In the short time that I’ve been enamoured with all things space our understanding of the universe has changed significantly. Just a few years ago we had no idea how common multi-planet systems like our own were but today we know that a star is far more likely to have several planets than just a few. At the same time we’ve discovered so many more exoplanets that their discovery is now just routine and the count has tripled from the couple hundred to well over 600 confirmed discoveries (not including the multitude of current candidates). At the same time our understanding of how planets form has also been called into question and today brings news that may just turn our understanding on its head yet again.
Astronomers at the Kavli Institute for Particle Astrophysics and Cosmology released a paper back in February that detailed a very interesting idea. Using the observable effects of gravity in our galaxy combined with the observable mass (detected via microlensing events) they’ve deduced that there needs to be many more planets than what can be accounted for. What’s really curious about these planets is that they would have formed without a parent star:
But how can this be? Every star can’t have tens of thousands of planets ranging from Pluto-sized to Jupiter-sized. This planetary “excess” actually suggests the existence of planets that were born without a star - nomad planets. These planetary vagabonds somehow went through the planet-forming process in interstellar space, not in the dusty proto-planetary disk surrounding a young star.
This astonishing number was calculated by extrapolating a dozen “microlensing” events of nomad worlds passing in front of distant stars. When these nomad planets drifted in front of distant stars, they briefly focused the starlight with their gravity, causing the star to brighten. This brightening was captured by astronomers and the microlensing events could be analysed to reveal the characteristics of the nomad planets.
The idea of planets forming sans a parent star is an interesting one as it turns our current ideas of planet formation on their head. The generally accepted idea of planet formation is that a large accretion disk forms a star first, sweeping away a lot of matter away from it. After that the left over accretion belt begins coalescing into planets, asteroids and other heavenly bodies. Nomad planets then would have formed in smaller accretion disks without the required matter to form a star. If the paper is anything to go by this happens extremely often, to the tune of 100,000 times more often than there are stars in our galaxy.
Such planets are incredibly difficult to detect as we have no beacon to observe for wobbles (the radial velocity method). The only way we have to detect them currently is via microlensing and that means that the planet has to pass between us and another star for us to be able to see it. Even with so many planets and stars out there the chances of them all lining up are pretty slim which explains why we haven’t detected any to date. What we have found though are Brown Dwarfs and they’re quite interesting yet again.
Brown Dwarfs are what you’d call failed stars (or over-achieving planets, take your pick) as whilst they’re quite massive, on the order of 13 times the size of Jupiter at minimum, they still don’t have enough mass to ignite and become a fully fledged star. They do however generate quite a bit of heat which they give off as infra-red light. We can detect this quite readily and have identified many of them in the past. What’s intriguing though is that these Brown Dwarfs (or other nomad planets) could be used as stepping stones to the rest of the galaxy.
There’s a couple things that such planets could be used for. We already know that such planets could be used as a gravity slingshot to give current interstellar craft a speed boost en route to their destination. Another highly theoretical use would be to use these planets as refuelling stops if you were using some kind of hydrogen/helium powered craft. Such planets would also make excellent observation posts as they’d be far away from strong sources of light and radio waves, allowing them an extremely clear view of the universe. Indeed nomad planets could be quite the boon for an interstellar civilization, all we need is the technology to access them.
I’m very interested to see where this theory takes us and hopefully we’ll star seeing some nomad candidates popping up in the exoplanet catalogues in the next couple years. We might not yet be able to make use of them but their mere existence would tell us so much about the formation of heavenly bodies in our universe. At the same time it also raises a lot of questions that we haven’t considered before, but that’s the beauty of science.