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Posted: 22 December 2011

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A tightly orbiting pair of smaller than Earth-sized planets has been discovered orbiting a star that has already passed through its red giant phase, providing insight into what might become of our own Solar System.

The planets, detected by NASA’s Kepler spacecraft, orbit the star at less than one percent of the Earth-Sun system, a distance that sees them reach temperatures of over 8,000 degrees Celsius. Had they begun their doomed lives at this distance, they would not have survived the red giant phase of their sun’s evolution, suggesting an alternative pathway for how they came to be residing around a hot subdwarf star of just 18 million years in age. One explanation is that they formed some considerable distance away from the star, and as it swelled into a red giant they were dragged into its inflated atmosphere. Such an event would have ripped the atmospheres from the planets, exposing a bare iron-rock core, as well as accelerated the demise of the star itself.

Artist's impression of hot planets orbiting close to a former red-giant core. Image: S. Charpinet.

“These planets were not necessarily hot-Jupiters,” says Valerie Van Grootel of Université de Liège, Belgium, a co-author on the paper lead by Stéphane Charpinet of the Institut de Recherche en Astrophysique et Planétologie in France and presented in the journal Nature this week. “In case of a unique planet, we should probably need a massive Jupiter to influence the evolution of the star. But we have here (at least) two planets, which act together to help to rip off the envelope of the star.”

The authors speculate that the planets transfered their orbital momentum to the stellar envelope, speeding up its rotation and perhaps even accelerating the star’s evolution.

“It is primarily the mass of the planet that is important to influence the evolution of the star, not the fact that it is gaseous or not,” explains Van Grootel. “The planets have to be sufficiently massive to influence the evolution of the star, that's why we privileged the explanation of former gas planets like Saturn or Jupiter, instead of smaller and much less massive telluric planets like the Earth.”

Alternatively, but perhaps less likely, is that the planets formed from the embers of the star’s debris. “But the subdwarf star is only 18.4 million years old after its red-giant phase, which is a very short time for planetary formation,” says Van Grootel. “We have also not observed a dust disc from which planets are formed around a subdwarf star.”

The star, known as KIC 05807616 (or KPD 194314058) was initially studied for its known pulsations as part of the Kepler Asteroseismic Investigation, but two weak patterns that were not found to be related to the star’s pulsations stood out in the observations, showing periodic dimming and brightening every 5.8 and 8.2 hours, corresponding to the changing amount of starlight reflected off the planets. The changes in brightness were found to be extremely small, corresponding to planets just 76 and 87 percent the size of Earth.

The team hope that further observations with Kepler, as well as measurements from ground-based telescope will help determine the finer details of the planets, such as mass and density. Meanwhile, the system presents an insight into one possible outcome of our Solar System when the Sun swells into a red giant in several billions of years time, likely engulfing everything up to Mars and maybe further. Perhaps the two planets seen here are a snap-shot of the fate of Jupiter and Saturn.

To learn more about asteroseismology pick up a copy of the January issue of Astronomy Now, which is on sale now!

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