The NASA TESS Mission opens a new frontier for the study of stars and planetary systems in our Galaxy

On 18 April, NASA’s Transiting Exoplanet Survey Satellite (TESS) was launched successfully on a SpaceX Falcon 9 rocket from Cape Canaveral into a beautiful, clear blue sky. TESS is now orbiting the Earth and will survey the brightest stars across the sky to detect thousands of planets orbiting the stars (so-called exoplanets), and to study the stars themselves. Its mission will provide a unique census of the local solar neighbourhood, our own cosmological back-yard, and astronomers at Birmingham are playing a leading role in the asteroseismology programme of TESS.

TESS is expected to detect thousands of exoplanets orbiting nearby stars in our Galaxy. But it will not observe exoplanets directly. Rather it will detect them indirectly by measuring the miniscule dimming of the stars as any planets they may harbour pass across their visible discs, blocking some of their starlight. The tell-tale dip in the amount of light received from the star provides a measure of the size of the planet relative to the size of the star. To calibrate the absolute sizes of discovered planets therefore requires we know the absolute sizes of their host stars. The same is true if we wish to estimate the masses and ages of exoplanets. This means that if we are to properly know the planets we have discovered, we need to know the stars in as much detail as possible.

Asteroseismology provides us with the means to obtain this information, to paint an exquisite portrait of the properties and characteristics of the stars and hence any planets they may host.

Asteroseismology is the study of stars by observation of their gentle oscillations. The oscillations arise because stars resonate like musical instruments. Sound is made naturally in the outermost layers of Sun-like stars. The sound is trapped, with some waves penetrating the centres of the stars. The waves are able to reinforce to make the stars resonate, like sound waves inside a clarinet or an oboe. Because stars are huge balls of hot gas, the trapped waves make the stars oscillate so that they breathe in and out in a rhythmic manner.

Detecting these oscillations allows us to measure properties of the stars – such as size, mass and age – to levels that cannot usually be reached in astrophysical observations. The oscillations also open a unique window on the usually hidden interiors of stars, providing crucial insights on what stars really look like inside and how they change as they age. We will detect oscillations in the TESS data by observing small, periodic changes in brightness as the stars breathe. They get ever so slightly hotter and brighter as they are compressed, and cooler and dimmer as they relax. The asteroseismology programme is being conducted by the TESS Asteroseismic Science Consortium (TASC), an international collaboration comprising more than 300 scientists around the world. Birmingham is leading the study of planet-hosting and Sun-like stars, in particular working closely with the TESS exoplanet leads to provide asteroseismic characterisation of those bright stars around which TESS finds planets, and also stars we already know host planets (discovered by ground-based telescopes).

NASA’s Kepler Mission made huge advances in space-based searches for exoplanets and studies of stars. But now TESS is opening fresh and exciting new opportunities. Not only will TESS survey stars over almost the entire sky, compared to the small patches that Kepler scanned, it will also look at the brightest stars. They are visible to the naked eye – unlike the significantly fainter stars that Kepler observed – and they present huge opportunities for characterisation and follow-up using a variety of other telescopes, opening a new discovery space on stars typically 50 to 100 times brighter than the stars observed by Kepler.

Professor Bill Chaplin
Professor of Astrophysics, School of Physics and Astronomy, University of Birmingham