Life in the 'Goldilocks zone' in space?

Their turn in the sunshine

Over the last few years, the term "Goldilocks zone" has come to be widely used to describe the range of distances from a star where its planets would be warm enough to have liquid surface water, but not too hot.

Obviously, our planet lies in the Sun's Goldilocks zone, with Mars orbiting at its outer edge. Planets lying in their stars' Goldilocks zones are prime candidates in our searches for life.

However, things have become much more complicated. There are other ways planets or moons can have oceans of liquid water that don't involve being at a precise distance range from a star.

If a moon orbits its host planet very closely, the gravitational pull of the planet on the side of the moon closer to the planet is larger than the gravitational pull on the other side of the moon. The result is a force trying to stretch the moon along a line joining it to the planet, so the moon gets stretched.

If that moon rotates, or if its distance from the planet changes, the interior of the moon is subject to a continuously changing stretching force. That produces heat. In the case of Io, the moon closest to Jupiter, that heat is enough to melt Io's interior, turning it into the most volcanic object in the Solar System.

Europa, the next moon out from the giant planet, produces enough tidal heat to maintain a deep ocean, protected from space by a thick layer of ice. The same situation applies to Iapetus, one of the moons of Saturn.

In the movie, Space Odyssey 2010, the writer Arthur C. Clarke explored the idea that extremely advanced alien beings judged that the life forms eking out an existence in Europa's oceans, in the darkness under the protective covering of ice, deserved a better chance.

So they used their advanced technology to turn Jupiter into a dwarf star, melting Europa's ice and providing a better opportunity for its life forms to advance and succeed. Now there is a possibility this could really happen, billions of years into the future. This will not need aliens, just the natural ageing of the Sun.

Our star, like all the other stars, formed from a big cloud of cosmic gas and dust. The amount of hydrogen fuel it picks up at the beginning of its life is what it will have to survive on for its lifetime.

We can divide the life of a star into four phases. First it is a young star, settling down, with its energy production processes getting into equilibrium. There follows a stable period, with the star brightening very slowly. Then, it starts to run out of fuel, expands into a red giant star, brightening a lot. Finally it runs out of fuel, either collapsing into a white dwarf star, or, for more massive stars, collapsing and exploding.

The longest phase is usually the stable period, known as the “Main Sequence Lifetime.” The Sun is at that phase of its life. Paradoxically, the more fuel the star starts with, the faster it gets through it. Less massive stars last longer.

One day, in maybe two billion years from now, the Sun will start running out of fuel. It will swell and brighten. Mercury and Venus will wind up being inside. The Earth may remain outside, but will still be roasted, ending up as a lifeless ball of hot rock.

However, the solar brightening could change everything for Europa and Iapetus. They could end up as temperate, ice-free worlds. However, this cosmic summer will be relatively short lived, because the Sun will then sneeze off its outer layers and become a really dim, white dwarf. There probably won't be time for life to develop from scratch, but if there is life eking out an existence out there, under the ice, it too might have "its time in the sun".

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Before dawn, Saturn is visible low in the southeast and Venus low in the east. In the evening, Mars lies low in the west. The Moon will be new on June 25.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.