Resource Library

ENCYCLOPEDIC ENTRY
ENCYCLOPEDIC ENTRY

Goldilocks Principle

Goldilocks Principle

In astrobiology, the Goldilocks Principle applies to the range of distances that a planet’s orbit can be from its star and maintain temperatures on the surface that are just right for liquid water. This range is known as the Goldilocks Zone.

Grades

5 - 8

Subjects

Astronomy, Biology, Chemistry, Climatology, Earth Science

Image

Earth Moon Sun

Looking at the evidence available to them, astrobiologists had assumed the distance between Venus to Mars from the sun was the distance in which liquid water, and therefore life, was possible. This became known as the Goldilocks Zone.

Illustration by NASA

In astrobiology, the Goldilocks Principle applies to the range of distances that a planet can be from its star and maintain surface temperatures that are just right for water to be liquid. This range is known as the Goldilocks Zone. Temperatures that allow for liquid water are considered “just right” because life as we know it requires water.

Based on the idea that liquid water on a planet’s surface makes life possible, the Goldilocks Zone of our solar system extends approximately from the orbit that Venus takes around the sun to the orbit that Mars takes around the sun. Earth’s orbit is farther from the sun than Venus but closer than Mars. In other words, Earth’s orbit is within the sun’s Goldilocks Zone. This is why Earth can maintain a vast ocean of liquid water, which makes Earth a place where life can thrive.

Venus and Mars are both at distances from the sun in which water could have been possible on the surface. Planetary scientists think that Venus may have had surface water, possibly even a shallow ocean, billions of years ago. However, at some point Venus’s atmosphere filled with very high amounts of carbon dioxide and other gases, which caused an extreme greenhouse effect, heating the planet’s surface to about 462° C (864° F). The stronger the greenhouse effect has in an atmosphere, the more the planet is heated. The hot temperature on Venus means that if water were to exist on its surface, it would exist only as a gas (water vapor or steam). Earth’s atmosphere also produces a greenhouse effect, which keeps the planet from getting too cold, but it is much weaker than Venus’s, so water is able to exist in all three states. Additionally, Venus’s atmosphere is so thick that the air pressure on the surface of Venus is about 92 times the air pressure on the surface of Earth. Mars also went through changes long ago and lost liquid water from its surface. Solar wind stripped away the ancient Mars atmosphere, leaving the air pressure too low for water to exist as a liquid for extended periods.

Despite the prior belief that liquid water could only exist within the sun’s Goldilocks Zone, there is now evidence that celestial bodies outside of the Goldilocks Zone can have liquid water as well. These include a moon of Saturn called Enceladus and a moon of Jupiter called Europa. These two worlds may hold an ocean of water underneath a surface of ice. This is possible because as the moons move around their massive planets, their insides are heated, a phenomenon known as tidal heating.

Astrobiologists think that the oceans of Europa and Enceladus are the best places to look for life away from Earth. This means that the “Goldilocks Zone” extends beyond the distance from a star where water can be liquid on a planet’s surface and into areas that are much colder.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Director
Tyson Brown, National Geographic Society
Author
National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Producer
Clint Parks
other
Last Updated

May 20, 2022

For information on user permissions, please read our Terms of Service. If you have questions about licensing content on this page, please contact ngimagecollection@natgeo.com for more information and to obtain a license. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. She or he will best know the preferred format. When you reach out to him or her, you will need the page title, URL, and the date you accessed the resource.

Media

If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.

Text

Text on this page is printable and can be used according to our Terms of Service.

Interactives

Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources