An Inclination to Search for a Dynamic Past

How did the 14 Herculis planetary system get into an unusual configuration? Did the system have a run-in with another star, or was there a troublemaker within the system itself? Astronomers peek deep into the characteristics of the system to find out! 

Planets Going the Distance 

Light curve from 14 Her, showing time (JD) vs. V (magnitude)

Light curve for 14 Herculis, as measured in the ASAS-SN (All-Sky Automated Survey for Supernovae) program. [Adapted from Bardalez Gagliuffi et al. 2021]

When a star forms, leftover material from the protostellar nebula creates a disk around the star and may eventually become planets. Once these planets form, their characteristics, such as mass and inclination, can determine how the system will evolve. Some planetary systems, like our own, have relatively circular orbits, but others have strange configurations. One example of an oddball system is 14 Herculis, a middle-aged K0 dwarf star that’s orbited by two giant planets with very high eccentricities. Exactly how did the planets end up on these peculiar orbits? A team led by Daniella C. Bardalez Gagliuffi at the American Museum of Natural History examined the full orbital configuration of the system to find out. 

When the Planets Don’t Align 

Corner plot showing distributions for mass, inclination, eccentricity, and semi-major axis

Distributions of parameters for planet b. Click to enlarge. [Bardalez Gagliuffi et al. 2021]

14 Her was one of the first stars targeted in radial-velocity searches for exoplanets because of its proximity and brightness. Astronomers detected the closer in of its two known planets, 14 Her b, in 2003, and its outer planet, 14 Her c, four years later. Using archival radial-velocity data from the high-resolution echelle spectrometer on the Keck Telescope and proper motions inferred from the Hipparcos–Gaia Catalog of Accelerations, the team was able to calculate the orbital parameters of the two planets.

They found that planet b orbits at a semimajor axis of ~3 au and has a moderate eccentricity, and that planet c is 27 au from the star and has a highly eccentric orbit. Because the authors only had data for ~15% of 14 Her c’s orbit, they were only able to get a broad distribution of inclinations relative to 14 Her b, but the data pointed to the two orbits being misaligned by nearly 70 degrees. There are only three other known systems with giant planets that have misaligned orbits. 

Corner plot showing distributions for mass, inclination, eccentricity, and semi-major axis

Distribution of parameters for planet c. Click to enlarge. [Bardalez Gagliuffi et al. 2021]

Feeling Inclined to Search for Answers 

What caused this strange misalignment? The answer surely lies in the system’s dynamic past, but what kind of dynamic past is still up in the air. The most likely explanation is planet–planet scattering: multiple planets of similar mass formed together in circular orbits and gravitationally influenced one another, eventually leading to the ejection of one planet and affecting the orbits of the others. It’s also possible that a passing star came close enough to the system that it perturbed the orbits and launched one planet out of the system; to reach a stable configuration, the surviving planets got farther apart, and their eccentricities grew. 

Imaging the system in the mid-infrared could reveal a hidden planetary sibling that might shed further light on 14 Her’s dynamic past, so we’ll just have to wait for JWST and the Nancy Grace Roman Space Telescope to help solve this mystery! 


“14 Her: A Likely Case of Planet–Planet Scattering,” Daniella C. Bardalez Gagliuffi et al 2021 ApJL 922 L43. doi: 10.3847/2041-8213/ac382c