On the Possibility of an Artificial Origin for Oumuamua

Brief

Published in Astrobiology 22(12):1392-1399 (2022), Harvard astronomer Abraham Loeb catalogs six anomalies of 'Oumuamua, detected by Pan-STARRS on October 19, 2017, and systematically rejects the four leading natural-origin models: porous dust aggregate, tidal-disruption fragment, hydrogen iceberg, and nitrogen iceberg. Each model is faulted on quantitative grounds: the hydrogen iceberg evaporates before completing interstellar transit, the nitrogen iceberg requires an implausible production rate, and the tidal fragment predicts a cigar shape rather than the disk inferred at ~90% confidence. The excess non-gravitational acceleration, which followed a smooth inverse-square law with no cometary jets or spin changes and was ruled out as outgassing by Spitzer's infrared non-detection, is argued to be most consistent with radiation pressure on a sub-millimeter-thick object. Loeb invokes the Galileo Project (announced July 26, 2021, with more than 100 scientists) as the mechanism to collect decisive data on the next such object.

Metadata

Category

Phenomenon

Venue

Astrobiology

Type

Peer-reviewed

Year

2022

Authors

Abraham Loeb

DOI

10.1089/ast.2021.0193

arXiv

2110.15213

Access

Open access

Length

852.3 K

Programs

Galileo Project

Instruments

Pan-STARRS, Spitzer Space Telescope, Vera Rubin Observatory, James Webb Space Telescope

Data sources

Micheli et al. 2018 astrometry, Trilling et al. 2018 Spitzer infrared photometry, Mashchenko 2019 light curve analysis

Tags

interstellar objects, SETI, technosignature, lightsail hypothesis, astrobiology, anomalous phenomena

Key points

• 'Oumuamua was at rest in the Local Standard of Rest frame, a kinematic state shared by only 1 in 500 stars, and fewer than 0.2% of all stars share its specific kinematic origin there, ruling out a nearby stellar parent.p.2
• Brightness varied by a factor of 10 (2.5 magnitudes) over an 8-hour rotation period; Mashchenko (2019) found the shape disk-like at approximately 90% confidence, contradicting the cigar depiction widely circulated in media.p.2
• Spitzer Space Telescope detected no carbon-based molecules or dust around 'Oumuamua, ruling out substantial cometary outgassing and constraining maximum size to approximately 200 meters.p.2
• Excess acceleration would have required ~10% mass loss via cometary evaporation (Micheli et al. 2018), which Spitzer absolutely ruled out; the repulsive force followed a smooth inverse-square law with no spin changes or localized jets.p.3
• All four natural-origin models require objects never previously observed, each with 'major quantitative shortcomings': porous aggregate 100x less dense than air, tidal fragment with wrong shape, hydrogen iceberg destroyed in transit, nitrogen iceberg with insufficient production rate.p.4
• 2020 SO, a thin-walled NASA rocket shell from 1966, was independently rediscovered by Pan-STARRS displaying the same two anomalies as 'Oumuamua (no cometary tail, sunlight-driven push), providing an existence proof that thin artificial shells produce exactly these dynamics.p.3
• For radiation pressure to explain the excess force, the object must be thinner than 1 mm, an area-to-mass ratio Loeb argues is inconsistent with any known natural formation process.p.3
• The Galileo Project, announced July 26, 2021, comprises more than 100 scientists and plans a multi-observatory ground network plus a space mission to intercept future 'Oumuamua-like objects flagged by Vera Rubin Observatory alerts.p.4

Verbatim

• “Each of these natural-origin models has major quantitative shortcomings, and so the possibility of an artificial origin for `Oumuamua must be considered.”

  p.1

• “only 1 in 500 stars is so much at rest as `Oumuamua was in that frame, before the Sun 's gravitational force deflected its trajectory”

  p.2

• “since fewer than 0.2% of all stars share `Oumuamua's kinematic origin at the Local Standard of Rest, it was unlikely to originate from a nearby star system.”

  p.2

• “Since the push away from the Sun was consistent with a smooth inverse-square law, I reasoned that it may result from the reflection of sunlight from a thin object (Bialy & Loeb, 2018).”

  p.3

• “For the reflection of sunlight to exert a strong enough force, the object had to be thinner than a millimeter, like a light sail. Since nature does not make thin objects, I suggested that it might be artificial in origin (Loeb, 2018a; 2018b; 2018c).”

  p.3

• “the nature of celestial objects must be found through our telescopes rather than philosophical prejudice.”

  p.6

Most interesting

• A 2009 paper co-authored by Loeb himself (Moro-Martin, Turner & Loeb 2009) had forecast that Pan-STARRS would detect zero interstellar rocks, 'Oumuamua's arrival falsified his own prior prediction.
• 2020 SO, a thin-walled shell from a 1966 NASA lunar mission, was rediscovered by Pan-STARRS in 2020 exhibiting the identical pair of anomalies as 'Oumuamua, no cometary tail and solar-radiation-driven excess push, demonstrating that thin artificial objects produce this exact observational signature.
• The best-fit disk/pancake shape is physically more consistent with a lightsail than a cigar; even a razor-thin flat sheet would project apparent width at random sky orientations, meaning the true aspect ratio could be far smaller than the 1:10 inferred from the light curve.
• The hydrogen iceberg model requires 'Oumuamua to have originated from molecular clouds roughly 5.2 kpc distant; Hoang & Loeb (2020) show destruction timescales over that distance make survival implausible, and no giant molecular cloud exists at the requisite proximity.
• The nitrogen iceberg hypothesis (Desch & Jackson 2021) requires a mechanism chipping N2 ice from Pluto-like exoplanets that, per three independent analyses (Levine et al. 2021, Siraj & Loeb 2021, Phan et al. 2021), cannot generate enough material to account for the inferred abundance of 'Oumuamua-like objects.
• Kepler data (Bryson et al. 2021) shows half of Sun-like stars host an Earth-sized planet at roughly Earth's orbital separation; Loeb uses this to note that technological civilizations could have been launching interstellar probes billions of years before the Sun formed.