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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 1 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 2 1. Formation of planetary systems - nebular hypothesis - ● Developed by Immanuel Kant in 1755 ● Modern variant: Solar nebular disk model – 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 3 1. Formation of planetary systems - nebular hypothesis - – How planetesimals form out of 1 μm - 1 cm grains is not yet fully understood – Stages of formation: – ● ● Runaway accretion (lasts 10 – 100k years) ● Oligarchic accretion (lasts another few 100k years) ● 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 – Gas giants tend to migrate inwards Jan Tenzer; Nov 29th, 2016 https://en.wikipedia.org/wiki/Nebular_hypothesis 4 2. Classification and examples of exoplanet discoveries Jan Tenzer; Nov 29th, 2016 5 2.1. Circumbinary and pulsar planets - by the example of PSR B1620-26 b (Methuselah) - ● Host stars: Pulsar (M = 1.35 M☉, ω = 100/s) and white dwarf (M = 0.34 M☉); distance: 1 AU; period: six months ● Method: Pulsar timing ● Constellation: Scorpius ● Distance: 12400 ly ● Age: 12.7 billion yr (!!!) ● Semi-major-axis: 23 AU ● Orbital period: 100 yr ● Mass: 2.5 ± 1 MJ ● Temperature: 72 K ● Eccentricity: Low Jan Tenzer; Nov 29th, 2016 https://en.wikipedia.org/wiki/PSR_B1620-26_b 6 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 7 2.2. Rogue planets - by the example of OTS44 ● Method: near-infrared photometry and spectroscopy ● Constellation: Chameleon ● Distance: 554 ly ● Mass: 11.5 MJ (0.011 M☉) ● Radius: 0.23 – 0.57 R☉ ● Luminosity: 0.0013 – 0.0024 L☉ ● Temperature: 1700 – 2300 K ● 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 8 2.3. Hot Jupiters - by the example of 51 Pegasi b - ● First known exoplanet orbiting main-sequence (G-)star ● Launched new field of astronomical research (orbital migration) ● Method: Radial velocity ● Constellation: Pegasus ● Distance: 50 ly ● Age: 6.1 – 8.1 billion yr ● Semi-major-axis: 0.053 ± 0.003 AU ● Orbital period: 101.5 h ● Rotation period: Synchronous ● (Minimum) mass: 0.472 ± 0.039 MJ ● Radius: probably > RJ ● Temperature: 1284 ± 19 K Jan Tenzer; Nov 29th, 2016 https://en.wikipedia.org/wiki/51_Pegasi_b 9 2.3. Hot Jupiters - by the example of 51 Pegasi b (Dimidium) ● Migration hypothesis – Planets can drive spiral density waves in the surrounding gas – Gaps move inwards due to inwards accretion – Tidal forces also may contribute – Orbits of smaller objects can reform after giant passed through ● Gas giants orbiting red giants (Jupiter in far future) ● Other characteristics: – Exotic atmospheres – More than half of hot Jupiters reported to have retrograde orbits – 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 10 2.4. Super Earths - by the example of COROT-7b - ● First known terrestrial exoplanet ● Method: Transit, follow-up radial velocity ● Constellation: Unicorn ● Distance: 489 ly ● Age: 1.2 – 2.3 billion years ● Semi-major-axis: 0.0172 ± 0.0003 AU ● Orbital period: 20.5 h ● Rotation period: possibly synchronous ● Mass: 2 - 9 M⊕ ● Radius: 1.58 ± 0.1 R⊕ ● Temperature: 1300 – 1800 K ● Possible atmosphere consisting of Na, O2, SiO due to silicate rock vaporization Jan Tenzer; Nov 29th, 2016 https://en.wikipedia.org/wiki/COROT-7b 11 2.4. Super Earths - by the example of Gliese 1214 b - ● Method: Transit, follow-up radial velocity ● Constellation: Snake-bearer ● Distance: 42 ± 3 ly ● Age: 6 billion years ● Semi-major-axis: 0.014 ± 0.002 AU ● Orbital period: 38 h ● Mass: 6.55 ± 0.98 M⊕ ● Radius: 2.678 ± 0.13 R⊕ ● Density: 1870 ± 400 kg/m³ ● Surface gravity: 0.91 g ● Temperature: 393 - 555 K ● 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 12 2.5. Planets with high ESI - Earth similarity index - ● Used to compare planets to Earth (ESIEarth = 1) ● It is a weighted geometric mean: ● – xi … ith property of planet – xi0 … ith property of Earth – ωi … weighted exponent of ith property Examples: – Gliese 581 g: 0.89 (unconfirmed) – Kepler-438b: 0.88 – Gliese 581 d: 0.74 – Mars: 0.697 – Moon: 0.56 Jan Tenzer; Nov 29th, 2016 https://en.wikipedia.org/wiki/Earth_Similarity_Index 13 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 ● Method: Radial velocity ● Constellation: Libra ● Distance: 20.37 ly ● Age: 7 - 11 billion years ● Semi-major-axis: 0.13 AU ● Orbital period: 32 d ● Rotation period: Synchronous ● Mass: 2.2 M⊕ ● Temperature: 242 (209) – 261 (228) K ● Liquid water could exist, if the atmosphere is thick enough to transport heat from the permanently exposed dayside ● 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 14 2.5. Planets with high ESI - by the example of Kepler-438b - ● Method: Transit ● Constellation: Lyra ● Distance: 20.37 ly ● Age: 4.4 ± 0.8 billion years ● Semi-major-axis: 0.166 AU ● Orbital period: 35.2 d ● Mass: 1.3 (+ 2.6, – 0.7) M⊕ ● Radius: 1.12 ± 0.16 R⊕ ● Temperature: 276 K ● Kepler-438b is orbiting within the habitable zone ● 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 15 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 16 3. The Fermi paradox - Drake equation - N = number of potential communicating civilizations in our galaxy and the product of: – R* = average rate of star formation in our galaxy – fp = fraction of stars that have planets – ne = average number of planets that can potentially support life per star that has planets – 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) – fc = fraction of civilizations that develop a technology that releases detectable signs of their existence into space – 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 17 3. The Fermi paradox - rare Earth hypothesis - ● Optimistic result: N = 156 million (we already could have detected them) ● 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: ● Right location in the right kind of galaxy ● Orbiting at the right distance from the right type of star ● ● ● With the right arrangement of planets ● A continuously stable orbit A terrestrial planet of the right size ● With plate tectonics ● A large moon ● 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 18 3. The Fermi paradox - other explanation attempts - ● Intelligent alien species lack advanced technology ● It is the nature of intelligent life to destroy itself and others ● Periodic extinction by natural events ● Intelligent civilizations are too far apart in space and time ● It is too expensive to spread physically throughout the galaxy ● They are too alien, humans are not listening properly ● Earth is deliberately not contacted or isolated (simulation) ● They are here unacknowledged (UFO conspiracy theories) Jan Tenzer; Nov 29th, 2016 https://en.wikipedia.org/wiki/Fermi_paradox 19 Exoplanet data bases ● Extrasolar Planets Encyclopaedia ● SIMBAD ● Exoplanet Archive ● Open Exoplanet Catalogue ● arXiv.org for abstracts and papers on discoveries Jan Tenzer; Nov 29th, 2016 20