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.
I can remember for the longest time being completely unaware of the asteroid belt between Mars and Jupiter. After learning about it however I never paid much more thought to it, although I was curious about how there seemed to be a line that separated the smaller, rocky planets from the large gas giants of our solar system (negating Pluto, of course). As my interest in space grew I began to wonder how any spacecraft that had ventured past Mars (there have been 9 of them) hadn’t managed to have a run in with a stray asteroid. As it turns out there’s a few reasons for that, and I find them quite fascinating.
The first is that the average density of the asteroid belt is extremely low with the total mass contained within the entire system being less than 4% of that of our Moon. Our best calculations then put the odds of a satellite coming into (unintended) contact with an asteroid in this region at about a billion to one, or so unlikely that you wouldn’t even consider it a risk. The images of the asteroid belt that many are familiar with make it look far more densely packed than it really is, much like this series of pictures that shows all the artificial satellites of Earth. That’s not to say the amount of junk we’ve sent up around ourselves isn’t an issue, but an accurate scale representation of each satellite wouldn’t look anywhere near as packed as they do.
What fascinates me the most about the asteroid belt however is how the majority of the mass is concentrated within 4 objects, with the two largest of these being 4 Vesta and an object big enough to be classed as a dwarf planet called Ceres. In astronomical terms they’re right in our backyard but even with our most powerful space based telescope we’re still only able to capture relatively blurry representations of them, shrouding these little heavenly bodies in mystery. Ceres especially so, with a series of images showing a massive bright spot moving across its surface of which its nature is still unknown.
So fascinating are these objects that NASA launched a mission to both of them, named Dawn, back in 2007. This particular spacecraft is something of a novelty in of itself as well as it is the first purely exploratory mission to use only ion thrusters for propulsion. It needs these highly efficient engines as it will be the first spacecraft to launch to its target, orbit it for a set amount of time and then set off again to approach yet another target. To do this it is carrying with it over 400kg of propellant enough for it to change its velocity by over 10km/s, a figure well above that of any other spacecraft that has come before it. It may take its time in doing so, but it’s still an incredible achievement none the less.
Dawn is scheduled to arrive at its first target, 4 Vesta, in just over a month and it has already begun sending back pictures and video of this strange mega-asteroid. They’re not much to look at right now but once its closer the imagery will become much clearer, revealing the nature of all the blurry spots we’ve as of yet only been able to speculate about. Dawn will spend a year surveying 4 Vesta before it sets off on its long journey for Ceres, for which it is not expected to reach until February 2015. Its a long wait to get a better look at something that’s so small compared to nearly everything in our solar system, but the prospect still excites me immensely.
Perhaps its the combination of their close proximity yet relative lack of information about these two little bodies that makes them so interesting, they’re just sitting there begging to be investigated. The next year will reveal all sorts of insights into the asteroid belt and its second largest contributer which will in turn tell us something about Ceres itself. We’re still a long, long way away from seeing Ceres in the flesh (or rock, as it were) but any information that Dawn sends back is valuable and I can’t wait to see what it brings us.