Using the Keck Planet Imager and Characterizer (KPIC) on the Keck II telescope at the W.M. Keck Observatory, astronomers have obtained and analyzed the high-resolution spectra of the four giant planets orbiting HR 8799.
An artist’s rendering of the planetary system HR 8799 at an early stage in its evolution, showing the planet HR 8799c, a disk of gas and dust, and interior planets. Image credit; Dunlap Institute for Astronomy & Astrophysics / Mediafarm.
HR 8799 is a 30-million-year-old star located approximately 129 light-years away in the constellation Pegasus
It hosts a massive debris disk and four super-Jupiters: HR 8799b, c, d, and e.
Unlike most exoplanet discoveries, which are inferred from analysis of data, these planets are directly visible through ground-based telescopes.
“HR 8799 is a notable planetary system, as it is the only known system with four directly imaged exoplanets,” said Caltech astronomer Dr. Jason Wang and colleagues.
“The four planets are either near or in mean-motion resonance, and dynamical modeling of their orbits have constrained their masses to be 7.2 Jupiter masses for the inner three planets and 5.8 Jupiter masses for HR 8799b.”
In the new study, the astronomers found that the minimum rotation speeds of HR 8799d and HR 8799e clocked in at 10.1 km/s and 15 km/s, respectively.
This translates to a length of day that could be as short as three hours or could be up to 24 hours such as on Earth depending on the axial tilts of the HR 8799 planets, which are currently undetermined.
For context, one day on Jupiter lasts nearly 10 hours; its rotation speed is about 12.7 km/s.
As for the other two planets, they were able to constrain the spin of HR 8799c to an upper limit of less than 14 km/s; HR 8799b’s rotation measurement was inconclusive.
“With KPIC, we were able to obtain the highest spectral resolution observations ever conducted of the HR 8799 exoplanets,” Dr. Wang said.
“This allows us to study them with finer granularity than ever before and unlocks the key to gaining a deeper understanding of not just how these four planets formed, but how gas giants in general develop throughout the Universe.”
How fast a planet spins gives insight into its formation history. Created out of gas and dust kicked up by a newborn star, protoplanets start spinning faster as they accumulate more material and grow — a process called core accretion.
It is believed that planetary magnetic fields then slow and cap their rotation speed. After the fully-formed planet is finished accreting and cools off, it spins back up.
“The spins of HR 8799d and HR 8799e are consistent with the theory that the planets’ magnetic fields put a brake on their spins in their natal years,” Dr. Wang said.
“The spin measurements also hint at the notion that lower mass planets spin faster because they are less affected by magnetic braking, which might tell us something important about how they form. I find this tantalizing.”
“With enough spin measurements, we’ll be able to identify trends that would reveal how the physical processes driving planet formation work,” said Dr. Jean-Baptiste Ruffio, also from Caltech.
“This is something that people have already started doing, but KPIC is allowing us to do this for the smallest, faintest, and closest imaged alien worlds.”
The results will be published in the Astronomical Journal.
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Jason J. Wang et al. 2021. Detection and Bulk Properties of the HR 8799 Planets with High Resolution Spectroscopy. AJ, in press; arXiv: 2107.06949
