Sub-Saturn Planet MOA-2008-BLG-310Lb: Likely To Be In The Galactic Bulge

We report the detection of sub-Saturn-mass planet MOA-2008-BLG-310Lb and argue that it is the strongest candidate yet for a bulge planet. Deviations from the single-lens fit are smoothed out by finite-source effects and so are not immediately apparent from the light curve. Nevertheless, we find that a model in which the primary has a planetary companion is favored over the single-lens model by \Delta\chi^2 ~ 880 for an additional three degrees of freedom. Detailed analysis yields a planet/star mass ratio q=(3.3+/-0.3)x10^{-4} and an angular separation between the planet and star within 10% of the angular Einstein radius. The small angular Einstein radius, \theta_E=0.155+/-0.011 mas, constrains the distance to the lens to be D_L>6.0 kpc if it is a star (M_L>0.08 M_sun). This is the only microlensing exoplanet host discovered so far that must be in the bulge if it is a star. By analyzing VLT NACO adaptive optics images taken near the baseline of the event, we detect additional blended light that is aligned to within 130 mas of the lensed source. This light is plausibly from the lens, but could also be due to a companion to lens or source, or possibly an unassociated star. If the blended light is indeed due to the lens, we can estimate the mass of the lens, M_L=0.67+/-0.14 M_sun, planet mass m=74+/-17 M_Earth, and projected separation between the planet and host, 1.25+/-0.10 AU, putting it right on the “snow line”. If not, then the planet has lower mass, is closer to its host and is colder. To distinguish among these possibilities on reasonable timescales would require obtaining Hubble Space Telescope images almost immediately, before the source-lens relative motion of \mu=5 mas yr^{-1} causes them to separate substantially.

💡 Research Summary

The paper reports the discovery of a sub‑Saturn‑mass exoplanet, designated MOA‑2008‑BLG‑310Lb, via gravitational microlensing. The event, observed by the MOA collaboration in 2008, initially appeared to be well described by a single‑lens model. However, a careful re‑analysis that incorporates finite‑source effects—where the angular size of the source star is comparable to the Einstein radius—reveals subtle deviations that are best explained by a planetary companion to the primary lens.

When a three‑parameter planetary model (mass ratio q, normalized separation s, and source‑trajectory angle) is fitted, the χ² improvement relative to the single‑lens fit is Δχ² ≈ 880 for three additional degrees of freedom. This large Δχ² is statistically decisive, confirming the presence of a planet. The derived planet‑to‑host mass ratio is q = (3.3 ± 0.3) × 10⁻⁴, corresponding to a planet mass of roughly 74 ± 17 M⊕ if the host is a typical main‑sequence star. The normalized separation is within 10 % of the Einstein radius, implying a projected physical separation close to the Einstein ring radius.

The measured angular Einstein radius is exceptionally small, θ_E = 0.155 ± 0.011 mas. Since θ_E = √(κ M_L π_rel), where κ is a constant, M_L the lens mass, and π_rel the lens‑source relative parallax, a small θ_E can be produced either by a low‑mass lens or by a large π_rel (i.e., a distant lens). Assuming the lens is at least a hydrogen‑burning star (M_L > 0.08 M☉), the authors infer a lower limit on the lens distance D_L > 6 kpc. This places the lens, and therefore the planetary system, firmly in the Galactic bulge—a rare and valuable case, as most microlensing planets have ambiguous host distances.

Adaptive‑optics imaging with VLT/NACO taken near the baseline of the event shows additional blended light within 130 mas of the source. This blend could be (1) the lens itself, (2) a companion to the lens, (3) a companion to the source, or (4) an unrelated foreground/background star. If the blend is the lens, its photometry yields a host mass M_L = 0.67 ± 0.14 M☉, a planet mass of 74 ± 17 M⊕, and a projected separation of 1.25 ± 0.10 AU. This separation lies right at the so‑called “snow line,” where water ice can condense and accelerate core accretion, making the system a benchmark for planet‑formation theories.

If the blended light is not the lens, the host would be fainter and less massive, the planet would be somewhat less massive, and the orbital separation would be smaller, implying a colder, more distant planet. Distinguishing among these scenarios is therefore crucial. The relative proper motion between source and lens is modest, μ ≈ 5 mas yr⁻¹. The authors argue that high‑resolution imaging with the Hubble Space Telescope (HST) should be obtained as soon as possible, before the source and lens separate by more than a few tens of mas. Such observations would resolve the blend, confirm whether it is the lens, and thus pin down the physical parameters of the system.

In summary, this work demonstrates that (i) finite‑source effects can mask planetary signals in microlensing light curves, (ii) careful modeling can recover those signals, and (iii) the combination of microlensing analysis with adaptive‑optics imaging can place strong constraints on the host’s distance and mass. MOA‑2008‑BLG‑310Lb stands out as the most compelling candidate for a bulge planet to date, and its location near the snow line provides a valuable data point for testing core‑accretion models in the dense, metal‑rich environment of the Galactic center. Future HST (or JWST) imaging will be decisive in confirming the lens’s identity and refining the planet’s mass and orbital characteristics.