The key factors that determine whether a planet can support life include being located within the circumstellar habitable zone (Goldilocks zone) around its star where liquid water can exist on the surface, having a planetary size that allows for an atmosphere to develop and be retained, and orbiting a stable star that provides a constant radiation level like our middle-aged G-type Sun. Other considerations include the planet having a chemistry that supports liquid water through a greenhouse gas atmosphere, and being located within the optimal area of its galaxy to avoid threats from supernovae while still forming necessary heavier elements.

1. What makes a planet habitable?

2. Circumstellar Habitable Zone (CHZ)
“Goldilocks Zone”– is the region…

3. …around a star within which
planetary-mass objects…

4. with sufficient atmospheric
pressure can support liquid water
at their surfaces.
Too little, just right,
too much!

5. “Temperature”
Not too hot, not too cold
Liquid water is needed to exist on the
surface of a planet.

6. Too Close – Water boils off the surface
Too far – Water is frozen, not allowing
for molecules to mix and interact

7. Our solar system has one planet in the
Goldilocks Zone (0.9-1.2 AU)

8. Earth (But Mars had liquid water at
one time)
Tons of evidence of liquid
water flowing on the surface
of Mars.

9. Extended habitable zone (0.75-3.0 AUextended to Ceres)
The light green zone

10. Could Venus’s atmosphere be thinned
to allow liquid water to exist?
After??????
Before

11. Not to big not too small (size)
Planets need to have sufficient size to hold
onto its atmosphere, Mars lost most of its
atmosphere.

12. If planet is too big, gravity could limit
the development of life.

13. What type of star is it?
To determine
the habitable
zone you need
to know total
radiation a
star emits.

14. OBAFGKM- Our star is G-class

15. Massive stars- hotter, blaze with
radiation, zone is further out.
Live for short time, not enough time for life
to evolve

16. Smaller stars- tighter belts than our
sun, closer to the star
Live for
longer time,
more chance
for life to
evolve.
Ex. Kepler-62f, takes 267 days to complete an orbit

17. Best stars to search are F-M class, O-A
don’t live long enough

18. Is that star stable?
Solar eruption from a star could bathe a planet
in radiation.

19. New stars/old stars- variations in
radiation
Middle-aged star – radiation tends to be
constant

20. Liquid water- absorbs high amount
of radiation, could protect life
underwater

21. A planet’s chemistry?
A planet’s atmosphere will absorb a
certain amount of energy from
starlight and radiate the rest back
out.

22. Atmosphere- Tends to trap heat, more
CO2 or methane can increase the
greenhouse effect and extend the
zone.

23. Energy that is trapped- difference
between turquoise sea vs. erupting
volcanoes

24. Atmosphere- look for oxygen,
water, carbon dioxide and methane
(could indicate life).

25. CHZ Controversy- Other ways for
liquid water • tidal heating
• radioactive decay
• or pressurized by other non-atmospheric
means

26. …basic conditions of life in
interstellar space on rogue planets
or their moons.

27. Non-water solvents to hypothetical life
based on alternative biochemistries.

28. Galactic Habitable Zone : Area in a
galaxy where life has the best chance
of occurring.

29. Too close to the center, star density
increases, greater chance of being
taken out by a supernova explosion.

30. Too far out, less stars to generate
the heavier elements necessary for
planets and life to form.