Posts Tagged ‘astronomy’
exoplanets – an introduction of sorts
Jacinta: So do you think we’ve hauled ourselves out of ignorance sufficiently to have a halfway stimulating discussion on exoplanets?
Canto: I think we should try, since it’s one of the most exciting and rapidly developing fields of inquiry at the moment.
Jacinta: And that’s saying something, what with microbiomes, homo naledi, nanobots and quantum biology…
Canto: Yes, enough to keep us chatting semi-ignorantly to the end of days. But let’s try to enlighten each other on exoplanets…
Jacinta: Extra solar planets, planets orbiting other stars, the first of which was discovered just over 20 years ago, and now, thanks largely to the Kepler Space Observatory, we’ve discovered thousands, and future missions, using TESS and the James Webb telescope, will uncover megatonnes more.
Canto: Yes, and you know, about the Kepler scope, l was blown away – this might be veering off topic a bit, but I was blown away in researching this by the fact that Kepler orbits the sun. I mean, I knew it was a space telescope, but I just assumed it was in orbit around earth, probably at a great distance to avoid interference from our atmosphere, but that we can position satellites in orbit around the sun, that really sort of stunned me, more I think than the exoplanet discoveries. Am I being naive?
Jacinta: No, not at all. Well, yes and no. Everything is stunning if you haven’t followed the incremental steps along the knowledge pathway. I mean, if you think, hey the sun’s a way away, and it’s really big and dangerous, best not go there, or something like that, you might be shocked, but think about it, we’ve been sending satellites around the earth for a long time now, and we know how to do it because we know about earth’s gravitational field and can calculate precisely how to harness it for satellite navigation. We’ve currently got a couple of thousand human-made satellites orbiting the earth and trying more or less successfully to avoid colliding with each other. So the sun also has a gravitational field and we’ve known the mathematics of gravitational fields since Newton. It’s the same formula for a star, a planet or whatever, all you need to know is its mass and its radius. And look at all the natural objects orbiting the sun without a problem. Can’t be that hard.
Canto: Okay… so how do we know the mass of the sun? Okay, forget it, let’s get back to exoplanets. What’s the big fuss here? Why are we so dead keen on exploring exoplanets?
Jacinta: Well the most obvious reason for the fuss is SETI, the search for extra-terrestrial intelligence, but to me it’s just satisfying a general curiosity, or you might say a many-faceted curiosity. And it’s all about us mostly. For example, is the solar system that we inhabit typical? We’ve mostly thought it was but we didn’t have anything to compare it with, but now we’re discovering all sorts of weird and wonderful planetary systems, and star systems, with gas giants like Jupiter orbiting incredibly close to their stars – it’s completely overturned our understanding of how planets exist and are formed, and that’s fantastically exciting.
Canto: So you say we discovered the first exoplanet about 20 years ago, and now we know about thousands – that’s a pretty huge expansion of our knowledge, so how come things have changed so fast? You’ve mentioned new technologies, new space probes, why have they suddenly become so successful?
Jacinta: Well I suppose it’s been a convergence of developments, but let’s go back to that first discovery, back in 1992. Two planets, the first ever discovered, were found orbiting a pulsar – a rapidly rotating neutron star. First discovery, first surprise. Pulsars with planets orbiting them, who would’ve thought? Pulsars are the remnants of supernovae – how could planets have survived that? But that first discovery was largely a consequence of our ability to measure, and the fact that pulsars pulse with extreme regularity. Any anomaly in the pulsing would be cause for further investigation, and that’s how the planets were found, and later independently confirmed. Now this was big news, in a field that was already becoming alert to the possibility of exoplanets, so you could say it opened the floodgates.
Canto: Really? But they didn’t discover any more for two or three years.
Jacinta: Well, okay it opened the gates but it didn’t start the flood, that really happened with the second discovery, the first discovery of a planet orbiting a main-sequence star like ours.
Canto: Main sequence? Please explain?
Jacinta: Well these are stars in a stable state, a state of balance or equilibrium, fusioning hydrogen – basically stars not too different from our own, within much the same range in terms of mass and luminosity. So 51 pegasus b was the first planet to be discovered by the radial velocity method, and radial velocity means the speed at which a star is moving towards or away from us. We can measure this, and whether the star is accelerating or decelerating in its movement, by means of the Doppler effect – waves bunch up when the object emitting them is moving towards us, they spread out when the object is receding from us, and the degree of the bunching or the spreading is a measure of their speed and whether it’s accelerating or decelerating. Now we can measure this with extreme accuracy using spectrometers, and that includes any perturbations in the star’s movement caused by orbiting bodies. That’s how 51 pegasus b was discovered.
Canto: So… how long have we had these spectrometers? Were they first developed in the nineties, or to the level of accuracy that they could detect these perturbations?
Jacinta: Well I don’t have a precise answer to that apart from the general observation that spectroscopes are getting better, and more carefully targeted for specific purposes. The French ELODIE spectrograph, for example, which was used to find 51 pegasus b, was first deployed in 1993 specifically for exoplanet searching, and since then it’s been replaced by another improved instrument, but of the same type. So it’s a kind of non-vicious circle, research leads to new technology which leads to new research and so on.
Canto: So – we’ve gotten very good at measuring perturbations in a star’s regular movements…
Jacinta: Regular perturbations.
Canto: And we know somehow that these are caused by planets orbiting around them? How do we know this?
Jacinta: Well we will know from the size of the perturbation and its regularity that it’s an orbiting body, and we know it’s not a star because it’s not emitting any light (though it may be a low-mass star whose light isn’t easily separated from its parent star). We also know – we knew from the results that it was a massive planet orbiting very close to its star – a hot Jupiter as they call it. And that was another surprise, but we’ve developed different techniques for discovering these things and we often use them to back each other up, to confirm or disconfirm previous findings. The ELODIE observation of 51 pegasus b was confirmed within a week of its announcement by another instrument, the Hamilton spectrograph in California. So there’s a lot of confirmation going on to weed out false positives.
Canto: So radial velocity is one technique, and obviously a very successful one as it got everyone excited about exoplanets, but what others are there, and which are the most successful and promising?
Jacinta: Well the radial velocity method was initially the most successful as you say, and hundreds of exoplanets have been discovered that way, but this method actually led to a kind of bias in the findings, because it was only able to detect perturbations above a certain level, so it was best for finding large planets close to their stars. But of course that was good too because we had never imagined that large gassy planets could exist so close to their stars. It’s opened up the whole field of planet formation. Then again, if the main aim is to find earth-like planets, this method is less effective than other methods. So let’s move on to the Kepler project. Kepler was launched in 2009, and since then you could say it has blitzed the field in terms of exoplanet detection. It uses transit photometry, which means that it measures the dimming of the light from a star when an orbiting planet passes between it and the Kepler detector.
Canto: So I get the idea of transit, as in the transit of venus, which we can see pretty clearly, but it’s amazing that we can detect transiting planets attached to stars so many light years away.
Jacinta: Well this is how we’ve expanded our world, from the infinitesimally small to the unfathomably large, from multiple billions of years to femtoseconds and beyond, through continuously refining technology, but let’s get back to Kepler. It orbits around the sun, and has collected data from around 145,000 main sequence stars in a fixed field of view – stars that are generally around the same distance from that dirty big black hole at the centre of our galaxy as our sun is.
Canto: Is that significant – that we’re focusing on stars in that range?
Jacinta: Apparently so, at least according to the Rare Earth Hypothesis, which puts all sorts of unimaginative limits on the likelihood of earth-like planets, IMHO, but no matter, it’s still a vast selection of stars, and we’ve reaped a grand harvest of planets from them – some 3000-odd, with over 1000 confirmed.
Canto: So… promising earth-like planets?
Jacinta: Yes, but I must point out that earth-like planets are difficult to detect. You see, Kepler was a kind of experiment, and we’ve learned from it, so that our next project will be much improved. For various reasons due to the photometric precision of the instrument, and inaccurate estimates of the variability of the stars in the field of view, we found that we needed to observe more transits to be sure we’d detected something. In other words we needed a longer mission than we’d planned for. And of course, Kepler has suffered serious technical problems, especially the failure of two reaction wheels, which have affected our ability to repoint the instrument. Having said that, we’ve been more than happy with its success.
Canto: Okay I just want to talk about these exoplanets. Can you summarise the most interesting discoveries?
Jacinta: Well, Kepler has certainly corrected the view we might’ve gotten from the earlier radial velocity method that large Jupiter-like planets are more common than smaller ones. We’ve had a number of reports from the Kepler group over the years, and over time they’ve adjusted downwards the average mass of the planets detected. And yes, they’ve discovered a number of planets in the ‘habzone’ as they call it. But that’s not all – only this year NASA confirmed the existence of five rocky planets, smaller than earth, orbiting a star that’s over 11 billion years old. I’m just trying to give you an idea of the explosion of findings, whether or not these planets contain life. And we’ve only just begun our hunt, and the refinement of instruments. It’s surely a great time to study astrophysics. It’s not just SETI, it’s about the incredible diversity of star systems, and working out where we fit into all this diversity.
Canto: Okay, I can see this an appropriately massive subject. Maybe we can revisit it from time to time?
Jacinta: Absolutely.
Some very useful sites:
http://www.planetary.org/explore/space-topics/exoplanets/
https://en.wikipedia.org/wiki/Kepler_(spacecraft)
why are our days getting longer?
I’ve just finished reading a book by the Welsh biologist and science communicator Steve Jones entitled Coral; a pessimist in paradise, which covers a helluva lot of ground and makes me feel inadequate as most science writers do, but one of the many things he has taught me about – something I didn’t know that I didn’t know – is that the days are getting longer, in an inexorable process of rotational slowing. This fact, and the reasons behind it, were further confirmed for me today in an episode of an elegant little podcast out of the University of Houston, called The engines of our ingenuity. I just happened to be browsing through the science and scepticism podcasts on my TV, and I sampled a few curiously titled ones…
Let me backtrack a bit. I’m very very poor (from an affluent western perspective of course) but I received a HD TV from my neighbour recently as part of a complicated deal, and now I can watch free-to-air channels I didn’t have access to before, and what’s more I’ve managed to buy a device which I’m sure many people out there know all about, called an Apple TV, which is so cheap that even I can afford it without too much suffering (what’s a few days without food? it’ll probably extend my lifespan). So now I can explore an almost endless variety of podcasts, vodcasts and classic film noir movies on youtube. That reminds me, one of the podcasts I’ve listened to, the Brain Science Podcast, was all about brain fitness – at least the episode I tuned into was – and inter alia the interviewee informed us that just about the worst thing for the brain was sitting around all day watching TV – Apple or no Apple, presumably…
Anyway I listened to this informative and also charmingly poetic three-minute episode of The engines of our ingenuity, entitled ‘How far the moon?’, narrated and presumably written by Dr John Lienhard. So I’ll share the info, if not the poetry, here.
Our earth spins at a pretty well constant rate because of the forces that set it in motion in the first place and because of Newton’s first law of motion which, put simply, states that an object will stay in the same state (resting or in motion) unless an external force acts on it. A ball spinning in the air will slow down because of air friction, but the earth is spinning in a vacuum, essentially – there’s nothing to slow it down.
Well, not quite. The earth is slowing down, and all in accordance with Newtonian physics. And it’s all due to the moon. Each day is about a twelfth of a second longer than it was when the Egyptians built the pyramids. Doesn’t sound that much, but 4000 years is a mere blip in geological and cosmological time. The moon drags at the earth gravitationally, creating high tides and low tides at a regular rate, and slowing our rate of rotation. But our earth has a much greater influence on the moon than vice versa, the moon having only an eightieth of earth’s mass. This gravitational effect slowed down the moon’s spin until it was in synch with the earth, and locked into the earth’s movement like a dancer being swung around by its partner. And so the moon faces us always. The slowing down of the earth due to the moon’s influence had the effect of loosening the embrace – the moon is slowly moving away from us. Just as a spinning dancer or skater extends her arms out to slow down or pulls her limbs in to speed up. The moon moves away from us so that our combined rotational inertia remains constant. The distance between earth and moon, and the speed at which the moon moves away from us, is being measured thanks to an instrument, placed on the moon by Apollo astronauts, which reflects laser beams from earth. Through measuring the time taken for the beam to return, we know that the moon is moving away from us at a little under 4 cms a year. Back in the dim distant past, days lasted only 12 hours, and the moon was half of today’s distance from us. This has affected the shape of the earth, which is gradually becoming more spherical. The earth’s diameter is at its greatest at the equator and at its smallest at the poles, because of centrifugal forces operating against the force of gravity…
Okay, let me get clearer on this, with the help of this source, among others. Isaac Newton accepted the mathematics and the accuracy of Kepler’s laws of planetary motion, but the great unanswered question was why planets – and moons – traced out these orbits. Newton’s own first law stated that an object will continue in its trajectory (that is, in a straight line) or in its resting state, unless some external force acted upon it to speed it up or slow it down. This state is called a state of inertia. Clearly planets and moons were being acted upon by some force, which could only be exerted by the object being orbited. This force might be called a centripetal force, though that doesn’t explain it in this case. If you swing a stone around on the end of a string, you apply a force to the stone to keep it going, but the string, and your hand holding the string, exerts a force on the string to keep it ‘in orbit’. Its motion will be circular, providing you keep your hand still, because the length of the string is constant. But there’s nothing obvious attaching the moon to the earth. Newton pondered this for some time, until one day the apple dropped.
I’m thinking that, if the moon is moving away from us, its orbit can’t be entirely circular, it must be spiralling outwards, ever so slightly. In any case, the moon pulls the earth out of shape, and that is due to a centrifugal force that balances the centripetal force exerted by the earth on the moon. The moon is moving away due to a reduction in both these forces, and a slowing of the earth’s rotation, and hence of the moon’s orbit.
But sadly, it gets more complicated than that! This is the Newtonian explanation of how these forces operate, but it doesn’t really answer the why question. I’m not going to go deeply into that here – as if I could – but I’ll end with a quote from an astronomer’s explanation, not so much about the earth’s slowing, but about the moon’s behaviour, in terms of Newtonian and then Einsteinian physics:
First case: – Why does the Moon orbit the Earth? It just does. And you can understand how it does by analyzing the forces on the Moon caused by its orbit and finding the forces pushing in and out are equal.
Second case: – Why does the Moon orbit the Earth? Because the Earth distorts spacetime in the vicinity of the Moon, and causes it to orbit the Earth the way it does and the balance of forces to come out the way it does.
So why do massive objects distort space-time? Apparently they just do?
is there life on enceladus?
a cool place – and note the tiger stripe
The Curiosity landing has been fabulously successful, and it’ll certainly be worth keeping tabs on the rover’s findings. I posted recently on the possibility of life on Mars, not a couple of billion years ago, as many Mars experts think probable, but right now. The Curiosity rover, as we know, will be investigating this possibility further, but meanwhile there are other possibilities of finding extra-terrestrial life in this solar system, and one of the best places to look, I’m reliably informed, is Enceladus, a tiny moon of Saturn.
Enceladus is only about 500 kilometres in diameter, but its surface has intrigued astronomers ever since Voyager 2revealed detailed features in the early eighties, indicating a wide range of terrains of varying ages. Data from the Cassini spacecraft that performed fly-bys in 2005 showed a geologically active surface, with the most spectacular feature being a large volume of material, mostly water vapour, issuing from the southern polar region. This indicated the existence of ice volcanoes, or cryovolcanoes, which have also been observed elsewhere, and were in fact first observed by Voyager 2 on Triton, Neptune’s largest moon. However, on Enceladus what we have are more like geysers spewing out material from an area known by observers as ‘the tiger stripes’, a series of prominent, geologically active ridges. This material is now known to account for much of the outermost E ring of Saturn, within which Enceladus has its orbit, though a certain amount falls back onto the moon as snow.
Finding water on any object in the solar system obviously excites the souls of astrobiologists. A report from a May 2011 conference on Enceladus stated that this moon “is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it”. However, there are plenty of sceptics, or I should say cautious questioners. First, the existence of water vapour spumes doesn’t necessarily entail liquid water below the surface – for, in spite of the thrill of detecting snow in large quantities on the surface, liquid water is generally regarded as essential to finding life. And even if we assume liquid water…
Some analysts argue that the spumes may be a result of sublimation – a change from a solid, icy state to a vapour, missing out on the liquid phase – or of the decomposition of clathrate deposits. A clathrate is a type of ice lattice that traps gas [methane clathrates are found at the polar regions of Earth]. However, the recent discovery of salt in these plumes has made these possibilities less plausible. Salt is more likely to be associated with liquid water, but hydrogen cyanide, also recently found, would have been expected to react with liquid water to form other compounds, not found as yet. In short, the jury is still out on the presence of liquid water.
And assuming there is liquid water, how could we test for life within it? With great difficulty, obviously. Analysts would be searching for biomarkers, ‘chemicals that appear to have biological rather than geophysical origins’ [Cosmos 44, p78]. Photosynthetic production wouldn’t be an option, so other systems are being hypothesised, including a methanogenic system in which methane is synthesised from carbon dioxide, or a system of metabolizing acetylene, which occurs on Earth. Traces of acetylene have been found on Enceladus. Other biomarkers include amino acids with the right chirality – that’s to say a strong chiral preference, one way [as found on Earth] or its opposite. Amino acids with no chiral preference are likely to be abiotic.
To test for such biomarkers would require new instrumentation and another visit to this intriguing moon. Something else to look forward to. What would we do without anticipation?