Circumbinary exoplanets and brown dwarfs with the Laser Interferometer Space Antenna

Abstract

Aims. We explore the prospects for the detection of giant circumbinary exoplanets and brown dwarfs (BDs) orbiting Galactic double white dwarfs binaries (DWDs) with the Laser Interferometer Space Antenna (LISA). Methods. By assuming an occurrence rate of 50%, motivated by white dwarf pollution observations, we built a Galactic synthetic population of P-type giant exoplanets and BDs orbiting DWDs. We carried this out by injecting different sub-stellar populations, with various mass and orbital separation characteristics, into the DWD population used in the LISA mission proposal. We then performed a Fisher matrix analysis to measure how many of these three-body systems show a periodic Doppler-shifted gravitational wave perturbation detectable by LISA. Results. We report the number of circumbinary planets (CBPs) and (BDs) that can be detected by LISA for various combinations of mass and semi-major axis distributions. We identify pessimistic and optimistic scenarios corresponding, respectively, to 3 and 83 (14 and 2218) detections of CBPs (BDs), observed during the length of the nominal LISA mission. These detections are distributed all over the Galaxy following the underlying DWD distribution, and they are biased towards DWDs with higher LISA signal-to-noise ratio and shorter orbital period. Finally, we show that if LISA were to be extended for four more years, the number of systems detected will be more than doubled in both the optimistic and pessimistic scenarios. Conclusions. Our results present promising prospects for the detection of post-main sequence exoplanets and BDs, showing that gravitational waves can prove the existence of these populations over the totality of the Milky Way. Detections by LISA will deepen our knowledge on the life of exoplanets subsequent to the most extreme evolution phases of their hosts, clarifying whether new phases of planetary formation take place later in the life of the stars. Such a method is strongly complementary to electromagnetic studies within the solar region and opens a window into the investigation of planets and BDs everywhere in the entire Galaxy, and possibly even in nearby galaxies in the Local Group. Evidences for sub-stellar objects around WDs White dwarf pollutionWhite dwarfs are expected to have a pure H or He atmosphere (Schatzman 1945) and their high surface gravity (∼ 10 5 denser than the Sun) makes the sinking metals diffusion timescale several order of magnitude shorter than the evolutionary period. Yet, observations show the presence of heavy elements in the spectra of 25% to 50% of all observed WDs (Zuckerman et al. 2003(Zuckerman et al. , 2010Koester et al. 2014), indicating that a continuous supply Article number, page 1 of 16

Will gravitational waves discover the first extra-galactic planetary system?

“…Note that, according to our estimates, circumbinary objects with masses heavier than 13 M J , alias the deuterium burning limit which distinguishes planets from more massive bodies, will be even easier to detect. This will possibly lead to the extragalactic observation of triple stellar systems or circumbinary brown dwarfs [16,17]. Our estimates above will considerably worsen if the third object appears with a much larger separation than 1 au, or with an orbital inclination far from edge-on, in which case only heavier circumbinary masses will be detectable.…”

“…By detecting and characterizing this Doppler modulation, LISA will have the potential to estimate the period and the mass of the third object if its mass is comparable or heavier than Jupiter's and its separation from the DWD is less than 10 au [15]. Our recent estimates predict that LISA could detect up to few hundreds exoplanets and a larger number of more massive circumbinary objects [16]. Such a population of exoplanets, which will be probed for the first time, can be detected all over the Milky Way, overcoming the horizon limitation of current and upcoming electromagnetic exoplanetary surveys, restricted to observe in the Solar neighbourhood.…”

“…Among this plethora of sources, LISA will be able to individually resolve tens of thousands of compact double white dwarf (DWD) binaries within and around the Milky Way [13,14]. Binary white dwarfs are the objects that will allow us to expand our understanding on planetary formation and planetary evolution, since they constitute a mean for detecting massive circumbinary exoplanets, as we recently demonstrated [15,16].…”

“…The same planet will be detected within 4 years for a higher frequency DWD with 3 min period, for which 10 years of LISA observations will even have the potential to detect a similar 5 M J planet. Given the fact that both circumbinary detections and extragalactic DWDs observed by LISA will be biased towards the higher possible DWD periods [15,19], we might expect reasonable chances for the detection of an extragalactic circumbinary planetary system already with LISA, if their rates are similar to the more optimistic ones estimated for the Milky Way [16]. In any case, even if LISA will not have the sensitivity required to detect such extra-galactic planets, a future LISA-like mission, with an improved sensitivity of at least one order of magnitude around the mHz frequency region, similarly to the envisaged µAres [22] or AMIGO [23] space missions, should be able to spot a 4 M J planet orbiting a 5 min period DWD with 10 years of observations.…”

Prospects for detecting exoplanets around double white dwarfs with LISA and Taiji

“…Strong tidal forces can crush the planetary cores during their migration or scattering processes (Farihi et al 2018), which may be associated with the WD pollution effect (Jura et al 2009;Farihi 2016;Brown et al 2017;Smallwood et al 2018). Therefore, as noted by Danielski et al (2019), the detection and study of these objects can help analyze planetary systems after CE phases and the planetary formation processes. .…”