What Makes a Planet “Habitable”?

Photo Credit: Space Engine by Wake Up Freeman

Kepler spacecraft observations indicate 20 billion habitable planets in our galaxy alone. What does that mean? Could humans someday infest 20 billion different planets?

Maybe, but not likely. And certainly not that many in our lifetime. The nearest potentially habitable planet, Proxima B, orbits one of our nearest stars, Proxima Centauri, which is four light-years away (or 24 trillion miles, better pack a lunch).

What are astronomers looking for when they decide what planets can support life?

Habitable Zone

The habitable zone is important for liquid water, and as far as we are aware, water begets life and absence of water eliminates it.  

For water to remain liquid on a planet the planet’s temperature must remain steady between 273K and 373K. There is no direct number of miles between every star and its habitable zone–the radiation of a star changes over time, thus changing the habitable zone. For example, four billion years ago, Venus was within our solar system’s habitable zone, and four billion years from now, Earth may not be.

Sounds like a catchy book title–Escape from the Habitable Zone!

Stable Stars

A star must be old enough to emit enough radiation for a long enough time that a planet reaches an ideal temperature.

It must not be too big–the bigger a star, the shorter its lifespan and the opportunity for life on its orbiting planets.

It must not be too small–the planets have to huddle close for warmth and when this happens, they cease to rotate. Dynamite comes in small packages, as well. Smaller stars tend to be more violent and therefore less stable for life on a planet. A quick rule is no less than half the mass of our Sun.

A star can’t be excessively violent. While the Sun has been known to flare up (literally) it is a relatively calm star. A violent star can destroy atmospheres and any burgeoning life on its nearby planets.

Similar elemental makeup to our Sun is key, as well. Too much metal in the star, or too little, produces planets vastly different from the silicate-based world we know. This is an interesting area of study–could we live on a diamond planet?

And lastly, for a solar system to be considered stable enough for a life-hosting planet, it shouldn’t be crowded with other jerk planets. Let’s not take any chances on interplanetary wrecking balls.

Habitable Mass

A planet with little mass is unsuitable for life because it also has little gravity. Gravity retains atmosphere–gases necessary for life and insulation to maintain stable temperatures. Small planets also lose their energy quickly, thus losing their volcanic activities which are necessary for recycling of a planet’s material. Mold grows on stale bread but most life requires fresh materials. Penicillin World.

A larger mass also indicates a larger magnetic core–crucial for gravity and for optimal rotation.

Axis Rotation

A planet must rotate at a steady pace–fast enough that the “dark side” doesn’t lower in temperature so far that life is destroyed, and slow enough that there is a balance of temperature. While different angles of rotation have the potential to create different versions of Earth’s seasons, the seasons are not necessarily crucial to all forms of life, although they do instigate the evolution of variety.

Molten Core

Our closest neighbors, Venus and Mars, do not have active planetary cores. They do not support life.

Earth’s molten core with its energetic volcanic activity recycles Earth’s materials, redistributes minerals and gases, changes landscapes, and stimulates evolution in the process. Earth is the only planet with identifiable plate tectonics.

Boring old plate tectonics may be the deciding factor in a “second” Earth. Pay attention, seventh graders!


Who likes breathing, raise your hand. Now put it down, freak. This is the internet. No one is watching.

Who likes not freezing to death? Who likes not burning to death? Great, that’s everyone. That’s why we need an atmosphere.

It needs to be thick enough to insulate but thin enough that it releases excess heat. It also needs to contain appropriate gases, like carbon dioxide, if humans are to exist there. Other life forms may have adapted to different gases and levels of those gases. We’ll wear astronaut suits there, they’ll wear them here. Guess that eliminates kissing aliens.

Now that’s a romance novel I’d read–Never Love an Alien.


Would you leave everything behind to colonize a new planet? Tell me in the comments.

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