Multimessenger Exoplanets: Combining Infrared Excess and Astrometry with Gravitational-wave Detections of Double White Dwarf Exoplanets

Abstract Multimessenger astrophysics provides a powerful framework for characterizing astrophysical systems through multiple, independent signals rather than relying solely on electromagnetic (EM) observations. While most prior multimessenger studies have focused on the properties of ultracompact objects, recent work has shown that gravitational waves (GWs) detectable with the upcoming Laser Interferometer Space Antenna (LISA) observatory can also be used to detect exoplanets orbiting stellar remnants. In particular, this includes circumbinary planets around tight double white dwarf (DWD) systems. This population is unique: no such planets are currently known around DWDs, and only a handful of circumbinary planets around main-sequence stars and planets orbiting white dwarfs (WDs) have been identified. Independent EM confirmation will be essential, especially for the first GW-detected exoplanets, to validate this method. While EM techniques are unlikely to efficiently survey large populations of DWDs, GW detection followed by targeted EM confirmation provides one pathway for identifying and characterizing these systems. Using a representative case, the known DWD ZTF J2243+5242 scaled to a fiducial distance of 150 pc, we compute the detectability of circumbinary exoplanets via LISA GW observations. We then demonstrate two complementary EM techniques—JWST infrared (IR) excess and Gaia or Habitable Worlds Observatory astrometry—that could independently confirm a GW-detected planet in this fiducial system. We present the first exploration of IR excess around DWDs as a viable exoplanet detection method, extending its established use for solitary WDs. Both EM techniques overlap with regions of parameter space where LISA is sensitive to DWD exoplanets, enabling robust confirmation and characterization.