Terrestrial Planet Bombardment & Habitability Jane Greaves St Andrews, Scotland

early bombardment & the Earth  violent formation by merger of planetesimals Moon formation impact at ~50 Myr –"steam atmospheres, magma oceans…" Zahnle et al Space Science Reviews

the late heavy bombardment  early Earth hit by 100-km class impactors –~4 Gyr-old zircons > always some livable surface? by ~0.8 Gyr, impact rate falls off (end of the LHB) Zahnle et al Space Science Reviews

Solar System timeline  100-km* impactors bad for (near-surface) habitability… but life appeared soon after bombardment tailed off ~0.05 Gyr: Earth completed 0.7 Gyr: end heavy bombardment; life 2 Gyr: oxygen catastrophe 4.5 Gyr: NOW ~10 Gyr: Sun-like star becomes red giant 'life like us' * cf. the K-T 'dinosaur' extinction, associated with a 10-km impactor

is Earth's history the norm?  no, probably not! theories of the LHB require migration of gas giants  Doppler wobble studies show ~20% of Sun-like stars host a gas giant at < 20 AU –norm is no giant(s) > comet belt persists for Gyr many have close-in or eccentric giants –an Earth may be ejected, or never form –some gas giants can increase rate of comets dropping onto Earth Horner & Jones 2008

David A Hardy Teterev et al  impacts faster than ~Myr timescales, or of ~100 km size = NOT GOOD

… but some impacts necessary build a terrestrial planet (of enough mass for plate tectonics and so land) icy outer-system comets can contribute (some of the) water for oceans  kick-start biology from chemical complexity, by delivery of organics  driver of evolution: new species dominate after an impact changes the environment

tracing comets around other Suns  destructive collisions between comets create dust showers  this debris absorbs stellar light and re-radiates it in the infrared to millimetre  map out the comet belts & perturbing influence of planets Derek Richardson

 destructive collisions amongst comets create dust showers  this debris absorbs stellar light and re-radiates it in the infrared to millimetre  map out the comet belts & perturbing influence of planets Greaves et al SCUBA image at 850 microns of epsilon Eridani tracing comets around other Suns

 Spitzer has discovered dozens of such systems

Spitzer results  confirm that many comets can exist at late times data: Trilling et al Beichman et al. 2006

 agrees with models with varied initial masses of planetesimals per star, and slow decline of dustiness without clearing out in LHB events Löhne et al. 2008

 agrees with models with varied initial masses of planetesimals per star, and slow decline of dustiness without clearing out in LHB events Booth et al. 2009

 can attempt to infer dustiness of systems below Spitzer's threshold Solar System (if viewed externally) would be ultra faint! Greaves & Wyatt, in prep.

exo-comet populations  bootstrapping the dustiness to the population of comets via a collisional cascade: present-day Sun is depleted compared to most stars

implications  Solar System is unusual  other Earths likely to have different impact histories catastrophe worlds? –gas giants perturb comets inwards; frequent massive impacts static worlds? –comets far from star; rare impacts, little evolution?

how rare are we? more comets distance to innermost gas giant ~10%: gas giant at < 3 AU ~10%: gas giant at 3-20 AU systems like ours?

catastrophe worlds  e.g. HD F8 star ~ 5Gyr old, with a Jupiter at 2 AU, 17 pc from Sun  70 micron excess is >2500 times that of the Sun! giant cold debris disk imaged Liseau et al Solar System to scale

closest analogue system  a close analogue host system for an exo-Earth should have moderate comets and gas giants external to the Habitable Zone? closest such system is HD (G8 V) –a Jupiter at 4 AU –> 2 Gyr old –no debris detected –18 pc from Sun

future outlook  high priorities are to search down to Sun's level of dustiness, and model infall of comets Herschel launch May 14; SCUBA-2 online in 2009!