How An Improbable Planet Escaped Death

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Planets are at the mercy of their stars. Their birth and evolution are shaped by the amount of light they receive, how close they have formed to the star, and many other factors. Their death is also tied to how the star changes. For example, a star becoming a red giant will suddenly become much larger and engulf any planets that orbit too close.

But a peculiar planet appears to have avoided this fate and astronomers are very curious about it. As reported in The Astrophysical Journal, researchers used the data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to study the pulsation of two red giant stars, HD 212771 and HD 203949. Both of them host planets and by using the stars' pulsations, a technique known as asteroseismology, the team was able to determine their properties such as their mass, age, and size.

"TESS observations are precise enough to allow measuring the gentle pulsations at the surfaces of stars. These two fairly evolved stars also host planets, providing the ideal testbed for studies of the evolution of planetary systems," lead author Tiago Campante, from the Portuguese Institute of Astrophysics and Space Sciences (IA), said in a statement.

Based on observations, HD 203949 should have devoured its planet. While now it is a cool red giant (in terms of temperature rather than style), it is expected to have expanded beyond the current orbit of the planet. So how did the planet avoid engulfment?

The team ran numerical simulations of what the system might have looked like in the past and found a straightforward solution to why the planet wasn’t destroyed. It simply didn’t orbit that close to the star when it expanded. The researchers believe that a tidal interaction with the evolved star slowly forced the planet to migrate inwards and eventually settle in its current orbit.

"This study is a perfect demonstration of how stellar and exoplanetary astrophysics are linked together,” added co-author Vardan Adibekyan, also at the IA and the University of Porto. “Stellar analysis seems to suggest that the star is too evolved to still host a planet at such a 'short' orbital distance, while from the exoplanet analysis we know that the planet is there!"

Asteroseismology has significantly improved our understanding of the inner workings of stars over the last decade. TESS and upcoming observatories such as the European Space Agency's PLATO will continue this important work.

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