Detecting Gravitational Waves from Exoplanets Orbiting Binary Neutron Stars with B-DECIGO and DECIGO

TL;DR

The paper addresses the problem of detecting exoplanets in extreme environments by leveraging gravitational waves from circumbinary planets around binary neutron stars, using decihertz detectors DECIGO and B-DECIGO. It extends the Doppler-modulation approach of Tamanini & Danielski (2019) and employs a Fisher-matrix framework to forecast detectability across planet mass, orbital period, distance, and carrier GW frequency. The results show that B-DECIGO can reach sub-Earth to super-Earth/Neptune-mass planets around Galactic BNSs, while DECIGO can probe sub-Jovian to Earth-mass planets out to ~1 Gpc, enabling cosmological-scale CBP surveys and insights into planet formation in post-main-sequence environments. These findings have significant implications for expanding exoplanet demographics into the gravitational-wave window and for multi-band GW–EM studies of planetary systems in extreme astrophysical settings.

Abstract

The first detection of a gravitational-wave (GW) signal in 2015 has opened a new observational window to probe the universe. This probe can not only reveal previously inaccessible binaries, black holes, and other compact objects, but also can detect exoplanets through their imprint on GW signals, thereby significantly extend current exoplanet surveys. To date, nearly 6000 exoplanets have been confirmed, yet most of them reside either in the solar neighbourhood or along the sightline toward the Galactic bulge, reflecting the range limits of traditional electromagnetic techniques. In this work, we follow the method proposed in N.Tamanini&C.Danielski(2019) to investigate frequency modulations in GW signals from early-stage binary neutron stars (BNSs) induced by circumbinary planets (CBPs) and obtain that CBPs can be detected by the future space-borne detector DECi-hertz Interferometer Gravitational wave Observatory (DECIGO). For BNS system with the masses of two components both being 1.4 $M_{\odot}$, DECIGO could detect CBPs with mass being dozens of times that of Jupiter out to distances of $\sim 1$ Gpc, well beyond the Local Supercluster, offering an unprecedented opportunity to study planetary formation and evolution for the post main-sequence stage.

Figures (11)

• Figure 1: Detector sensitivity adopted in this work. We show the characteristic noise $h_n(f)\equiv\sqrt{f\,S_n(f)}$ for B-DECIGO and DECIGO. The two overlaid source tracks indicate the pre-merger signal strength of the reference compact binaries (BNS and DWD), and the vertical lines mark the GW frequencies reached $100\,\mathrm{years}$ before coalescence.
• Figure 2: Illustration of a three-body system co mposed of a BNS and a CBP.
• Figure 3: Source–frame coordinates $(\hat{x}',\;\hat{y}',\;\hat{z}')$; detector–frame coordinates $(\hat{x},\;\hat{y},\;\hat{z})$; and ecliptic coordinates $(\bar{x},\;\bar{y},\;\bar{z})$.
• Figure 4: Normalized 1$\sigma$ errors on the circumbinary-planet parameters as a function of the planetary period $P$ for BNS and DWD sources observed by B-DECIGO and DECIGO. The curves show the error on $K$ and on $P$, each rescaled by the GW SNR and by $M_p$ (in $M_J$), so that lower values indicate tighter constraints. Errors remain small when multiple orbital cycles are sampled, develop a minimum at intermediate $P$ (where the Doppler imprint strengthens), and then rise sharply once $P$ approaches or exceeds the mission duration; a feature near $P\!\sim\!1\,\mathrm{year}$ reflects degeneracy with the detector’s annual motion. Default source and observing assumptions are used throughout.
• Figure 5: Selection function in the plane of planetary mass versus orbital separation for CBPs around compact binaries. The solid curves give, for each detector and source class (BNS/DWD), the minimum detectable mass $M_p^{\rm min}$ at a given orbital distance $a$, adopting the detection criterion $\sigma_K/K<0.3$ and $\sigma_P/P<0.3$. The horizontal dashed line marks the deuterium-burning limit at $13\,M_J$. DECIGO outperforms B-DECIGO across the board, and BNS hosts yield substantially lower $M_p^{\rm min}$ than DWDs due to their higher carrier GW frequencies.