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Dr Karl: G'day. Dr. Karl here. Now, we all know there are eight major planets in our Solar System (sorry again Pluto). These eight planets all orbit the Sun in roughly circular orbits.

But, how's this for weird?

Imagine a planet that does not "belong" to a solar system and does not orbit a star!

Welcome to the free-wheeling life of a "rogue planet". And according to the latest "guesstimates", there could be tens of billions of "rogue planets" in the Milky Way.

Just like the 300 billion stars in our galaxy, these rogue planets are in some kind of orbit around the common centre of gravity of all the mass in the Milky Way. So, they roam freely through interstellar space, on their own solitary intrepid missions.

Astronomers have worked out a few ways that a planet can 'go rogue' — in other words, how it could be expelled from its home solar system.

In the first case, another star passes close by the system and pull one or more planets out of orbit.

In the second case, the planets within the solar system interact with each other via gravity — and again, one of the planets slips loose.

A third pathway is when a star loses a huge amount of mass as it evolves into a red giant — in its later life. Under certain conditions, this mass being thrown out from the star, could push a planet into interstellar space.

The fourth possible pathway can happen when the solar system is being formed. Over time, the planets will naturally grow bigger, as the material in the so-called "protoplanetary disc" clumps together. It's possible that when this disc is disturbed, some big fragments might be ejected, and that they could then coalesce into a planet outside of the star system.

But it's still early days for this research.

So far, we seem to have discovered about twenty-or-so of these Rogue Planets. In 2006, we found two in orbit around each other, with no star nearby. These planets were seven and 14 times the mass of Jupiter. All the other candidates are single rogue planets. And we may have even discovered a rogue planet with its own moon.

One rogue planet has been discovered as close as 100 light years from our solar system.

The main method used to find them is "microlensing".

"Microlensing" is when the brightness of a distant star suddenly changes, because another object has briefly passed between the star and us. And what can that object be?

Yup, you guessed it — often, a rogue planet! And with that star halo behind it, we get a simple and short-lived brightening of the distant object.

And it was this technique that led to the "guestimate" of tens of billions of rogue planets in our galaxy.

In 2011, a study looked at 50 million stars in our Milky Way galaxy, using the 1.8 metre MOA-II telescope at New Zealand's Mount John Observatory. They observed all these stars at least once per hour, for two whole years. They found 474 incidences of microlensing. Ten of these appeared to be planets roughly the size of Jupiter. However, 5 of these 10 cases were probably still in orbits around their stars, but just at a great distance. This left us with 5 where there was no star nearby.

Look at how quickly the numbers drop — 50 million stars, 474 microlensing incidents, and only 5 possible rogue planets. Such a small final number makes it hard to extrapolate what the total number of rogue planets in the Milky Way might be. Plus, we looked only for a few years, not centuries, and not millions of years.

But when the astronomers took all this into account, they came up with the rough "guesstimate" of billions of possible rogue planets in the Milky Way.

Now, this study actually measured what was happening to the brightness of 50 million stars in our galaxy, waiting for a rogue planet to pass exactly in front of one of them.

But another study took a theoretical approach. They set up a computer model and came up with surprisingly similar results — and I'll talk more about that next time...

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