Exploring the diversity of exoplanets

In the last two decades, the way we view the Universe has been transformed. Scientists have discovered many hundreds of exoplanets – that is, planets that orbit stars other than our Sun – some of which may well harbour life. In this lecture, which took place after the SUPA Cormack Astronomy Meeting, Dr Suzanne Aigrain described the journey to discover and characterise planets outwith our own Solar System – and gave an intriguing glimpse into future possibilities.

For millennia, astronomers and others have looked to the stars to try to understand our place in the Universe. Many discoveries have been made – for example, our understanding of our own Solar System, with its eight planets orbiting the Sun, was transformed from the 17^th Century onwards, when the invention of effective telescopes allowed us to see the first four moons of Jupiter (1610), Uranus (1781) and Neptune (1846). We have also been able to observe other kinds of objects, such as comets and asteroids, as well as dwarf planets such as Pluto in the Kuiper Belt, all of which provide important information about the formation of the Solar System.

For a long time, astronomers suspected that ours was not the only Solar System – that there were other suns, orbited by their own planets, in the Universe. But it was only as recently as the mid 1990s, with the discovery of the first exoplanets (or extra solar planets), that we received confirmation. Since then, many hundreds of exoplanets have been discovered: some are large, resembling Jupiter or Uranus; some are smaller and may share properties similar to those of Earth – including the ability to harbour life. Dr Aigrain said she had been very lucky to start her PhD in 2001, when the nature of the first exoplanets had been confirmed. It was an exciting time, she said, because the whole area was transformed from a ‘pipe dream’ to an endeavour of physical characterisation.

Dr Aigrain gave a summary of where we are and how we got here. There are some 100 billion stars in the Milky Way (our Galaxy), including our Sun, with its own planetary system around it. These planets were formed in the disk of dust and gas which surrounded the infant Sun. Although the Sun has the vast majority of the mass in the Solar System, the planets contain most of the angular momentum or ‘spin’, and can be seen as a natural by-product of the formation of the Sun itself. Our Solar System is the best studied around, and provides a useful frame of reference when looking at others. Unless our Solar System is in some way special or unusual – and Dr Aigrain doesn’t subscribe to that view – it’s likely that other solar systems formed in similar ways to ours. In other words, a home star created the conditions in which planets could form.

The first definitive discovery of an exoplanet orbiting a star was reported in 1995 (51 Pegasi) and this was followed by more and more discoveries. To date, there are around 800 confirmed exoplanets and many more thousand candidates. But how do we find exoplanets and, if we do discover them, how can we learn more about them? There are several crucial differences between planets and stars, said Dr Aigrain. For example, planets emit almost no light, which makes them very difficult to see unless you can ‘cancel’ the light from the home star. You can look for light from planets, for example, light that is reflected or absorbed from the Sun, and finding the infrared gives the best chance, but as planets tend to be close to their own suns, it is difficult to make them out. It’s also difficult to cancel the light from the star perfectly.

Direct imaging is one of the ways to find exoplanets, although it is still relatively rare. Dr Aigrain described her delight when a colleague brought her a photograph taken in 2008, which showed three red dots – basically massive planets in a scaled-up version of our Solar System. “This was the most exciting photo I’d ever seen, the first picture of another planetary system.”

Although some exoplanets have been found by direct observation, most have been discovered using indirect methods. Dr Aigrain described two of these in some detail. The first is the radial velocity method. This involves looking for the small wobble in the star’s position as both the planet and the star orbit around the centre of mass of the system. The radial velocity is the velocity along the line of sight. Using a dispersing element such as a slit or a grism, the light from the star is dispersed into its constituent colours, or wavelengths, The resulting spectrum contains dark lines at specific wavelengths, which are due to absorption by various elements, such as hydrogen. Because we know where these lines should be if the star is at rest, but they are shifted towards the blue or the red if the star is moving towards or away from us, we can measure the star’s radial velocity, and infer the presence of one or more planets. It’s possible to measure the radial velocity of stars to less than a metre per second, she said.

The amplitude of the star’s motion depends on how massive the planet is, and how close to the star it orbits. Hot Jupiters – so called because they are large, gaseous planets, like our Jupiter, and because their proximity to their star heats them up (especially the ‘day side’ facing the star) – are easier to find via the radial velocity method because they create more of a ‘wobble’ in the motion of the star.

Dr Aigrain also works with the transit method, which involves looking at the small flux that happens when a planet moves in front of the star. You have to be lucky to see it because it will only happen if the orbit of the planet is aligned just right, she said. What’s interesting is that the depth of the transit tells you the size of the planet relative to the star. You also know the inclination, so with the radial velocity method you get the mass, and if you put the two together you get the density of a planet. This in turn can give a good idea of the planet’s physical composition. Very specialist equipment isn’t needed to observe transits, so it can even involve an army of lay or amateur astronomers, who can examine large swathes of sky from their back gardens. “You don’t need a big telescope, just ten centimetres,” said Dr Aigrain. “Many dedicated amateurs make very significant contributions to the field.”

Dr Aigrain showed a slide of around 750 of the known exoplanets, which also showed how many are in multi-planet systems – and described how the first transitting planet significantly smaller than Jupiter – a so-called hot-Neptune – had been found almost by chance by colleagues using a small telescope designed primarily for outreach. This planet probably has a thick water layer on top of a rocky code and below a gaseous atmosphere. Exotic planets like this, which have no equivalent in our Solar System, defy our imagination. Although this planet is much hotter than the Earth, because it is much closer to its star, much of the water inside the planet is likely to be frozen, because at high pressure water freezes up to much higher temperatures, forming a kind of ‘hot ice’. As the pressure drops towards the outside of the planet, there may be ocean floating on an ice layer, rather than the other way around.

If, however, like Dr Aigrain, your interest lies in Earth-like planets – which are terrestrial in nature, and have temperate surface temperatures – you really need to go into Space, she said, because their transits are so shallow and so rare. A number of missions doing just this are either in progress or in the pipeline. CoRoT, a mission led by the French Space agency CNES, has been used since 2007 to search for planetary transits from orbit, and has detected a number of exoplanets. But the NASA mission Kepler (on an Earth-trailing orbit, which makes it very stable) is a more powerful successor to CoRoT and is providing very precise information on stars across a large part of the sky. It has already detected 2,300 candidate transitting planets so far – “a bewildering array of planets of different sizes and temperatures”, including numerous multi-planet systems, which are proving to be one of the most interesting results from Kepler.

Excitingly, some are deemed to be in the ‘habitable’ zone of their home star, which means that the computed temperature of the planet suggests that it’s somewhere that water (essential for formation of life as we know it) would remain liquid on the surface.

Although Dr Aigrain’s work includes coming up with algorithms for finding transits, she acknowledged that even the best method won’t capture them all. For some of them, you just need a human eye, she added. Projects such as planethunters.org have been set up to enable the public to participate in the exoplanet discovery process by visually examining Kepler light curves and spotting transit- like events. The transit method (using spectrometry) can also be used to probe the make up of the planet’s atmosphere; this is a very difficult process, but has been done thoroughly for a small number of exoplanets. Dr Aigrain described one such observation done with the Hubble Space Telescope, which led to the discovery of a haze of small particles in the atmosphere of one of the hot Jupiters.

Dr Aigrain concluded with some glimpses into the future. As well as ongoing research, such as the characterisation of the atmospheres of hot giant planets, and the Kepler mission to measure the incidence of Earth-like planets (and others), it will shortly be possible to use direct imaging on large ground-based telescopes to view young solar systems. Within the next decade, new insights are likely from other Space telescopes such as TESS or PLATO (looking for transiting planets around bright stars) or EChO and JWST (characterising atmospheres of hot Jupiters, Neptunes and Super-Earths), while the ground-based E-ELT (European Extremely Large Telescope) will be used to detect and characterise the atmosphere of even smaller, cooler planets – possibly a bit like Earth.

“We now know that planets are common, possibly more numerous than stars in the Galaxy,” said Dr Aigrain, adding that within the next decade, we should know how common Earth-like planets are. “Ultimately we might look for signs of life,” although there is some debate about what precisely will be looked for. There are also exiting indications of planets closer to home, with new discoveries all the time – all in all, a fascinating subject and a fascinating time to be involved in it.

Questions

Questions ranged over topics from the laws of physics to religion and our place in the Universe. Asked about whether different galaxies in different Space time zones were affected by different laws of physics, Dr Aigrain said the laws of physics would still apply, but that different environments might have an impact on the types of planets that form.

Is there a message for “us humble residents of Earth”, one audience member asked. Dr Aigrain said that one thing about astronomy is that it makes us feel very small and insignificant, but that it is also reassuring to know that there are other planets out there that are potentially like ours. “We’re not in a special place”, she said, “so although we don’t know how common life is [on other planets], and certainly not life in as sophisticated a form as has developed on Earth, it’s likely that other planets which have the potential for life are common”. Pressed a little further, she refused to give a concrete ‘academic’ answer on life on other planets, but said that if she were betting on it, she would put quite a lot of money on the notion that there was life [on other planets] but wouldn’t bet so much on there being complex life.

Asked about the impact that recent and further discoveries would have on Hindus, Muslims and Christians, Dr Aigrain said that this question should really be posed to someone who is religious, but that religions have had a history of coping with changes in what we know of the world, even if they sometimes initially obstructed these changes. She added that one should recognise that religious astronomers have made some important contributions in the past.

The last question concerned how long it would be until we could examine planets in different galaxies. Dr Aigrain said that microlensing – using a star to act as a gravitational lens – should mean it is possible to see things at large distances, so there is no reason why it shouldn’t be possible to see them in neighbouring galaxies. Where you can’t distinguish individual stars, it would be difficult to look for planets, however.