How Many Habitable Zone Planets Are Out There?

Numworks Python and recent science results provide an amazing answer

Earth is unique in the cosmos, just like every person on this planet is unique. But there are many, many more habitable zone, or "Goldilocks" planets just like our planet, in that they too could support life as we know it. The evidence is overwhelming.

Just what is a "habitable zone", or "Goldilocks" planet? The core concept is that these planets are at the right distance from the star they orbit to have liquid water, the most fundamental requirement for carbon-based life forms.

Current thinking by the experts is tending to expand the habitable zone possibilities. Extremophile life is being discovered in many harsh ecosystem niches here, such as deep within the Earth's rocks, at the deepest points in the oceans, in acidic volcanic vents, and at the very top edges of our atmosphere. Life finds a way to exist just about everywhere.

Also, even though the moons of Saturn and Jupiter are outside the conventionally defined habitable zone, they have liquid water oceans under their ice crusts, and current speculation is that they could easily harbor life. There are likely many other planets in the known universe with similar icy ocean conditions, in addition to the planets included in conventionally defined habitable zones.

Let the calculations begin!

Recently, while authoring the book Python for Numworks , I created a program for high school students that uses the very latest numbers and estimates from credible scientific sources to make an estimate of the number of Goldilocks planets. The results were astounding, and hard to grasp.

I decided to create a mental visualization by equating each Goldilocks planet with a U.S. penny. By stacking these pennies a person could get a feel for the big numbers we're talking about.

For example, how much is one million? Well, a million pennies in tightly-packed-and-stacked rows and columns creates a mass roughly the size of a desk, or a small refrigerator. Keep this image in mind as we proceed with some even bigger numbers.

It could be more, but a conservative estimate is that one in every four stars, on the average, has a habitable zone planet. There are 250 billion stars in our average-sized galaxy, and the number of galaxies was recently bumped up to about two trillion. When you multiply all this out, there should be about 1.25 x 10²³ habitable zone planets in the known universe. Just how big is that number?

I'll include the Numworks Python source code here. Notice that multiple links are provided in the comments to resources where the latest numbers and estimates were obtained for the astronomical data as well as the penny dimensions and total land area of the continental United States.

# Pennies.py

# http://bit.ly/2Poq48E
# Dimensions of U.S. penny
diameter = 19.05 # mm
thickness = 1.52 # mm

# cubic millimeters per stacked penny
rect_vol = diameter * diameter * thickness

# cubic meters for one stacked penny
penny_vol = rect_vol * 1e-9  

# http://bit.ly/2Pqf8ao
# One in 4 stars has a goldilocks planet
goldilocks_factor = 0.25

# http://bit.ly/2VmUkoc
# Two trillion galaxies  
galaxies = 2e12

# http://bit.ly/2TitCdt
# 250 billion stars in our average sized galaxy  
milky_way_stars = 250e9

# Total stars in the known universe
stars = galaxies * milky_way_stars

# Total habitable goldilocks planets
habitable_planets = stars * goldilocks_factor

# Cubic meters of pennies if same
# number as habitable planets
cubic_meters_pennies = habitable_planets * penny_vol

# http://bit.ly/2HUkZRd
# Area of continental United States
us_square_kilometers = 8_080_464.3

# http://bit.ly/32qvRjj
# Convert area to square meters
us_square_meters = us_square_kilometers * 1000 * 1000

# Meters height of all those pennies if covering U.S.
height_in_meters = cubic_meters_pennies / us_square_meters

# Convert to kilometers
height_in_km = height_in_meters / 1000

# http://bit.ly/3c7NdFT
# Also convert to U.S. miles...  
height_in_miles = height_in_km * 0.621371

# Output results
print("\nNumber of habitable zone")
print("planets: ",habitable_planets)
print()
print("If every earth-like planet")
print("were a U.S. penny, you could")
print("stack them to cover the total")
print("continental U.S. land area to")
print("a height of",round(height_in_km,1),"km!")
print("(or",round(height_in_miles,1),"miles)")

I would encourage verifying, and updating these numbers as continuing research refines the data. I've been doing this for a few years now, and almost always the numbers get bigger rather than smaller as more accurate information is gleaned from the Kepler Space Telescope and other sources.

Okay, are you ready for the big pile of pennies? It turns out that if each of the 1.25x10²³ predicted habitable zone planets were a penny, they'd stack up to cover every square inch of the entire continental United States land area to a depth of over 5 miles!

"The universe is a pretty big place. If it's just us, seems like an awful waste of space."

― Carl Sagan, Contact

IMPORTANT UPDATE: November 2, 2020

This week a new estimate of the number of habitable planets was reported by NASA. It's a smaller number that I calculated before, but this study provides a very in-depth, highly reliable and much more accurate estimate based on the very latest research efforts. Their admittedly conservative estimate puts the number of habitable planets in our Milky Way galaxy at 300 million. Let's go with this much more reliable estimate and revise the program...

# Pennies.py

# http://bit.ly/2Poq48E
# Dimensions of U.S. penny
diameter = 19.05 # mm
thickness = 1.52 # mm

# cubic millimeters per stacked penny
rect_vol = diameter * diameter * thickness

# cubic meters for one stacked penny
penny_vol = rect_vol * 1e-9

# https://tinyurl.com/y6cqbaz5
# 300 million in Milky Way galaxy
habitable_milky_way = 300e6

# http://bit.ly/2VmUkoc
# Two trillion galaxies  
galaxies = 2e12

# Total habitable goldilocks planets
habitable_planets = habitable_milky_way * galaxies

# Cubic meters of pennies if same
# number as habitable planets
cubic_meters_pennies = habitable_planets * penny_vol

# http://bit.ly/2HUkZRd
# Area of continental United States
us_square_kilometers = 8_080_464.3

# http://bit.ly/32qvRjj
# Convert area to square meters
us_square_meters = us_square_kilometers * 1000 * 1000

# Meters height of all those pennies if covering U.S.
height_in_m = cubic_meters_pennies / us_square_meters

# Convert to feet
height_in_ft = height_in_m * 3.28084

# Output results
print("\nNumber of habitable zone")
print("planets: ",habitable_planets)
print()
print("If every earth-like planet")
print("were a U.S. penny, you could")
print("stack them to cover the total")
print("continental U.S. land area to")
print("a height of",round(height_in_m,1),"meters")
print("(or",round(height_in_ft,1),"feet)")

It's still a mind-boggling pile of pennies

This result dropped our big tall stack of pennies down to size. It's "only" enough pennies to cover the entire land area of the continental United States to a depth of 41 meters, or about 134 feet. That's still a huge, huge number! More importantly, it's a much more accurate and reliable estimate than ever before.

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