Compact planetary system TOI-1533 hosts two unique super-Neptune planets

Star system: TOI-1533 | Distance from the Sun: 326 light years

The planetary system TOI-1533 consists of two planets orbiting an active orange dwarf star. Located in a compact arrangement, this system features an inner sub-Neptune planet and an outer, larger hot giant planet. Such systems, where a large gas-rich planet is accompanied by a smaller companion, are rare and provide insight into how planetary systems migrate and evolve over time.

The research reveals that both planets transit, or pass in front of, their host star. The inner planet, TOI-1533 b, completes an orbit every 3.63 days, while the outer planet, TOI-1533 c, orbits every 8.06 days. The outer planet is classified as a super-Neptune with a mass of approximately 40 times that of Earth, and it contains a significant amount of hydrogen and helium.

To identify these worlds, the data from the TESS space telescope was combined with radial velocity measurements from the HARPS-N spectrograph. By using advanced statistical modeling to account for the star's own activity, which can mimic planetary signals, the team successfully isolated the distinct gravitational tugs exerted by both planets on their host star.

The unusual mass ratio and the disparate sizes of these two planets make the TOI-1533 system a significant find compared to other known systems containing hot giant planets. Its existence suggests that these planets may have formed through a gentle migration process within the original gas disk surrounding the star.

Key findings
- TOI-1533 hosts two transiting planets, an inner sub-Neptune and an outer super-Neptune.
- The outer planet has a mass of approximately 40 Earth masses and a low density, indicating a rich hydrogen and helium atmosphere.
- The orbital periods for the inner and outer planets are 3.63 and 8.06 days, respectively.
- Advanced statistical modeling allowed researchers to distinguish planetary signals from the host star's surface activity.

Scientific study

The GAPS programme at TNG ?. TOI-1533: a compact system hosting a super-Neptune-mass pair with disparate radii (external link)