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Detecting and characterizing exoplanets around evolved stars with NASA's TESS mission

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Ringing stars help us sound out distant planets

Researchers used NASA’s TESS satellite to survey 100 000 red giant stars, discovering a planet that had survived against the theoretical odds.

Space

Asteroseismology is an emerging field revolutionising astrophysics by focusing on the soundwaves that move through stars to deepen our knowledge about the galaxy. During the PULSATION project, asteroseismologists from the Institute of Astrophysics and Space Sciences at the University of Porto in Portugal used NASA’s Transiting Exoplanet Survey Satellite (TESS) to survey about 100 000 stars, detecting a hot Saturn and a warm sub-Saturn, and discovering more about previously known planets including one that should theoretically never have survived. “The information contained in the oscillations of stars allows fundamental stellar properties like the mass, radius and age of a star to be precisely determined,” explains Tiago Campante, a researcher from the Institute whose work was supported by the Marie Skłodowska-Curie Actions programme. His team is assembling an online catalogue of the planets and their host stars to help scientists answer some of the most important outstanding questions about how stars and their planetary systems evolve.

Mission bonus

TESS was primarily designed to find new planets around bright, nearby stars. Its photometer continuously collects starlight, observing a strip of the sky for 27 days before moving to another vista. That precision offered asteroseismologists like Campante an opportunity. Stars are traversed by soundwaves excited by convective motions near their surface, making them ring ever so slightly. This causes tiny changes in their brightness. “With a photometer that is precise enough, one can measure these brightness fluctuations and decompose them into a star’s natural modes of oscillation,” adds Campante. The researchers conducted an automated transit search on TESS light curves of about 100 000 red giant stars, with a focus on giant planets with orbital periods from their stars of less than 20 days. Their wide-field survey is the first to systematically combine asteroseismology with transit photometry to characterise exoplanets.

Planetary survivor

As well as discovering two Saturn-like planets, the survey also shed light on several that were known before the launch of TESS in 2018. The researchers looked at the orbital evolution of one in particular, HD 203949 b, after inferring its mass, size and age through asteroseismology. They were surprised this planet had not been engulfed by the envelope of the star, which would have expanded beyond the current planetary orbit during the red giant phase of evolution. They put forward a hypothesis in ‘The Astrophysical Journal’ on how tidal interactions between the planet and sun may have led to its survival. Campante shared his findings with a wider public, in particular during a secondment to Ciência Viva, an agency set up to promote science and technology. He used astronomy to increase interest in science among the old and young, commenting: “The public was usually very keen to know how one gets involved in a major space mission like TESS!”

Keywords

PULSATION, asteroseismology, Transiting Exoplanet Survey Satellite, TESS, red giant stars, giant planets

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