I doubt there are alien spacecraft in the outer Solar system


You might have seen a fairly overblown article in the Daily Express about some things I said at the British Science Festival in Bradford (Thanks to @BritishSciFest for tweeting the above pic).  This article is probably one I would have addressed at Research the Headlines, but I thought I would put it here on my personal blog so things were clear.

The article seems to imply I’m convinced that there are alien spacecraft lurking in the Outer Solar System.  I’m not – in my opinion, the odds on that are extremely small.  However, we still need to map out the Kuiper belt and the other asteroid regions to rule it out rigorously using the scientific method.

This in fact was the thrust of my argument.  I’m of the opinion that the search for extraterrestrial intelligence (SETI) is likely to fail, but the process of failing will tell us something important about the human condition, and give hints at the ultimate fate of our civilisation.  What’s more, we’re already getting data from other astronomical surveys which would help, as I’ve blogged about before.

Amusingly, the Express article states:

There are some astronomers who are convinced there is no other intelligent life within a close enough distance that will ever allow us to make contact

I would include myself in the above “some astronomers”! The football pitch description isn’t quite right either.  The blue part in the penalty box is a crude representation of how many stars the $100m Breakthrough Listen programme will eventually look at, using a small range of radio and optical frequencies.  The little orange box in the top left corner post is an approximate estimate of the number of stars we’ve looked at previously.

The Express article cherry-picked some images of my slides to back up their bombastic statements – you can see the whole lot here.  Note my subtitle is “A Pessimist’s Plea for SETI”.  Also note that I was never approached for comments directly, despite giving out an email and Twitter handle.  Finally, as was pointed out by a colleague:

It’s a shame that these mistakes were made – other aspects of the article are quite reasonable.  I guess it would help matters if I blogged here more often, and I will try to do that (look out for a piece on our latest ideas about searching for dead civilisations).

In the meantime, a pinch of salt is always a good idea…

Under the Radar: Is it the Weed or the Nicotine that’s Helping You Lose Weight?

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Duncan Forgan:

Here’s my latest piece for Research the Headlines, for those of you interested in weight loss, smoking illicit substances, or both…

Originally posted on :

It’s a touchy subject in most of the Western world – should marijuana be legal? Both sides marshal various arguments for and against legalisation.

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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.