So close, so different: characterization of the K2-36 planetary system with HARPS-N
Bibliographic record
Abstract
Context. K2-36 is a K dwarf orbited by two small (Rb = 1.43 ± 0.08 R⊕ and Rc = 3.2 ± 0.3 R⊕), close-in (ab = 0.022 au and ac = 0.054 au) transiting planets discovered by the Kepler/K2 space observatory. They are representatives of two distinct families of small planets (Rp < 4 R⊕) recently emerged from the analysis of Kepler data, with likely a different structure, composition and evolutionary pathways. Aims. We revise the fundamental stellar parameters and the sizes of the planets, and provide the first measurement of their masses and bulk densities, which we use to infer their structure and composition. Methods. We observed K2-36 with the HARPS-N spectrograph over ~3.5 yr, collecting 81 useful radial velocity measurements. The star is active, with evidence for increasing levels of magnetic activity during the observing time span. The radial velocity scatter is ~17 m s−1 due to the stellar activity contribution, which is much larger that the semi-amplitudes of the planetary signals. We tested different methods for mitigating the stellar activity contribution to the radial velocity time variations and measuring the planet masses with good precision. Results. We find that K2-36 is likely a ~1 Gyr old system, and by treating the stellar activity through a Gaussian process regression, we measured the planet masses mb = 3.9 ± 1.1 M⊕ and mc = 7.8 ± 2.3 M⊕. The derived planet bulk densities ρb = 7.2−2.1+2.5 g cm−3 and ρc = 1.3−0.5+0.7 g cm−3 point out that K2-36 b has a rocky, Earth-like composition, and K2-36 c is a low-density sub-Neptune. Conclusions. Composed of two planets with similar orbital separations but different densities, K2-36 represents an optimal laboratory for testing the role of the atmospheric escape in driving the evolution of close-in, low-mass planets after ~1 Gyr from their formation. Due to their similarities, we performed a preliminary comparative analysis between the systems K2-36 and Kepler-36, which we deem worthy of a more detailed investigation.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".