Stellar evolution models go back decades.  Ever since the Hertzsprung-Russell diagram came out in 1910 (graphing temperature against luminosity), showing most stars fell on a line called the “main sequence”, astrophysicists have sought to understand the life cycle of stars from birth to death.  In general, the story goes, collapsing clouds of gas and dust produce main-sequence stars that burn nuclear fuel till they run out.  Depending on their masses, they end up as supernovae, red giants or slowly-cooling cinders.  While red dwarfs cool down slowly into the darkness, supernovae and red giants eject mass outward into space .  Two new planets found close to a red giant are among new headaches for theorists.

Hot survivors:  The paper by Charpinet et al. in Nature1 was discussed by Eliza M. R. Kempton in the same issue,2 who said, “The prospect of planets being detected in close proximity to an evolved star is certainly of great interest, because it was previously assumed that such objects would be destroyed during the stars evolution.” While other sites like the BBC News and Live Science focused on how scorching hot these planets must be, having once been inside the envelope of the red giant star, Science Daily put the deeper issue into the headline: “Discovery of Two Earth-Size Planets Raises Questions About the Evolution of Stars.”

In the paper, Charpinet et al. discuss two possibilities.  One is that the planets are cores of gas giant planets that lost their gaseous envelopes. The authors believe this implies the startling idea that planets can affect the evolution of their parent stars.  If they are indeed survivors, the rocky core remnants must have endured hellish conditions that astronomers believed would have vaporized any planet.  Another problem is how they migrated inward so far when the red giant phase involves mass movement outward.

Alternative scenarios may also be considered. Another way to form single sdB stars is through the merger of two helium white dwarfs, and planet formation following this event may be possible. We could speculate that the collapse of the extended envelope resulting from this merger could produce a circumstellar disk, where second generation planets may form. However, it seems unlikely that new, sufficiently dense, planets could have formed within a rather short period of time (less than ~18 Myr) in an environment that close to this hot star….

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