Ten Years of Observations Reveal the Complex Planetary System of HIP 41378
Artist's impression of the planetary system around the star HIP 41378. The rings around HIP 41378 f have not been confirmed. Credit: Salomé Grouffal
A decade-long international observing campaign, including key observations with HARPS-N at the Telescopio Nazionale Galileo, has revealed the architecture of one of the most complex planetary systems known.
Located about 346 light-years away in the constellation of Cancer, HIP 41378 hosts at least six confirmed planets, with a possible seventh still awaiting confirmation. The system includes three inner planets smaller than Neptune and three outer, low-density worlds reaching sizes comparable to Saturn.
First discovered in 2015 by NASA's Kepler space telescope, HIP 41378 is particularly well suited for exoplanet studies. Almost all of its planets transit in front of their host star, allowing astronomers to combine transit observations, which reveal their sizes, with radial-velocity measurements, which provide their masses.
HARPS-N at the TNG played a key role in this effort, being among the first instruments to begin the radial-velocity campaign, almost one year before the other spectrographs involved in the study. This early start was crucial for characterising the system's outer planets, whose long orbital periods require many years of observations.
"With HARPS-N at the Telescopio Nazionale Galileo, we were the first to start the observations needed to characterize the non-transiting planets and measure the masses of all the planets in the system", says Luca Malavolta, Associate Professor at the University of Padova and co-author of the study.
The campaign combined HARPS-N observations with data from HARPS, HIRES, ESPRESSO, and the space telescopes TESS and CHEOPS, providing one of the most detailed views yet of a multiplanetary system hosting long-period planets.
The outermost planet, HIP 41378 f, is particularly intriguing. This "super-puff" planet is large but has an exceptionally low density. It takes about one and a half years to orbit its star, while its transit lasts around 19 hours. Gravitational interactions with its planetary companions also shift the timing of its transits by several hours, making long-term and coordinated observations essential.
The HIP 41378 system demonstrates how patient, long-term monitoring and high-precision spectroscopy can reveal the architecture of distant planetary systems, and highlights the crucial contribution of HARPS-N and the TNG to the characterisation of their planets.
Planetary radial velocity signals for the the seven planets in the system. See paper for more details.