Life in the Universe Part 3: How many planets are capable of supporting life?

This is part of the Life in the Universe series of posts. Go here for the first one.

The previous post in this series looked at how many suitable stars form planetary systems, so in this post we'll try to find out, on average, how many planets in each of those systems are capable of supporting life.

It's worth mentioning that so far the vast majority of extra-solar planets that have been discovered are in solar systems that differ in a number of ways from our own. For starters, most of the planets that we know of in other systems are much bigger than the Earth-like stars we want to look at: they're Jupiter-sized and bigger. Also, these Jupiter-sized planets tend to be closer to their parent star than Jupiter is to ours, as well as having distinctly more elliptical orbits (the orbits of planets in our system are closer to being circular). Both of these factors work against the possibility of an Earth-like planet forming with a stable orbit at the right distance from its star.

However (and this is a big however), the techniques and equipment that we're currently using to find planets outside our own solar system are much better at detecting large planets than small ones so it makes sense that, at the moment, we're only finding massive planets. As techniques and equipment develop and improve we may well see an increase in the number of Earth-like planets orbiting stars in solar systems similar to our own.

What makes a planet suitable?
Remember that in this series we're looking at finding life that is likely to be similar to us and able to reach similar or greater levels of technological ability. In looking for species that fit the bill, it makes most sense to look for planets similar to our own, so a promising planet in a suitable system would have to have the following characteristics.

  • At the right distance from its parent star for liquid water to exist- not too hot, and not too cold, this region around a star is called its 'goldilocks zone';
  • In a stable, nearly circular orbit around its star;
  • Big enough to retain heat from its formation for long enough, and to keep hold of its atmosphere;
  • Has a molten iron core which can rotate to produce a magnetic field strong enough to protect any life that develops from the solar wind;
  • Not so big that its atmosphere is too thick, blocking light from reaching its surface.

What is the chance of finding a suitable planet in orbit around a suitable star?
Back in the sixties when these questions were first being asked, it was suggested that there would be somewhere between one and five such planets in orbit around any given candidate star: even in our own solar system there are places other than the Earth in which scientists are hoping to find evidence of life, if not as advanced as that on our own planet.

With advances in knowledge and technology, however, we have discovered that solar systems with planets that fit the bill may be rarer than originally thought. Now, estimates for suitable planets range from one in every five solar systems having a suitable planet to one in fifty. That means that if you pick any fifty suitable star systems at random, you could expect to find somewhere between 1 and 10 suitable planets in total.

Life in the Universe
This is the third in a series of posts about Life in the Universe, culminating in a discussion about the Drake Equation. The first post concentrated on the number of suitable stars formed each year, the second looked at how many of these stars form planetary systems. The next one will follow on from this one and discuss the question of what fraction of these planets will see the emergence of life.


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