National Geographic- "Goldilocks Worlds"

Goldilocks worlds: just right for life?

Of the 1,780 confirmed planets beyond our solar system, as many as 16 are located in their star’s habitable zone, where conditions are neither too hot nor too cold to support life. Size also matters: A planet that’s too small can’t maintain an atmosphere; one that’s too large will have a crushing atmosphere. A recently detected planet 493 light-years from Earth, Kepler-186f, is close to Earth's size and is located in its solar system's habitable zone.

Graphic by John Tomanio and Xaquín G.V., NGM staff.
Source: Abel Méndez, Planetary Habitability Laboratory, University of Puerto Rico at Arecibo
Note: Exoplanet mass estimated from mass-radius relationship when not available.

Star

Hot orbital zone

Warm

Cold

Too hot

On planets orbiting close to their respective suns, surface water evaporates into space.

Just right

Any water present on a planet orbiting here can remain liquid, given the right atmospheric pressure.

Too cold

Here planets orbit far from their suns, so any surface water remains frozen.

EARTH MASS

EARTH MASSES

EARTH'S MASS

EARTH MASSES

EARTH MASSES

Life in a box

Planets in the box have the right atmospheric pressure and the right temperature to keep surface water in a liquid state. In our solar system Earth and Mars are in the box, Venus and Mercury outside. The cold gas giants are literally off the chart.
The logarithmic scale used initially here allows for an easier comparison of the exoplanets. The linear scale shows how shallow the habitability zone is.

Logarithmic

Linear

Most confirmed exoplanets are much larger and hotter than Earth, characteristics that make them relatively easy to detect.

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