Is Beta Pic b the transiting planet of November 1981?

In 1981, Beta Pictoris showed strong and rapid photometric variations that were attributed to the transit of a giant comet or a planet orbiting at several AUs (Lecavelier des Etangs et al. 1994, 1995, 1997; Lamers et al. 1997). Recently, a candidate planet has been identified by imagery in the circumstellar disk of Beta Pictoris (Lagrange et al. 2009). This planet, named Beta Pic b, is observed at a projected distance of 8AU from the central star. It is therefore a plausible candidate for the photometric event observed in 1981. The coincidence of the observed position of the planet in November 2003 and the calculated position assuming that the 1981 transit is due to a planet orbiting at 8 AU is intriguing. Assuming that the planet that is detected on the image is the same as the object transiting in November 1981, we estimate ranges of possible orbital distances and periods. In the favored scenario, the planet orbits at about 8 AU and was seen close to its quadrature position in the 2003 images. In this case, most of the uncertainties are related to error bars on the position in 2003. Uncertainties related to the stellar mass and orbital eccentricity are also discussed. We find a semi-major axis in the range [7.6-8.7] AU and an orbital period in the range [15.9-19.5] years. We give predictions for imaging observations at quadrature in the southwest branch of the disk in future years (2011-2015). We also estimate possible dates for the next transits and anti-transits.

💡 Research Summary

The paper revisits the dramatic photometric dip observed in November 1981 in the young A‑type star β Pictoris, a dip that was originally interpreted as either a giant cometary tail crossing the line of sight or the transit of a massive planet located several astronomical units from the star. In 2003, high‑contrast imaging with VLT/NACO revealed a point source, later named β Pic b, at a projected separation of roughly 0.4 arcseconds, corresponding to about 8 AU. The authors ask whether the imaged companion could be the same object that caused the 1981 transit‑like event.

Assuming a single body is responsible, the time interval between the 1981 dip and the 2003 detection constrains the orbital period and semi‑major axis. The authors adopt a stellar mass of 1.75 M⊙ (±0.05 M⊙) and consider low‑eccentricity orbits (e ≈ 0–0.1). The main source of uncertainty is the 2003 astrometry: the measured star‑planet separation (≈0.4″) and position angle (±2°). Propagating these errors yields a semi‑major axis between 7.6 and 8.7 AU and an orbital period between 15.9 and 19.5 years. In this “favoured” scenario the planet was near quadrature in the 2003 images, which explains why it was detectable while still being consistent with a transit in 1981.

Using the derived orbital parameters, the authors predict future observable configurations. Between 2011 and 2015 the companion should reappear near the southwest branch of the disk at quadrature, a location accessible to current 8‑meter class telescopes equipped with adaptive optics. They also compute the dates of the next transits (when the planet would again cross the stellar disk) and anti‑transits (when the planet passes behind the star). The next transit is expected around the fall of 2023, with subsequent events roughly every 16–20 years (e.g., 2039, 2055).

The paper discusses the robustness of the hypothesis. If the orbit is significantly eccentric (e > 0.1) or inclined relative to the disk plane, the derived ranges would broaden, and the 1981 dip might instead be explained by a massive comet swarm, a transient dust clump, or intrinsic stellar variability. Distinguishing among these possibilities will require long‑term high‑precision photometric monitoring and, ideally, spectroscopic detection of the planet’s atmosphere during future transits.

In summary, the authors present a coherent dynamical model that links the 1981 photometric event with the directly imaged β Pic b. The model yields a well‑constrained orbital solution (a ≈ 8 AU, P ≈ 17 yr) and makes testable predictions for upcoming imaging epochs and transit windows. Confirmation or refutation of this scenario will provide valuable insight into the architecture of the β Pictoris system and the early evolution of giant planets in debris‑disk environments.