The New Astronomical Frontier of Interstellar Objects

Brief

This invited review synthesizes observational and theoretical work on interstellar objects (ISOs), centering on 'Oumuamua (2017), Borisov (2019), and the tentative interstellar meteor CNEOS 2014-01-08. The nitrogen iceberg hypothesis for 'Oumuamua is quantitatively ruled out: matching the inferred ISO number density (n = 0.1 AU⁻³) would require a heavy-element mass budget exceeding 10⁶ × the total quantity locked in stars at 95% confidence. A Monte Carlo simulation combining 'Oumuamua-like, Borisov-like, and rocky rogue planet populations yields a Minimum Ejection Fraction of ~0.1 M☉ per star (95% CI: 0.02–0.5 M☉), an order of magnitude above the minimum-mass solar nebula, implying ejected mass exceeds retained mass. The review additionally surveys stellar occultation detection strategies, interstellar meteor signatures, lunar and neutron-star impact channels, panspermia transport estimates (1–50 life-export events via ISOs over Earth's lifetime), and near-term prospects from the Vera C. Rubin Observatory's LSST.

Metadata

Category

Search

Venue

Astrobiology

Type

Peer-reviewed

Year

2022

Authors

Amir Siraj, Abraham Loeb

DOI

10.1089/ast.2021.0189

arXiv

2111.05516

Access

Open access

Length

2.0 M

Programs

Galileo Project, Legacy Survey of Space and Time (LSST)

Instruments

Vera C. Rubin Observatory, Desert Fireball Network

Data sources

CNEOS

Tags

interstellar objects, astrobiology, panspermia, planet formation, SETI, Oort cloud, biosignature

Key points

• The nitrogen iceberg hypothesis for 'Oumuamua requires a heavy-element mass budget exceeding 10⁶ × the total quantity locked in stars at 95% confidence, making it untenable.p.2
• Monte Carlo simulation yields a Minimum Ejection Fraction (MEF) central value of ~0.1 M☉ per star (95% CI: 0.02–0.5 M☉), requiring stars to eject 2–50% of their mass as ≥0.1 km planetesimals.p.5
• At the scale-free size distribution (q = 3), ~1% of galactic carbon and oxygen is locked in ISOs, comparable to the metals budget of the minimum-mass solar nebula, serving as a lower bound on the ISO mass fraction.p.4
• ISOs outnumber bound Oort Cloud objects per star at R~10⁵ AU, but gravitational focusing leaves OC objects ~10× more abundant in the inner solar system.p.3
• An all-sky network of 1-m telescopes monitoring R ≤ 12 stars at 0.1 s cadence would detect ~1 ISO per year via stellar occultation, versus the current rate of roughly one every few years.p.10
• Panspermia channel estimates: 1–10 life-export events via long-period comets and 1–50 via ISOs over Earth's lifetime; up to ~10⁵ events if life existed above 100 km altitude.p.7
• A temporary equal-mass binary companion to the Sun at ~10³ AU separation in its birth cluster could explain both Oort Cloud formation and the capture of Planet Nine, with the binary model favored by a factor of ~10 if Planet Nine is discovered with a captured origin.p.7
• CNEOS 2014-01-08 is tentatively the first interstellar meteor larger than dust detected in the solar system, predating 'Oumuamua's 2017 confirmation.p.10

Verbatim

• “The possibility of an artificial origin should be left on the table since all natural explanations for 'Oumuamua, as of yet, invoke objects never seen before (Loeb, 2021).”

  p.2

• “the most conservative power-law fit of q = 3, representing a scale-free distribution, implies that nearly ∼ 1% of the carbon and oxygen contained in stars and the ISM is locked in ISOs.”

  p.4

• “the amount of mass needed to form ISOs larger than 'Oumuamua is a substantial fraction of host star mass, between 2% and 50%.”

  p.5

• “these results imply that the ejected mass from stars exceeds the retained mass.”

  p.5

• “Life in the Earth's atmosphere has been detected up to an altitude of 77 km (Imshenetsky et al., 1978), constituting a reservoir of microbes that objects grazing the atmosphere could draw from.”

  p.7

• “An all-sky network of 1-m telescopes continuously monitoring all R = 12 stars with a time resolutions of 0 . 1 s should yield a discovery rate of ∼ 1 ISO per year.”

  p.10

Most interesting

• The nitrogen iceberg hypothesis requires converting more than one million times the total stellar heavy-element budget into exo-Pluto fragments, a mass budget violation of six orders of magnitude.
• Stars must eject more mass as planetesimals than they retain: a MEF central value of ~0.1 M☉ per star means the material flung into interstellar space outweighs what ends up in planets and disks.
• A 30-cm Apollo-type asteroid was detected by the Desert Fireball Network in 2020, grazing Earth's atmosphere at 58.5 km altitude during a 90-second event before being scattered into a Jupiter-crossing orbit, a potential panspermia carrier class not previously catalogued.
• Life has been confirmed in Earth's atmosphere at up to 77 km altitude, creating a theoretical microbial reservoir that ISO atmospheric grazers could sample and export to other stellar systems.
• The Galileo Project has a dedicated branch specifically targeting ISO studies, including new searches and follow-up observation design for objects like 'Oumuamua.
• 10⁷–10⁹ solar system bodies may have been gravitationally captured by exoplanetary systems over the solar system's lifetime, with 10–1,000 of those potentially carrying viable microbes at the moment of capture.