Pumping up a Planet’s Orbital Eccentricity

HD 8673 is an F-type main sequence star located approximately 120 light years away. In 2010, HD 8673 was confirmed by Hartmann et al. (2010) to have a massive planet in a highly eccentric orbit around it. The planet is identified as HD 8673b, with the suffix “b” indicating its planetary nature. HD 8673b has at least 14 times the mass of Jupiter and such a high mass suggests HD 8673b could also be a low-mass brown dwarf rather than a planet. HD 8673b orbits 3 AU from its host star in a highly eccentric orbit with a period of approximately 1600 days. At closest approach, HD 8673b is 0.83 AU from its host star. Its highly elongated orbit also takes it out as far as 5.17 AU.

Roberts et al. (2015) report the discovery of a faint companion star around HD 8673. The companion star is an M dwarf star with an estimated 0.33 to 0.45 times the Sun’s mass and orbits HD 8673 at a distance of roughly 35 to 60 AU. Simulations show the companion star strongly influences the orbit of HD 8673b. Initially, the planet was on a more circular orbit and perturbations from the companion star elongated the planet’s orbit into its current high eccentricity. Such a process may also explain the high mass of HD 8673b since the large increase in the planet’s orbital eccentricity, especially during its formative period, would have enabled it to plough through and accrete material from a larger area of the protoplanetary disk. 

References:
- Hartmann et al. (2010), “A Sub-stellar Companion around the F7 V Star HD 8673”, ApJ Vol 717: 348-356
- Roberts et al. (2015), “Know The Star, Know the Planet. IV. A Stellar Companion to the Host star of the Eccentric Exoplanet HD 8673b”, arXiv:1502.06630 [astro-ph.SR]