The anomalously large transiting planet around Boyajian's star: evidence for a circumplanetary ring

Wei Zhu and Quanyi Liu investigate whether circumplanetary rings could explain the unexpectedly large apparent size of a transiting companion to Boyajian’s star, also known as Tabby’s star. An earlier study reported a companion mass of 9.4 Jupiter masses, with uncertainties of +4.9 and −4.1 Jupiter masses, and a radius of 1.7 ± 0.1 Jupiter radii. This radius is difficult to reconcile with standard models: a cool, massive giant planet with an estimated equilibrium temperature of 270 K and an age of approximately 1.2 billion years should have a radius of about 1.0–1.1 Jupiter radii. The authors therefore explore whether opaque rings around a smaller planet could provide enough additional occulting area to make the companion appear 60–70% larger than expected. The analysis uses two-minute-cadence TESS observations from Sector 15, covering a single, approximately 21-hour transit on September 3–4, 2019, with a depth of about 1.1%. The authors remove the local baseline trend using a quadratic polynomial fitted to out-of-transit data, then compare conventional spherical-planet and ringed-planet models using Markov chain Monte Carlo sampling. Because a single transit cannot establish the orbital period or eccentricity, both models assume a 1,030-day period and a circular orbit. The spherical-planet fit reproduces the earlier results, yielding a planet-to-star radius ratio of 0.1090, an impact parameter of 0.77, and a companion radius of approximately 1.74 Jupiter radii. For the ring calculations, the authors extend the JoJo transit code to calculate the light blocked by a planet and a projected elliptical annulus across a limb-darkened stellar disk. They report that the ring module is more than 20 times faster than PYPPLUSS at comparable precision. To limit model flexibility, they fix the planet’s radius at 1.1 Jupiter radii, assume fully opaque rings, and test inner ring edges at either 1.0 or 1.1 planetary radii. They assess physical plausibility using the Roche limit, within which tidal forces inhibit ring particles from assembling into satellites. Both ring models fit the observations acceptably. Their inferred outer ring radii range from approximately 3.1 to 4.4 planetary radii, and their projected shapes are strongly flattened, with flattening parameters of about 0.82–0.89. The projected ring orientations range from approximately 74° to 81°, implying nearly polar configurations if the inferred geometry is correct. Representative true obliquities are about 80° or 100°. The rings would extend nearly to the estimated Roche limit. Compared with the spherical-planet model, the ring models reduce chi-squared by approximately 14 while adding two free parameters. However, the Bayesian information criterion improves by only 0.4–1.3, indicating marginal statistical support and leaving the two interpretations nearly indistinguishable. The authors conclude that rings offer a physically motivated explanation for the radius anomaly, but the results do not constitute a confirmed detection. Several limitations remain. The normal planetary radius is imposed in the ring fits rather than independently measured, and the inferred ring geometry has substantial uncertainties and degeneracies. Much of the fit improvement depends on a feature approximately 0.25 days after mid-transit that could instead reflect a starspot crossing. Excluding this feature reverses the model preference. A rocky debris ring might connect the companion to the dusty environment implicated in the star’s historic irregular dimmings, but this possibility remains speculative. Precise observations of repeat transits, particularly during ingress and egress, could test predicted differences of a few parts in ten thousand. Multiband photometry and future astrometric constraints on the companion’s mass and orbit would provide additional tests.

Could Giant Rings Explain the Oversized Companion to Boyajian’s Star?

A planet’s silhouette does not always reveal its true size. If rings surround a transiting world, they can block extra starlight and make the planet appear larger than it really is. That possibility lies at the heart of “The anomalously large transiting planet around Boyajian's star: evidence for a circumplanetary ring,” a study by Wei Zhu and Quanyi Liu.

The researchers investigate whether rings could explain the unexpectedly large apparent radius of a companion to Boyajian’s star, also known as Tabby’s star. Their answer is cautiously encouraging: rings can reproduce the observed transit while allowing the planet itself to have a more conventional size. But the available evidence falls well short of confirming their presence.

A Planet That Looks Too Large

An earlier analysis estimated the companion’s mass at 9.4 Jupiter masses, with uncertainties of +4.9 and −4.1 Jupiter masses, and its radius at 1.7 ± 0.1 Jupiter radii. The radius is the puzzle. At an estimated age of about 1.2 billion years and an equilibrium temperature of roughly 270 K, such a massive, cool giant should measure around 1.0–1.1 Jupiter radii according to standard models.

The companion therefore appears roughly 60–70% larger than expected. Rings offer a straightforward geometric explanation: the planet need not be unusually inflated if surrounding material contributes substantially to the area blocking the star.

One Transit, Two Interpretations

The study uses two-minute-cadence observations from TESS Sector 15. These captured a single transit on September 3–4, 2019, lasting approximately 21 hours and dimming the star by about 1.1%. Before fitting models, Zhu and Liu removed the local brightness trend with a quadratic polynomial fitted to observations outside the transit.

They then compared a conventional spherical planet with a ringed planet, using Markov chain Monte Carlo sampling to explore possible parameter values. Because one transit cannot establish the orbital period or eccentricity, both interpretations assume a 1,030-day period and a circular orbit. Those assumptions are important: the observations do not independently establish the companion’s full orbital geometry.

The spherical-planet model closely reproduces the earlier findings. It gives a planet-to-star radius ratio of 0.1090, an impact parameter of 0.77, and a companion radius of approximately 1.74 Jupiter radii. In other words, treating the entire silhouette as a planet preserves the size anomaly.

Building a Ringed Silhouette

To model rings, the authors extended the JoJo transit code to calculate the light blocked by both a planet and a projected elliptical annulus across a limb-darkened star. They report that their ring module runs more than 20 times faster than PYPPLUSS at comparable precision, making repeated calculations more practical.

The ring models deliberately limit flexibility. The planet’s radius is fixed at 1.1 Jupiter radii, the rings are assumed to be fully opaque, and their inner edges are placed at either 1.0 or 1.1 planetary radii. This tests whether rings can rescue a normal-sized planet, rather than independently measuring that planet’s size.

Both versions fit the observations acceptably. Their inferred outer ring radii span approximately 3.1–4.4 planetary radii. The projected rings are strongly flattened, with flattening parameters around 0.82–0.89, and projected orientations of roughly 74°–81°. If the inferred geometry is correct, the rings would occupy nearly polar configurations, with representative true obliquities around 80° or 100°.

The rings would also extend nearly to the estimated Roche limit—the region where tidal forces inhibit ring particles from assembling into satellites. That provides a useful physical plausibility check, although it does not establish that such rings actually exist.

A Better Fit, but Only Marginal Evidence

The ring models reduce chi-squared by approximately 14 while adding two free parameters. That sounds promising, but extra flexibility can improve a fit even without identifying the correct physical explanation. The Bayesian information criterion, which penalizes additional parameters, improves by only 0.4–1.3. Statistically, the two interpretations remain nearly indistinguishable.

More importantly, much of the improvement depends on a feature about 0.25 days after mid-transit. That feature could instead be a starspot crossing. Removing it reverses the model preference, illustrating how sensitive the conclusion is to a small part of this single event. Substantial uncertainties and degeneracies in the inferred ring geometry add further caution.

What Would Settle the Question?

Repeat transits offer the clearest route forward. The predicted differences between the models are only a few parts in ten thousand, so precise observations—especially during ingress and egress—would be particularly valuable. Multiband photometry could provide additional tests, while future astrometric constraints could clarify the companion’s mass and orbit.

A rocky debris ring might also connect the companion to the dusty environment implicated in Boyajian’s star’s historic irregular dimmings, but that remains speculative. For now, Zhu and Liu have demonstrated a physically motivated alternative to an anomalously oversized planet: a smaller world wearing an enormous silhouette. Whether that silhouette truly contains rings awaits better data.