Astrobiology: Exploring Other Worlds

Week 1 - Planets in the Solar System and Beyond

Planets beyond our the solar system
- Planets not orbiting our sun
Our situation in cosmic space
- Earth is ~26,000 light years away from the galactic center
- The Milky Way is 100,000 light years in diameter
- The Milky Way is part of a Local Group which is 10 million light years in diameter
- There is nothing special about our place in the Solar System
A planet is a solid (or partially liquid) body orbiting a star
- Too small to generate energy by nuclear reactions
Until recently we couldn't see exoplanets directly
First confirmed exoplanet was 51 Pegasi b
- Not an Earth-like planet
We have discovered nearly 4000 exoplanets as of 2018
Many of the first exoplanets were hot Jupiters (close to their star but massive)
- How could they form so close to thier stars?
Exoplanets tend to have more eccentric orbits than in the Solar System
- Makes us wonder if the Solar System is typical in the universe or not

Stars are born in molecular clouds
- These clouds collapse and become dense (gravitational collapse)
- Accretion disk is formed
  - Small objects gather and stick together
The type of exoplanet which a protoplanet will develop into depends where it is positioned in the accretion disk
- Rocky planets
  - Inside the frost line
  - Available material is primarily rock and dust
- Gas giant planets / ice giant planet
  - Outside the frost line
  - Protoplanetary disk is mostly ice and gold gas (still has rock and dust)
Differentiation
Summary
1. Molecular cloud collapse
2. Protoplanetary disk
3. Accretion
4. Differentiation
There are ~100-400 billion stars in the Milky Way
- Some live very short lives
- More long-lived than short-lived
- Recent results from kepler space telescope suggest that between 1/5 to 1/2 of all stars have at least one planet
  - 0.2 * 1000 billion stars = 20,000,000,000 --> conservative estimate is 20 billion stars in the Milky Way

The surface of rocky planets is constantly changing
- Geologically active
Magnetic shields are a product of differentiation
- Shield the surface from harmful rays of space
- Helps a planet retain its atmosphere
Planets near their star:
- Hot
- Cannot keep light gasses in their atmospheres
- If they are very close, they tidally lock their star (one side always facing the star)
Planets far from their star:
- Cold
- May not have liquids on their surface
- Can retain a thick atmosphere
- May have subterranean liquids
Ices --> mostly frozen hydrogen compounds
- Water
- Methane
- Ammonia
- Carbon dioxide
Moons form around planetesimals
We haven't detected moons around exoplanets
- Too small to detect

The Solar System might not be typical but we do believe planet/moon formation is universal
Planets
- Mercury
  - Geologically dead; no atmosphere
- Venus
  - Almost identical to Earth in size, mass, and interior structure
  - Runaway greenhouse effect
  - Might have been habitable 3 billion years ago
- Earth
  - Thick atmosphere helps with water retention
- Mars
  - Smaller than Earth
  - Old, cratered surface
  - Evidence it was wet in the past
- Jupiter
  - 71% of the mass of all planets in the Solar System
- Saturn
- Uranus
  - Large axial tilt (probably from being hit)
- Neptune

Planets like the Earth
Ice giant planet
- First ice giant exoplanet was discovered in 2014
Small exoplanets typically come in two sizes
- Mini-Neptunes
  - Smaller or less massive
- Super-Earths
  - Like Earth but larger or more massive
Measuring mass of planets
- Minimum mass
- Detected using the radial velocity method (Doppler spectroscopy)
- Real orbital inclinations and true masses are generally unkown (sin i degeneracy)

How do we find exoplanets?
- Small and trillions of miles away
Two methods for finding
- Wobble observation
  - Observing red and blue shift as it moves towards/away from us
- Brightness of its star will dip as it orbits in front

Uses reflex motion to detect exoplanets
Magnitude of the Doppler shift indicates the speed of the reflex motion (or radial velocity) of the star
Limitations
- Limiting mass
  - Bigger planets --> obvious shifts in the light from the star
  - Smaller planets --> smaller shifts
    - Some planets are so small the shift is imperceptible
- Orbital distance
  - Gravitational influence decreases over distance
  - Large orbital distances --> barely detectable reflex motion (even for massive planets)
- Only measures mass, not size
- Can't determine orbital inclination

Observing the light of a star
The light will dim as an exoplanet moves in front of the star (transiting)
Very difficult because the dimming is very short
Strongly prefers finding hot Jupiters
Limitations
- Size
  - The larger the planet radius, the easier it will be to observe dips
- Orbital distance
  - Planets with greater orbital distance from their star crease smaller dips

Relies on Johannes Kepler's laws of planetary motion
From the radial velocity method:
- With radial velocity and time (radial velocity method) and Keper's first law, we can determine the orbiutal period and distance
- With orbital distance and Kepler's first law, we can calculate the velocity
- Once a planet's velocity is known, we can calculate the mass of an exoplanet
From the transit method:
- Exoplanet radius (how much dimming is occurring --> radius)
From both:
- Exoplanet density (once we know the size + mass)

We can't take images of exoplanets because they are too faint compared to their stars
- Need to block out the star first if you want to take an image of an exoplanet (easier to do in IR)
Coronagraphs
- Designed to block out the central cirlce of its field of view
- Used to block out a central star so we can image exoplanets
Gravitational lensing
- Massive objects warp the fabric of space-time
- Light travels through this warped space
- Better for more distant exoplanets
- Alignments are very rare