Life in the Universe Part 2: How many life-friendly stars have planets orbiting them?

Stars are formed when clouds of gas and dust start to rotate and collapse in on themselves. As the cloud shrinks it starts to rotate faster and faster, much like an ice-skater who draws their arms inwards as they spin. When the cloud of matter collapses far enough, a star is born, and this star will spin with a speed based on something called the angular momentum of the collapsing cloud. If everything in the cloud goes into making the star, all of that system's angular momentum goes into making that star spin, and it does so quite rapidly.

Stars that spin relatively slowly, then, must have farmed out their angular momentum elsewhere. It turns out that most stars spin more slowly than would be expected, so where is that extra angular momentum? It is stored in other objects orbiting the star.

Around half of all the slowly spinning stars that we can observe have at least one stellar companion- a secondary star. Such systems are called 'binary' star systems, and the two stars orbit one another, sharing the angular momentum of the system.
What about the other half? We can't see any evidence of a single large companion with the rest of these slow-spin stars, so scientists suspect that the missing angular momentum in these cases is locked up in a number of smaller bodies: a planetary system. Improvements in the science of planet detecting are so far corroborating these ideas, with over four hundred extrasolar planets (exoplanets) having been discovered in recent years. Also, studies of the Orion Nebula using the Hubble Space Telescope and other instruments have shown that around half of the young stars in this region have protoplanetary disks; the disks of matter from which planets are thought to form.

How many suitable stars have planetary systems?
From the information above we can make the assumption that the answer is that about half of all sun-like stars have planets in orbit around them, though this is a very loose educated guess: In reality, with much more detailed studies, scientists currently predict that somewhere between 20% and 60% of suitable stars have planetary systems.

Life in the Universe
This is the second in a series of posts about life in the universe, culminating in a discussion about the Drake equation. The first post in the series centred on the number of suitable stars formed in a year, and the next one will follow on from this one, asking how many of the planets discussed above are capable of supporting life.


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