Discovery Of An Icy And Nitrogen-rich Extrasolar Planetesimal

White dwarfs accreting planetary debris provide detailed insight into the bulk composition of rocky exo-planetesimals. However, only one Kuiper-Belt analogue has been identified in that way so far.

Here, we report the accretion of an icy extra-solar planetesimal onto white dwarf WD 1647+375 using ultraviolet spectroscopy from the Hubble Space Telescope. The accreted material is rich in the volatiles carbon, nitrogen, and sulphur, with a chemical composition analogous to Kuiper-belt objects (KBOs) in our solar system.

It has a high nitrogen mass fraction (5.1±1.6 per cent) and large oxygen excess (84±7 per cent), indicating that the accreted planetesimal is water-rich (a water-to-rock ratio of ≃2.45), corroborating a cometary- or dwarf planet-like composition.

The white dwarf has been accreting at a rate of ≈2×10⁸ g s⁻¹ for the past 13 years, implying a minimum mass of ∼10¹⁷ g for the icy parent body. The actual mass could be several orders of magnitude larger if the accretion phase lasts ∼10⁵ yr as estimated in the literature from debris disc studies.

We argue that the accreted body is most likely a fragment of a KBO dwarf planet based on its nitrogen-rich composition. However, based on the chemical composition alone, it is difficult to discern whether this icy body is intrinsic to this planetary system, or may have an interstellar origin.

Snehalata Sahu, Boris T. Gänsicke, Jamie T. Williams, Detlev G. Koester, Jay Farihi, Steven J. Desch, Nicola Pietro Gentile Fusillo, Dimitri Veras, Sean N. Raymond, Maria Teresa Belmonte

Comments: Published in MNRAS on 18th September 2025, see this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2509.13422 [astro-ph.EP] (or arXiv:2509.13422v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.13422
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Related DOI:
https://doi.org/10.1093/mnras/staf1424
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Submission history
From: Snehalata Sahu
[v1] Tue, 16 Sep 2025 18:01:17 UTC (950 KB)
https://arxiv.org/abs/2509.13422

Astrobiology,