Exoplanets come in various types, from gas giants to rocky worlds like Earth. Their characteristics, such as size, composition, and distance from their star, play a crucial role in determining their potential habitability.

The search for habitable exoplanets focuses on finding worlds with the right conditions for liquid water. Factors like a planet's position in the "habitable zone," its atmosphere, and its ability to retain heat all contribute to its potential for supporting life.

Exoplanet Characteristics and Classification

Classification of exoplanets

• Gas giants primarily composed of hydrogen and helium feature large radius and low density (Jupiter, Saturn)
• Ice giants contain hydrogen, helium, and ices (water, ammonia, methane) smaller than gas giants but larger than terrestrial planets (Uranus, Neptune)
• Terrestrial planets exhibit rocky composition with solid surfaces smaller size and higher density compared to gas and ice giants (Earth, Mars, Venus)
• Super-Earths larger than Earth but smaller than ice giants can be rocky or have thick atmospheres
• Hot Jupiters gas giants orbiting very close to their host stars experience high surface temperatures and short orbital periods

Exoplanet distance vs habitability

• Inverse square law of radiation intensity decreases with the square of distance $I = \frac{L}{4\pi r^2}$, where I is intensity, L is luminosity, and r is distance
• Equilibrium temperature planet would have if it were a perfect blackbody depends on distance from star, star's luminosity, and planet's albedo
• Atmospheric effects greenhouse effect can increase surface temperature while circulation distributes heat
• Tidal heating gravitational interactions cause internal heating more significant for planets close to their stars or in elliptical orbits

Habitable zones around stars

• Habitable zone region around a star where liquid water can exist on a planet's surface
• Inner boundary determined by runaway greenhouse effect (Venus)
• Outer boundary determined by CO2 condensation and ineffective greenhouse warming (Mars)
• Factors affecting habitable zone boundaries include:
  • Star's luminosity and spectral type
  • Planet's atmospheric composition and pressure
  • Planet's mass and ability to retain an atmosphere
• Continuous habitable zone remains habitable as the star evolves over time

Atmospheric effects on exoplanet climate

• Greenhouse gases CO2, water vapor, and methane trap heat extend the habitable zone outward
• Atmospheric pressure affects temperature distribution and phase transitions of water higher pressure broadens the temperature range for liquid water
• Biosignature gases oxygen and ozone potential indicators of life methane in combination with oxygen stronger biosignature
• Atmospheric escape low-mass planets struggle to retain atmospheres stellar wind and radiation can strip away atmospheres
• Albedo reflectivity of the planet's surface and atmosphere affects the amount of energy absorbed from the star
• Atmospheric circulation distributes heat from day side to night side on tidally locked planets creates habitable regions on otherwise uninhabitable worlds