New Multi Planet System Discovered Around HD 224018

Astronomers have unveiled a compelling planetary system around the Sun like star HD 224018, combining data from he HARPS N spectrograph at the Telescopio Nazionale Galileo (TNG), Kepler/K2, CHEOPS, and TESS missions. The system hosts three close in transiting planets and evidence for an outer, eccentric cold Jupiter candidate, offering new clues into how systems hosting both small inner worlds and distant gas giants form and evolve.

The star HD 224018 is a G type dwarf about 106 parsecs away, with physical properties that closely resemble an older Sun. Originally, Kepler/K2 observations in late 2016 to early 2017 revealed two transiting sub Neptune planets, one with a single transit event. Since then, HARPS N has been collecting radial velocity data for over six years (more than 200 high precision measurements), revealing additional companions (the innermost and outermost planets). CHEOPS and TESS follow ups helped sharpen transit timings and gather further evidence for a second transit of one of the planet in the system.

The results for this complex system were striking: the innermost planet, HD 224018 b, is a super Earth with a mass of 4.1 Earth masses and a period of 10.64 days confirmed via RV. The team discovered that this planet is transiting, but its radius cannot be precisely constrained due to the shallow transit signals.

The second planet in the system, HD 224018 c, is a warm sub Neptune with a period ~36.6 days, radius ~2.42 Earth radii, and mass ~10.4 Earth masses, yielding a bulk density around ~4.0 g/cm³. These parameters were derived using a combination of HARPS N RVs and transit observations (CHEOPS, K2, TESS). Models suggest that its composition could include an iron-rich core, a silicate mantle and a thin hydrogen/helium envelope or a richer volatile layer, depending on its formation inside or outside the ice line.

Planet d orbits its star roughly every 138 days, even though the orbital period must be confirmed with follow-up observations, and has a radius of about 2.4 times that of Earth, similar to that of planet c. Its mass could not be precisely constrained from the RV measurements.

The outermost companion HD224018 e, detected via RVs, is a candidate cold giant planet with a minimum mass of ~ 0.5 Jupiter masses that moves on a highly eccentric orbit.

HD 224018 stands out because systems that host both small, close in planets and outer gas giants are relatively rare and crucial for testing theories of planet formation and migration. These results help us understand how the presence of a cold Jupiter might influence the formation and evolution of inner planets. Further monitoring with HARPS N is ongoing, especially to better constrain the mass and orbital period of the planets in the system that are not yet well characterised.

Doppler signals of HD 224018 induced by the four planetary companions. For planets b,c and d, phase folded data are shown. The best-fit models are indicated by a red curve. More information in the paper.

Snapshot of the triple transit of planet b, c, and d as seen by K2 at the end of 2016. This artistic representation has been created with the software tango (https://github.com/oscaribv/tango).

Link to the paper.