🪐Intro to Astronomy Unit 21 – Star Formation and Exoplanet Discovery

Key Concepts and Definitions

• Interstellar medium (ISM) consists of gas and dust between stars in a galaxy
• Molecular clouds are dense regions within the ISM where star formation occurs
• Protostars are the earliest stage of star formation when the molecular cloud begins to collapse under its own gravity
• Main sequence stars are stable stars that fuse hydrogen into helium in their cores (Sun)
• Stellar evolution describes the life cycle of a star from birth to death
• Protoplanetary disks are rotating disks of gas and dust around young stars where planets form
• Exoplanets are planets that orbit stars other than our Sun
  • Terrestrial exoplanets are rocky planets similar to Earth, Mars, Venus, or Mercury
  • Gas giant exoplanets are large planets primarily composed of hydrogen and helium (Jupiter, Saturn)
• Transit method detects exoplanets by measuring the dimming of a star's light as a planet passes in front of it
• Radial velocity method detects exoplanets by measuring the wobble of a star caused by the gravitational pull of an orbiting planet

The Interstellar Medium and Molecular Clouds

• ISM is not uniform and consists of regions with varying densities and temperatures
• Composition of the ISM includes hydrogen (atomic and molecular), helium, and trace amounts of heavier elements
• Molecular clouds are the coldest and densest regions of the ISM with temperatures around 10-20 Kelvin
• Molecular clouds are composed primarily of molecular hydrogen (H2) and dust grains
  • Dust grains are important for shielding the interior of the cloud from ultraviolet radiation and allowing molecules to form
• Molecular clouds can be light-years in size and contain enough mass to form thousands of stars
• Turbulence within molecular clouds creates high-density regions called cores
• Cores that exceed the Jeans mass (critical mass needed to overcome internal pressure) will collapse under their own gravity to form protostars
• Magnetic fields in molecular clouds can help regulate the collapse of cores and influence the formation of stars and planets

Stages of Star Formation

• Gravitational collapse of a molecular cloud core marks the beginning of star formation
• As the core collapses, it fragments into smaller clumps called protostars
• Protostars continue to accrete matter from the surrounding cloud, increasing in mass and temperature
• Accretion disk forms around the protostar as material with too much angular momentum to fall directly onto the protostar
  • Accretion disks are the site of planet formation
• Bipolar outflows (jets) are launched from the poles of the protostar, carrying away excess angular momentum
• As the protostar contracts and its core temperature reaches ~2000 K, deuterium fusion begins, marking the birth of a star
• T Tauri phase is a period of instability as the young star continues to contract and accrete matter
• Once hydrogen fusion begins in the core, the star reaches the main sequence and becomes stable
• Entire process of star formation takes millions of years, with more massive stars forming more quickly than lower-mass stars

Stellar Evolution and the Main Sequence

• Main sequence is the longest stage of a star's life where it fuses hydrogen into helium in its core
• Stars on the main sequence fall along a diagonal line on the Hertzsprung-Russell (H-R) diagram, which plots luminosity vs. temperature
• Mass of a star determines its position on the main sequence and its ultimate fate
  • Low-mass stars (<0.5 solar masses) are cooler, redder, and less luminous (M-type stars)
  • Intermediate-mass stars (0.5-8 solar masses) have moderate temperatures and luminosities (Sun, A-type stars)
  • High-mass stars (>8 solar masses) are hotter, bluer, and more luminous (O and B-type stars)
• Main sequence lifetime depends on mass, with more massive stars burning through their fuel more quickly
• Once a star exhausts its hydrogen fuel, it evolves off the main sequence and becomes a red giant or supergiant
• Low and intermediate-mass stars end their lives as white dwarfs, while high-mass stars explode as supernovae and become neutron stars or black holes

Planetary Formation in Stellar Systems

• Planets form in the protoplanetary disks around young stars
• Dust grains in the disk collide and stick together through electrostatic forces, forming larger particles
• As particles grow, they settle into the midplane of the disk and continue to accrete matter
• Planetesimals are kilometer-sized objects that form through gravitational interactions and collisions
  • Planetesimals are the building blocks of planets
• Terrestrial planets form through the collisions and mergers of planetesimals in the inner disk
• Gas giant planets form further out in the disk where ices can condense onto planetesimals
  • Core accretion model suggests that gas giants form when a solid core reaches ~10 Earth masses and rapidly accretes gas from the disk
• Planetary migration can occur as planets interact gravitationally with the disk, causing them to spiral inward or outward
• Protoplanetary disks dissipate after a few million years, leaving behind a fully formed planetary system
• Impacts and collisions continue to shape planets and their surfaces even after the disk dissipates (Late Heavy Bombardment)

Exoplanet Detection Methods

• Transit method is the most successful method for detecting exoplanets
  • Measures the dimming of a star's light as a planet passes in front of it
  • Provides information about the planet's size and orbital period
  • Kepler Space Telescope used the transit method to discover thousands of exoplanets
• Radial velocity method detects the wobble of a star caused by the gravitational pull of an orbiting planet
  • Measures the Doppler shift of the star's spectrum as it moves towards and away from Earth
  • Provides information about the planet's mass and orbital period
• Direct imaging method captures images of exoplanets directly
  • Difficult due to the glare from the host star and the small angular separation between the star and planet
  • Works best for young, massive planets orbiting far from their host stars
• Gravitational microlensing method detects the bending of light from a background star by the gravity of a foreground star and its planet
  • Sensitive to planets at larger orbital distances and can detect low-mass planets
  • Rare and non-repeatable events make follow-up observations difficult

Notable Exoplanet Discoveries

• 51 Pegasi b was the first exoplanet discovered orbiting a Sun-like star in 1995
  • Hot Jupiter orbiting very close to its host star
• Kepler-186f was the first Earth-sized planet discovered in the habitable zone of its star in 2014
• TRAPPIST-1 system contains seven Earth-sized planets, three of which are in the habitable zone
• Proxima Centauri b is the closest known exoplanet, orbiting our nearest stellar neighbor Proxima Centauri
• HD 209458 b was the first exoplanet observed to have an atmosphere in 2001
  • Hubble Space Telescope detected sodium in the planet's atmosphere during a transit
• Kepler-16b was the first circumbinary planet discovered, orbiting a pair of stars in 2011
• Kepler-10b was the first confirmed rocky exoplanet, with a density similar to that of Earth
• Kepler-452b is a super-Earth orbiting in the habitable zone of a Sun-like star
  • Sometimes called Earth's cousin due to its similar size and orbital period

Implications for Life Beyond Earth

• Discovering exoplanets in the habitable zones of their stars raises the possibility of finding life beyond Earth
• Habitable zone is the range of distances from a star where liquid water could exist on a planet's surface
  • Depends on the star's luminosity and the planet's atmospheric composition
• Presence of liquid water is considered essential for life as we know it
• Biosignatures are indicators of past or present life that can be detected in a planet's atmosphere
  • Examples include oxygen, ozone, methane, and carbon dioxide in disequilibrium
• James Webb Space Telescope (JWST) will be able to study the atmospheres of exoplanets and search for biosignatures
• Missions like the Transiting Exoplanet Survey Satellite (TESS) and the PLAnetary Transits and Oscillations of stars (PLATO) will continue to search for potentially habitable exoplanets
• Discovering a habitable or inhabited exoplanet would have profound implications for our understanding of life in the universe
  • Could indicate that life is common and arises readily given the right conditions
  • Would provide a new perspective on our place in the cosmos and the potential for life beyond Earth