Two of the Longest-Period Young Transiting Exoplanets in the HD 114082 System
The HD 114082 system has been studied: two transiting planets with record-long periods for young stars have been discovered. Using data from TESS, NGTS, CHEOPS, and ASTEP+, orbits were determined: planet b's period is 225.5504±0.0004 days, planet c's about 400 days. From photometric and radial velocity observations, parameters were derived: for b, semi-major axis 0.791±0.008 AU, eccentricity ~0, inclination 89.791°, radius 1.046 R_J, mass upper limit 1.6 M_J (95%); for c, semi-major axis 0.99 AU, inclination 89.701°, radius 1.36 R_J, mass ≤2.0 M_J (or 0.24 M_J considering transit timing variations). The system contains a debris disk with two dust components. Dynamical modeling (N-body) indicates nearly resonant, coplanar, circular orbits. The results suggest planet formation in situ or beyond the snow line with subsequent migration that affected the disk.
Context
Studying young exoplanets is crucial for testing planet formation models. In early epochs (less than 100 million years), planets still retain accretion heat, and their radii can be inflated, making them 'puffy.' The transit method, advanced by missions like the one launched by William Borucki (the visionary behind Kepler), allows us to measure planet radii, while spectroscopy determines masses via radial velocities. However, strong stellar activity in young stars complicates such measurements. The star HD 114082, an F-type star about 15 million years old in the Scorpius–Centaurus OB association, challenges standard methods with its rapid rotation and hot atmosphere, composed mostly of hydrogen and helium — a fact first firmly established for stars by Cecilia Payne-Gaposchkin. The discovery of two transiting giant planets with record-long periods opens a window into a previously inaccessible parameter space.
Methods
To determine the orbits and properties of the planets, photometric data from space telescopes TESS and CHEOPS, along with ground-based surveys NGTS, ASTEP+, and the LCO network, were used. The key challenge was distinguishing the true orbital period of planet b from its alias (pseudo-period) due to rare transits. With strategic observations at predicted times and the use of transit method, supplemented by high-resolution spectroscopy (FEROS and HARPS), the periods were reliably determined. The spectra measured the star's radial velocities, where planet signals were sought, taking into account correlations with the bisector of the cross-correlation function to filter out activity-induced false signals. Spectral lines of hydrogen, such as the Balmer series, were used for calibration and analysis.
Results
Modeling data with Gaussian processes and Bayesian inference showed planet b has a radius of 1.046 ± 0.014 R_J and a period of 225.5504 ± 0.0004 days, while planet c has 1.36 ± 0.03 R_J and 314 (+11/-18) days. Upper mass limits (95% confidence) are 1.6 M_J for b and 2.0 M_J for c, implying low densities — less than 1.7 and 1.0 g/cm³ respectively. The orbits are nearly circular and coplanar, inclined about 89.8° to the line of sight. The system also contains a debris disk, including a warm inner belt (possibly analogous to the asteroid belt) and a cold outer belt (analogous to the Kuiper Belt). Dust particles in the disk likely originate from destructive collisions of comets and asteroids. The location of the water snow line, where water condenses into ice, may have played an important role in forming these helium-hydrogen giants.
Implications
The discovery of this pair fills a gap in exoplanet demographics: so far, very little was known about young gas giants on wide orbits. Existing formation models suggest giant planets can be born beyond the snow line and migrate inward, but the details remained unclear. The observed configuration — planets in near-resonant orbits, tilted about 7° relative to the outer disk — indicates dynamical interaction that may have shaped the system's architecture. This allows linking properties of exoplanets with dusty cometary structures.
Future development
Future observations, particularly with high-resolution spectroscopy on new instruments like ANDES at the ELT, will refine planet masses and possibly detect atmospheric lines of hydrogen and helium. Continued transit monitoring will refine periods and reveal possible transit timing variations (TTV), providing independent mass estimates. Studying these planets at a 'teenage' age will help understand how quickly gas giants cool and contract. The distribution of water and other volatiles in the disk will also be investigated to reconstruct the formation history.
Impact
The results will influence models of exoplanetary system formation, the theory of debris disk dynamical evolution, and the atmospheric physics of young giants.
Next steps
Next steps include precisely determining planet c's period with additional transits, TTV analysis for independent planet weighing, and deep spectroscopic study of the atmospheres.
Key open problems
Connection to unsolved problems: The HD 114082 system is directly relevant to questions of giant planet formation and migration. It may clarify the 'puffy' planet paradox — why some gas giants have radii larger than predicted by cooling models. The observed resonant pattern also ties into the long-standing problem of planetary system stability and the role of cometary bombardment in delivering water to inner planets.
🎯 Fun fact: The star HD 114082 lies in the Scorpius–Centaurus OB association — the same group of young stars as the HR 8799 system, where exoplanets were first directly imaged.
Key numbers
- Period of planet b: 225.5504 days
- Age of the star: 15 million years
- Radius of planet c: 1.36 R_J
- Mass of planet b (upper limit): 1.6 M_J
- Distance to the system: 95.06 pc