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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…

Friday, January 17, 2014 / Duncan Forgan

Dear Better Together: I was going to vote No to Independence, but you’re pushing me away


When the referendum was announced, I felt quite strongly that Scottish independence was a silly idea.  I’m hardly a fan of the current UK Government, but I believed in the Union as something that, on balance, Scotland benefited from, and contributed positively to.  Yes campaigners may be quick to point out that statistic about how many Westminster election results bore no relation to Scottish voting data, but I’m fairly sure that I could find more than one Scottish-sized sample of voters in the UK who were “irrelevant” in the final tally.  Anyway, with such a poor voter turnout, how can any of us complain that British politics has stopped being representative? And how would an independent Scotland do any better? Would rural communities be in thrall to the Central Belt, as the UK seems to be to the South of England?

So I was moderately certain of my answer: I didn’t want to see a positive relationship binned because of some recent, admittedly moronic decision-making.  As the Scottish Government announced their policies in dribs and drabs, I scoffed along with the Better Together campaign.

Then the Yes campaign’s White Paper, Scotland’s Future, was released.  At over 600 pages, it’s a fairly large time sink to go from cover to cover, but given my major complaint against voting Yes was a lack of facts or answers to policy questions, I felt that I should listen to what they had to say.

Most of the facts in the document refer to the present, and rightly so – facts pertaining to the future violate the laws of physics.  It is easy to criticise it as

a) an SNP manifesto (because parts of it are), and

b) as a “wishlist” for Scotland’s future political framework, predicated on negotiations with bodies like the UK government and the EU, apparently because people seem to have forgotten “wishlist” is another word for manifesto.

All political decisions are based on uncertainty – it seems to me the Better Together voices are hooting about uncertainty after independence, without dealing with the uncertainties about where the UK is going.  I believe that Scotland and the UK as a whole should remain in the EU, but that is potentially subject to an in-out referendum in the future.  Any ballot is embedded in uncertainty, regardless of which way you vote (and even if you don’t vote at all).

The thing that bugs me most about the whole debate so far is its depressingly predictable trajectory of positive and negative campaigning.  The whole situation is admittedly biased from the start (“Yes” being a positive word and “No” being a negative word), but it seems to me like Better Together have accepted their fate, and much of their campaign appears to be deconstructing the Yes campaign’s White Paper.  While the Yes campaign have dedicated pages and pages of positive copy to Scotland’s Future (with a healthy side of negative jabs at Westminster), all I could find from the Better Together side was a few paragraphs on a website.

The White Paper is a vision for the future of Scotland.  You might not like aspects of it (I found parts I wasn’t sure of), some parts may simply not stand up to the evidence, but you may like some parts of it.  One thing I particularly welcomed was the discussion of a written constitution for an independent Scotland.  It’s all well and good to point out where the numbers don’t add up, but the Yes campaign can point to this paper (plus its supporting documents on fiscal and defence policy) and say to the No campaign: “where’s yours?”

At that point, it doesn’t matter if the White Paper is made of congealed pixie dust – its mere existence shows the electorate that one side is more engaged than the other.  Better Together need to produce a vision for the future of Scotland inside the UK.  Given their cross-party membership, they probably need to produce several.

At the beginning of the debate, the electorate were hungry for facts and policies, so that they could make an informed choice.  The Yes campaign have made their case well, couched in the language of hope (and hope used well is a powerful political tool).  There is a growing feeling of malaise and distrust towards politicians, and a sense that any figures can be massaged to look better or worse.  Exchanges like this, between Yes Prime Minister’s PM Jim Hacker and Cabinet Secretary Humphrey Appleby, are practically aphorisms:

Hacker: The statistics are irrefutable…

Humphrey: Statistics? You can prove anything with statistics.

Hacker: Even the truth.

Humphrey: Yes… No!

Thirty years later, TV’s most famous Scottish political animal is the foul-mouthed Malcolm Tucker, which does not really help matters.  My laboured point is that the electorate don’t care about the details, mostly because they don’t trust them.  They care about the message that the Yes campaign is sending – that an independent Scotland would move away from unpopular policies, stash its vast oil money in an Energy Fund and become an affluent Nordic clone, with mountains of free childcare, bus passes, university degrees and puppies.

The No campaign needs a message of its own.  Do you want a written constitution? Let’s discuss it at UK level, I suspect there are a lot of people who might like the idea.  Tuition fees? Let’s find a better solution.  Rural broadband and 4G distribution are not problems unique to Scotland.  Improvements to the distribution of Common Agricultural Policy rebates? Of course that’s a good idea.

In short? Better Together, stop scoring own goals, and give me some powerful reasons to convince me that the UK is the place to be long-term, not simply reject something that doesn’t yet exist.


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