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Detection of Exoplanets
- Discoveries, Formation and Habitability -
Jan Tenzer; Nov 29th, 2016
https://www.nasa.gov/jpl/spitzer/pia19333/map-of-exoplanets-found-in-our-galaxy
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Detection of Exoplanets
- Discoveries, Formation and Habitability -
1. Formation of planetary systems (nebular hypothesis)
2. Classification and examples of exoplanet discoveries
2.1. Circumbinary and pulsar planets (PSR B1620-26 b)
2.2. Rogue planets (OTS 44)
2.3. Hot Jupiters (51 Pegasi b)
2.4. Super Earths (COROT-7b, Gliese 1214 b)
2.5. Planets with high ESI (Gliese 581 g, Kepler-438b)
3. The Fermi paradox
Jan Tenzer; Nov 29th, 2016
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1. Formation of planetary systems
- nebular hypothesis -
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Developed by Immanuel Kant in 1755
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Modern variant: Solar nebular disk model
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Protoplanetary (accretion) disks are a natural byproduct of star formation
–
Volatile material evaporates outwards (frost line)
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Nebular_hypothesis
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1. Formation of planetary systems
- nebular hypothesis -
–
How planetesimals form out of 1 μm - 1 cm grains is not yet fully
understood
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Stages of formation:
–
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Runaway accretion (lasts 10 – 100k years)
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Oligarchic accretion (lasts another few 100k years)
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Merger stage (completed after 10 – 100 million years)
Protoplanetary disk fades after up to 10 million years
The formation of gas giants appears to be a lot more complicated
–
Their cores need to reach 5 – 10 Earth masses in order to accrete gases,
which might be feasible with 4:1 ice to rock ratio
–
Gas accretion may accelerate in a second runaway stage
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Gas giants tend to migrate inwards
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Nebular_hypothesis
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2. Classification and examples of exoplanet discoveries
Jan Tenzer; Nov 29th, 2016
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2.1. Circumbinary and pulsar planets
- by the example of PSR B1620-26 b (Methuselah) -
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Host stars:
Pulsar (M = 1.35 M☉, ω = 100/s) and white dwarf (M = 0.34 M☉);
distance: 1 AU; period: six months
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Method:
Pulsar timing
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Constellation:
Scorpius
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Distance:
12400 ly
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Age:
12.7 billion yr (!!!)
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Semi-major-axis:
23 AU
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Orbital period:
100 yr
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Mass:
2.5 ± 1 MJ
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Temperature:
72 K
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Eccentricity:
Low
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/PSR_B1620-26_b
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2.1. Circumbinary and pulsar planets
- by the example of PSR B1620-26 b -
In a supernova planetary orbits most probably get unstable due to rapid loss of
gravitational pull. A theoretical explanation for PSR B1620-26 b is as follows:
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/PSR_B1620-26_b
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2.2. Rogue planets
- by the example of OTS44 ●
Method:
near-infrared photometry and spectroscopy
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Constellation:
Chameleon
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Distance:
554 ly
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Mass:
11.5 MJ (0.011 M☉)
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Radius:
0.23 – 0.57 R☉
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Luminosity:
0.0013 – 0.0024 L☉
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Temperature:
1700 – 2300 K
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Also known as interstellar planets, free-floating planets, orphan planets, …
- Either got ejected during merger stage, by the close encounter of another star or in
a supernova,
- Or form as brown or sub-brown dwarfs with protoplanetary disk (like OTS44)
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Rogue_planet; https://en.wikipedia.org/wiki/OTS_44
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2.3. Hot Jupiters
- by the example of 51 Pegasi b -
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First known exoplanet orbiting main-sequence (G-)star
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Launched new field of astronomical research (orbital migration)
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Method:
Radial velocity
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Constellation:
Pegasus
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Distance:
50 ly
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Age:
6.1 – 8.1 billion yr
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Semi-major-axis:
0.053 ± 0.003 AU
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Orbital period:
101.5 h
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Rotation period:
Synchronous
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(Minimum) mass:
0.472 ± 0.039 MJ
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Radius:
probably > RJ
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Temperature:
1284 ± 19 K
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/51_Pegasi_b
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2.3. Hot Jupiters
- by the example of 51 Pegasi b (Dimidium) ●
Migration hypothesis
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Planets can drive spiral density waves in the surrounding gas
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Gaps move inwards due to inwards accretion
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Tidal forces also may contribute
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Orbits of smaller objects can reform after giant passed through
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Gas giants orbiting red giants (Jupiter in far future)
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Other characteristics:
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Exotic atmospheres
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More than half of hot Jupiters reported to have retrograde orbits
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In some cases, atmosphere gets stripped away by the star (Roche limit)
=> Hypothetical Cthonian planet (core of gas giant)
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Planetary_migration; https://en.wikipedia.org/wiki/Hot_Jupiter
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2.4. Super Earths
- by the example of COROT-7b -
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First known terrestrial exoplanet
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Method:
Transit, follow-up radial velocity
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Constellation:
Unicorn
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Distance:
489 ly
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Age:
1.2 – 2.3 billion years
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Semi-major-axis:
0.0172 ± 0.0003 AU
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Orbital period:
20.5 h
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Rotation period:
possibly synchronous
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Mass:
2 - 9 M⊕
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Radius:
1.58 ± 0.1 R⊕
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Temperature:
1300 – 1800 K
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Possible atmosphere consisting of Na, O2, SiO due to silicate rock vaporization
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/COROT-7b
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2.4. Super Earths
- by the example of Gliese 1214 b -
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Method:
Transit, follow-up radial velocity
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Constellation:
Snake-bearer
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Distance:
42 ± 3 ly
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Age:
6 billion years
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Semi-major-axis:
0.014 ± 0.002 AU
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Orbital period:
38 h
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Mass:
6.55 ± 0.98 M⊕
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Radius:
2.678 ± 0.13 R⊕
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Density:
1870 ± 400 kg/m³
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Surface gravity:
0.91 g
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Temperature:
393 - 555 K
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Speculated to be “Failed” gas giant (~75% water) and result of planetary migration
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Gliese_1214_b
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2.5. Planets with high ESI
- Earth similarity index -
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Used to compare planets to Earth (ESIEarth = 1)
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It is a weighted geometric mean:
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–
xi … ith property of planet
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xi0 … ith property of Earth
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ωi … weighted exponent of ith property
Examples:
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Gliese 581 g:
0.89 (unconfirmed)
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Kepler-438b:
0.88
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Gliese 581 d:
0.74
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Mars:
0.697
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Moon:
0.56
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Earth_Similarity_Index
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2.5. Planets with high ESI
- by the example of Gliese 581 g (Zarmina's world) ●
Still unconfirmed and disputed, existence depends on more high-precision data
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Method:
Radial velocity
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Constellation:
Libra
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Distance:
20.37 ly
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Age:
7 - 11 billion years
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Semi-major-axis:
0.13 AU
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Orbital period:
32 d
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Rotation period:
Synchronous
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Mass:
2.2 M⊕
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Temperature:
242 (209) – 261 (228) K
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Liquid water could exist, if the atmosphere is
thick enough to transport heat from the permanently exposed dayside
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Given the age of the system, extremophiles might exist on the surface
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Gliese_581_g
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2.5. Planets with high ESI
- by the example of Kepler-438b -
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Method:
Transit
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Constellation:
Lyra
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Distance:
20.37 ly
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Age:
4.4 ± 0.8 billion years
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Semi-major-axis:
0.166 AU
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Orbital period:
35.2 d
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Mass:
1.3 (+ 2.6, – 0.7) M⊕
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Radius:
1.12 ± 0.16 R⊕
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Temperature:
276 K
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Kepler-438b is orbiting within the habitable zone
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Kepler-438 emits violent stellar flares every 100 d, which would sterilize life on Earth
=> It is suggested to resemble a cooler version Venus
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Kepler-438b
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3. The Fermi paradox
- the big question -
Steven S. Vogt (discoverer of Gliese 581 g):
“But if GJ 581g is confirmed by further RV scrutiny, the mere fact that a
habitable planet has been detected this soon, around such a nearby
star, suggests that [the fraction of stars with potentially habitable planets]
could well be on the order of a few tens of percent, and thus that either
we have just been incredibly lucky in this early detection, or we are truly
on the threshold of a second age of discovery.”
On the one hand we just surveyed a negligible amount of planetary systems
in the Milky Way (roughly 3000 out of 100 - 200 billion), on the other hand
there is no hard evidence of extraterrestrial, let alone intelligent life, so ...
“Where is everybody?”
Jan Tenzer; Nov 29th, 2016
https://arxiv.org/abs/1009.5733
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3. The Fermi paradox
- Drake equation -
N = number of potential communicating civilizations in our galaxy and the
product of:
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R* = average rate of star formation in our galaxy
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fp
= fraction of stars that have planets
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ne
= average number of planets that can potentially support life per
star that has planets
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fl
= fraction of planets that could support life that actually develop
life at some point
–
fi
= fraction of planets with life that actually go on to develop
intelligent life (civilizations)
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fc
= fraction of civilizations that develop a technology that releases
detectable signs of their existence into space
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L
= length of time for which such civilizations release detectable
signals into space
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Drake_equation
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3. The Fermi paradox
- rare Earth hypothesis -
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Optimistic result:
N = 156 million (we already could have detected them)
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Pessimistic result:
N = 9.1 x 10⁻11 (we are alone in the universe)
Rare Earth hypothesis states that life is unique to Earth and heavily depends on
the following requirements:
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Right location in the right kind of galaxy
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Orbiting at the right distance from the right type of star
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With the right arrangement of planets
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A continuously stable orbit
A terrestrial planet of the right size
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With plate tectonics
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A large moon
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Atmosphere
One or more evolutionary triggers for complex life (right time in evolution)
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Rare_Earth_hypothesis
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3. The Fermi paradox
- other explanation attempts -
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Intelligent alien species lack advanced technology
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It is the nature of intelligent life to destroy itself and others
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Periodic extinction by natural events
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Intelligent civilizations are too far apart in space and time
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It is too expensive to spread physically throughout the galaxy
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They are too alien, humans are not listening properly
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Earth is deliberately not contacted or isolated (simulation)
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They are here unacknowledged (UFO conspiracy theories)
Jan Tenzer; Nov 29th, 2016
https://en.wikipedia.org/wiki/Fermi_paradox
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Exoplanet data bases
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Extrasolar Planets Encyclopaedia
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SIMBAD
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Exoplanet Archive
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Open Exoplanet Catalogue
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arXiv.org for abstracts and papers on discoveries
Jan Tenzer; Nov 29th, 2016
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