The MIRI Exoplanets Orbiting White dwarfs survey is a cycle 2 JWST program to search for exoplanets around dozens of nearby white dwarfs via infrared excess and direct imaging. In this Letter, we present the detection of mid-infrared excess at 18 and 21 μm toward the bright (V = 11.4) metal-polluted white dwarf WD 0310-688. The source of the IR excess is almost certainly within the system; the probability of background contamination is <0.1%. While the IR excess could be due to an unprecedentedly small and cold debris disk, it is best explained by a 3.0 − 1.9 + 5.5 M _Jup cold (248 − 61 + 84 K) giant planet orbiting the white dwarf within the forbidden zone (the region where planets are expected to be destroyed during the star’s red giant phase). We constrain the source of the IR excess to an orbital separation of 0.1-2 au, marking the first discovery of a white dwarf planet candidate within this range of separations. WD 0310-688 is a young remnant of an A- or late B-type star, and at just 10.4 pc, it is now the closest white dwarf with a known planet candidate. Future JWST observations could distinguish the two scenarios by either detecting or ruling out spectral features indicative of a planet atmosphere.

Aims. We investigate whether volcanic exomoons can be detected in thermal wavelength light curves due to their phase variability along their orbit. The method we use is based on the photometric signal variability that volcanic features or hotspots would cause in infrared (IR) wavelengths, when they are inhomogeneously distributed on the surface of a tidally heated exomoon (THEM). Methods. We simulated satellites of various sizes around an isolated planet and modeled the system’s variability in two IR wavelengths, taking into account photon shot noise. The moon’s periodic signal as it orbits the planet introduces a peak in the frequency space of the system’s time-variable flux. We investigated the THEM and system properties that would make a moon stand out in the frequency space of its host’s variable flux. Results. The moon’s signal can produce a prominent feature in its host’s flux periodogram at shorter IR wavelengths for hotspots with temperatures similar to the ones seen on the Jovian moon, Io, while the same moon would not be identifiable in longer IR wavelengths. By comparing observations at two different wavelengths, we are able to disentangle the signal of an exomoon with transiting and non-transiting orbital inclinations from the planet’s signal in the frequency domain for system distances up to ∼10 pc for Mars-sized exomoons and even further for Earth-sized ones. Conclusions. This method enlarges the parameter space of detectable exomoons around isolated planetary mass objects and directly imaged exoplanets, as it is sensitive to Io- to Earth-sized exomoons with hot volcanic features for a wide range of non-transiting orbital inclinations. Exomoon transits and the detection of outgassed volcanic molecules can subsequently confirm a putative detection.

Context. Though efforts to detect them have been made with a variety of methods, no technique can claim a successful, confirmed detection of a moon outside the Solar System yet. Moon detection methods are restricted in capability to detecting moons of masses beyond what formation models would suggest, or they require surface temperatures exceeding what tidal heating simulations allow. Aims. We expand upon spectroastrometry, a method that makes use of the variation of the centre of light with wavelength as the result of an unresolved companion, which has previously been shown to be capable of detecting Earth-analogue moons around nearby exo-Jupiters, with the aim to place bounds on the types of moons detectable using this method. Methods. We derived a general, analytic expression for the spectroastrometric signal of a moon in any closed Keplerian orbit, as well as a new set of estimates on the noise due to photon noise, pointing inaccuracies, background and instrument noise, and a pixelated detector. This framework was consequently used to derive bounds on the temperature required for Solar System-like moons to be observable around super-Jupiters in nearby systems, with ϵ Indi Ab as an archetype. Results. We show that such a detection is possible with the ELT for Solar System-like moons of moderate temperatures (150–300 K) in line with existing literature on tidal heating, and that the detection of large (Mars-sized or greater) icy moons of temperatures such as those observed in our Solar System in the very nearest systems may be feasible.

Context. Exomoons are expected to orbit gas giant exoplanets just as moons orbit Solar System planets. Tidal heating is present in Solar System satellites, and it can heat up their interior, depending on their orbital and interior properties. Aims. We aim to identify a tidally heated exomoon's (THEM) orbital parameter space that would make it observable in infrared wavelengths with MIRI/JWST around Ïμ Eridani b. We study the possible constraints on orbital eccentricity and interior properties that a successful THEM detection in infrared wavelengths can bring. We also investigate what exomoon properties need to be independently known in order to place these constraints. Methods. We used a coupled thermal-tidal model to find stable equilibrium points between the tidally produced heat and the heat transported within a moon. For the latter, we considered a spherical and radially symmetric satellite with heat being transported via magma advection in a sublayer of melt (asthenosphere) and convection in the lower mantle. We incorporated uncertainties in the interior and tidal model parameters to assess the fraction of simulated moons that would be observable with MIRI. Results. We find that a 2RIo THEM orbiting Ïμ Eridani b with an eccentricity of 0.02 would need to have a semi-major axis of 4 planetary Roche radii for 100% of the simulations to produce an observable moon. These values are comparable with the orbital properties of the satellites of the Solar System gas giants. We placed similar constraints for eccentricities up to 0.1. We conclude that if the semi-major axis and radius of the moon are known (e.g., with exomoon transits), tidal dissipation can constrain the orbital eccentricity and interior properties of the satellite, such as the presence of melt and the thickness of the melt-containing sublayer.

NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows, and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimized for planets near quadrature, but HabWorlds’ coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet’s orbital inclination and/or the coronagraph’s inner working angle (IWA). We use a recently created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ∼20^◦ − 60^◦ would be accessible with HabWorlds for a planet with an Earth equivalent instellation in ∼46 systems, while the ocean glint signature at phase angles of ∼130^◦ − 170^◦ would be accessible in ∼16 systems, assuming an IWA = 62 mas (3λ/D). Improving the IWA = 41 mas (2λ/D) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability.