Yesterday’s Pluto flyby’s been one of the most amazing unmanned space moments of the last half century – up there with the Voyager probes and with the Mars rovers. It’s also only the beginning. Over the next sixteen months the probe will transmit the data it picked up during that super-fast cruise through the Pluto system.
None of which, I suppose, will dislodge the decision by the IAU to make Pluto not-a-planet, after new discoveries revealed it was but one of many ice-worlds in the Kuiper belt. Add to that the flood of data about other solar systems, and I can’t help thinking there’s a bigger problem in hand. A lot of what we’re finding doesn’t fit the old idea that solar systems consist of (a) rocky planets close to the star, (b) gas giants further out, and (c) asteroids and comets. We’ve found:
- Lots of extra-solar planetary systems – more than 1000 as I write this, and more coming fast because their existence is buried inside data that’s already been collected. Very few are anything like ours.
- A lot of ‘brown dwarfs’, planets dubbed ‘failed stars’ because they’re too small to trigger nuclear fusion – but they’re plenty big enough to glow red-hot under their own heat of compression. Some have moons around them. Some are free-floating in space, away from other planetary systems.
- Planetary systems that are just forming – Formalhaut, for instance. The Hubble telescope directly photographed what looks like a planet ‘under construction’.
- Multi-star systems with planets (yeah – ‘Tattooine’).
- Lots of ‘hot Jupiters’ – planets orbiting within roasting distance of their stars. The nature of orbital dynamics suggests their systems wouldn’t have planets further out. Maybe. There’s also evidence these worlds migrate – and this is common.
- Super-Earths (‘hot Neptunes’) within the ‘Goldilocks’ zones of stars. We’ve also found giant worlds in the same place, including one pair orbiting in lock-step around a star 120 light years away (I’ve set a sci-fi story there- watch this space).
To me that suggests going back to first principles to find a way of classifying them. We know solar systems form from dust clouds. Rather than getting hung up over what is a planet, what is a star and so forth, why don’t we look for broader patterns? Starting with the way they are formed.
A lot depends on the size and composition of the dust cloud. The earliest universe was hydrogen. Everything heavier emerged from fusion inside stars. It happened in two steps, because normal stellar fusion can’t produce anything heavier than iron. Heavier elements than that require a net input of energy to form.
So the first stars in the universe, Population I stars, had solar systems with nothing heavier than iron. However, stars that fuse hydrogen in their core into each element up to iron usually explode – as a supernova. In the process of that explosion, the iron is fused into the rest of the elements and flung into space, along with the usual hydrogen, helium and so forth. Clouds of the sort that formed our own solar system (and a lot of others) are the detritus of supernova explosions – stardust.
New stars that form out of these clouds are known as Population II stars, and their planetary systems have the full range of elements we know and love. A good deal depends on the size of the cloud – or on the coalescing swirl within a larger cloud that allows a system to form. The evidence is that, more often than not, these clouds produce brown dwarfs or M-class red dwarfs, the most common star in the universe. And it seems to me that we could usefully consider them as being on a continuum with Jupiter-type worlds – the differences come down to the amount of matter accumulated, and what boils off.
As for planets – well, they’re down the same continuum. During the process, all sorts of things influence the final planetary mix – including migration. But in principle, the outer edge of the formation disk remains icy and likely to coalesce into Pluto-type objects – and, further out, into looser-formed things like comets.
All these things are formed by the exact same laws of physics, and similar mixes of chemicals. Yet – depending on the size of the cloud and the specific blend of gravitational forces and radiation pressures it’s subject to, dramatically different styles of world and solar systems form. What’s more – although we’re at the beginning of the learning curve – there’s evidence ours is the exception rather than the rule.
That’s why I think we need to return to first principles and reconsider how we conceptualise all of this.
One thing’s certain, though – whatever we discover, the fact remains that we’re made, quite literally, of stardust.
Copyright © Matthew Wright 2015