Detectability of continuous gravitational waves from planetary-mass companions orbiting compact stars

¹ State Key Laboratory of Radio Astronomy and Technology, Xinjiang Astronomical Observatory, CAS, 150 Science 1-Street, Urumqi, Xinjiang, 830011, PR China
² Xinjiang Key Laboratory of Radio Astrophysics, Urumqi, Xinjiang, 830011, PR China
³ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, PR China
⁴ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
⁵ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, PR China
⁶ School of Physics and Electronic Engineering, Sichuan University of Science & Engineering, Zigong 643000, PR China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 September 2025
Accepted: 15 February 2026

Abstract

Binary systems with ultrashort-period planetary-mass companions are expected to radiate continuous gravitational waves (GWs). However, earlier studies found that the detectability of such systems by the Laser Interferometer Space Antenna (LISA) is unlikely. In this study, we investigate the detectability of GWs from planetary-mass companions orbiting pulsars (PSRs) or white dwarfs (WDs) whose fundamental parameters, essential for calculating GW properties, have been measured. We compare the GW signals from our sample with the sensitivity curves of space-based GW detectors. We find that fourteen sources achieve a signal-to-noise ratio (S/N) of ≳ 5 within four years of observations. Among these, three sources have PSR primaries (2S 0918-549 b, 4U 0513-40 b, and 4U 1543-62), and eleven systems possess WD primaries (BW Scl b, CP Eri b, CR Boo b, EF Eri b, GP Com b, GW Lib b, SDSS J0926+3624 b, SDSS J1507+5230 b, SMSS J1606-1000 b, SRGeJ0453 b, and WZ Sge b). We note that their detectability is less probable with near-term missions such as LISA, TianQin, and Taiji. Nevertheless, they could be detected by more advanced, future-generation observatories, such as the Deci-hertz Interferometer Gravitational wave Observatory (DECIGO) and the Big Bang Observer (BBO). This offers the potential to investigate the formation and evolution of ultrashort-period planetary-mass companions around compact stars through joint GW and electromagnetic surveys.