Lithopanspermia in Star-Forming Clusters

Abstract

This paper considers the lithopanspermia hypothesis in star forming groups and clusters, where the chances of biological material spreading from one solar system to another is greatly enhanced (relative to action in the field) due to the close proximity of the systems and lower relative velocities. These effects more than compensate for the reduced time spent in such crowded environments. This paper uses ∼ 300, 000 Monte Carlo scattering calculations to determine the cross sections σ cap for rocks to be captured by binaries and provides fitting formulae for other applications. We assess the odds of transfer as a function of the ejection speed v eject and number N ⋆ of members in the birth aggregate. The odds of any given ejected meteroid being recaptured by another solar system are relatively low, about 1 in 10 3 − 10 6 over the expected range of ejection speeds and cluster sizes. Because the number of ejected rocks (with mass m > 10 kg) per system can be large, N R ∼ 10 16 , virtually all solar systems are likely to share rocky ejecta with all of the other solar systems in their birth cluster. The number of ejected rocks that carry living microorganisms is much smaller and less certain, but we estimate that N B ∼ 10 7 rocks can be ejected from a biologically active solar system. For typical birth environments, the capture of life bearing rocks is expected to occur N bio ≈ 10 -16,000 times per cluster (under favorable conditions), depending on the ejection speeds. Only a small fraction (f imp ∼ 10 −4 ) of the captured rocks impact the surfaces of terrestrial planets, so that N lps ≈ 10 −3 − 1.6 lithopanspermia events are expected (per cluster). Finally, we discuss the question of internal vs external seeding of clusters and the possibility of Earth seeding young clusters over its biologically active lifetime.

Planet seeding through gas-assisted capture of interstellar objects

“…These ejections occur both during the early planet formation phase, or on longer timescales throughout the stellar and dynamical evolution of the system, long after the PPD dissipates. Adams & Spergel (2005) estimate that for each young star at least M⊕ of solids are ejected into the interstellar medium (ISM) with a typical ejection velocity of veject = 6.2 ± 2.7 km/s. They consider a mass function of dNeject/dm ∝ m −p , whence the total number of ejected planetesimals up to mass m is (Adams & Spergel 2005)…”

Requirements and limits for life in the context of exoplanets

“…Napier (56) has proposed that life could be carried on dust between stars (see also ref. 57), and others have suggested rocks could travel between star systems (58,59). If such dust grains or rocks were incorporated into the preplanetary nebula, then every planet and moon that formed would be infected with life.…”

The Interplanetary Exchange of Photosynthesis

“…The probability of a terrestrial rock ejected from our Solar System colliding with a terrestrial-type extrasolar planet is 10 −4 in a total period of 4.5 Gyr, making transfer a low probability event. Similarly, Adams and Spergel (2005) considered the transfer of rocks between stellar systems in star clusters where star densities are much higher than our local galactic neighbourhood. They arrive at similarly low probabilities.…”