Final Exam Review
Exoplanets and the Search for Life
Transiting Exoplanet Survey Satellite (TESS):
A NASA mission launched in 2018 to discover Earth-sized exoplanets around nearby stars.
Primary goal: Identify promising planets for subsequent follow-up observations by future telescopes to assess their potential for life.
Discovery of Exoplanets:
Over 5,000 exoplanets (planets beyond our solar system) have been confirmed, with thousands of additional candidates.
The first exoplanets were discovered in the 1990𝑠, confirming that planets orbit other stars.
Water on Mars and Clues to Life:
Recent news (2018-2024) indicates signs of buried liquid water reservoirs on Mars.
This strengthens the case for Mars having harbored habitable conditions in the past or present, linking space missions with astrobiology.
Proxima Centauri-b:
Orbits Proxima Centauri, the closest star to Earth (about 4.2 ly away).
Located in its star's habitable zone, meaning liquid water could exist given a suitable atmosphere.
Distances are measured in light-years (1 ly=9.4×1012 km).
TRAPPIST-1 System:
A system believed to have up to 3 planets in its habitable zone.
TRAPPIST-1e is considered a prime candidate for habitability studies due to its location and proximity to Earth (around 39.12 ly or 12 pc).
Planets in Binary Star Systems:
Planets can orbit around two stars (circumbinary) or one star within a binary system.
This adds complexity to habitability due to varying insolation and dynamic stability.
James Webb Space Telescope (JWST):
Has capabilities to peer into exoplanet atmospheres.
Primary goal: Search for biosignature gases (gases indicating biological activity, unlikely to exist abiotically) in exoplanet atmospheres.
K2-18b Case Study:
A sub-Neptune (𝑀𝑝≈8.6 𝑀⊕) located in its star's habitable zone, approximately 120 ly away.
Atmospheric indicators like methane (CH4), carbon dioxide (CO2), and dimethyl sulfide ((CH3)2S) are discussed.
The presence of these gases can suggest biological activity, but requires careful follow-up to rule out abiotic false positives.
Key Concepts
Detection and Exploration Pipeline:
TESS identifies candidates.
JWST performs atmospheric spectroscopy for biosignatures.
Ground and space observations refine properties.
Habitable Zone:
The region around a star where liquid water can exist on a planet's surface, given the right atmospheric conditions.
Biosignature Caveats:
Potential biosignature gases (e.g., CH4,CO2,(CH3)2S) can also arise from non-biological processes, requiring robust, multi-spectral data for confirmation.
Atomist View
Core Concepts: Atomists like Leucippus, Democritus, Epicurus, and Lucretius believed that all existence is made of infinite, imperishable atoms in random motion. They argued there's nothing special about Earth, leading to the conclusion of an infinite number of universes and, consequently, ET life. This was a naturalistic, materialistic view without a divine creator.
Principle of Plenitude: Their argument for infinite universes relied on this principle, stating that given inexhaustible sources (infinite atoms), all possible forms of existence will occur, including infinite worlds with living creatures similar to ours.
Universe Structure: Atomists envisioned independent, geocentric universes, each with its own Earth-centered system (Moon, Sun, similar planets), where the central Earth(s) were inhabited.
Aristotle and Plato's Counterarguments
Plato: Argued against plurality based on the uniqueness of the creator and the idea that a composite universe would decay.
Aristotle: Grounded his argument in physics, proposing a finite, geocentric universe composed of five elements (Fire, Air, Water, Earth, Aether). Elements sought their 'natural place,' making multiple Earth-centers (and thus multiple Earths) contradictory. The heavens were perfect and unchanging, contrasting with the changeable sub-lunary realm.
Other Ancient Greek Views
Pythagoreans: Proposed life on the Moon, with inhabitants superior to Earthlings, and advanced speculative cosmologies about Mondial life.
Lucian of Samosata's A True History: A satirical work from the 2nd century AD, featuring a voyage to the Moon and beyond, describing alien inhabitants and anticipating modern science fiction themes like space travel, planetary colonization, and interplanetary imperialism.
Modern Connections
Modern Definitions: The current definition of a planet (orbits the Sun, spherical, cleared its orbit) and classification of dwarf planets (e.g., Ceres, Kuiper belt objects) inform the search for ET life.
Relevance: Atomist thought, particularly the principle of plenitude, is seen as a historical precursor to modern ideas about multiverses and the Copernican Principle (Earth is not special).
Implications: Historical debates frame modern questions about the prevalence of life, influencing ethics, epistemology, and public understanding of ET life, while fiction (like Lucian's) shows how imagination can foreshadow scientific exploration.
### History of the Extraterrestrial Life Debate: Medieval Views
#### Early Christian Perspectives on Plurality of Worlds
Augustine of Hippo (354-430 AD) attacked the concept of "plurality of the worlds."
13th Century saw Christian scholars begin to reconsider multiple worlds, with renewed access to ancient philosophy.
Albertus Magnus (1200-1280 AD) acknowledged the debate but largely rejected plurality.
#### Aristotelian Philosophy and Early Christian Theology
Thomas Aquinas (1225-1274), a student of Magnus, rejected plurality based on Aristotelian texts.
A key theological dilemma: Did rejecting multiple worlds limit God's omnipotence (unlimited power)?
Omnipotence: Unlimited power of God.(c)(c)
Other divine attributes: Omniscience (knowing everything), Omnibenevolence (unlimited benevolence), Omnipresence (God everywhere at once).
#### The Condemnations of 1277 and Their Impact
Etienne Tempier, Bishop of Paris, condemned 219 statements perceived as limiting God's power in 1277.
Condemnation Number 34 stated: "that the First Cause cannot make many worlds," which opened the door for further inquiry into plurality.
#### Post-1277 Developments and Proponents of Plurality
The condemnations spurred re-examination; many conceded God could create multiple worlds, even if He had not.
William of Ockham (1280-1347) hypothesized distinct "natural places" in various locales and that God could create a world superior to ours.
Jean Buridan (1295-1358) proposed God could create worlds with different elements and unique natural places.
Primary elements in known Earth life: Carbon, Hydrogen, Oxygen, Nitrogen (Silicon-based life is a hypothetical alternative).
#### Theological Implications for Extraterrestrial Life
William Vorilong (1390-1463) explored if extraterrestrials would live in sin, referencing the Doctrine of Atonement.
Doctrine of Atonement: Forgiveness of original sin through Christ's death/resurrection.
He argued Christ could redeem infinite worlds but it would not be "fitting" for Him to die again on another world. This doctrine was a common reason for denying populated alien worlds.
Nicolas of Cusa (1401-1461) was among the earliest to attribute life to celestial bodies (Moon, Sun), envisioning extraterrestrials as fundamentally different.
Philip Melanchthon (1497-1560) strongly rejected plurality, also based on the Doctrine of Atonement (Christ died once, on Earth).
#### The Great Chain of Being
Concept: A philosophical and theological hierarchy of all existing entities from God to inanimate matter.
Hierarchy: God, Angels, Extraterrestrials (hypothetical), Humans, Animals, Plants, Matter.
An entity's position was determined by its proportion of spirit and matter.
Ramon Llull (1232-1315) was associated with this concept
Geocentric Universe: Earth-centered model, proposed by Claudius Ptolemy, where celestial bodies orbit Earth. It was mathematically complex with shortcomings.
Night Sky Appearance: Celestial objects rise in the East and set in the West, following concentric paths.
Retrograde Motion: The apparent backward motion of planets against background stars.
Geocentric Explanation: Used epicycles (planets orbit a small circle, which orbits a larger one).
Heliocentric Explanation: Arises from Earth overtaking other planets in orbit.
Copernican Heliocentrism: Sun-centered model, proposed by Nicolaus Copernicus (1543).
It places the Sun at the center with planets, including Earth, orbiting it.
Philosophically, it suggested Earth might not be unique, raising questions about habitable planets.
Giordano Bruno (1548-1600):
Early proponent of Copernicanism, arguing stars are suns with their own planetary systems.
Believed in the principle of plenitude (universe's vastness implies possibilities of existence).
Executed for heresy in 1600.
Tycho Brahe (1546-1601):
Acquired extensive observational data.
Proposed a geo-heliocentric model (Earth central, but planets orbit the Sun).
Rejected heliocentrism due to the inability to detect stellar parallax.
Objections to Heliocentrism: The primary objection was the lack of observable stellar parallax.
Galileo Galilei's Discoveries (1610):
First to use a telescope for astronomical observations.
Observed the Moon's rugged surface and Jupiter's four moons, contradicting Aristotelian views.
Documented findings in Sidereus Nuncius.
On Extraterrestrial Life: Expressed skepticism for life on the Moon or other bodies due to extreme temperature variations.
Tidal Heating and Inner Moons of Jupiter:
Process: Gravitational pull from Jupiter causes its moons to flex, generating internal heat.
Effects: Despite cold surfaces, moons like Europa have subsurface global oceans beneath icy shells, indicating geological activity.
Notable Moons:
Io: Most volcanically active body in the solar system, heated by tidal forces.
Europa: Important candidate for microbial life due to its subsurface ocean.
Johannes Kepler and His Contributions:
Assistant to Tycho Brahe; analyzed planetary data.
Discovered planets move in elliptical orbits.
First Law: Planets move in elliptical orbits with the Sun at one focus.
Believed in the plurality of worlds and used teleological reasoning (moons serve purposes, e.g., illuminating life).
Kepler's Insomnia ("Somnium"):
An early science fiction work advocating for the Copernican universe.
Explores Earth's motion through a story of travel to the Moon, illustrating shifting perspectives.
Philosophical Implications:
Teleology: Explaining existence through the purposes things serve.
Anthropocentrism: Belief that Earth is uniquely suitable for human life, reflecting human bias.
Rene Descartes
Lifespan: 1596-1650
Key Contribution: Introduced vortex cosmology, where each star is a sun orbited by planets.
Vortex: Large circling band of material particles forming stratified bands.
Mechanism: Explains planet motion; tiny particles keep planets in orbit.
Ambiguity on Pluralism: His cosmology raised questions but he left the existence of intelligent creatures in stars "undecided."
Bernard Fontenelle
Lifespan: 1657-1757
Key Work: Conversations on the Plurality of the Worlds (1686).
Utilized literary techniques (science fiction-like dialogues between a marquise and a philosopher) for a broad audience.
Argued for extraterrestrial life, linking Descartes’s cosmology and telescopic observations.
Faced censorship by the Catholic Church.
Content: Explored habits and nature of extraterrestrial beings (e.g., "little black people, scorched with the Sun" Venusians; solemn Jupiterians).
Philosophical Implications: Integrated Teleology and The Principle of Plenitude.
Christian Huygens
Lifespan: 1629-1695
Noted for discoveries concerning Saturn (rings, moon Titan).
Key Work: Cosmotheoros (1698, posthumously).
Advocated for extraterrestrial life, proposing "other planets are not inferior in dignity to us."
Critiqued notions of lifeless planets.
Assumed extraterrestrial life would share similarities with Earth life (senses, rationality).
Speculated on the use of scientific instruments, geometry, and arithmetic by extraplanetary beings.
Modern Discoveries Related to Huygens’s Theories
Titan (Saturn's moon):
Largest moon of Saturn, second-largest in solar system.
Atmosphere: Nitrogen, hydrocarbons (ethane, methane), carbon dioxide.
Surface: Lakes of liquid hydrocarbons, raising questions about alternative biochemical pathways.
2017 Discovery: Vinyl cyanide, a molecule for membrane formation.
Searching for Extraterrestrial Life
Challenges: Huygens noted difficulty in observing exoplanets due to stars' glare.
Modern Detection Techniques (Indirect):
Radial Velocity Method: Detects star wobble caused by orbiting planets.
Transit Method: Identifies periodic dimming of starlight as planets pass in front.
Over 6000 extrasolar planets discovered.
Classification of Extrasolar Planets:
Mini Neptunes: More massive than Earth (greater than 10 Earth masses) but less than Uranus/Neptune.
Hot Jupiters: Gas giants orbiting very close to their stars (orbital periods less than 10 days), resulting in extreme temperatures.
Super-Earths: Planets with masses ranging from 1 to 10 times that of Earth. For instance, 55 Cancri e has an estimated mass of 8.6 and a radius twice that of Earth.
Majority of discovered exoplanets belong to the super-Earth category, contrasting with no such planet in our solar system