The Mass Budget Necessary to Explain Oumuamua as a Nitrogen Iceberg
Amir Siraj · Abraham Loeb
New Astronomy · 2021
Siraj & Loeb (2021) show that explaining 'Oumuamua as a nitrogen iceberg requires converting heavy-element mass exceeding total stellar mass by a factor of ~20 at the central value, ruling the hypothesis out on energetic grounds at 95% confidence.
Brief
Using Pan-STARRS detection statistics (n = 0.1 AU⁻³, 95% Poisson bounds), the inferred 'Oumuamua mass of 2.4×10¹¹ g, and a nitrogen surface fraction of ~0.5% per Pluto-like body, Siraj & Loeb compute the fraction f★ of stellar heavy-element mass that must be converted into exo-Plutos to make the detection probable. The 95% confidence interval spans 0.7 < f★ < 120, with a central value of f★ ≈ 20, meaning the required exo-Pluto mass typically exceeds the total mass locked in stars. Accounting for cosmic-ray erosion (Phan et al. 2021), which demands an initial 'Oumuamua radius of 10 km rather than ~0.1 km, inflates the mass budget by an additional factor of 10⁶. The authors conclude the nitrogen iceberg scenario is physically untenable; a light-sail geometry (mass < 1 mm thick) would reduce the heavy-element shortfall to roughly the mass of a few-kilometer asteroid per star.
Metadata
- Category
- Phenomenon
- Venue
- New Astronomy
- Type
- Peer-reviewed
- Year
- 2021
- Authors
- Amir Siraj, Abraham Loeb
- arXiv
- 2103.14032
- Access
- Open access
- Length
- 294.4 K
- Programs
- Breakthrough Prize Foundation
- Instruments
- Pan-STARRS
- Data sources
- Pan-STARRS survey (Do et al. 2018), Galactic stellar mass estimates (Licquia & Newman 2015), local stellar mass density (Bovy 2017)
- Tags
- interstellar objects, Oumuamua, planetary science, mass budget, nitrogen iceberg, SETI-adjacent
Key points
- Central value of f★ ≈ 20: the mass converted into exo-Plutos must typically exceed total stellar heavy-element mass by a factor of 20.p.2
- 95% Poisson confidence interval on f★ is 0.7 to 120, with even the lower bound corresponding to a mass two orders of magnitude larger than the minimum mass solar nebula converted entirely into exo-Plutos.p.2
- Cosmic-ray erosion (Phan et al. 2021) requires an initial 'Oumuamua radius of 10 km, raising the mass budget by an additional factor of fCR ≈ 10⁶ beyond the fiducial calculation.p.1
- Nitrogen ice comprises only ~0.5% of a Pluto-like body's mass; even a factor-of-100 enhancement in N₂ ice posited by Desch & Jackson (2021) still leaves a shortfall of 10⁴ when cosmic-ray erosion is included.p.2
- The 0.45 Gyr inferred age of 'Oumuamua restricts its source to ~1% of the Galactic stellar population (≈7×10⁸ M☉), raising the required production rate per star by two orders of magnitude.p.1
- A light-sail geometry with thickness < 1 mm reduces the required heavy-element mass to that of a few-kilometer-scale asteroid per star, consistent with known planetary budgets.p.2
Verbatim
“the mass budget in exo-Pluto planets necessary to explain the detection of 'Oumuamua as a nitrogen iceberg chipped off from a planetary surface requires a mass of heavy elements exceeding the total quantity locked in stars with 95% confidence, making the scenario untenable because only a small fraction of the mass in stars ends in exo-Plutos.”
p.1“The nitrogen fragment hypothesis is strongly disfavored, since no known physical process could accomodate such a mass budget.”
p.2
Most interesting
- Even the most conservative dynamical efficiency factor (ϵ = 0.2, twice the value implied by Desch & Jackson 2021) fails to rescue the hypothesis; using perfect efficiency ϵ = 1 'would still present a significant challenge for the N₂ hypothesis.'
- The 95% lower bound on the required exo-Pluto mass (f★ ≈ 0.7) already implies converting a mass two orders of magnitude larger than the entire minimum mass solar nebula exclusively into nitrogen-rich Pluto analogs.
- Cosmic-ray erosion alone inflates the required mass budget by six orders of magnitude, dwarfing every other uncertainty in the calculation.
- The light-sail alternative (Bialy & Loeb 2018) is the only 'Oumuamua model that brings the implied heavy-element mass into a physically plausible range, roughly a few-kilometer asteroid worth of material per star.
- This paper was supported in part by a grant from the Breakthrough Prize Foundation, the same philanthropic entity that funds Breakthrough Listen SETI searches.
- No new observational data were generated; the entire refutation is derived analytically from previously published Pan-STARRS survey parameters and Pluto compositional measurements.