Betting on Planet Formation, and Century-old Insider Tips


In my field of work, it’s the biggest game in town.  How exactly do you predict how massive a planet will be before it’s even fully formed? I may not have the sure bet, but we’ve been working on ways to shorten the odds.

OK, so let’s explain this a little bit.  There are quite a few betting systems out there.  Our system only works on one breed of planets – those formed by direct break-up of the protostellar disc into fragments under gravity (see some of my earlier posts here and here).  These fragments are typically really massive planets, or very low mass stars.

So how can you predict the mass of fragments?  Our system uses a method that’s been around for nearly a hundred years, and comes from Sir James Jeans.  He looked at waves in gas, and showed that if the perturbation caused by the wave was strong enough, then the wave could collapse under its own gravity.  This is the underlying reason why stars form in molecular clouds – the turbulence in these clouds makes perturbations, some of which collapse to form stars.

But we’re not looking at turbulent molecular clouds – we’re looking at rotating discs of gas, with spiral waves like the ones in the image above. We redesigned Jeans’ equations to see if the perturbation caused by the spiral wave would be sufficient to cause collapse.

These equations also tell us how much mass is inside the perturbation when it starts to collapse.  This is the key part of our betting system – we know the starting mass of the planet.  It doesn’t tell us the final mass, because a lot can happen in the early stages of a planet’s life.  The outer layers of the planet can be stripped away, the dust that’s mixed in with the gas will begin to sink to the centre, forming a rocky core, and the planet can continue to feed from the gas disc, growing in mass.  But knowing the starting point of this whole process gives us a much better chance of guessing the mass of extrasolar planets.

Sadly, betting on planet formation doesn’t attract high stakes…at least, not yet…

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