Rogue Planets: Lost in space
Planets. There are lots of them in our Universe. They orbit around stars, have atmospheres, some have water and might hold life. But do they all orbit stars? Actually no. Some planets don’t orbit stars, instead they just float out in space, in complete darkness. It’s hard to find them, but there are thousands of them in our galaxy, and their surface would be radically different to almost any other environment in our Universe. Let’s dive in, to rogue planets.
Rogue Planets: An introduction
We generally think of planets as orbiting stars. All the planets in our solar system, the Earth, Mars, Jupiter orbit the Sun. Many other stars have large numbers of planets orbiting them in exactly the same way. In fact, it seems like most stars in the Milky Way have at least a few planets. Seeing as there are hundreds of billions of stars in the Milky Way and hundreds of billions of galaxies out there, there are probably trillions of planets across our Universe.
However, planets don’t all orbit a star. Some planets just sit out in space far away from any stars. These planets are called rouge planets and we have evidence for thousands of them existing in our galaxy.
Not a lot is known about how rogue planets are formed. Some are probably formed at the very edge of a solar system and then ejected out into space when their star explodes. Others probably just form from dust off on their own, not having quite enough mass to turn into a star. We’ve never really seen one form in our galaxy, so it’s hard to be sure.
The environment on a rogue planet would be very different from the environment on Earth. Our planet, and all the other planets in our solar system are heated by the Sun. For the Earth this means the air temperature on most of the planet stays pretty consistent and we get lots of light. This light is then absorbed by plants and algae and is the basis for life on Earth (see our post on photosynthesis here).
The environment on most rogue planets would be very cold. Because they would not be near any stars, they would not receive any light or heat from a Sun. Hence, most would be roughly the temperature of surrounding space, about -270 degrees. Similarly, they would be dark, not have any tieds.
Some rogue planets may have some heat. Some scientists have theorized those planets with very thick atmospheres might be able to keep heat. Similarly, a rogue planet with a satellite could be heated by the satellite through a process called tidal heating. So there is still some possibility that rogue planets should be dynamic environments and maybe even support basic forms of life. But for the most part, they’d be cold, lonely dead rocks.
Rogue Planets: How’d we find them?
The problem with talking about rogue planets, is how little we know about them. It’s easy to study normal planets – they’re right next to a star. This means, lots of light bounces off them and we can see them really easily. Depending on what light the planet is absorbing, we can tell what sort of atmosphere they have and what elements are present on its surface and in its atmosphere. For the ones in our solar system, we can even send probes and collect samples.
None of this is true for rogue planets. They don’t emit any light or have any big bright light sources near them. This means it’s almost impossible to even find rogue planets, let alone work out what they’re made of. We just can’t see them. It’s like trying to point a pair of binoculars to see a grain of sand in a dark room.
The only way we’ve been able to find these rogue planets is if they get in the way of other stars. The leading study in the area was by Astrophysicist Takahiro Sumi of Osaka University in Japan. They observed 50 million stars in the galaxy and for ‘microlensing’, basically a small distortion in the light coming from the star implying that something was in front of it. After eliminating all the planets the stars themselves had, and any other stars or dust getting in the way, they identified 474 incidents with no other explanation, that is, it was just some small weird planet getting in the way far away from the star. About 10 of these were then conclusively identified to be large planets (around Jupiter’s size). From these observations, this means there are properly two Jupiter-sized rogue planets for every star in the Milky Way.
Since that initial study, we’ve conclusively identified about 20 rogue planets. There’s a list of them on the Wikipedia page. We’ve managed to learn a little bit about a few of them, for instance OGLE-2016-BLG-1928, is roughly Earth-sized and made of rock.
Clearly, our understanding of these rogue planets is in its infancy. We don’t have any pictures of any of them. We don’t know what most of them are made of. We have wildly different estimates of how many of them there are, or how exciting they are. We need to do a lot more study.
Rogue Planets: Final Thoughts
Rogue planets make you realize how much of our thinking is bound up in pictures. Even the smartest astrophysicists, when they think of “planets”, think of a pretty basic primary school diagram of the planets around our Sun.
But that’s not what all planets are like. Some just float out in space. And there are thousands of them. And some very small number might even have dynamic environments, that could potentially hold life.
How many other pictures, that you have in your head about the Universe are wrong? Probably a lot of them. It’s good to approach our Universe with a great degree of humility. We know very little about it. And what we know, we certainly don’t always understand. Mystery abounds. There’s much more to do.
