Authors: Aldo G. Sepulveda, Daniel Huber, Timothy R. Bedding, Daniel R. Hey, Simon J. Murphy, Zhoujian Zhang, and Michael C. Liu
First Author’s Institution: Institute for Astronomy, University of Hawai‘i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
Status: Accepted for publication in AJ [open access on arXiv]
What is HIP 65426?
HIP 65426 is a star relatively close to Earth, and it has a giant planet, HIP 65426 b orbiting around it that has been directly imaged. You might recall HIP 65426 b from a JWST early science release, as being the first exoplanet directly imaged by JWST.
The star itself is part of a group of young stars called the Lower Centaurus-Crux (LCC) moving group, which is around 10 to 23 million years old, but scientists estimate the age of HIP 65426 to be around 14 million years using different methods. This star rotates very quickly and shows signs of potential pulsations in its brightness. Confirming these pulsations, known as delta-scuti pulsations, could help determine the star’s age more precisely. Determining ages of stars is actually surprisingly difficult, so any method that can accurately predict ages is very intriguing to astronomers.
Now switching gears briefly, planet HIP 65426 b is located relatively far from the star, between 62 to 120 times the distance between the Earth and the Sun. Its orbit is tilted at a significant angle relative to our line of sight. This is particularly interesting because the alignment between a star and its orbiting companions, like planets or brown dwarfs, can tell us more about how these systems formed and evolved.
A recent work revealed that misalignments are common with brown dwarfs, but giant planets tended to be aligned or nearly aligned with their host stars. Understanding whether planets like HIP 65426 b are aligned with their stars helps us understand planet formation and the history of these systems.
Observing with and using data from TESS
Time-series photometry from the Transiting Exoplanet Survey Satellite (TESS) has provided a lot of data about the rotation of stars and any variations in brightness caused by features on their surfaces or by orbiting objects passing in front of them. Time-series photometry also probes for other phenomena, including stellar pulsations and transit events. In this paper, the authors use this data, along with some data from direct imaging of the HIP 65426 system, to investigate the orbital inclination of the exoplanet HIP 65426 b. They aim to determine whether there is evidence for misalignment between the planet and its host star.
The star was observed by TESS in three different time periods called sectors (Fig 1). These sectors spanned from April 2019 to May 2019, April 2021 to May 2021, and April 2023 to May 2023. Data was collected from the star every 2 minutes during these time periods. The data was analyzed using a software called lightkurve, which helps process and analyze the light curves of stars. To ensure the data clean and free from contamination, the authors first removed any unusual or outlier data points from the light curves. Then, they examined a region around the star within a radius of 80 arcseconds to see if any nearby objects were affecting the measurements. This is important because contamination from other sources can affect the accuracy of the analysis.
Identification of the delta Scuti pulsations for Mass and Age Estimations
Several pulsation modes, spanning 28-131 cycles per day, were identified in the star, which are consistent with a high-frequency Scuti pulsator. The presence of these high-frequency Scuti pulsations confirms the young age of HIP 65426 and may even provide an opportunity to estimate its age through detailed asteroseismic modeling, which is beyond the scope of the paper.
The authors also investigated the possibility of pulsation timing variations caused by mutual gravitation with an orbital companion. This is typically measurable only for sufficiently massive planets with long enough periods. No such variations were detected, which places an upper limit on the mass of 12.8 Jupiter masses for HIP 65426 b.
Stellar Inclination of the Host Star
Using a known relation between the star’s rotation period, its radius, and a measure of its rotational velocity, one can constrain the angle between the star’s rotational axis and our line of sight, also known as stellar inclination. This paper uses a Bayesian framework that properly computes the inclination using these parameters. Based on their analysis using values of these parameters from literature (radius from isochrones and rotational velocity from spectroscopy) and TESS measurements (rotation period), the authors place statistical limits on the inclination difference between the star and the planet, the median value being 
Orbital Inclination of the Giant Planet
The orbit of the planet was measured out using astrometric measurements from various sources, including high-precision measurements from VLTI/GRAVITY. From MCMC fitting of Keplerian orbits using the python package orbitize, the median orbital inclination is estimated to be 
Is there a misalignment?
Fig 3 says no! Here the authors compared the inclination of HIP 65426 b with the inclination of its host star. As the plot shows, the stellar and planetary orbital inclinations line up within their uncertainties, and hence there’s a lack of evidence for a misalignment, just a small star-planet obliquity as suggested by the ~3 degree difference in inclination.
This seems to be in line with the general trend of alignment where directly imaged long-period giant planets appear aligned with their host stars, as shown by the plot in Fig 4, where the orbital and host star inclinations for 6 directly imaged exoplanet systems is being compared. This type of perfect alignment also extends to debris disks, which are analogous to our solar system’s Kuiper belt.
If the observed trend of relatively aligned orbits between stars and their imaged giant planets continues, it goes against recent understanding from a 2023 work that suggests misalignments begin common in brown dwarf systems. These differences between giant planets and brown dwarfs could extend to other key characteristics, like their orbital shapes, which might indicate that they form through different processes.
Now, what can we tell about the formation of HIP 65426 b?
There are two key models which explain how planets could form– core accretion and disk instability. Core accretion does not really explain how this planet is born because it is further away from the host star than the region where core accretion would take place. The lack of evidence for misalignment also disfavors the core-accretion scenario. Given the large orbital eccentricity, planet-planet scattering could be a possible mechanism. This scenario suggests that the planet formed closer to its star via core-accretion and was then scattered to its current position by the gravitational interactions with other planets in the system. However, planet-planet scattering typically results in orbits being tilted relative to each other, which isn’t the case here, so the lack of significant misalignment between the HIP 65426 b’s orbit and its star’s rotation axis doesn’t strongly support this idea.
It is important to note that these theories are not conclusive. The current data doesn’t provide complete information about the system’s geometry, so it’s still possible that the star’s actual tilt might be larger than what’s currently estimated. Additionally, the orbital eccentricity is not yet concretely determined, so further astrometric measurements can change our current geometric understanding of the system.
The Big Picture
This paper describes yet another work that combined space-based brightness data and direct imaging data to understand other solar systems well after they have formed, and understand the implications of their obliquity. With new missions and exoplanet surveys, new systems will be discovered which will also usher in more similar studies of inclinations, and orbital architecture.
Astrobite edited by Amaya Sinha
Featured image credit: NASA, ESA, CSA, Alyssa Pagan (STScI)
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