January 9, 2013 | 2
It’s been an exciting few days for exoplanetary science. A slew of refined statistical measurements of the abundance of other worlds have made it clearer than ever that our galaxy is crammed with planets.
One in six stars should host at least one Earth-sized object in an orbit smaller than that of Mercury, implying that in total there are tens of billions of planets this size across the Milky Way.
But for me one of the most exciting announcements has come from new observations of the Fomalhaut system, a ‘mere’ 25 light years from us. Fomalhaut is a star some ninety percent more massive than the Sun and only a few hundred million years into its time as a hydrogen burning ‘main-sequence’ object.
Back in 2008 Hubble Space Telescope imagery confirmed that not only is Fomalhaut surrounded by a remarkable hoop-like structure of dusty material – encircling this system at more than 120 times the distance of the Earth from the Sun – but revealed that an object possibly of Jupiter size is orbiting at the apparent inner edge of this hoop.
This was quite a claim. It made this planet, Fomalhaut b, one of a mere handful of worlds directly imaged (as opposed to being detected indirectly as the majority of exoplanets are). It also raised a huge question – how could such a planet be on such an enormously large orbit, more than 4 or 5 times further out from its star than Neptune is in our solar system?
For nature to build a planet in situ at these distances confounds many of our theories about planet formation. The problem was exacerbated by the estimates of the shape of the orbit that suggested it was relatively circular – indicating that the planet had not been flung to this position by earlier dynamical jostling with other planets.
It was indeed puzzling. My colleague Kristen Menou and I had recently prepared a paper where we estimated the rate at which astronomers might expect to find such large worlds on such distant orbits from their stars. The mechanism for their placement that we simulated was an earlier episode of intense gravitational interaction between the original planets around such a star. If planets form efficiently (and they certainly seem to do so) they can start off crammed into the orbital space close to their parent star. But this can result in dramatic gravitational instability, the systems are dynamically ‘hot’. To cool down they collide, eject, and fling planets onto larger orbits.
There was a snag though. Our simulations, along with those from another group working on this problem led by Dimitri Veras, indicated that the remnants of such chaos – planets on large orbits – should also have highly non-circular, strongly elliptical orbits. Fomalhaut b did not seem to fit this pattern.
Flash forward to this week. The most recent Hubble imagery of Fomalhaut b, combined with the passage of time and an opportunity to refine the measurements of Fomalhaut b’s orbital path, now suggests that its orbit is in fact elliptical. At its closest passage to its parent star this giant world is estimated to be a ‘mere’ 4.6 billion miles away. At its furthest it reaches about 27 billion miles. Running these numbers indicates an orbital ellipticity, or eccentricity of about 0.7 – a 70% deviation from a circular path.
Our simulations tell us that this kind of orbit is almost precisely what you’d expect in about 5-10% of systems where strong dynamical cooling has taken place (yes, I’m patting myself on the back, along with congratulating my colleagues).
The new observations also reveal possible evidence for signs of another planet in the system that may be ‘slicing’ through this outer dusty hoop of material – it could be a candidate for the world that helped push Fomalhaut b to this orbit. As for Fomalhaut b itself, its mass is unknown, it could be anything between the size of tiny Pluto and Jupiter. It could also be surrounded by a disk or ring of its own, enhancing its apparent brightness.
Time will tell what’s really going on in this system, but it’s turning out to be a wonderful testbed for many ideas.