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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 46:
The Search for Life in the Universe

Key Ideas:

A Habitable Planet is Defined By:

Stars with rocky planets in their habitable zones are the best places to look for life

Planets are hard to find

Detecting life on other planets

Habitable Zone

The range of distances from the Sun, or more generally the parent star, where liquid water can be stable on the surface (with the appropriate atmosphere).

Inner edge of the habitable zone:

Outer edge of the habitable zone:

Habitable zone: 0.84-1.7 AU (for the Sun!)

Size of the Planet

Too small:

Too big:

Planet size limits: About 0.2 Earth Masses to 10 Earth Masses.

The "Goldilocks" Principle

Planets with a=0.84-1.7 AU and masses between 0.2 - 10 Earth masses are 'just right' in terms of their physical properties enabling liquid water on the planet over a long time scale. So where are the best places to look for life?

Stars with rocky, Earth-sized planets in their habitable zones are the best places to look for life

Planets are very small compared to their parent stars




Detecting Exoplanets

Direct Detection:

Indirect Detection:

Doppler Method

Detect the change in velocity of the primary star due to the indirect effect of the star orbiting around the center of mass of the planet-star system.

Doppler Method:

Known Extrasolar Planets:

Solar systems?

Pale Blue Dots

What is the ultimate goal?

Direct Detection of Earthlike Planets

Determining the properties of the planets

Features in Earth's Spectrum

The Earth's spectrum has many different features, but the most important in this context are: The spectrum of unresolved Earthlike planets may indicate the habitability of the planet or even the presence of life!
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