The Fate of Top Down Planets

Avid readers may remember a few years ago I blogged about the statistics of objects formed by disc fragmentation. In our paper, we looked at tidal downsizing, to see if big fluffy disc fragments with masses bigger than Jupiter could form solid cores and lose their gas to become objects of Neptune, maybe even Earth masses.  We created what’s known as a population synthesis model – this is essentially a way of running lots of simulations very quickly to get lots of output data, in our case lots of fragments which have evolved to form objects (or not, as the case may be).

We discovered that making terrestrial planets is very hard – after producing millions of disc fragmentation events in our model, only one of those objects went on to make an Earthlike body. In fact, we lost about half of the fragments to tidal disruption – they were ripped to shreds by getting too close to their star before they had become fully formed. Most of the surviving fragments were giant planets or brown dwarfs orbiting at about Neptune’s distance from the Sun. 

Our one in a million Earthlike planet formed by tidal downsizing, all alone at the bottom of the plot.
Our one in a million Earthlike planet formed by tidal downsizing, all alone at the bottom of the plot.

But, our model was incomplete. We wanted to run millions of simulations, so we had to simplify the physics quite a bit. This is a common problem for population synthesis models, as they need to run to completion without it taking millions of years! The only way to beat it is by using clever algorithms and lots of supercomputer power. 

One of the most important pieces of physics we jettisoned early on was fragment-fragment interactions. Our discs were fragmenting to form multiple objects with relatively strong gravitational fields – they should be pulling on each other and changing their orbits.   Also, the systems were likely to still be in their parent star cluster. All those nearby stars would have a gravitational effect as well! 

Instead, our fragments coast along on nice circular orbits.  We didn’t add gravitational interactions to the model because it would have needed extra computational power, and would have taken much longer to produce results. Also, we knew that in the simulation, the disc would have been the biggest nearby source of gravity, and would have acted to smooth out these orbital changes. 

But eventually, the disc will disappear. Then what happens? We decided to run our models through N Body simulations to find out. With the help of Richard Parker, we placed our systems into his star cluster simulations, and we ran separate simulations to find out whether the planetary systems the model produced were stable. 

An example of an unstable planetary system produced by the population synthesis model.  Four bodies orbit the star initially - two bodies get launched away from the host star, leaving two in stable orbits.
An example of an unstable planetary system produced by the population synthesis model. Four bodies orbit the star initially – two bodies get launched away from the host star, leaving two in stable orbits.

So what did happen? One thing that happened quite a lot (about 25% of the time) is that a planet or brown dwarf gets ejected from the system. This means that if disc fragmentation happens regularly, we should see lots of free floating planets! Many of the bodies still orbit quite far from the star, but they now have very eccentric orbits. We know that some exoplanets have very eccentric orbits, like HD 8606b, so that is a sensible outcome.  Our simulations can now give us some predictions for what we should see with the next generation of direct imaging surveys, and we are working with observers to figure out whether our predictions match up with what they are now seeing.

Some, but not many, of the bodies get pushed very close to the star, probably close enough to become Hot Jupiters. This is where it gets interesting, because these will look very similar to Hot Jupiters formed by other planet formation mechanisms, like core accretion. How do we tell them apart?  This is something we’re working on right now, and I hope to tell you more about it soon. 

Making Sweet Planetary Music at NAM Hack Day 2014

I’ve been in Portsmouth this week at the RAS National Astronomy Meeting.  The weather has been extremely pleasant – bagged lunches on the steps of the Guildhall were very pleasant, as Bob Nichol notes:

I gave a quick 8 minute talk in the “IMF: Facts and Myths” session on the properties of brown dwarfs – those awkward objects that are too small to be stars, but too big to be planets.  These in-betweeners turn out to be a very sensitive probe of planet formation theories, and observing the mass distribution of brown dwarfs should tell us whether they are more starlike than planetlike (more on that in a future post).

Alongside my usual conference activities, I took part in the first ever NAM hack dayHack days are an opportunity for programmers and like-minded people to spend a day creating something useful or fun from scratch.  “Hack” is the operative word here – throwing together something in a few hours is never that polished 🙂

My effort was inspired by Pythagoras’ musica universalis, or “music of the spheres”.  Pythagoras, and others like him, were convinced that there was a deep relationship between mathematical concepts and music.  Music theory depends heavily on mathematics, but Pythagoras believed that mathematics itself was inherently musical, and that the Universe moved to a deeply beautiful set of rhythms and harmonies.  For example, he believed the motions of the planets produced a music that, if humans could hear it, they would not only consider it beautiful, but discover a deeper understanding of how the Universe worked.

So, I thought about the music in planetary systems.  We have the benefit of knowing many more planets than Pythagoras did, orbiting stars other than our Sun.  Even for a musical dunce like myself, it’s easy to create musical notes from the properties of planets.  And that’s exactly what I did for my hack: I took exoplanet data from the Open Exoplanet Catalogue, and made repeating notes for each planet.  The period of the planet’s orbit dictates how frequently a note is played.  If a planet orbits its star once a year, then its note will play once per second.  The pitch of the note is determined by the planet’s size – small planets play a high pitched tone, and large planets play a low pitched tone.

So here’s what the Solar System sounds like as a song (headphones recommended for the full bass experience):


The inner planets orbit the Sun quickly, and make a series of high pitched ringing sounds, with the giant planets beating out a slow, ponderous bass line.

The code I wrote to make this music is open-source on Github – you can find it here.  It’s written in Python, and has a reasonable user interface (remember it’s a work in progress!).  Happy music making!

What makes a Brown Dwarf and a Massive Gas Giant Different? Have Your Say

I’m off to the University of Hertfordshire in Hatfield, to speak at a conference on what makes exoplanets and brown dwarfs different (if anything).

mind_the_gap_v1.0Brown dwarfs are objects that don’t have enough mass to fuse hydrogen into helium (like our Sun does).  We know that a star can fuse hydrogen when its mass is about 0.08 times the mass of the Sun (or around 80 times the mass of Jupiter), and this sets a rather neat upper mass limit for brown dwarfs.  Where it gets really messy is the lower mass limit.

Traditionally, the lower mass limit for a brown dwarf is taken to be 13 Jupiter masses, as theory tells us this mass is the minimum required to fuse deuterium.  Below this mass, no nuclear fusion of any consequence can happen, and the object is “merely” a rather big gas giant planet.

Planet - or Brown Dwarf?
Planet – or Brown Dwarf? via Discovery News

This is the usual line taken by astronomers who aren’t in the brown dwarf game. It is a very simple rubric, but a little misleading, as it doesn’t reflect the arguments currently brewing in the field, fuelled by the latest infrared observations of very faint objects.  Glibly, the arguments are split into two camps – the formation argument, or the composition argument.  Is a pizza a pizza because of how it is made, or because of its ingredients?  As I tend to form what appear to be both planets and brown dwarfs in my simulations, I tend to feel the ingredients are more important, but there are probably several ways to make a brown dwarf, which muddies the waters significantly.

This conference will be laying out the current observational evidence from exoplanets and very low mass stars (or very massive planets, depending on your view), as well as new work on the theory behind how they are made.  As we learn more about the composition and interior structure of these objects from spectroscopy, and the many different formation mechanisms available, the old mass arguments are starting to look a bit too simple.

Interestingly, the organisers of the conference are polling the community, testing how they distinguish between planets and brown dwarfs.  They’re very keen for this poll to be completed by as many specialists and nonspecialists as possible, so click the link to find out more!

Alternatively, you can click here for some recent, tangentially related pedantry of my own…