Goldilocks Zone

The "Goldilocks zone" is the distance from a star where liquid water can exist on a planet's surface. Based on the nursery tale, it is not too hot, nor too cold, but it is just right. It is sometimes known as the 'habitable zone'.

Stellar variations

The size of the star will effect the size and distance from the star of the goldilocks zone. Red dwarfs (the smallest stars) are cool, so their Goldilocks zone is close to the star. Stars brighter than Sol, our sun, will have larger zones, which extend further from the star.

Other bodies in the system might change the size of the Goldilocks zone. Imagine a cool star with a larger than Jupiter sized gas giant orbiting it just outside of the zone. Such gas giants give off a great deal of infrared light (heat), and a moon of such a planet might have liquid water despite being a bit too far away. If there is more than one star in this star system, the Goldilocks zone might be larger.

The age of the star also changes the zone. As a star ages, it becomes more dense and contracts, becoming hotter & brighter. Thus the golidocks zone will gradually move outwards as the star ages. For example, in our solar system, when the sun first formed it was only 70% as bright as it is now. In 500 million years, the sun will get too hot so the Earth will have a runaway greenhouse effect, and will lose its oceans. All multicellular life will die. Mars will become warmer, as the sun ages.

In our solar system, it would appear that the Earth is at the inner edge of this zone. Mars can have, briefly, liquid water on its surface, so it is almost in the Goldilocks zone. Venus is a hell planet with a surface temperature which can melt lead, so it is too close to the sun.

Planet atmospheres & the size of the Goldilocks zone:

However, the size of the planet, and especially the thickness of its atmosphere, can effect if a planet can keep liquid water. Let us imagine that Venus and Mars were swapped. Mars' thin atmosphere would retain little heat, but it would be so close to Sol that liquid water would likely exist on its surface. Venus with a atmosphere 90 times the mass of Earth's atmosphere would be much cooler in Mars orbit, and so it might have liquid water on it. (The run away greenhouse effect that boiled away its oceans would likely not have happened, so it probably would still have its starting oceans.)

In a solar system with Mars and Venus swapped, the Goldilocks zone would seem to be very large indeed.

Terraforming & the Goldilocks zone:

People have considered terraforming Mars by warming it. One method which would certainly work, would add massive amounts of super greenhouse gases to Mars' atmosphere. This would allow Mars to retain more heat from the sun, and warm up. As the planet warms, more gas (including water vapour which is a powerful green house gas), would enter the atmosphere and further warm the planet.

Thus, human effort can warm planets (and possibly cool them), adjusting the size of the Goldilocks zone.

Life & liquid water outside the Goldilocks zone:

The Goldilocks zone is where liquid water can exist on the surface of the planet. However, Mars has liquid ground water, so life can exist there, deep in cracks in the rocks. Europa (around Jupiter) and Enceladus (around Saturn) have frozen surfaces, but liquid oceans under many kilometres of ice. So it is likely that life can exist outside of the Goldilocks zone of stars.

In other solar systems, the Goldilocks zone may be expanded by gas giants, or brown dwarfs. A large body may radiate enough heat to make an otherwise too cold moon, have liquid water. Strong tidal heating may help keep liquid water (perhaps under a layer of surface ice).

Biography:

"The Birth and Death of the Sun", by George Gamow.

"Worlds Beyond Our Own: The Search for Habitable Planets", by Susan Sengupta.