertainties regarding their location. Selecting quiescent target stars or monitoring the light curves of stars would be a helpful way to get rid of contaminates associated with violent stellar activities.