Transcript unidentified flying object

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?
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 know 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)
Where to search for life? - 2
Main phases of life on Earth
t (GYr)
h
Event
−4.6
0h
Formation of Earth
−4.5/−4.0
1h/3h
−4.0?
3h?
−1.3
17h
−0.6
21h
Formation of oceans
Caution !
First unicellular organisms
The adopted point of view in this
First multicellular plants
table (last lines) is very
Cambrian
explosion (1st animals)
anthropomorphic
!
−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)
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
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? - 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 50% of stars in our Galaxy
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
θ = ang. Dist. = 5.2/260000 rad = 4″
d (Jupiter – Sun) = 5.2 AU
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
Exoplanets - 2
First discoveries
• 1992: discovery of 2 planets around the pulsar
PSR B1257+12 by Aleksander Wolszczan
M = 4.3 & 2.8 ME
d = 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
51 Peg b: M > 150 ME
M = 1.05 M
d = 0.05 AU
T = 4 days
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 50 pc
5 MJup planet at 40 AU
• AB Pic, K2V at 46 pc
13 MJup planet at 275 UA
Brown dwarf 2M1207 and its planet (ESO)
Exoplanets - 4
Orbital motion
generalized 3rd Kepler law:
T2
4π 2

3
( A  a)
G ( M  m)
(obtained by gravitational force = centripetal force for M and m)
Velocity of star:
MV 2
GMm

A
( A  a) 2
GmA
2
a  A  V  2
a
A
m
Gm 2
2

V 
a
M
Ma
V
C
M
A
a
m
v
Exoplanets - 5
Detection methods: radial velocities
2
1
T2
4π 2
1
3
M >> m & a >> A → Kepler:


 K1T M 3
3
a
GM
a
V
2
 K2m T
2
2
3
M
4
 Vrad  K m sin i T
3
1
3
M
2
3
i = angle between orbital plane and sky
V in km/s
T in years
m in MJup
M in M
V
C
 Vrad  28.4 m sin i T
1
3
M
2
M
A
a
m
3
v
→ more sensitive to large planet
masses and short periods
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
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
Exoplanets - 8
Detected exoplanets
October 2016: ~3500 planets discovered
~2600 planetary systems
Method
Planets
Systems
2692
2015
687
518
Microlensing
51
49
Imaging
72
67
Pulsars
23
18
Transits
Radial velocities
Source: www.exoplanet.eu
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
Formation of planetary systems
Contraction of protostellar nebula
→ star at the center, surrounded by a disk of gas and dust
Collisions between dust grains → aggragates
→ 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
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 et big cleaning of
the planetary system
Formation of planetary systems - 3
Planetary differentiation
• Gravitational contraction 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
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
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?
Formation of planetary systems - 6
Atmosphere 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
Search for intelligent life
We estimated that ~ 10 to 50 % 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
Search for intelligent life - 2
Fraction of stars with planets fp:
Most 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!)
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 intelligence
fi:
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
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 lifetime L of a technological civilisation
(if L < 500 ans, we are probably alone)
Search for intelligent life - 5
First SETI programme
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)
Search for intelligent life - 6
Difficulties of SETI programmes
• At which frequency searching?
• Discard parasitic signals (mostly emitted by humans)
• How to separate artificial signals from natural ones?
• Reproducibility
t
Opposite:
an apparently artificial signal
detected in 2002
(= unusual interference
between a GPS satellite and a
ground station?)
ν
Search for intelligent life - 7
Most ambitious SETI programme
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
Search for intelligent life - 8
Results of SETI programmes
• 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)
Search for intelligent life - 9
Types of detectable signals
• 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?
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…
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
colonized
Search for intelligent life - 12
Time it would take to colonize the Galaxy
In 100 000 years, 10 planets are colonised
In 200 000 years, 100 planets
In 300 000 years, 1000 planets
… (exponential growth)
En 1 100 000 years, 10 billion habitable planets = the whole Galaxy
Yet, stars more than 5 billion years older than the Sun should 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!!!
Search for intelligent life - 13
Solution proposed by Fermi
• To solve his `paradox´, Fermi suggest that, when a civilisation gets
the proper technology for space travel, it also gets the power for selfdestruction
(cold war context at the time of Fermi)
• If the mean lifetime of a technological civilisation is lower than the
mean time it takes for such a civilisation to appear in the Galaxy
(~ 500 years in my estimate)
many technological civilisations may appear
but, on average, there is only one
technological civilisation at a time in the
Galaxy
civilisation
Are we visited?
Are we really sure the ETs are not here?
Many people pretend that:
– not only extraterrestrial civilisations exist
– we do not need SETI programmes to detect them
because they visit is:
ETs are among us!
Are we visited? - 2
Proofs of extraterrestrial visits?
• Unidentified Flying Objects
• Crop Circles
• Ancient Astronauts
• Contacts
• Abductions
→ until now, none of these `proofs´ has even resisted a serious exam
→ encounters with extraterrestrials = modern version of ancient myths
→ Are we visited?
Probably not…
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 ~10 million ET civilisations existed in our galaxy, il would
take ~ 2 million years to reach the nearest one!
Are we visited? - 4
Possible solutions to the Fermi paradox
1. Huge distances (continued)
If we could multiply by 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
Even with ~ 107 ET civilisations in the Galaxy, it would take:
~ 500 years to reach the nearest one
→ distances too large, travels too long and too risky!
Are we visited? - 5
Possible solutions to the Fermi paradox
2. Low probability of intelligent life
• Life seems to appear quite easily in favorable conditions
• But maybe it needs very special conditions for intelligence to emerge
(= to become an asset in natural selection)
• On Earth, it took more than 2 billion years for life to go from
unicellulars to more complex cratures!
→ our Galaxy is full of life, but not of intelligence
And on Earth?…
Are we visited? - 6
Possible solutions to the Fermi paradox
3. Short lifetime of technological civilisations
• Darwin: the best adapted species survive
• In a 1st step, intelligence has been a favorable factor for the human
species survival on Earth
of the mankind
story: has become its own ennemy, and
• But, from a `Moral´
certain stage,
by far the most dangerous
We have only one Earth to live on and it is
probably
not tomorrows
that
weon?
will have
→ how will natural
selection
play from
now
others
→ extrapolation:
→ we should take care of it
Technological civilisations do not survive long enough
to colonize the Galaxy