NASA's Transiting Exoplanet Survey Satellite (TESS) will be launched from Cape Canaveral in Florida on Wednesday 18 April, 2018.

TESS will survey the brightest stars in the sky to search for planets orbiting stars and to also study the stars themselves. Birmingham physicists are playing a leading role internationally in the asteroseismology programme of TESS; the study of stars by observing their resonant oscillations, which are caused by sound trapped inside the stars. By measuring the tones of this ‘stellar music’, it is possible to determine the properties of individual stars in exquisite detail.

Professor Bill Chaplin, from the University of Birmingham's School of Physics and Astronomy, is leading the international work that will characterise bright, newly discovered exoplanet host stars – and hence their planets – using asteroseismology. Professor Chaplin said: ‘Perhaps the most exciting results from TESS are going to be from the brightest stars in the sky, which we can observe at night with the naked eye. There will be stars with well-known names around which TESS discovers planets - we will be able to study them in unprecedented detail.’

He added: ‘Previous space missions dedicated to these studies have not looked at anything like the amount of sky TESS will cover, nor have they observed stars as bright at this. On a clear night we will be able to point out these newly discovered stellar systems directly by hand.’

The mission will produce results by the end of 2018.

Kate Chapple, Press Office, University of Birmingham, tel +44(0)121 414 2772, email Press Office.

The Transiting Exoplanet Survey Satellite (TESS) is NASA’s new exoplanet and stellar astrophysics mission. It will survey the brightest stars across the sky to detect thousands of planets orbiting the stars, and to study the stars themselves. TESS will as such provide a unique census of stellar systems in the local solar neighbourhood, our own cosmological back-yard. Its nominal mission will last 2 years, with exciting opportunities for extending the mission for a further few years.

Astronomers at Birmingham are playing a leading role in the asteroseismology programme of TESS. Asteroseismology is the study of stars by observation of their gentle oscillations, the natural resonances of the stars (of which more below). It provides us with a unique window on the interiors of the stars and an ability to paint an exquisite portrait of the properties and characteristics of the stars, and any planets they may host.

The asteroseismology programme is being conducted by the TESS Asteroseismic Science Consortium (TASC), a large international collaboration comprised of a few hundred scientists from around the world. Birmingham’s Professor Chaplin is one of the seven members of the TASC Board, which oversees the collaboration. He also co-leads the TASC studies of planet-hosting and Sun-like stars.

Stars resonate like musical instruments. 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 the stars. The sound waves are able to reinforce to make the stars resonate at their natural frequencies, like waves inside a wind instrument. The compressions of the trapped sound makes the stars oscillate and we are able to detect this gentle breathing by observing small, periodic changes in brightness as stars get slightly hotter as they are compressed, and then cooler as they relax.

By measuring the oscillations, we may use this “music of the stars” to not only estimate the fundamental properties of the stars (e.g., size, mass and age, to level of precision and accuracy that cannot usually be reached in astrophysical observations) but we may also peel away their surface layers to probe the structure and dynamics of their normally hidden interiors. One can perform the equivalent of a CT scan on a star, something that is not possible by other means.

TESS will measure the brightness of millions of stars. It will detect planets by measuring the miniscule dimming of light from the star as any orbiting planet passes in front of the visible disc of the star, blocking some of the light we receive from it. Exactly the same data are used to detect the oscillations, and do asteroseismology. TESS will detect these gentle oscillations by observing changes in brightness as stars get slightly hotter and brighter as they are compressed and then cooler and darker as they relax.

To “know thy planet” one must “know thy star”. When a planet is discovered by the transit method the tiny dip in the amount of light received from the star provides a measure only of the size of the planet relative to the size of the star. Thanks to asteroseismology we can measure the size of the star extremely precisely, allowing the absolute size of the planet to be fixed with a high level of confidence. Next, we want to check if the planet lies in the habitable zone of the star, not too close or too far away, but just right for liquid water to potentially be present on the surface of the planet. The location of this so-called “Goldilocks Zone” depends on the intrinsic brightness of the star, for which we again need the precise asteroseismic size of the star. Should the planet be an Earth-like or rocky body orbiting in the habitable zone, we would of course want to know its age. This we can get from the age of the star, a property that asteroseismology is uniquely equipped to estimate since the waves that penetrate the core provide a direct probe of the beating heart of the star, its central “ticking clock”.

We can also go further: asteroseismology allows us to measure how rapidly the insides of the stars are spinning, and how the spin axes are oriented with respect to the orbital planes of the planets. This can help us better understand how planetary systems evolve over time, and whether the configuration of planets in our solar system (and our place within it) is likely to be common or atypical. We can also use asteroseismology to measure the amount of surface activity on the stars, and hence better understand the physical impact that the stars have on their local environments, which must be taken into account when considering if life might be possible.

There are two main differences in terms of the stars observed. First, whilst Kepler has observed stars in only a fraction of the sky, TESS will look at stars across almost the entire sky. Second, TESS will observe the brightest stars in the sky. These are stars that are visible to the naked eye, and they present huge opportunities for exquisite characterisation, opening a new discovery space on stars several magnitudes brighter than the stars observed by Kepler (which were too faint to be visible with the naked eye).

Observing from space means we are above the Earth’s turbulent atmosphere, which makes stars twinkle and can hide the tiny variations in the brightness of stars – due to oscillations and exoplanets – that we are trying to detect. Another advantage is that we can observe the stars continuously with one telescope from space, without there being gaps due to daylight hours on Earth. This is important because gaps in the observations can make interpreting data much more challenging. We can of course arrange for observations on the ground to be made by a network of telescopes, but we are still vulnerable to the effects of weather (including cloud!).

Birmingham has led the selection of the Sun-like stars that will be observed by TESS for asteroseismology. We focus on stars predicted to show solar-like oscillations because each will have a rich spectrum of resonant overtones, which provides huge diagnostic potential for our studies. The brightest stars are also amongst the most exciting targets: they will generally give the highest-quality data.

What are you likely to discover with TESS that will be exciting? What would be the Holy Grail in terms of a discovery?
The most exciting initial discoveries are likely to come on the brightest stars. We will be discovering new stellar systems in our own back-yard, i.e. planets orbiting the closest stars to the solar system. The host stars will be visible with the naked eye. Just think: anyone will be able to go out at night at point directly at some of these stars!

When you look up at the sky on a clear night do you ever ask yourself: how many of the twinkling stars have planets, like the planets orbiting our own Sun? And how many of those planets may be capable of harbouring life, like the precious planet we live on? Is there some special combination of properties that a star must possess to elevate the chances of it hosting a habitable planet? Or are some Sun-like stars just too unsafe for their planets?

The discovery and characterisation of other exoplanet systems in our Galaxy allows us to address these questions, and place our own solar system (and our place within the Universe) in a better context.

TESS will provide a census of exoplanet systems in our local solar neighbourhood. The new exoplanetary systems it discovers that have bright host stars will also be high-priority targets for the James Webb Space Telescope (JWST), which will be launched between March and June next year. JWST will have the ability to make exquisite characterisations of the atmospheres of the planets TESS discovers!