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• Where to search for life? • Exoplanets • Formation of planetary systems • Seach for intelligence

Life in the Universe. • Where to search for life? • Exoplanets • Formation of planetary systems • Seach for intelligence • Are we visited?. Where to search for life?.

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• Where to search for life? • Exoplanets • Formation of planetary systems • Seach for intelligence

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  1. Life in the Universe • Where to search for life? •Exoplanets • Formation of planetary systems • Seach for intelligence • Are we visited?

  2. Where to search for life? In our solar system: there is a low probability that we can still find some form of primitive life on other planets or moons • Mars (in the past?) • Titan, Europa,…??? Search for evolved life forms → go further away What to look for? Only known life forms: on our Earth Possibility of exotic life forms (science fiction) but we wouldn’t know what to search for, and how… → search for life forms similar to ours → on planets: • with solid crust • with liquid water (excellent solvent)

  3. Caution ! The adopted point of view in this table (last lines) is very anthropomorphic ! Where to search for life? - 2 Main phases of life on Earth t (GYr)hEvent −4.6 0h Formation of Earth −4.5/−4.0 1h/3h Formation of oceans −4.0? 3h? First unicellular organisms −1.3 17h First multicellular plants −0.6 21h Cambrian explosion (1st animals) −0.4 22h Life gets out of oceans −0.1 23h30m First mammals −0.0005 23h59m50s Homo Sapiens (−100 ans 23h59m59s.998 Invention of radio)

  4. Star Habitable zone at the beginning of star’s life Habitable zone at the end of star’s life Continuously habitable zone Where to search for life? - 3 Habitable planets Habitable zone (HZ) =zone around the star where liquid water can be found L* increases during the main sequence phase → the habitable zone moves Ideal location: in the continuously habitable zone (CHZ) Complication by possible greenhouse effect → depends on the planet’s atmosphere

  5. Where to search for life? - 4 Around which stars? • O, B, A, F stars: life too short < 3 GYr • M stars: very long life but low luminosity stars → (1) HZ very narrow and no CHZ (but 200 GYr not necessary) (2) HZ very close to the star → synchroneous rotation → deadly radiation from stellar corona? • G stars: good compromise • K stars: maybe same problems as M stars Non binary main sequence G stars are privileged targets, the case of K and M stars in open and under deep investigation → ~ 10 to 90% of stars in our Galaxy

  6. Exoplanets Exoplanet = extrasolar planet = planet orbiting a star that is not the Sun • Imagine a planetary system like ours around α Cen D (α Cen) = 4.2 LY = 260 000 AU d (Jupiter – Sun) = 5.2 AU θ = ang. Dist. = 5.2/260000 rad = 4″ Luminosity LP/L* ~ 10−9 • Other stars: further away → even tougher problem ex: ε Eri: D = 10.5 LY d (planet – star) = 3.2 AU θ = 3.2/650000 rad = 1″ → direct detection generally out of reach of present-day instruments → detection by indirect methods

  7. Exoplanets - 2 First discoveries • 1992: discovery of 2 planets around the pulsarPSR B1257+12 by Aleksander Wolszczan M = 4.3 & 2.8 MEd = 0.36 & 0.47 AU • 1995: discovery of the first exoplanet orbiting a `normal´ star by Michel Mayor and Didier Queloz 51 Peg: G2IV D = 48 LY M = 1.05 M 51 Peg b: M > 150 MEd = 0.05 AU T = 4 days

  8. Exoplanets - 3 Detection methods: direct imaging Only in peculiar cases and with the best instruments available (space, adaptive optics…) → nearby low luminosity stars massive planets wide orbits Examples: • 2M1207, brown dwarf at 70 pc 5 MJup planet at 55 AU • AB Pic, K2V at 46 pc 13 MJup planet at 275 AU Brown dwarf 2M1207 and its planet (ESO)

  9. V C a m A M v Exoplanets - 4 Orbital motion generalized 3rd Kepler law: (obtained by gravitational force = centripetal force for M and m) Velocity of star:

  10. V C r m R M v Exoplanets - 5 Detection methods: radial velocities M >> m & a >> A → Kepler: i = angle between orbital plane and sky V in km/s T in years m in MJupM in M → more sensitive to large planet masses and short periods

  11. Exoplanets - 6 Detection methods: gravitational microlensing Amplification of a background star by a star crossing the line of sight (deflection of light with pseudo-focussing) If a star and its planet cross the line of sight: → secondary maximum in the light curve Detection of low mass planets (ex: 5.5 ME) but no further check possible Low probability events → necessary to observe a large number of sources

  12. Exoplanets - 7 Detection methods: transits If the planet passes in front of its star → partial eclipse Apparent luminosity drop: ΔL/L~ (RP/R*)2 → requires high precision + favors large planets orbiting small stars Prob(transit) ~ R*/a + necessary to observe several transits → favors short periods Low probability events → need to observe a large number of sources

  13. Exoplanets - 8 Detected exoplanets October 2017: ~3700 planets discovered ~2700 planetary systems Source: www.exoplanet.eu

  14. Exoplanets - 9 Hot Jupiters The first exoplanets discovered were very massive planets orbiting close to their stars → they have been called Hot Jupiters (M > ~MJup, d < 0.05 AU) Their discovery came as a surprise and forced astronomers to reconsider their planetary systems formation theories However, these planets were the easiest to detect: • large Vrad, short period • deep and frequent transits → observational bias

  15. Formation of planetary systems Contraction of protostellar nebula → star at the center, surrounded by a disk of gas and dust Collisions between dust grains → aggregates → size increases and may reach a few km: planetesimals Gravitation starts to play a role → even more collisions with: • fusion and size increase • or destruction of aggregates • eccentric orbits → even more collisions

  16. Formation of planetary systems - 2 Protoplanets • The most massive planetesimals tend to grow further by capturing bodies on similar orbits • Size ~ 1000 km → protoplanets • The most massive can be surrounded by a disk of matter that will give birth to their satellites • Perturbation of the orbits of small bodies by the most massive planets → heavy bombardment and big cleaning of the planetary system

  17. Formation of planetary systems - 3 Planetary differentiation • Gravitationalcontraction of the star → luminosity maximum soon after its formation • In the inner system: – vaporisation of ices contained in dust grains – radiation pressure → pushes gases away from the star (→ only ~ 2% of initial matter remains) → planetesimals composed of rocks + metals → telluric objects

  18. Formation of planetary systems - 4 Planetary differentiation • In the outer system: – planetesimals of rocks + metals + ices → ganymedian objects – mass of ices ~ 3 or 4 times mass of rocks + metals → much more massive protoplanets and lower temperature → possibility to capture gas (H, He) → jovian planets • How to explain the existence of hot Jupiters? – formation in the outer system followed by migration towards the inner system (gravitational interactions in the disk or with other planets) – during migration: probable ejection of smaller planets → probably no telluric planets in these systems

  19. Formation of planetary systems - 5 Our solar sytem: representative or peculiar? Exoplanets: – significant proportion of hot Jupiters (~ 10%) – many high eccentricities (→ ejections) → what is the frequency of solar systems similar to ours? → consequences for life in the Universe?

  20. Formation of planetary systems - 6 Atmospheres and oceans of telluric planets The components of atmospheres (and oceans) of telluric planets were in the ices of the protoplanetary disk → how to explain their presence today? 2 hypotheses: • outgassing of a small fraction of ices that might have survived in the planetary interiors (gases ejected by volcanoes) • heavy rain: after the solar luminosity maximum, impact of ice-rich comets: – originating from outer regions – deflected by jovian planets

  21. Search for intelligent life We estimated that ~ 10 to 90 % of stars in our Galaxy can provide an adequate environment for life 1960: Frank Drake tries to estimate the number of technological civilisations in our Galaxy Rate of formation of suitable stars R*: ~2 1011 stars in our Galaxy ~1011 suitable stars Age of Galaxy ~ 1010 years → birth of 10 suitable stars per year (on average) Frank Drake and `his´ equation

  22. Search for intelligent life - 2 Fraction of stars with planetsfp: Recent searches indicate that most stars have planets → fp ~ 1 → 10 stars per year Number of habitable planets per star with planets ne: Telluric planets in the HZ, massive enough to retain an atmosphere, and no hot Jupiter in the system → let us assume this happens in 10 % of the systems: ne ~ 0.1 → 1 star per year (10 billion habitable planets!)

  23. Search for intelligent life - 3 Fraction of habitable planets on which life emerges fl: Life appeared quite fast on Earth when the conditions were adequate → we can assume fl ~ 1 (say 0.5) → one star every 2 years Fraction of planets where life evolves towards intelligencefi: We don’t really have any information… Probability that life evolves towards multicellular organisms? Probability that a complex life form develops intelligence? What is intelligence? → I assume fi ~ 0.01 → one star every 200 years

  24. Search for intelligent life - 4 Fraction of intelligent life forms that develop a technological civilisation fc: Personally, I find that quite probable → I assume fc ~ 1 (say 0.4) → one star every 500 years Our galaxy is ~ 10 billion years old → according to my estimate, ~ 107 technological civilisations could have emerged in our galaxy [and the nearest could have been at ~ 100 L.Y.] How many civilisations could be out there right now? It depends on the mean lifetimeL of a technological civilisation (if L < 500 ans, we are probably alone)

  25. Search for intelligent life - 5 First SETIprogramme SETI = Search for ExtraTerrestrial Intelligence = search for radio signals emitted (deliberately or not) by extraterrestrial intelligences 1960: Frank Drake points the Green Bank radiotelescope towards: – τ Ceti: no signal – ε Eridani: strong signal but non reproducible (in fact, signal emitted by an U-2 spy airplane flying 20 000 m above USSR)

  26. t ν Search for intelligent life - 6 Difficulties of SETIprogrammes • At which frequency searching? • Discard parasitic signals (mostly emitted by humans) • How to separate artificial signals from natural ones? • Reproducibility Picture: an apparently artificial signal detected in 2002 (= unusual interference between a GPS satellite and a ground station?)

  27. Search for intelligent life - 7 Most ambitious SETIprogramme Starts in 1992 (500th anniversary of discovery of America) Uses the Arecibo radiotelescope (300 m) on Porto Rico Aimed at analyzing signals from 1000 stars similar to the Sun Interrupted one year later by the Congress, after having been ridiculed by two senators Continued thanks to private funding Enormous quantity of data to be analyzed → SETI@home

  28. Search for intelligent life - 8 Results of SETIprogrammes • Detections of artificial signals • Often identified (human sources) • Sometimes unidentified but still not securely confirmed • Sometimes reproducible (2 – 3 detections) • Strict methodology: no announcement before sufficient confirmation (way of proceeding in sharp contrast with ufology)

  29. Search for intelligent life - 9 Types of detectablesignals • Signal intentionally emitted towards us → powerful and structured But what would be the motivation? • Radio communications escaping into space → weaker (3D) and more confuse → would we be able to detect it and to recognize its `intelligent´ structure? • Would a technological civilisation necessarily use radio communications?

  30. Search for intelligent life - 10 And us, what did we send? • 16/11/1974: 169 seconds message sent by the Arecibo radiotelescope towards globular cluster M13 at 25000 L.Y.: – numbers from 0 to 10 binary coded – atomic numbers of H, C, N, O, P (elements on which terrestrial life is mostly based) – chemical formulae of the 4 DNA bases – spatial structure of DNA – little man – position of Earth in the solar system –Arecibo telescope…

  31. Search for intelligent life - 11 Are we alone? The Fermi `paradox´ Among all extraterrestrial civilisations, if only one of them has an expansionary policy, our Galaxy should already be fully colonized How? Let us assume that they send space missions to 10 habitable planets and that each of the 10 colonies, when ready, sends missions to 10 new planets, and so on… It can take a very long time, but it doesn’t matter Let us assume that it takes 100 000 years to reach a new planet, settle, build a new civilisation and send 10 new missions → in one million years, the whole Galaxy is colonised

  32. Search for intelligent life - 12 Time it would take to colonise the Galaxy In 100 000 years, 10 planets are colonised In 200 000 years, 100 planets In 300 000 years, 1000 planets … (exponential growth) In 1 100 000 years, 10 billion habitable planets = the whole Galaxy Yet, stars more than 5 billion years older than the Sun most probably have habitable planets → among the ET civilisations, some could be several billions of years more advanced than we are → these ET should already be here Fermi’s conclusion: as the ETs are not here, we are alone!!!

  33. Search for intelligent life - 13 Solution proposed by Fermi To solve his paradox, Fermi suggested that, at the same time as a civilisation acquires space travel technology, it also acquires means to self-destruct (context of cold war) If the mean lifetime of a technological civilisation is less than the mean time it takes for such a civilisation to appear in the Galaxy (~ 500 years in my estimate) → many civilisations may appear but, on average, there would be a single one at a time in the Galaxy

  34. Are we visited? Are we really sure they are not here? Many people pretend that: – not only extraterrestrial civilisations exist – but we don’t need SETI programmes to detect them as they are already here: ETs are among us!

  35. Are we visited? - 2 `Proofs´ of extraterrestrial visits? • Unidentified Flying Objects • Crop Circles • Ancient Astronauts • Contacts • Abductions → until now, none of these `evidences´ survived serious examination → encounters with extraterrestrials = modern version of old myths → Are we visited? Probably not…

  36. Are we visited? - 3 Possible solutions to the Fermi paradox 1. Huge distances With a modern rocket (20 000 km/h) it takes: 1 day to reach the Moon, 1 year to reach Mars or Venus, 20 years to cross the solar system, 200 000 years to reach the nearest star, 4 billion years to cross the Galaxy Even if there are ~ 10 000 000 ET civilisations in our galaxy, it would take ~ 4 million years to reach the nearest one!

  37. Are we visited? - 4 Possible solutions to the Fermi paradox 1. Huge distances (continued) By multiplying by another factor 4000 the speed of our rockets (0.1c), it would take: 40 years to reach the nearest star, 1 million years to cross the Galaxy If there are ~ 107 ET civilisations in our galaxy, it would still take: ~ 1000 years to reach the nearest one → distances too large, travels too long!

  38. Are we visited? - 5 Possible solutions to the Fermi paradox 2. Low probability of intelligent life • Life seems to appear rather easily when conditions are favorable • But it may need very special conditions for intelligence to develop (= to be a positive factor in natural selection) • On earth, it took more than 2 billion years for life to go beyond unicellular forms and become more complex! → Universe is full of life, but not of intelligence

  39. Are we visited? - 6 Possible solutions to the Fermi paradox 3. Short lifetime of technological civilisations • Darwin: best adapted species survive • In a 1st stage, intelligence has been a factor favoring the human species on Earth • But, from a certain stage, human beings have become their own enemies, and by far the most dangerous → how will natural selection occur from then? → extrapolation: Technological civilisations do not survive long enough to colonise the Galaxy « Moral » of that story: We have only one Earth and we probably won’t have other ones for a long long time → let’s take care of it

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