The Wide Angle Search for Planets
Ben - This year, Professor Andrew Collier Cameron, from the University of St Andrews, announced some surprising discoveries from the Wide Angle Search for Planets, or WASP project.
Andrew - We've announced the discovery of nine new transiting planets from the WASP project which brings our grand total up to about 28 and among those, we've also done additional observations aimed at measuring the tilt of the planet's orbit relative to the star's equator. It turns out that two of the nine have orbits that are so strongly tilted that they're actually going around the star in the opposite direction to the stars spin - which is a bit of a surprise. So, these two odd balls join another three previously discovered WASP planets to which we've applied the same technique and which we've also found are going around in the sense opposite to the star's spin. Now, there's another group who've also found a 6th retrograde planet like this and this brings the grand total up to six planets out of 27 whose orbital tilts have been studied so far, that are going around in the wrong direction. So, these close orbiting planets which previously everybody expected would be orbiting neatly in the star's equatorial plane actually turn out to be all over the place.
Ben - Why was this result so unexpected?
Andrew - It came as a surprise because for many years, we've always considered that hot Jupiters form in the cold outer reaches of the planetary system, much as the gas giants in our own system did, but that they underwent unbalanced forces in those discs which very rapidly, within a few million years, drove them in through the disc, and then left them parked in orbits very close around their stars. The trouble is that the star itself has formed out of the material from that same disk. So, rather like a ping pong ball sitting on the plug hole, you would expect it to be spinning in the same direction as the water is going down the plug hole. So to find a planet going around in the opposite direction suggests that it's been through some violent interaction with something else which is completely reversed its motion.
Ben - We tend to have the idea that the early life of any solar system is quite violent anyway. There's lots of interactions and lots of collisions. Why should we be surprised that we're seeing evidence of violent action in other solar systems?
Andrew - Well, the violence that happened in our own planetary system all happened in more or less a single plane because it was driven by Jupiter and Saturn, having formed in the outer reaches of the solar system and there's a beautiful theory whereby, they migrated very slowly in towards the sun. They never got very far but they reached a point where Jupiter was going around the sun twice for every once that Saturn went around, and that played dynamical havoc with the rest of the system. It pushed Uranus and Neptune from relatively small orbits out to where they are today and at the same time, it sent planetesimals bombarding the inner solar system and bodies like Mercury and the moon still bear the cratering record of the so-called late heavy bombardment which happened about 700 million years into the solar system's history.
But the sort of violence we're talking about here is of the kind where something like that, Jupiter or Saturn event might have happened, but it was so violent that it actually resulted in one planet being thrown clean out of the system, and the other one stranded in a highly eccentric orbit. Even that might not be enough to give you a backward planet and there is a third flavour of the theory which invokes something that's called the Kozai mechanism. Now this was actually originally developed to explain why some comets have very highly tilted orbits because of the influence of Jupiter going around the sun. Now if you scale that picture up and you imagine a star that has say, a distant low mass, stellar or brown dwarf companion and if you've got a perfectly normal Jupiter, you do computer simulations and you find that the orbit of the Jupiter suddenly begins to oscillate and become very, very elongated. The planet passes very close to its star and moreover, the orbit begins to tumble. The upshot of this is that every time the planet goes close to the star, it raises a tidal bulge and that gradually leads to shrinkage of the orbit and the eventual formation of a hot Jupiter, but a hot Jupiter that isn't necessarily in the plane of this star's equator. It can be parked at just about any angle at all and that is actually remarkably close to the picture that seems to be emerging from our observations.
Ben - So if these systems have had a rampaging gas giant flying about at all sorts of angles throughout their history, would we expect to find other planets there?
Andrew - It would be very difficult to find terrestrial planets because the old theory whereby a Jupiter migrates in through the disk requires that process to happen while the disk is still there, obviously. And since the lifetime of the disk is only about 3 million years, then you have absolutely no trouble forming terrestrial planets out of the debris that's left in its wake. Now we think from meteoritic evidence and radio isotope evidence and from astrophysical evidence, looking at other planetary systems, that it takes about a hundred million years for terrestrial planets to form. So, Jupiter-like planets form very quickly but earth-like planets form very slowly. So, if you have a process that takes hundreds of millions of years as this Kozai mechanism does, then as you say, the rampaging gas giant is going to disperse all of those planetesimals long before they have a chance to build a terrestrial planet. So, if this is the correct picture for the formation of hot Jupiters, then we would certainly expect that hot Jupiters and Earths don't mix.
Ben - So the search for habitable planets may have to ignore systems with a hot Jupiter in place. That was Andrew Collier Cameron, from St Andrews University.