Masses and atmospheric evolution of planets in the young K2-233 system

The three-planet system orbiting the young orange dwarf K2-233 shows signs of dramatic atmospheric change. New analysis of radial velocity data confirms that these worlds are undergoing significant mass loss, with planet K2-233 d expected to shrink into either a thin-envelope sub-Neptune or a bare super-Earth over billions of years. Meanwhile, the inner planets K2-233 b and c are likely dominated by iron cores.

Star system: K2 233 | Distance from the Sun: 221 light years

The multi-planet system orbiting K2-233, an active orange dwarf star only 400 million years old, provides a clear view into the early evolution of planetary worlds. By studying the masses of the three transiting planets, researchers gain insight into how young stars influence the atmospheres and compositions of their orbiting companions. This is particularly important for planets residing near the radius valley, a theoretical transition point where planets either retain a thick atmosphere or lose it entirely to become bare rocky cores.

To determine the properties of these planets, data from the HARPS-N and HARPS instruments were combined with transit observations from the K2 and TESS space telescopes. Radial velocity measurements, which track the tiny back-and-forth wobbles of a star caused by the gravitational tug of its planets, were used to estimate planetary masses. Because the host star is young and magnetically active, its surface activity can mask planetary signals. Consequently, the analysis employed complex statistical modeling techniques to filter out this stellar interference, which resulted in a range of physically possible mass solutions rather than a single definitive value.

Modeling the long-term atmospheric future of the outermost planet, K2-233 d, shows that it is currently losing a significant amount of its atmosphere to space. Over the next few billion years, this process will transform the planet into one of two states: either a smaller sub-Neptune that keeps a thin layer of gas, or a solid super-Earth that has lost its gaseous envelope completely. While the outer planet currently maintains a substantial hydrogen and helium atmosphere, the inner planets K2-233 b and c appear to be much denser and likely contain primarily iron cores.

Key findings
K2-233 d is currently losing its atmosphere and will evolve into either a smaller sub-Neptune or a bare super-Earth.
The inner planets K2-233 b and K2-233 c are likely iron-dominated worlds.
Stellar activity from the young K2-233 star complicates mass measurements, leading to multiple viable models.
K2-233 d currently retains a significant hydrogen and helium atmosphere.

Scientific study

The GAPS Programme at TNG. LXXX. Characterisation of the 400 Myr old multi-planet system K2-233 (external link)