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Published in Belbruno, Moro-Martín, Malhotra, Savransky (Astrobiology 2012)
Chaotic exchange of solid
material between planetary
systems: implications for
lithopanspermia
Amaya Moro-Martín
Centro de Astrobiología (INTA-CSIC) & Princeton Univ.
Collaborators: Edward Belbruno (Princeton Univ.), Renu Malhotra (Univ. of
Arizona), Dmitry Savransky (Princeton Univ. and Lawrence Livermore National
Laboratory)
Giant planets are common
Approx. 20% of stars harbor giant planets < 20 AU
Planetesimal disks are common
•Protoplanetary disks of gas and dust (100:1 mass ratio) are
present around most stars; they dissipate in ~ 6 Myr.
•But there is evidence of dust around older stars (debris disks).
 The dust is not primordial but it must be generated
by planetesimals
dust lifetime << stellar age
0.01-1 Myr
10 Myr-10,000 Myr
 How common are they?
- A (26%), F (24%), G (19%), K (9.5%), M (1.3%)
(Kennedy in
prep.)
- Also present around white dwarfs (Jura et al. 2006, 2007)
Planetesimal formation takes places
under a wide range of conditions
(Jewitt 2010)
Solar System debris disk
extra-solar debris disk
β-Pictoris
(Schultz, HST)
Giant planets eject planetesimals efficiently
(Raymond, Armitage, Moro-Martin et al. 2011)
• Giant planets are common
• Planetesimal disks are common
• Giant planets eject planetesimals efficiently
The interstellar medium must be filled with planetesimals
Is the exchange of solid material possible
between planetary systems?
Transfer of solid material between single stars in an open star cluster
The Sun was born 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
Cluster properties (Adams 2010)
(similar to Orion’s Trapezium)
- Number of stars: N = 4300 (N=1000-10000)
- Cluster mass: M = <mstar> N = 3784 Msun
- Cluster size: R ~1pc (N/300)0.5 = 3.78 pc
- Average stellar distance: D = n-1/3 = 0.375 pc
- Cluster lifetime: t = 2.3Myr M0.6 = 322.5 Myr
(135-535 Myr for N=1000-10000)
Weak transfer using quasi-parabolic orbits
•Minimum energy; maximizes transfer probability
•Assume both planetary systems harbor a
Jupiter-like planet
planetary system of
destination
weak stability
boundary for
capture (σ = 1 km/s)
•The transfer takes place between two
weak stability boundaries:
star
giant
planet
(relative velocity
between stars)
planetary
fragment
- 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.
•Typical ejection velocity ~ 0.1 km/s
weak stability
boundary for
escape (σ = 0.1
km/s)
giant
planet
star
(ejection
velocity)
•Stars relative velocity ~ 1 km/s (determining capture velocity)
planetary system of
origin
Monte Carlo simulations
(Belbruno, Moro-Martín, Malhotra, Savransky, 2012)
Monte Carlo simulations
(Belbruno, Moro-Martín, Malhotra, Savransky, 2012)
Weak capture probabilities
M* source (Msun)
M* target (Msun)
Capture probab.
1.0
1.0
0.15%
1.0
0.5
0.05%
0.5
1.0
0.12%
Comparison to previous work
•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 ~109 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 ~103 times smaller than with weak transfer
Number of weak transfer events
(between the Sun and its closest cluster neighbor)
Adopt a planetesimal
size distribution
Number of
bodies > 10 kg
Number of
bodies >10 kg
that populated
the WSB
(using an Oort Cloud
(from KBO observations and
(adopting a MMSN) formation efficiency of 1%,
coagulation models)
Brasser et al. 2012).
−q1
dN/dD ∝ D for D > D0
dN/dD ∝ D−q2 for D < D0
Dmax = 2000 km (Pluto)
Dmin = 1 μm (blow-out size)
Number of
bodies >10 kg
may have been
transferred
(using a capture
probability of 0.15%)
Number of weak
transfer events:
O(1014)-O(1016)
Timeline
Birth cluster lifetime,
dispersed over approx.
135–535 million years
star cluster
135
Myr
322
Myr
535
Myr
(Adams 2010)
Heavy bombardment; planetesimal end of
clearing; population of the sun’s WSB LHB
with planetary fragments
solar system
Cooling ofEvidence of
Earth’s liquid water
crust
on Earth’s
Moon
formation
Earth
t=0
44
Myr
solar system
(Kleine et al.
(CAI)
2005)
formation
(4.57 Ga)
surface
70
Myr
164 288
Myr Myr
(Harrison et (Wilde et al.
(Mojzsis et
al. 2005)
al. 2001)
2001).
700
Myr
1st evidence of
microbiological
activity
718 (shortly after end
Myr
end of LHB)
(Mojzsis et
al. 1996)
window of opportunity of
lithopanspermia from Earth
1st
microfossils
1170
Myr
(Wacey et al. 2011)
(Schopf, 1993)
How much material may have been ejected from Earth?
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) ~
~ 1% populated the Oort Cloud (WSB of the Solar System) ~
5‧105 ‧
~ 0.15% may have been transferred to the nearest solar-type stars ~
l(km)
Comparison between transfer and life survival timescales
Size
Max. survival time
0-0.03 m
12-15 Myr
0.03-0.67 m
15-40 Myr
0.67-1 m
40-70 Myr
1-1.67 m
70-200 Myr
1.67-2 m
200-300 Myr
2-2.33 m
300-400 Myr
2.33-2.67
400-500 Myr
Time for ejection
4 Myr min.
50 Myr median.
6 Myr time of flight to Resc
Time for transfer
5 Myr (at 0.1 km/s)
Time for capture by
terrestrial planet
10’s Myr
Valtonen et al. (2009)
Survival of microorganisms could be viable
via meteorites exceeding 1m in size
In a nutshell
•We study the transfer of meteoroids between two
planetary systems embedded in an open star cluster.
•We use chaotic, quasi-parabolic
orbits of minimal energy that
increase greatly the transfer
probability.
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).