An exoplanet survived near a red giant

Artist's concept of exoplanet near its star (Image ESA)
Artist’s concept of exoplanet near its star (Image ESA)

An article published in “The Astrophysical Journal” reports the study of exoplanets that orbit two red giant stars: HD 212771 and HD 203949. A team of researchers led by Tiago Campante of the Instituto de Astrofísica and Ciências do Espaço (IA) of the University of Lisbon, Portugal, applied the astrosismology technique to observations conducted with NASA’s TESS space telescope. The result was surprising in the case of the HD 203949 system because the planet wasn’t swallowed by the star during its expansion even though it orbited very close to it.

The TESS telescope was launched on April 18, 2018. Its observations also include stars that have already been studied, in certain cases which were already known to host planets. Both HD 212771 and HD 203949 fall into this category since an exoplanet had already been discovered for each of them using the radial velocity method. The hope is that TESS can find more planets or provide more information on the already known exoplanets and these two stars represent interesting cases because they’re in their red giant phase in which they expand and contract. It’s an agony that represents the last phase of their life in which they can destroy any planets too close.

The precision of the observations conducted using the TESS space telescope allowed to apply the astrosismology technique, in simple terms its internal structure was studied through the interpretation of its pulsations and its spectrum. This allowed to accurately measure the characteristics of the stars HD 212771 and HD 203949 such as mass, size and age, useful in order to calculate their planets’ orbits. In the case of HD 212771, the planet is a gas giant similar to Jupiter with an orbit of about 373 days. The surprise came from HD 203949, whose planet is more massive than the other studied and, with an orbit of about 184 days, closer to its star, to the point that the planet should have been swallowed during the red giant’s expansion phase.

To try to solve the mystery of the survival of exoplanet that was cataloged as HD 203949b, the researchers conducted some simulations of planetary evolution. One possible explanation is that the exoplanet formed farther away from its star, which is why it survived when it reached its maximum expansion and only later, due to the tidal forces, migrated into its system. Planet migration is one of the topics studied in recent years and the effect of the changes of a star in its red giant phase can explain it in these cases. Vardan Adibekyan of IA, another of the authors of the research, explained that stars and planets not only form together but evolve together.

The history of the exoplanet HD 203949b is a case that shows the progress of astrosismology, which in recent years had a growing impact on the study in this case of red giants but in other cases of stars similar to the Sun as well. Progress also came thanks to increasingly sophisticated planet hunters, from CNES/ESA’s CoRoT to NASA’s Kepler to TESS and in the future PLATO, another ESA space telescope.

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