In Our Time: Exoplanets

The first planet orbiting a star other than the Sun wasn’t discovered until 1992 and since then the subject of exoplanets has gone from being something you argue about the existence of to a rapidly expanding field with new discoveries all the time. The experts who discussed exoplanets on In Our Time were Carolin Crawford (University of Cambridge), Don Pollacco (University of Warwick) and Suzanne Aigrain (University of Oxford).

One of the reasons it took so long to discover any extra-solar planets, despite people speculating about their existence for centuries, is that they are very hard to directly see. In fact I think they were saying that none of the known ones have actually been seen. Instead a variety of more indirect techniques are used to detect them, and these required both sophisticated technology & sophisticated knowledge of physics before they could be used. The technology needed to develop to a point where small differences in stars could be measured accurately and consistently over time. And the physics is required to both predict how a star without planets would behave and then to figure out what the differences from this prediction mean.

In the programme they ran through a variety of techniques used to detect planets. One of these is to look at the colour of the star’s light and see if it’s changing between blue-shifted & red-shifted over time. If the star has no planets then you won’t detect that. When there’s a planet orbiting the star it’s not quite as straightforward as the planet circling the star, actually the star and the planet are both circling a point between them (that’s a lot lot closer to the centre of the star than it is to the planet). So the star will seem to move back & forth relative to us observing it. This is biased towards detecting more massive planets, as they’ll move the centre of gravity from the centre of the star more – so-called “hot jupiters” for instance, which are planets the mass of Jupiter that orbit close to their star.

Another method is to look for the changes in the star’s light caused by the transiting of a planet across the face of the star. Obviously this is only possible to detect if the planet is orbiting in the right plane for us to see it. But if you have one transiting where we can detect it then you can detect the existence of other planets in that system by looking at the perturbations of the orbit of the one that transits. You can also detect things about a planet’s atmosphere with this method. The changes in the light of the star can be used to tell you something about the size of the planet (in terms of diameter), and if you look at different wavelengths of light then you’ll see varying diameters. This tells you when the atmosphere of the planet is thin enough to be transparent to that wavelength, and different gases absorb different wavelengths differently so you can figure out the gases that are present. Apparently you can even detect the presence of clouds using this technique.

Another method uses the phenomenon of gravitational lensing. If the light from a distant star passes by a closer to us star on it’s way to the telescope then it will be bent by the gravity of the middle star. A planet orbiting that middle star will affect the lensing effect, and you can figure out things about the size & distance from the star by exactly how the lensing is affected.

If you use the first two methods together you can tell things about the density of the planet. Is it small & heavy? Is it big & fluffy? Or even small & fluffy? There seem to be a wide variety of planet types out there, not all of which are represented in our own solar system. There are also a wide variety of types of solar system out there – Pollaco pointed out that one reason there was argument about the reality of the first exoplanet discovered was because people were assuming that our own solar system was a good model for “all systems everywhere”. It turns out it’s not. The example they used in the programme was systems that have hot jupiters – the first exoplanet was one of these, and the very idea of a Jupiter type planet orbiting with a periodicity of only 4 days was almost unthinkable. They also talked about planetary systems detected around brown dwarves – stars which weren’t quite massive enough to ignite at the end of the formation process. And planets around pulsars (again like the first ones detected) – and one of the experts (I think it was Crawford?) made a throwaway remark about how these are probably not the first planetary system for the star in question. Before a star becomes a pulsar it goes through a supernova explosion, which would probably destroy any original planets – the ones orbiting afterwards are probably secondary captures.

They also discussed looking for planets which might be habitable. Bragg asked if we are thinking about life like ourselves, or germs. The answer was (paraphrasing) “yes”. At the moment no-one knows enough to know what we’re looking for in terms of life on other planets, and at first we’re obviously limited to things we know about life on Earth as a starting point for what to look for. So looking for rocky planets which are neither too big nor too small, that are in the right zone for liquid water. And other things about our own solar system might’ve been necessary – like the presence of Jupiter which draws away some of the comets that could bombarded Earth & wipe out all life. I think it was Aigrain who talked about other ways of detecting life – looking at what we can tell about the atmospheres of the planets. If there are very reactive gases present then they must be being made constantly – some of these we only know of biological processes that make them. So if one could detect such gases that’d be a sign of life.

It was a little bit of an odd In Our Time episode, because there was less of a sense of a narrative than they normally have. It felt like this is because the study of exoplanets is in its infancy – we’re at a point where most of the work is data gathering. I mean in the sense that a lot of planets are being discovered and categorised, but as yet they’re not classified and grouped into types. Nor are there overall theories about how solar systems in general work or were formed – it’s now clear that the one we know isn’t the only sort there can be, nor is it particularly typical of what we’re detecting now.

In Our Time: Comets

Comets are an astronomical event/phenomenon that have exerted quite a hold on the imagination of people in the past & it’s only relatively recently that we have any understanding of what they are or why they happen. The In Our Time programme that discussed them primarily focused on the astronomy but did touch on the omens and portents side of them as well. The experts on the programme were Monica Grady (Open University), Paul Murdin (University of Cambridge) and Don Pollacco (University of Warwick).

They discussed what is known about comets and what the current theories are about where they come from etc. Comets were formed at the same time as the rest of the solar system – when the nebula that formed the sun and planets coalesced at a particular distance from the sun is what is known as the snow line, and beyond this small lumps of ice formed. These are the comets. Grady told us about the Oort cloud, which is a spherical region around the outside of the solar system where the comets orbit. When something perturbs this – gravitational changes due to the relative movements of our solar system & other parts of the galaxy, for instance – a comet might get jostled free and plunge in towards the sun. I specified that it was Grady that discussed the Oort cloud because one of the others (Pollacco, I think) was of the opinion that it wasn’t so much a sphere around the solar system, but more that this is how the spaces between the gravitational wells of different stars are filled (if that makes sense).

Once a comet is jostled free it still orbits the sun, but now the orbit is an eccentric one as compared to the planets. All the planets orbit in the same plane, in the same direction, in roughly circular orbits. But comets can be in any plane, and often move very close to the sun before returning to a much further out position. Comets are split into two classes – short-period and long-period. Murdin (I think it was) said that they’d like to be able to classify them by composition or something like that, but sadly we just don’t know enough about them to do that. So long-period comets take a long time to come back – this might be a few hundred years, or it might be forever. Some comets break up when they get close to the sun, due to the heat & gravitational pull. Some comets swing round the sun once and then go back to the Oort cloud (or whatever the true situation out there is). Short-period comets come back more often – Halley’s Comet is an example of this sort of comet.

They were saying that we only actually know the orbits & can predict 150 comets out of the many millions that there are. And Pollacco was crediting Halley’s prediction about his comet’s return as being one of the factors that helped to get the Enlightenment going. Basically he was saying that it was a very good demonstration of the power of science – Halley predicted the return of the comet despite this occurring after his death via scientific observations & mathematics, and then this prediction came true.

There is a little known about the composition of comets – due to space missions that have flown past comets and through their tails. One of those missions was named Stardust and it brought back some of the particles in a comet tail. They know that comets are lumps of ice, that are pretty small by cosmic standards – up to a few hundred kilometres across. They aren’t white like you expect when you say “ball of ice”, they’re black due to all the dust and rocky particles in them. Bragg asked what the difference was between an asteroid and a comet & the answer was partly the place you find them orbiting, and partly it’s a continuum where asteroids are icy bits of rock, but comets are rocky bits of ice. As a comet gets closer to the sun (inside the orbit of Mars) it develops a coma, which is gas that has sublimated out of the ice. A comet has two tails, both created by the effects of the sun. One of these tails is lots of bits of dust – melted out of the comet by the sun’s heat & left behind as the comet moves. The other tail is the coma being pushed back by the solar wind & radiation – this is the ion tail. The Stardust mission brought back bits of the first type of tail, and they found that these are little bits of rock much like rocks on earth – made up of silicon, plus some carbon, some nitrogen etc.

Bragg brought up Fred Hoyle’s theory that life on Earth was seeded from outer space by comets – discredited some time ago – and was slapped down by Grady (politely, but firmly). Hoyle was postulating that bacteria were present on comets and this is where life came from, but at best comets may’ve brought some of the water and minerals needed for life to the Earth.

While on the subject of how astronomy is a science where you might have things you want to know but you have to live with the things you can find out, they talked about the Shoemaker-Levy 9 impact on Jupiter. This comet was discovered in 1993, and it shortly afterwards became apparent that it had not just been captured by Jupiter’s gravity but was going to crash into Jupiter. In 1994 this happened – sadly not quite in full view of all the telescopes, but the aftermath was clearly visible. Despite the relatively small size of the comet the marks it left were spectacular – about 80% the size of Earth! The comet broke up into about 25 pieces, and these hit in turn generating a straight line of marks. As each piece hit it ploughed through the atmosphere leaving a hole behind itself, and once it had hit the lower region of the atmosphere spurted back up the hole leaving a dark mark on the surface of Jupiter. Having seen the pattern of marks astronomers looked at craters on other planets/moons, and could see other examples of a row of craters in a straight line – presumably also from being hit by comets or asteroids that fragmented before they hit.

Thinking about comets is one of those things that makes it clear just how fragile life is on this planet …