What happens to planets when their stars are dying?
Many astronomers today are trying to understand how planets form. There are numerous research groups dedicated to planet searches, and these groups' findings continually add to our collective inventory of known extrasolar planets. Other astronomers study younger stars that still possess disks capable of forming planets. Still others create detailed physical models with computers to try to reproduce the diversity of planetary systems we have found so far.
My research, on the other hand, is probably better described as trying to understand how planets die. When stars grow old, they eventually exhaust their central reserves of hydrogen. While the stars' inert cores (primarily composed of helium) shrink, the stars' outer layers expand and cool. These evolved stars are called "red giants." A natural question to ask is "what happens to planets that are orbiting these expanding stars?" Planets that were once in relatively stable orbits may now find themselves in peril of being consumed by their host stars.
This idea is illustrated in the figure below. The dark blue circles represent known exoplanets with measured planet masses (y-axis) and distance from their stars (x-axis). The other colored circles indicate the masses and orbital distances of planets in the Solar System, as a reference. On the top of the diagram is an illustration of how a sun-like star's radius grows during different stages of red giant evolution. The left end of each colored arrow (and the bottom point of each star symbol) is aligned to show the corresponding distance in AU. Any planet to the left of the arrow would be inside the star. Furthermore, when the distances between stars and planets are small enough, they can interact with each other----especially through tidal interactions. Any planet that is less than 4--5 times the stars radius may be "doomed" to be dragged into the star by tidal interactions. The right tip of each colored arrow shows the edge of the tidal interaction zone. Planets to the right of each arrow tip may still safely orbit its giant star. You can read more about these tidal interactions on my detailed research page or in my published paper.
A planet that is consumed by its star can contribute angular momentum to the star (making it rotate faster) and can change the abundances of elements in the stellar atmosphere. I mainly study the abundances of red giant stars that are rapidly rotating to better understand this process of planet destruction. You can read more about this project (and some of previous project) by visiting the links below. Of course, I am not alone in my study of planetary demise. Other ways that researchers are studying the fate of planets include searching for planets around evolved stars or stellar remnants (to see planets that survive and infer which planets cannot), modeling the interaction between a planet and its evolving host star, and studying planetary nebulae (the stellar corpses of sun-like stars).
- Observable Signatures of Planet Accretion in Red Giant Stars I: Rapid Rotation and Light Element Replenishment , Carlberg, J. K., Cunha, K., Smith, V. V. Majewski, S. R., 2012, ApJ, 757, 109
- The Frequency of Rapid Rotators Among Red Giant Stars , Carlberg, J. K., Majewski, S. R., Patterson, R. J., Bizyaev, D., Smith, V. V., Cunha, K., 2011, ApJ, 732, 39
- The Super Lithium-rich Red Giant Rapid Rotator G0928+73.2600: A Case for Planet Accretion? , Carlberg, J. K., Smith, V. V., Cunha, K., Majewski, S. R., Rood, R. T., 2011, ApJL, 723, L103
- The Role of Planet Accretion in Creating the Next Generation of Red Giant Rapid Rotator, Carlberg, J. K., Majewski, S. R., Arras, P., 2009, ApJ, 700, 832