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Physics professor and alumni publish article about alien suns

John Lindner, emeritus professor of physics at ������˾��, and two ������˾�� alumni, Xinchen (Ariel) Xie �21 and Hwan (Michelle) Bae �19, recently published an article called �Alien suns reversing in exoplanet skies� in the journal Scientific Reports. Their research describes the motion of suns for planets that orbit other stars.

�On Earth, we�re used to the sun rising in the east and setting in the west day after day, but for planets orbiting other stars (or Mercury orbiting our own star, the sun) the motion can be more complicated, including days when the sun appears to reverse its motion in the sky,� Lindner said.

In the article, the authors modeled how suns can travel backwards. �We not only found that the sunappears to move backwards in the sky in some eccentric orbits but alsomathematically derived the conditions for reversals for some important special cases,� Xie explained.

Lindner further explained the work that brought them to these conclusions. �We discovered a simple but nonlinear criterion for solar reversals in the important special case of planets whose spins are perpendicular to their orbits,� he said. �Apparent solar reversals occur when a planet’s spin angular speed is between its minimum and maximum orbital angular speeds, which depend on the eccentricity (or non-circularity) of its orbit.� To their knowledge, the team was the first to do this kind of research in detail. Their conclusions have significant and fascinating implications.

�My imagination is excited by the possibility of a civilization living on an exoplanet whose sun appears to move backward in the sky once (or more) per year. The cultural implications would surely be enormous, with reversal days connected to myths, legends, and festivities!� Lindner said.

Lindner began working on this research with Bae for her senior Independent Study during the 2018-2019 school year. Xie picked up the work for her I.S. two years later. Both students were physics and mathematics majors and Robert Kelvey, visiting assistant professor of mathematics, served as their other advisor. �This is the work from my senior I.S. and also my first published paper, so it means a lot to me,� Xie said. Due to the COVID-19 pandemic, Xie spent her senior year at her home in China, working remotely with Lindner.

�Despite a pandemic keeping us a planet apart and meeting via video conference at simultaneously very early and very late hours, we completed the project,� Lindner said. He remembered a particularly exciting moment during the research process. �During one of our early morning, late evening sessions, we mathematically derived the above condition for solar reversals in the important special case of no planetary tilt,� he said. �Working with Michelle and Ariel was a privilege and a joy, although I wish the pandemic had not kept Ariel and I so far apart.�

Published June 9, 2022.

Posted in News on June 9, 2022.