Much like the stars observed in the TESS, NASA’s new exoplanet and stellar astrophysics mission, the future for the field of asteroseismology has never looked so bright, building on the discoveries from Kepler to open up unique insights into stellar systems in our own cosmological back yard.
Professor Bill Chaplin is leading international efforts to reveal the hidden secrets of the Sun and stars using observations of their gentle oscillations. Seismic data analysed in the fields of asteroseismology (stars) and helioseismology (the Sun) provide a unique window on stars’ interiors and allow us to paint an exquisite portrait of the properties and characteristics of the stars, and any planets they may host.
“Our ability to see inside stars using these techniques allows us to perform stringent tests on stellar evolution theory, and also enables us to better understand how stars interact with their local environments, which of course include planets ”, commented Professor Chaplin.
The legacy of Kepler
Since its launch in 2009, the NASA Kepler mission has revolutionised our ability to be able to study stars, discovering thousands of planets orbiting other stars in our Galaxy. “Kepler has provided data of exquisite quality on unprecedented numbers of stars. It really has changed what we can do with asteroseismology.,” said Professor Chaplin.
Professor Bill Chaplin’s Sun, Stars and Exoplanets Research Group2 has been using Kepler to detect the natural oscillations of solar-type stars and the larger, cooler red-giants that stars like the Sun will go on (swell up) to become. The oscillations in all these stars arise from trapped standing waves in their interiors, and recording the resulting tiny, periodic changes in stellar brightness allows us to probe the structure and dynamics of their normally hidden interiors and to estimate the fundamental properties of the stars – such as size, mass and age – to levels not usually reached in astrophysical observations.
This exciting work is set to continue, but on a different and much larger set of stars, thanks to the recent launch of TESS.
The promise of TESS
TESS, NASA’s new exoplanet and stellar astrophysics mission3 will observe the brightest stars in the sky, to detect thousands of planets orbiting the stars, and to study the stars themselves. 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. Whilst Kepler has observed stars in only a fraction of the sky, TESS will look at stars across almost the entire sky. TESS will as such provide a unique census of stellar systems in the local solar neighbourhood, our own cosmological back-yard.
TESS was launched on April 18 from Cape Canaveral on a SpaceX Falcon-9 rocket. Professor Chaplin observed the launch first hand and commented: “seeing and actually feeling the launch was uqite an experience, and it was of course a huge relief to see everything go like clockwork..”
The spacecraft took about a month to reach its final science operations orbit around the Earth. This orbit involves a complex resonant “dance” with the Moon, and as a result it is incredibly stable; so stable in fact that it is now clear that TESS has the potential to continue to operate for comfortably in excess of 10 years. The nominal TESS mission will last for two years, with each celestial hemisphere taking a year to map but Bill is already looking ahead, “We are already formulating plans for what to do in an extended mission”.
The brightest stars are expected to provide exciting discoveries
The commissioning of the four telescopes and cameras that comprise TESS has now been successfully completed and TESS began full science operations on July 25. It is monitoring the brightness of stars in a thin strip of the sky, and after roughly one month the spacecraft will shift its focus slightly to an adjacent strip, gradually working its way around the southern celestial hemisphere before flipping over to observe the north from summer 2019 onwards.
“At Birmingham, we led the selection of solar-type stars to be observed by TESS for asteroseismology, and at the time of writing the initial set of targets is being observed. We focussed on stars predicted to show solar-like oscillations delivering a rich spectrum of resonant overtones. The brightest stars will generally give the highest-quality data; and we expect them to provide the most exciting initial discoveries of the mission.”
TESS will find new stellar systems in our own back-yard with host stars visible to the naked eye but we will have to wait until January 2019 for the first release of analysis-ready data. Before then the team will be prioritising well-known targets of interest as they prepare for the analysis, one example being a cohort of around 100 already known planet hosts where asteroseismology from TESS promises to provide much improved estimates of the fundamental properties of the stars. This is important, since this information is needed to constrain the properties of their planets.
TESS alerts to detect planets by Doppler and transit techniques
Planets detected simultaneously by the transit and Doppler techniques are a precious commodity. With both you have estimates of the planet radius (from the TESS transits) and mass (from the ground-based Doppler signal), hence its average density from which one may infer information about its bulk composition. The brightest TESS Objects of Interest will be prime targets for asteroseismology.
The TESS exoplanet team have just begun to issue “TESS alerts” on stars where an initial “quick-look” at the data has revealed possible newly discovered planets from miniscule dimming of starlight as any orbiting planets transit (pass in front of) the stars. The alerts will allow teams of astronomers to train large ground-based telescopes on these so-called TESS Objects of Interest to try and detect the periodic Doppler wobble of the stars’ light due to the gravitational pull exerted by any planets TESS may have discovered.
TESS will reveal its first ground-breaking results soon, and has the potential to transform our picture of the local solar neighbourhood.
- Main header image: Cutaway showing the paths followed by sound waves inside a Sun-like star. The resonant oscillations these sound waves give rise to make it possible to characterize in detail both the star and any planets it may be hosting. (Credit: Gabriel Perez Diaz/IAC)
- The asteroseismology programme was conducted through the Kepler Asteroseismic Science Consortium (KASC) – over 500 scientists from around the world working together with Birmingham leading coordinated activities for solar-type stars, including working directly with the Kepler exoplanet Science Team to deliver asteroseismic characterisation of stars discovered by Kepler to be hosting planets.
- Based in the School of Physics and Astronomy, Professor Chaplin’s Sun, Stars and Exoplanets research group, runs its own global network of automated helioseismology telescopes – the Birmingham Solar Oscillations Network (BiSON) – but also has international leadership roles in the NASA Kepler and Transiting Exoplanet Survey Satellite (TESS) missions.
- The international asteroseismology community has organised to coordinate efforts, and Birmingham is leading the programme of work in the TESS Asteroseismic Science Consortium (TASC) on studies of planet-hosting and solar-type stars.