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Friday, June 27, 2014 / Duncan Forgan

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!

Monday, March 3, 2014 / Duncan Forgan

Life on Planets where 3 Spins = 2 Orbits

It’s been my pleasure to help on a recent paper to be published in the International Journal of Astrobiology, about how life might be on a planet with a peculiar spin.

Imagine a world where the planet’s spin was so slow, that one day took two thirds of a year.  Well, actually we don’t have to, as we can see a world in our Solar System that does this – Mercury:

MercuryNow, it’s immediately obvious that Mercury is extremely inhospitable, as it is so close to the Sun, and has no atmosphere to control its temperature (Mercurian days are 600 degrees C hotter than Mercurian nights!).  It is because Mercury is so close that it has this unusual relationship between its day and its year.  The Sun’s gravity causes tidal forces that twist and crush the planet, slowing down its rotation.  It just so happens that these tidal forces act in a rhythmic way, just as people do when they push each other on swings.  This rhythm allows the planet to enter what is known as a 3:2 spin-orbit resonance.  This means that there are 3 spins for every 2 orbits, 3 days for every two years!

Now imagine we take a planet like the Earth, and put it around a dim star.  For it to still be warm enough for liquid water, we have to put this pseudo-Earth closer to the star, close enough that it might fall into one of these 3:2 spin-orbit resonances.  What would it be like for life?

This is what we set out to discover.  Firstly, we had to think about how the sunlight would be distributed across our planet’s surface.  Now on a planet spinning quickly, it doesn’t matter whether you live in the West or the East, you get the same amount of sun.  Not on this 3:2 world:

2 years of sunlight on our planet's surface.  This planet's orbit is elliptical, like Mercury's.

2 years of sunlight on our planet’s surface. This planet’s orbit is elliptical, like Mercury’s.

When the planet’s orbit is elliptical, the sunlight tends to fall in hotspots.  This is because the star undergoes retrograde motion on the sky – this means that depending on where you stand on the planet’s surface, the star can rise in the east, change its mind, and set in the east! This happens because during an elliptical orbit, the orbital speed changes quite a bit, so sometimes the speed of spin outpaces the orbital speed, and sometimes it doesn’t.

Thanks to this (and the planet moving closer to and away from the star), the amount of light received at a point on the planet’s surface varies drastically, and according to a very unusual schedule.  Plants trying to use sunlight to carry out photosynthesis will need to take heed of this schedule, working frantically while the sun is up, and laying dormant for a very long time during prolonged periods of darkness.  The circadian rhythm for life on Earth is set to around 24 hours, and easily readjusted when it goes out of sync.  Imagine how complicated circadian rhythms would be on our imagined planet!

So what’s the point of all this? Well, we know that small dim stars are much more common than stars like our Sun, and we are getting closer to identifying Earth-sized worlds in the habitable zone of these stars.  So far, the only world we know of in a 3:2 resonance is Mercury, but that could soon change.  And when it does, we’ll continue our work, thinking carefully about how we might detect signs of life on these worlds.

Monday, February 24, 2014 / Duncan Forgan

The Case of the Missing Star

When stars are born and grow in close proximity to each other, the presence of such massive neighbours can have significant consequences, especially while the stars are young.  The gravity of a neighbour can trigger flows of gas and energy off a star’s surface, spraying it far into space.

The XZ Tau star system has these impressive outflows (see below).  These messy, bubbly flows fizz out far beyond the orbits of XZ Tau A and XZ Tau B, two stars long known to exist in the system (you can see them as the two bright spots in the lower left corner of the images).  XZ Tau B orbits XZ Tau A at about the same distance that Pluto orbits the Sun.  The structure inside the bubble suggests that there was a strong pulse at some point in the 1980s that launched a vast chunk of stuff.  At one time, it was thought that the pulse was caused by XZ Tau B coming close to XZ Tau A at a specific stage in its orbit, but it was later shown that the timings didn’t match.

XZ Tau's outflows of hot gas, as imaged by the Hubble Space Telescope (image Credit: John Krist (STScI) et al.)

XZ Tau’s outflows of hot gas, as imaged by the Hubble Space Telescope (image Credit: John Krist (STScI) et al.)

So what is causing the outflows? Recently, a third star was detected in the XZ Tau system, XZ Tau C. This star was detected using the Very Large Array of radio telescopes.  It was thought to have been hidden from optical telescopes like Hubble because it was shrouded in a very thick cloud of dust and gas.  It was found to be much closer to XZ Tau A, so perhaps it collected camouflaging material direct from the source of the outflow.

The first detection of XZ Tau C at radio wavelengths.  XZ Tau A and C are in the bottom left corner of the right hand image, and B is in the top right corner.  (Image Credit: Gonzalez et al, VLA)

The first detection of XZ Tau C at radio wavelengths. Each image shows the system at different wavelengths – XZ Tau C is only detected at the longest wavelength (7 mm).  XZ Tau A and C are in the bottom left corner of the right hand image, and B is in the top right corner. (Image Credit: Gonzalez et al, VLA)

Here’s where I come in.  The original observation of XZ Tau C was made in 2004, and we wanted to see if we could detect it again.  Our observations (using the upgraded Very Large Array) were made in 2012, so we expected to see XZ Tau C move from its previous position.  Comparing 2004 and 2012 data would allow us to estimate the orbit of C around A.  When XZ Tau C was detected, the astronomers who made the detection speculated that its orbit would have the right period to be the cause of the pulse in XZ Tau A’s outflows.

So, we made our observations, and found… no XZ Tau C.

Our radio observations of XZ Tau.  Despite observing with even higher sensitivity, we saw no sign of XZ Tau C (Image Credit: Forgan et al., VLA)

Our radio observations of XZ Tau. Despite observing with even higher sensitivity, we saw no sign of XZ Tau C (Image Credit: Forgan et al., VLA)

Despite using a much improved version of the VLA, we only saw XZ Tau A and B.  So what is going on?

One thing we know for a fact is that XZ Tau C hasn’t just left the star system.  If XZ Tau C was orbiting XZ Tau A and was somehow kicked out, we would see XZ Tau A and B moved in their orbits, and the whole system would recoil, like a gun does when it fires a bullet.  We found that the positions of A and B are where we would expect them to see, given previous studies of their orbits.

Could XZ Tau C have moved in front or behind of XZ Tau A? Yes, possibly.  In fact, we found that XZ Tau A was a little dimmer than we thought it would be.  Hardly conclusive proof, though.  It’s still possible that XZ Tau C was some kind of transient phenomenon – a distant object flaring up during the first observation in 2004.

We don’t have enough data to decide which of these reasons is the correct reason we didn’t see XZ Tau C.  We’ll just have to try and look again in a few years’ time.  If we see XZ Tau C peeking out from its hiding place as it moves away from XZ Tau A, we’ll know its orbit incredibly well.  If we don’t see it, then there’ll be one less star in the universe…


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