HD 33142 is a solitary 8th-magnitude red giant located about 394 light-years (121 pc) away in the southernconstellation of Lepus. It is orbited by three confirmed exoplanets, namely the Jupiter-sized planets HD 33142 b and c, and a Saturn-like planet, d, located closer to the star.
Stellar characteristics
HD 33142 belongs to a class of "retired A stars," meaning it was likely once an A-type main-sequence star but has since evolved past the main sequence.[5][6] Now, it is entering the red-giant branch with a spectral type of K0 III, a radius of 4.17 R☉, and a mass of 1.52 M☉.[3] The effective temperature of HD 33142 is estimated to be about 5,025 K (4,752 °C; 8,585 °F), giving it an orange color. The star is ten times as bright as the Sun, which, combined with its distance from Earth, places its apparent magnitude at 7.96,[2] making it too faint to be visible by the naked eye under most circumstances, but it can be observed using binoculars.[7] It has a solar-like metallicity of 0.06, which translates to an iron abundance 15% higher than the Sun. The star is aged approximately 2.72 billion years,[3] making it three-fifths as old as the Solar System (4.568 billion years old[8]).
The star's rotation period has been measured to be about 106 days from rotational broadening (i.e., Doppler broadening caused by the star's rotation), but this value is very uncertain as only the upper limit of the rotation velocity is known and the axial tilt is entirely undetermined. Light curves obtained by the TESS reveal no transit signals and suggest that the star is photometrically quiet. Archived data from Hipparcos photometry generally agree with this, with no indication of variability.[3]
Planetary system
In 2011, Johnson et al. reported the discovery of a Jovian planet, HD 33142 b, alongside 17 other planets orbiting retired A stars.[5] It revolves around the star in an Earth-like circular (eccentricity 0.049) orbit that lasts 330 days (0.90 years) each, and has an estimated minimum mass of 1.26 MJ.
The second planet, c, was discovered by Bryan et al. in 2016, and was initially described as a super-Jupiter with a minimum mass of 5.97 MJ and an orbital period of 834 ± 29 days (2.283 ± 0.079 years).[9] A 2019 follow-up study by Luhn et al., independently reported radial velocity signals that indicated the existence of a planet with a similar period of 809 ± 26 days (2.215 ± 0.071 years), but with a far smaller mass of 0.62 MJ.[6] A 2022 study by Trifonov et al. seems to agree more with the latter, confirming an 810-day period planet weighing at least 0.89 MJ. Its orbit has a low eccentricity and a semi-major axis of 1.955 AU, roughly twice that of planet b.[3]
In 2022, at the same time Trifonov et al. confirmed the previous two planets, they also reported another smaller object, d, with a mass of 64 ME, two-thirds that of Saturn (95 ME[10]). With a period of 89.9 days, an eccentricity of 0.191, and a semi-major axis of 0.452 AU, its orbit closely resembles that of Mercury, which has a period of 88.0 days, an eccentricity of 0.2056, and a semi-major axis of 0.3871 AU.[11]
Stability and future
The HD 33142 system features massive planets in closely packed orbits, which makes it prone to orbital instability due to gravitational perturbations. While the system is presently stable, out of the 1,000 simulations conducted by Trifonov et al. with randomly generated initial conditions, one-third of them resulted in orbital destabilization within the million-year simulation span. For the unstable runs, the planetary system's median survival time was a mere 8,500 years.[3]
Having left the main-sequence stage, HD 33142 is in the midst of developing into a red giant, and is expected to undergo two periods of rapid expansion in the next 300 million years: first during the red-giant branch, when it will reach a diameter of ~0.75 AU; and then in the asymptotic giant branch, ballooning to around 1.45 AU; all before ceasing nucleosynthesis and shriveling up into a white dwarf. In both phases, tidal forces from the bloated star cause orbital decay. According to simulations, the two inner planets, d and b, will be engulfed by the star before the tip of the red-giant branch as their orbits contract. The outermost planet, c, is predicted to survive the red-giant branch and migrate outward to ~2.5 AU, but will ultimately either succumb to the same fate during the asymptotic giant branch, or be ejected from the system entirely.[3]
