Amaya Moro-Martín Centro de Astrobiología (INTA-CSIC) & Princeton Univ. Chaotic exchange of solid material between planetary systems: implications for lithopanspermia Collaborators: Edward Belbruno (Princeton Univ.), Renu Malhotra (Univ. of Arizona), Dmitry Savransky (Princeton Univ. and Lawrence Livermore National Laboratory) Published in Belbruno, Moro-Martín, Malhotra, Savransky (Astrobiology 2012)

Approx. 20% of stars harbor giant planets < 20 AU Giant planets are common

✴ How common are they? - Also present around white dwarfs (Jura et al. 2006, 2007) - A (26%), F (24%), G (19%), K (9.5%), M (1.3%) (Kennedy in prep.) Planetesimal disks are common Myr 10 Myr-10,000 Myr dust lifetime << stellar age ✴ The dust is not primordial but it must be generated by planetesimals Planetesimal formation takes places under a wide range of conditions But there is evidence of dust around older stars (debris disks). Protoplanetary disks of gas and dust (100:1 mass ratio) are present around most stars; they dissipate in ~ 6 Myr.

(Jewitt 2010)

Solar System debris disk

extra-solar debris disk β-Pictoris (Schultz, HST)

(Raymond, Armitage, Moro-Martin et al. 2011) Giant planets eject planetesimals efficiently

Is the exchange of solid material possible between planetary systems? The interstellar medium must be filled with planetesimals Giant planets are common Planetesimal disks are common Giant planets eject planetesimals efficiently

Transfer of solid material between single stars in an open star cluster Solar System properties that depend on birth environment: - evidence of short-lived radionuclides in meteorites - dynamical properties of outer planets and Kuiper Belt The Sun was born in an open star cluster - Number of stars: N = 4300 ( N= ) - Cluster mass: M = N = 3784 M sun - Cluster size: R ~1pc (N/300) 0.5 = 3.78 pc - Average stellar distance: D = n -1/3 = pc - Cluster lifetime: t = 2.3Myr M 0.6 = Myr ( Myr for N= ) (similar to Orion’s Trapezium) Cluster properties (Adams 2010)

Weak transfer using quasi-parabolic orbits - Region where the particle is tenuously and temporarily captured. - Created by the gravitational fields of the central star, the giant planet and the rest stars in the cluster. - The particle slowly meanders between both planetary systems. The transfer takes place between two weak stability boundaries: planetary fragment weak stability boundary for capture (σ = 1 km/s) weak stability boundary for escape (σ = 0.1 km/s) Stars relative velocity ~ 1 km/s (determining capture velocity) (relative velocity between stars) star giant planet planetary system of destination planetary system of origin star Assume both planetary systems harbor a Jupiter-like planet (ejection velocity) Typical ejection velocity ~ 0.1 km/s Minimum energy; maximizes transfer probability

(Belbruno, Moro-Martín, Malhotra, Savransky, 2012) Monte Carlo simulations

(Belbruno, Moro-Martín, Malhotra, Savransky, 2012)

M * source (M sun )M * target (M sun )Capture probab % % % Weak capture probabilities Melosh (2003): - transfer between single stars in the solar local neighborhood ( after cluster dispersal ) ( ours: before cluster disperses ) - stars velocitiy dispersion: 20 km/s ( ours: 1 km/s ) - hyperbolic trajectories with median ejection speed of 5 km/s ( ours: 0.1 km/s ) - capture probability ~10 9 times smaller than with weak transfer Adams & Spergel (2005) - transfer between binary stars in an open cluster (ours: single stars like the Sun) - hyperbolic trajectories with median ejection speed of 5 km/s (ours: 0.1 km/s) - capture probability ~10 3 times smaller than with weak transfer Comparison to previous work

(between the Sun and its closest cluster neighbor) Number of weak transfer events (from KBO observations and coagulation models) D max = 2000 km (Pluto) D min = 1 μm (blow-out size) dN/dD ∝ D −q1 for D > D 0 dN/dD ∝ D− q2 for D < D 0 Adopt a planetesimal size distribution (adopting a MMSN) Number of bodies > 10 kg (using an Oort Cloud formation efficiency of 1%, Brasser et al. 2012). Number of bodies >10 kg that populated the WSB (using a capture probability of 0.15%) Number of bodies >10 kg may have been transferred Number of weak transfer events: O(10 14 )-O(10 16 )

Timeline window of opportunity of lithopanspermia from Earth Birth cluster lifetime, dispersed over approx. 135–535 million years star cluster 135 Myr 535 Myr (Adams 2010) 322 Myr Moon formation 44 Myr Cooling of Earth’s crust 70 Myr 1st microfossils 1170 Myr t = 0 solar system (CAI) formation 718 Myr Earth (4.57 Ga) (Kleine et al. 2005) (Mojzsis et al. 1996) (Wacey et al. 2011) (Harrison et al. 2005) (Schopf, 1993) (shortly after end end of LHB) Evidence of liquid water on Earth’s surface Myr (Wilde et al. 2001). (Mojzsis et al. 2001) 1st evidence of microbiological activity solar system 700 Myr end of LHB Heavy bombardment; planetesimal clearing; population of the sun’s WSB with planetary fragments

Assuming l (km) of the Earth surface was ejected, this correspond to a mass of... adopting a power-law size distribution, the number of bodies > 10 kg is ~ 1% remained weakly shocked (allowing microorganisms to survive) ~ How much material may have been ejected from Earth? ~ 1% populated the Oort Cloud (WSB of the Solar System) ~ 5 ‧ 10 5 ‧ l(km) ~ 0.15% may have been transferred to the nearest solar-type stars ~

Time for ejection 4 Myr min. 50 Myr median. 6 Myr time of flight to R esc Time for transfer 5 Myr (at 0.1 km/s) Time for capture by terrestrial planet 10’s Myr Comparison between transfer and life survival timescales SizeMax. survival time m12-15 Myr m15-40 Myr m40-70 Myr m Myr m Myr m Myr Myr Valtonen et al. (2009) Survival of microorganisms could be viable via meteorites exceeding 1m in size

In a nutshell We use chaotic, quasi-parabolic orbits of minimal energy that increase greatly the transfer probability. We study the transfer of meteoroids between two planetary systems embedded in an open star cluster. Orion’s Trapezium cluster (2.2 μm) We find that significant quantities of solid material are exchanged. If life on Earth had an early start (arising shortly after liquid water was available on the surface), life could have been transferred to other systems. And vice versa, if life had a sufficiently early start in other planetary systems, it could have seeded the Earth (and may have survived the LHB).