Astronomy

Listening to the ‘music of the stars’ and the search for new worlds

The possibility of finding planets just like the Earth, on which life might exist, is coming closer – thanks to out-of-this-world research by astronomers in Birmingham. We listen to the ‘music of the stars’ elsewhere in our galaxy, around which new planets are being discovered.

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Our scientists are working with the Kepler Mission, a space observatory launched by NASA to find potentially habitable planets like our own, orbiting stars like our Sun, outside our solar system.  Already it has produced dramatic results.

So far, 115 such planets – known as extrasolar or exoplanets – have been found by Kepler in the Cygnus-Lyrae constellations of our galaxy, the Milky Way, some of which may be capable of supporting life.

Kepler recently announced the discovery of two of the most compelling candidates yet – planets of the right size and distance from their parent star to support liquid water and, therefore, the possibility of life.

Our astronomers are studying the planets’ ‘suns’, because the more information we can gather about the neighbouring star, the more we will know about the planet.

Professor of Astrophysics Bill Chaplin from the School of Physics and Astronomy leads an international team of scientists, which is playing a leading role in this exciting international project.

Our task is to characterise the host stars around which new planets are discovered, using a powerful, rapidly-growing field of astronomy called asteroseismology, which is the study of the stars by observation of their natural resonances which manifest as surface oscillations.

Stars are able to make sound naturally in their interiors and that sound gets trapped, just as it does in a musical instrument. So we listen to this ‘music’ to measure a star’s properties and look at what’s going on inside. And as we shall see below, by measuring the properties of the stars we get a much more complete picture of the planets, and whether some of them might turn out to be a new Earth.

How we identify distant planets

To detect potentially habitable planets, a technique called the ‘transit method’ is used by Kepler. If there is a planet orbiting a star, it will block some of the light from the star during its transit. This results in a slight dimming of light from the star, which will brighten again when the planet has passed.

To definitively identify a distant planet, we have to measure its parent star’s brightness – and dimming – very accurately over a long period of time. The Kepler Mission is currently monitoring the brightness of around 170,000 stars in our galaxy.

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The ‘music of the stars’

Using data recorded by Kepler, our role is to listen to the ‘music’ of the stars to measure their properties and uncover their otherwise hidden interiors.

Stars are like musical instruments. Take an oboe, for example: To produce music, you blow over the reed of the oboe, which makes the reed vibrate. This changes the pressure of the air to make sound; and the sound is then trapped in the body of the instrument. This sets up what we call standing waves or resonances, resulting in a series of nice, crisp notes.

Sound is made naturally in the outermost layers of Sun-like stars. This sound is trapped, with some sound waves penetrating all the way to the centres of stars. The sound waves are able to reinforce to make the stars resonate at their natural frequencies, like waves inside a wind instrument.

So are we really listening to the stars? Not directly to the sound – rather, we detect the effects of the vibrations. Because a star is a huge ball of gas, the compression of the sound wave makes the star gently breathe in and out. So the star oscillates or pulsates, and it is these oscillations that we are able to track.

What does this tell us about neighbouring planets?

Because these stars, with their newly discovered planets, are hundreds of light years away, it is difficult to examine them. We need a lot of information about the parent stars to fully understand the planets.

For example, if you find a planet from the miniscule dimming as it passes in front of the star, the dip in the amount of light received from the star tells you about the size of the planet relative to the size of the star. So you need to know the size of the star.

Sizes of stars may be measured very accurately using seismology. Using the analogy of musical instruments again, if you listen to someone playing a piccolo trumpet and someone playing a tuba, the tone tells you which is which, because the tuba resonates at a lower pitch. The same is true of stars. Bigger stars resonate in lower tones and smaller ones in higher tones. 

Once you've worked out the size of the star and, from that, possibly deduced that the planet is a small, Earth-like planet, the next question is: does the planet lie in the habitable zone of its star – the so-called Goldilocks Zone (not too hot, not too cold, but just the right distance from the star so that the temperature will allow liquid water)? The answer lies in how intrinsically bright the star is. This can also be measured using seismology.

To determine how old a planet is, again you look to the age of the star. Like the Sun, other similar stars having a beating heart – the central ‘ticking clock’ – where nuclear reactions drive its evolution. The sound waves penetrating the core provide a direct probe of the star’s beating heart.

We can also say something about how active the star is – if, for instance, it is producing lots of harmful radiation, which wouldn't be conducive to life on an orbiting planet.

The advance of technology means we are now at the point of being able to measure the atmospheres of some exoplanets, and in the future we may be able to uncover so-called ‘biomarkers’ to detect signs of life.

Will we really find life on another planet?

Prof Chaplin says: ‘Kepler has detected planets which are very likely to lie in the habitable zones of their stars. In the coming years, those stars and planets will become a focus for study by the international community.

‘It’s hard to say how many there are likely to be, but the candidates at the moment number in the tens. And scientists are starting to discuss how one might go about detecting possible signatures of life – biomarkers – although finding evidence will be hard.’