HAT-P-13b,c: a transiting hot Jupiter with a massive outer companion on an eccentric orbit

We report on the discovery of a planetary system with a close-in transiting hot Jupiter on a near circular orbit and a massive outer planet on a highly eccentric orbit. The inner planet, HAT-P-13b, transits the bright V=10.622 G4 dwarf star GSC 3416-00543 every P = 2.916260 \pm 0.000010 days, with transit epoch Tc = 2454779.92979 \pm 0.00038 (BJD) and duration 0.1345 \pm 0.0017 d. The outer planet, HAT-P-13c orbits the star with P2 = 428.5 \pm 3.0 days and nominal transit center (assuming zero impact parameter) of T2c = 2454870.4 \pm 1.8 (BJD) or time of periastron passage T2,peri= 2454890.05 \pm 0.48 (BJD). Transits of the outer planet have not been observed, and may not be present. The host star has a mass of 1.22 \pm ^0.05_0.10 Msun, radius of 1.56 \pm 0.08 Rsun, effective temperature 5653 \pm 90 K, and is rather metal rich with [Fe=H] = +0.41 \pm 0.08. The inner planetary companion has a mass of 0.853\pm ^0.029_-0.046MJup, and radius of 1.281 \pm 0.079 RJup yielding a mean density of 0.498\pm +0.103_-0.069 gcm^-3. The outer companion has m2 sini2 = 15.2 \pm 1.0 MJup, and orbits on a highly eccentric orbit of e2 = 0.691 \pm 0.018. While we have not detected significant transit timing variations of HAT-P-13b, due to gravitational and light-travel time effects, future observations will constrain the orbital inclination of HAT-P-13c, along with its mutual inclination to HAT-P-13b. The HAT-P-13 (b,c) double-planet system may prove extremely valuable for theoretical studies of the formation and dynamics of planetary systems.

💡 Research Summary

The paper reports the discovery and characterization of a two‑planet system orbiting the bright (V = 10.622) G4 dwarf GSC 3416‑00543, designated HAT‑P‑13. The inner planet, HAT‑P‑13b, was identified through the HATNet photometric survey as a shallow (≈0.009 mag) transit recurring every 2.916260 ± 0.000010 days, with a duration of 0.1345 ± 0.0017 days. Follow‑up high‑resolution spectroscopy with Keck/HIRES confirmed the planetary nature and yielded a radial‑velocity (RV) semi‑amplitude consistent with a mass of 0.853 +0.029/‑0.046 M_J. Combined with the transit depth, the radius is 1.281 ± 0.079 R_J, giving a mean density of 0.498 +0.103/‑0.069 g cm⁻³, typical of inflated hot Jupiters.

In addition to the short‑period signal, the RV data revealed a second, long‑period modulation with a period of 428.5 ± 3.0 days, a large eccentricity e = 0.691 ± 0.018, and a minimum mass (m sin i) of 15.2 ± 1.0 M_J. This outer companion, HAT‑P‑13c, does not show any detectable transits; the nominal transit center (assuming zero impact parameter) would be at BJD 2454870.4 ± 1.8, while the time of periastron passage is BJD 2454890.05 ± 0.48. The geometric probability of a transit for such an eccentric, distant orbit is only a few percent, and none have been observed to date.

Stellar parameters were derived from spectral analysis and Yonsei‑Yale isochrones: mass M★ = 1.22 +0.05/‑0.10 M_⊙, radius R★ = 1.56 ± 0.08 R_⊙, effective temperature T_eff = 5653 ± 90 K, and a high metallicity