[Paper Review] Interstellar planetesimals

Amaya Moro‐Martín|arXiv (Cornell University)|May 9, 2022

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper proposes that interstellar planetesimals—bodies ejected during planetary system formation—serve as pristine relics of planet formation, with 1I/ʻOumuamua and 2I/Borisov providing observational evidence of their existence. The study argues these objects, preserved since ejection, offer unprecedented insights into planetary system formation and could be captured in star and planet-forming regions, overcoming the meter-size barrier in planetesimal growth.

ABSTRACT

During the formation of our solar system, a large number of planetesimals were ejected into interstellar space by gravitational encounters with the planets. Debris disks observations and numerical simulations indicate that many other planetary systems, now known to be quite common, would have undergone a similar dynamical clearing process. It is therefore expected that the galaxy should be teeming with expelled planetesimals, largely unaltered since their ejection. This is why astronomers were perplexed that none had been detected passing through the solar system. Then, in 2017, the discovery of1I/'Oumuamua transformed the situation from puzzlement to bewilderment. Its brief visit and limited observations left important questions about its nature and origin unanswered and raised the possibility that 1I/'Oumuamua could be a never-seen-before intermediate product of planet formation. If so, this could open a new observational window to study the primordial building blocks of planets, setting unprecedented constraints on planet formation models. Two years later 2I/Borisov was discovered, with an unquestionable cometary composition, confirming that a population of icy interstellar planetesimals exists. These objects have remained largely unchanged since their ejection, like time capsules of their planetary system most distant past. Interstellar planetesimals could potentially be trapped into star and planet formation environments, acting as seeds for planet formation, helping overcome the meter-size barrier that challenges the growth of cm-sized pebbles into km-sized objects. Interstellar planetesimals play a pivotal role in our understanding of planetary system formation and evolution and point to the possibility that one day, we will be able to hold a fragment from another world in our hand.

Motivation & Objective

• To understand the origin and distribution of interstellar planetesimals as remnants of planetary system formation.
• To resolve the paradox of why interstellar objects were undetected before ʻOumuamua's discovery.
• To explore the role of interstellar planetesimals in seeding planet formation by overcoming the meter-size barrier.
• To assess the feasibility of detecting and studying these objects via telescopes, meteor observations, and space missions.
• To determine the physical and dynamical evolution of interstellar planetesimals during their galactic journey.

Proposed method

• Analyzing observational data from 1I/ʻOumuamua and 2I/Borisov to infer physical properties and composition.
• Using numerical simulations to model gravitational ejection of planetesimals during planetary system dynamical clearing.
• Evaluating the impact of galactic processes—such as stellar flybys, molecular clouds, and YORP effects—on interstellar object lifetimes.
• Assessing the detectability of interstellar objects via ground-based surveys, space telescopes (e.g., JWST), and all-sky camera networks.
• Proposing fly-by missions to study interstellar objects up close, with mission design considerations for high-velocity targets.
• Modeling the flux of interstellar meteors and micrometeorites to compare with Earth-based observations and constrain size distributions.

Experimental results

Research questions

• RQ1 What is the origin and physical nature of 1I/ʻOumuamua, and how does it differ from known solar system bodies?
• RQ2 How do interstellar planetesimals survive ejection and transit through the galaxy, and what processes alter their properties?
• RQ3 What is the expected size and velocity distribution of interstellar objects, and how can they be detected before they pass through the inner solar system?
• RQ4 Can interstellar planetesimals be captured in star and planet-forming regions to seed planet formation?
• RQ5 What is the likelihood of detecting interstellar meteorites or micrometeorites on Earth, and how do observed fluxes compare to predictions?

Key findings

• 1I/ʻOumuamua, with a radius of 55–130 m and axis ratio >6:1, exhibits extreme elongation and a red color consistent with space-weathered, iron-rich surfaces.
• 2I/Borisov confirmed the existence of icy interstellar planetesimals with cometary composition, validating theoretical predictions of ejected planetesimals.
• Spitzer observations set a 3σ upper limit on ʻOumuamua’s thermal emission, constraining its radius to <70 m for albedo >0.1.
• The observed flux of meteorites and micrometeorites on Earth exceeds predictions by many orders of magnitude, making it unlikely that interstellar meteorites are already in sample collections.
• Proposed all-sky camera networks could detect mm-sized interstellar meteors with asymptotic speeds ~42 km/s, distinguishable from typical meteors by their high velocity.
• Fly-by missions to interstellar objects are feasible with scaled versions of existing technology, though high-velocity targets require careful trajectory planning and long storage orbits, as in the Comet Interceptor mission concept.