Three young planets around the K-dwarf K2-198: high-energy environment, evaporation history and projected future

Future radii of planets b, c, d at 5 Gyr age. The current radii of planets c, d and b are shown as dashed lines, with the current and final radii for the outer two planets d and b in the range. The core is represented by a black circle, while the envelope is shown as a colored circle around the core. Core size increases with increasing mass. Note that the scaling of the envelope relative to the core is arbitrary. This is chosen so that one can easily compare the radii of the current and final planets and immediately see which structure can hold part of the envelope and stay above the gap. 1.8 R for all masses survives over an envelope and gap. Low-mass planets experience enough mass loss to lose their envelopes entirely, while high-mass planets have only very thin envelopes that match the observed radius due to their large core size, although their thinner envelopes are easily lost. large mass. Planet C is currently thought to be rocky and will not change its size significantly. c The current radius of the planet matches well with predicted radii for Earth-like rocky cores in the 3–5 M⊕ mass regime. – astro-ph.HE

Planets orbiting young stars are thought to experience atmospheric evaporation as a result of the high magnetic activity of their host stars. We study the evaporation history and expected future of three known transiting exoplanets in the young multiplanet system K2-198.

Based on spectroscopic and photometric measurements, we estimate the age of the K-dwarf host star to be between 200 and 500 Myr, and calculate the high-energy environment of these planets using eROSITA X-ray measurements. Regardless of the age at which the star exits the saturated X-ray regime, K2-198c’s inner planet likely lost its initial envelope within the first few tens of Myr.

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For the two exoplanets, a range of initial envelope mass fractions is possible, depending on the as-yet-unmeasured planetary mass and spin-down history of the stars. Regarding the future of the system, we find that the outer planet K2-198b is stable against photoevaporation for a wide range of planetary masses, while the middle planet K2-198d can only retain an atmosphere for a mass range between ~7 and . 18 Earth masses. Low-mass planets are more susceptible to mass-loss, and the thin present-day envelope for high-mass planets is easily lost with estimated mass-loss rates.

Our results suggest that all three planets started above the radius valley in the Neptune regime and were later transformed to their current positions by atmospheric evaporation, but emphasize the importance of measuring planetary masses for (young) multiplanet systems. Before conducting more detailed photoevaporation simulations.

Laura Ketcher, Katja Pappenhager, Martina Baratella, Ekaterina Ilin

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); High Energy Astrophysical Events (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)
Citation: arXiv:2311.06897 [astro-ph.EP] (or arXiv:2311.06897v1 [astro-ph.EP] for this version)
Journal reference: Monthly Notices of the Royal Astronomical Society, Volume 527, Issue 1, January 2024, Pages 374-385
Corresponding DOI:
https://doi.org/10.1093/mnras/stad3197
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Submission history
Posted by: Laura Ketcher [view email]
[v1] Sun, 12 Nov 2023 16:51:43 UTC (500 KB)
https://arxiv.org/abs/2311.06897
Astronomy

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