Billy Quarles

A variety of effects can perturb the orbital properties of single planets in close orbits around their host stars. HD80606 b is a highly eccentric ( ϵ = 0.93) exoplanet orbiting its host G5V star, HD 80606. HD 80606 b became a compelling target for exoplanet research after G. Laughlin et al. discovered that the 4.16 M _Jup planet transits and is eclipsed by HD 80606, thereby enabling photometric and spectroscopic observations to yield a wealth of information about the object. The exquisite precision of recent JWST eclipse timing offers an opportunity to investigate whether the orbit of HD 80606 b is modified due to a variety of mechanisms, including General Relativity, tidal torques, or the presence of a second perturbing planet. We have used over 25 yr of radial velocity data plus eclipse and transit observations to place limits on the precession of HD 80606 b’s orbit and to assess which of these effects, if any, are observable. The new models are consistent with constant values of period, eccentricity and the argument of periastron, ω , with a limit on ω ̇ at the level of the predicted general relativistic drift. The precision radial velocity and timing data limit the mass and location of a second planet external to HD 80606 b. Timing offsets between JWST eclipses relative to the K. A. Pearson et al. predictions are attributed to the poor constraints on ϵ ( cos ω , sin ω ) with only the single eclipse measurement available in that analysis and to an over-weighting of the eclipse timing in that earlier analysis.

While missions have long targeted terrestrial exoplanets within the habitable zone of their host stars, the number of exomoon candidates is expected to grow as next generation space-based observatories achieve the photometric sensitivity required to detect their transit signals. Constraining the stability limits of tightly packed moon systems is therefore essential for transit searches and predicting the number of moons around terrestrial planets. In our solar system, only three moons orbit the terrestrial planets, motivating the question of whether Earth-mass exoplanet systems can sustain long-lived, tightly packed satellites. We investigate the stability limits of an Earth-mass planet orbiting a Sun-mass star, where the planet hosts multiple moons. We use the REBOUND N -body integrator along with the tides_spin module in REBOUNDx to assess the stability of tightly packed systems of Luna-, Pluto-, and Ceres-mass moons across a range of tidal dissipation parameters, up to 10 ⁷ dynamical orbits of the innermost moon. We find that an Earth-mass planet can stably host up to two Luna-mass moons, three Pluto-mass moons, or five Ceres-mass moons. Under Earth-like dissipation, the Luna, Pluto, and Ceres packed systems survive within narrow regions of orbital spacing. These results imply that long-lived multimoon systems around Earth-mass planets are possible but strongly depend on tidal dissipation; for these architectures to exist on billion-year timescales, tides must be weaker than those of the present-day Earth–Moon system.

Earth-like planets in the habitable zone (HZ) of M dwarfs have recently been targeted in the search for exomoons. We study the stability and lifetime of large (Luna-like) moons, accounting for the effects of three-body interactions and tidal forces using the N -body simulator rebound and its extension library reboundx . We find that those moons have a notably different likelihood of existence (and, by implication, observability). Large moons orbiting Earth-like planets in the HZs of M4 and M2 dwarfs become unstable well before 10 ⁷ and 10 ⁸ yr, respectively, and in most cases, those orbiting M0 dwarfs become unstable in much less than 10 ⁹ yr. We conclude that HZ planets orbiting M dwarfs are unlikely to harbor large moons, thus affecting the total number of possible moons in our galaxy and the Universe at large. Since moons may help enhance the habitability of their host planet, besides being possibly habitable themselves, these results may have notable implications for exolife, and should also be considered when seeking solutions to the Drake equation and the Fermi paradox.

Nearby giant exoplanets offer an opportunity to search for moons (exomoons) orbiting them. Here, we present a simulation framework for investigating the possibilities of detecting exomoons via their astrometric signal in planet-to-star relative astrometry. We focus our simulations on α Centauri A, orbited by a hypothetical giant planet consistent with candidate detections in Very Large Telescope and James Webb Space Telescope observations. We consider a variety of observatory architectures capable of searching for exomoons, including upcoming facilities and also a hypothetical dedicated facility—e.g., a purpose-built space telescope with diameter = 3 m, central observing wavelength of 500 nm, and contrast-limited performance of ∼10⁻⁹ in 1 hr observations. We find that such a facility would be capable of detecting ∼Earth-mass moons in a 5 yr campaign, assuming a Saturn-mass planet. More generally, we simulate expected detection limits for a variety of levels of astrometric precision. We find that moons as small as ∼0.2 M _⊕ on orbital periods of 4−30 days can be detected with astrometric precision of 0.1 mas and observing cadence of 1 hr over a 5 yr campaign. Additionally, we find that a 39 m ground-based telescope can detect Earth-sized exomoons orbiting the same hypothetical planet with a more modest observing cadence of 1 day. We discuss these results as motivation for a dedicated space observatory as well as a more detailed study of the physical parameters of a greater variety of star–planet–moon systems.

We report on coronagraphic observations of the nearest solar-type star, α Centauri A (α Cen A), using the MIRI instrument on the James Webb Space Telescope. The proximity of α Cen (1.33 pc) means that the star’s habitable zone is spatially resolved at mid-infrared wavelengths, so sufficiently large planets or quantities of exozodiacal dust would be detectable via direct imaging. With three epochs of observation (2024 August, 2025 February, and 2025 April), we achieve a sensitivity sufficient to detect T _eff ≈ 225–250 K (1–1.2 R _Jup) planets between 1″–2″ and exozodiacal dust emission at the level of >5–8× the brightness of our own zodiacal cloud. The lack of exozodiacal dust emission sets an unprecedented limit of a few times the brightness of our own zodiacal cloud—a factor of ≳10 more sensitive than measured toward any other stellar system to date. In 2024 August, we detected an F _ν (15.5 μm) = 3.5 mJy point source, called S1, at a separation of 1 . ″ 5 from α Cen A at a contrast level of 5.5 × 10⁻⁵. Because the 2024 August epoch had only one successful observation at a single roll angle, it is not possible to unambiguously confirm S1 as a bona fide planet. Our analysis confirms that S1 is neither a background nor a foreground object. S1 is not recovered in the 2025 February and April epochs. However, if S1 is the counterpart of the object C1, seen by the Very Large Telescope/New Earths in Alpha Centauri Region program in 2019, we find that there is a 52% chance that the S1 + C1 candidate was missed in both follow-up JWST/MIRI observations due to orbital motion. Incorporating constraints from the nondetections, we obtain families of dynamically stable orbits for S1 + C1 with periods between 2 and 3 yr. These suggest that the planet candidate is on an eccentric (e ≈ 0.4) orbit significantly inclined with respect to the α Cen AB orbital plane (i _mutual ≈ 50^∘, prograde, or ≈130^∘, retrograde). Based on the photometry and inferred orbital properties, the planet candidate could have a temperature of 225 K, a radius of ≈1–1.1 R _Jup, and a mass between 90 and 150 M _⊕, consistent with radial velocity limits. This Letter is first in a series of two papers: Paper II discusses the data reduction strategy and finds that S1 is robust as a planet candidate, as opposed to an image or detector artifact.

K2-18b closely orbits a nearby M3 dwarf within its habitable zone, where this planet could be either a super-Earth or a mini-Neptune. Recent studies using transit spectroscopy suggest that it is Hycean in nature, but this classification is currently controversial. We use the N-body integrator rebound and its extension library reboundx to investigate the possibility of exomoons around K2-18b. Due to tidal interactions that induce outward migration, we find that any moons would be extremely unlikely. If formed, their lifetimes would be relatively short, not exceeding 10 Myr assuming Earth-like or Neptune-like tidal parameters for K2-18b. Recent studies estimate the stellar (and system) lifetime as 3 Gyr, which is significantly longer than the tidal migration timescale. We show that exomoons are unlikely to survive around K2-18b due to rapid tidal-driven migration, casting doubt on moon-based habitability scenarios for short-period M-dwarf planets in general.

K2-18b closely orbits a nearby M3 dwarf within its habitable zone, where this planet could be either a super-Earth or a mini-Neptune. Recent studies using transit spectroscopy suggest that it is Hycean in nature, but this classification is currently controversial. We use the N-body integrator rebound and its extension library reboundx to investigate the possibility of exomoons around K2-18b. Due to tidal interactions that induce outward migration, we find that any moons would be extremely unlikely. If formed, their lifetimes would be relatively short, not exceeding 10 Myr assuming Earth-like or Neptune-like tidal parameters for K2-18b. Recent studies estimate the stellar (and system) lifetime as 3 Gyr, which is significantly longer than the tidal migration timescale. We show that exomoons are unlikely to survive around K2-18b due to rapid tidal-driven migration, casting doubt on moon-based habitability scenarios for short-period M-dwarf planets in general.

We present F1550C (15.5 μm) coronagraphic imaging observations of the nearest solar-type star α Cen A using the James Webb Space Telescope (JWST) Mid-InfraRed Instrument (MIRI). The observations, executed in 2025 February, were compromised by having only one successful roll and degraded performance due to a position mismatch (Δr ∼ 10 mas) between α Cen A and the best-matching reference observation behind the MIRI coronagraph. We set preliminary upper limits on both the presence of a planet and an exozodiacal dust disk. The observations are sensitive to a planet heated by α Cen A (200–250 K) with a radius ≳1 R _Jup at a separation of 1 . ″ 5 (2 au) and zodiacal dust emission at the level of ≳5–10 times the brightness of our own zodiacal cloud. A complete analysis of all JWST/MIRI observations of α Cen A is forthcoming in Paper I (Beichman & Sanghi et al. 2025, in preparation) and Paper II (Sanghi & Beichman et al. 2025, in preparation).