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Tuesday, September 9, 2014 / Duncan Forgan

Life on Tattoine Planets: More Complicated Than You Might Think

We all know that life on Earth depends on the Sun, and not just to keep the cold out. Plants convert sunlight into energy through photosynthesis, giving us oxygen to breathe, and in one way or another, the whole food chain relies on our parent star for support.

But what if our solar system had two stars? We know that there are several exoplanet systems out there that have the luxury of multiple host stars. In the case of the Kepler 47 system, there is a planet in the habitable zone (confirmed by several teams of astrobiologists, including myself). Kepler-47c orbits a star quite like our Sun and a cool red dwarf star (see graphic below), and so we refer to it as a circumbinary planet.

The Kepler-47 system, compared to our Solar System.  Credit: NASA

The Kepler-47 system, compared to our Solar System. Credit: NASA

But what would it be like to live on a planet like Kepler-47c, and gaze at a double sunset like Luke Skywalker? Would life be all that different? In a paper recently accepted for publication by the International Journal of Astrobiology, we explored this question.

Strictly speaking, it would probably be quite hard to stand on Kepler-47c, as it’s probably a gas giant comparable to Neptune, so we imagined that Kepler-47c was in fact an Earthlike planet – after all, chances are that there is a planetary system out there like this. We considered how the radiation from the two stars hit the planet’s surface, mapping patterns of light and darkness.

Because the two stars in the Kepler-47c system are so different in mass, they produce radiation at very different wavelengths – the sunlike star emitting a spectrum that terrestrial plants would happily photosynthesise, and the other star emitting much more red and infrared radiation, which some forms of anaerobic bacteria would photosynthesise (see more here). So depending on the time of year and time of day, different organisms would take the lead in converting starlight to energy.

But it’s not just the light patterns that are interesting. The darkness patterns show that above the polar circles (on earth, these are the Arctic and Antarctic circles), summer and winter become rather peculiar.

Above the arctic circle on earth, winter begins when the sun sets, and stays set until winter ends a few months later. On our Tattoine planet, there are two stars in the sky, so the arctic winter begins when both stars drop below the horizon. But the planet orbits the centre of mass of the system, as do the stars. This means that depending on the arrangement of all three bodies, some years have a winter that is a few days too short, and others have winters that are a bit too long. If you’re an animal counting on the end of winter to end your hibernation cycle, you need to know whether this year’s winter will be long or short!

Judging by life on earth, it seems likely that animals will be able to develop instinctive and biochemical rhythms to cope with these fluctuations, just as we have circadian rhythms to cope with day and night time. In fact, some organisms on Earth  already obey the influence of a second star – except it’s not really a second star, it’s just the Moon!

In short, life on circumbinary planets will be a slave to the rhythm, just like life on Earth. But there will be many more rhythms to choose from!

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.

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