A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge (1312.3951v1)

Abstract: We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M_host ~ 4 Jupiter masses hosting a sub-Earth mass moon. The data are well fit by this exomoon model, but an alternate star+planet model fits the data almost as well. Nevertheless, these results indicate the potential of microlensing to detect exomoons, albeit ones that are different from the giant planet moons in our solar system. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product M pi_rel, where M is the lens system mass and pi_rel is the lens-source relative parallax. If the lens system is nearby (large pi_rel), then M is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, mu_rel = 19.6 +- 1.6 mas/yr, which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, the alternate solution has a lower proper motion, which is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam's razor favors a lens system in the bulge with host and companion masses of M_host = 0.12 (+0.19 -0.06) M_solar and m_comp = 18 (+28 -100 M_earth, at a projected separation of a_perp ~ 0.84 AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the low mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.

• The paper demonstrates that microlensing can detect potential exomoon systems by identifying a sub-Earth-mass body around a gas giant primary.
• The paper employs detailed proper motion measurements and Bayesian analysis to resolve the degeneracy between a nearby exomoon model and a bulge-located star-planet configuration.
• The paper highlights the need for high-cadence, multi-telescope observations and AI-enhanced data processing to overcome uncertainties in microlensing event interpretations.

Overview of "MOA-2011-BLG-262Lb: A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge"

The paper presented by Bennett et al. introduces a microlensing candidate for a free-floating exoplanet-exomoon system, specifically MOA-2011-BLG-262. This event is notable for suggesting a primary lens mass approximately four times that of Jupiter, with a sub-Earth-mass moon. The analysis by the researchers focuses on two main interpretations: the exomoon model and an alternate star+planet system model. Despite both models fitting the data closely, evidence favors the exomoon interpretation, contingent on several conditions, including proximity to the Sun.

Key Findings

1. Microlensing Potential: The paper underscores the capacity of microlensing to identify exomoons, which represents a significant advancement in the ability to detect celestial bodies beyond conventional methods focused on planets orbiting stars.
2. Proper Motion Analysis: The paper provides a detailed examination of lens system characteristics, relying on measurements indicating a large lens-source relative proper motion, a crucial data point that helps discern between nearby and distant lens scenarios.
3. Degeneracy Issue: The researchers highlight a degeneracy in solutions related to lens mass and distance, complicating definitive conclusions about the lens system's nature. Bayesian analysis does not preferentially support the exoplanet+exomoon model over a bulge-located star+planet configuration.
4. Numerical Analysis and Results: The Bayesian analysis suggests two potential outcomes with respective probability distributions. For a nearby option, the host mass is in the range of a few Jupiter masses, while a bulge-situated model involves a host star with planetary characteristics. Projected separations are accordingly 0.13 AU for the former and 0.84 AU for the latter.
5. Implications for Detection: Given the degeneracy, the analysis foresees the need for future high-cadence observations from multiple terrestrial locations to resolve ambiguities using terrestrial parallax effects.

Practical and Theoretical Implications

Practically, the paper reinforces microlensing as a potent tool for detecting non-stellar objects in space, which is paramount in expanding the inventory of celestial bodies well beyond our solar system, significantly increasing scope for moons around free-floating planets. Theoretically, this paper serves as a pioneering step, elucidating potential challenges in parsing microlensing signals from exomoons and free-floating planetary systems.

Future Prospects in AI and Astrophysics

The findings substantiate the requirements for high-precision, multi-telescope data collection strategies, complemented by AI-enhanced data processing to manage and analyze the substantial datasets these observations generate. AI algorithms could prove indispensable in distinguishing between nearly degenerate solutions and integrating parallax data, thus paving the way for more automated, highly sensitive exomoon detection processes.

Though the paper abstains from declaring groundbreaking results due to degeneracies and current model limitations, it sets meaningful parameters and a blueprint for advancing microlensing research. Future missions, potentially utilizing space-based observational platforms like the WFIRST mission or integrating microlensing capabilities with existing spacecraft, promise to enhance detection fidelity, extend range, and reduce uncertainty in lens mass and distance estimations. These future extensions represent essential strides towards systematically cataloging exomoons and understanding their formation and maintenance in diverse astrophysical environments.