Study reveals mystery of decaying planetary orbits


A new study has uncovered a mechanism that could solve a long-standing mystery about decaying planetary orbits around stars like our Sun. 

The study, published today (Mon April 29th) in The Astrophysical Journal Letters, proposes that stellar magnetic fields play a crucial role in dissipating the gravitational tides responsible for the orbital decay of ‘hot Jupiter’ exoplanets. 

The findings are the result of an international collaboration between University of Leeds, Northwestern University, USA, and Durham University – who led the study. 

Hot Jupiters are massive, gaseous planets similar to Jupiter that orbit extraordinarily close to their parent stars, taking only a few days to complete one orbit. 

Inward waves 

This close proximity subjects both the planet and star to powerful gravitational tides that transfer orbital energy to tidal waves that are dissipated inside the star, causing the planets to slowly spiral inwards over millions to billions of years until they are eventually consumed. 

Current tidal theories cannot fully explain the observation of orbital decay in the system WASP-12b, a hot Jupiter whose decaying orbit will send it into its host star WASP-12 in a few million years. 

It’s exciting to have a discovered a plausible solution to this mystery.” 

Professor Adrian Barker, School of Mathematics

According to the research, strong magnetic fields within certain Sun-like stars can dissipate the gravitational tides from hot Jupiter planets very efficiently.  

The tides create inward waves inside the stars. When these waves encounter the magnetic fields, they get converted into different types of magnetic waves that travel outwards and eventually disappear. 

Exciting discovery 

One of the authors of the study, Professor Adrian Barker, Professor of Applied Mathematics from the School of Mathematics at Leeds, said: "The observation that this “hot Jupiter” is spiralling into its host star and will be engulfed within a few million years, is one of the most fascinating recent discoveries in exoplanet science.  

“Our study provides a new way — involving magnetic fields deep inside the star — for gravitational tides to act in extrasolar planetary systems, and which appears able to neatly explain the decaying orbit of WASP-12b. 

“It’s exciting to have a discovered a plausible solution to this mystery.” 

Nils de Vries, PhD researcher in Leeds’ School of Mathematics and the study’s second author, said: “What is really interesting about this mechanism is that it only starts after the star has a reached a certain age.  

“At the moment, the only planet we know for certain to be spiralling into its star - and in the far future, possibly being destroyed - is WASP-12b.  

“With this new insight we might actually be able to predict when certain planets will start that process and our findings will help guide observational astronomers wanting to witness orbital decay.”  

The team conducted data analysis and calculations such as estimating how quickly the orbits of the planets around such stars would decay and comparing their results with recent observations. 

The study’s findings suggest certain nearby stars may be good targets to search for additional hot Jupiter planets on decaying orbits.  

If found, they could provide more evidence about how magnetic fields impact the tides from these alien worlds.  

The research could also reveal where the dissipated tidal energy goes within the star's interior. 

Further Information  

An efficient tidal dissipation mechanism via stellar magnetic fields’, (2024), C Duguid, N De Vries, D Lecoanet and A Barker, The Astrophysical Journal Letters.   

Picture caption: Artist’s concept of the exoplanet WASP-12b. Credit: NASA/ESA/G. Bacon 

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