How many planets typically surround one star?
Answers
That’s difficult to answer. We don’t really know.
On average, each of the 100 billion or so stars in our galaxy hosts at least 1.6 planets, according to the study, bringing the number of likely alien worlds to more than 160 billion.
To date, only a few thousand possible planets have been discovered outside our solar system. At present we do not have the technology to look at those planets like we would a planet in our solar system.
Statistically, it is nearly a sure thing that life exists on many other planets in probably every galaxy (300 billion) in the universe. Within that likelihood exists the probability that intelligent life exists elsewhere in the universe.
For one thing, it is not the case that larger stars tend to have more planets: A red giant may have swallowed some of its inner planets, whereas even the smallest stars can have a lot of planets (like the recently discovered example of TRAPPIST-1, an ultra-small red dwarf star with at least seven planets, possibly many more).
The number of planets around a star depends on how much material is available to form them, and how compact you can make the system without making it gravitationally unstable. Both of these restrictions mean that a star with many planets likely tends to have many small planets, not many big ones (this may seem obvious, but it’s important). Giant planets will take away a lot of material and they might prevent additional planets to form in a large portion of the system.
It is also worth to notice that there were probably about 100 planet-sized objects in the inner Solar System just after the Sun formed. The four inner planets we see today are simply those that survived. According to the official definition of a planet, it must dominate its orbit: Mercury, Venus, Earth and Mars cleared their orbit from all the other planet-sized objects, which is why they are actually “planets”. This in turn means that the Sun did not actually have 100 planets in its early days. Just 100 planet-sized objects.
It is probably possible to find a star with dozens of planets in a stable configuration. How likely that is to form is something we still have to figure out!
Unknown. We only detect the easiest and closese planets in other solar systems.
It also looks like our own solar system, with eight confirmed planets and a ninth suspected, is not typical.