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Journal 2/15/17
What requirements did we say were necessary for life?
How would we look for those in other star systems?
Objective
Tonight’s Homework
To learn about regions around
stars that can support human
life
Check your system for planets
within the habitable zone
The Habitable Zone
When we look at other star systems, one of the
first things we look for is what’s called the
“habitable zone”. This is the area in the star
system where you’d be just the right distance
for water to be liquid. Why? Because it might
mean a planet like Earth that we could someday
colonize.
So what factors influence the temperature of a
planet? (Let’s make a list as a class)
The Habitable Zone
For purposes of this class, we’re only going to
deal with 2 factors for finding habitable zone:
distance and atmosphere. There are dozens of
very complex equations to find this sort of
thing, but Mr. C. has done some math and is
boiling everything you need down to the simple
equation below:
Distance (in AU) = 73,632
(
L _ ½
(T-30)4
)
Or, rearranged…
Temperature (in Kelvin) = 271.35
L ¼
+ 30
D2
( )
The Habitable Zone
Example: Find the habitable zone range for the
sun.
The Habitable Zone
Example: Find the habitable zone range for the
sun.
We want to use the first equation as it will tell
us distances given temperatures. We said the
habitable zone is where the temperature would
give liquid water. Liquid water exists between
273 and 373 kelvin. So all we have to do is plug
those in to get our range.
Distance (close) = 73,632
1 _ ½
(373-30)4
(
Distance (close) = 73,632
Distance (close) = 0.63 AU
)
½
(7.22e-11)
The Habitable Zone
Example: Find the habitable zone range for the
sun.
1 _ ½
Distance (far) = 73,632
(273-30)4
(
)
½
-10
(6.86e )
Distance (far) = 73,632
Distance (far) = 1.25 AU
So for an earth-like planet with an atmosphere
like ours, we can expect to find liquid water at
any distance between 0.63 AU and 1.25 AU.
Earth is at 1 AU, so this makes sense!
The Habitable Zone
Problems with this idea:
We have a few places in our own solar system
that are outside the habitable zone but could
still have liquid water. Like Europa.
The major problem with the calculations for the
habitable zone is that they don’t take any odd
factors into account. Like tidal stretching,
volcanic activity, or ultra-thick atmospheres. We
have to keep in mind that this model is just an
estimate.
Habitable Zone Diagram
We’re going to do an activity where you find the habitable
zone range for each of the 7 major kinds of stars. Mr. C.
is going to give you a handout. On the handout, find the
close and far range for each type of star, given their
luminosities (replace L with each star’s luminosity). This
will tell you where liquid-water planets can be found for
each kind of star.
Exit Question
As a star gets bigger and brighter, its habitable zone will
move....
a) Farther from the star
b) Closer to the star
c) It depends
d) None of the above