The massive nature of WD 1856 b indicates that transiting rocky planets around white dwarfs are rare.

WD 1856 b


White dwarfs (WDs) have radii roughly the size of Earth—which has long been recognized to facilitate the discovery of sub-Earth-sized planets via transits.

Despite this, the first (and still only) transiting planet discovered in 2020 was a roughly Jupiter-sized world, detected using TESS photometry. This is somewhat surprising given the relative paucity of giant planets compared to terrestrial ones represented by both exoplanet demographics and theoretical simulations (“bottom-heavy” radius distributions).

Here, we account for the surprise of this fact by considering the geometry bias and detection bias of 1) the bottom-heavy Kepler derived radius distribution and 2) the top-heavy radial velocity inspired radius distribution. Both are related, the latter being more unusual for rocky planets and the former being a more surprising event at WD 1856 b < 0.5%.

Using an HBM, we infer the conditional implicit power-law radius distribution of WD 1856 b, and arrive at a less common order-of-magnitude distribution of 0.1–2 REarth planets than 2–20 REarth planets. The time range is 0.1-10 days. The implicit hypothesis is that WDs rarely transit rocky planets. We discuss ways to reconcile this with other evidence for small bodies around WDs, and ultimately argue that it should be easily tested.

Three models for the radius distribution of WD exoplanets (each column) and the resulting detection around a WD 1856-binary star with the TESS data field. The bottom-heavy Kepler-inspired model (left) produces a yield map (top row) that shifts WD 1856 b outward. In contrast, the top-heavy model (middle) naturally explains WD 1856 b, but predicts very few terrestrial planets. The right column shows a simple power-law assumption using the HBM, which favors a high-weight distribution. The middle row shows the marginal radius dimension of the top row, and the bottom row shows the intrinsic distributions drawn from them. – astro-ph.EP

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David Kipping

Comments: Accepted in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Citation: arXiv:2310.15219 [astro-ph.EP] (or arXiv:2310.15219v1 [astro-ph.EP] for this version)
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Submission history
Posted by: David Kipping
[v1] Monday, 23 October 2023 18:00:00 UTC (952 KB)

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