22 December 2011

Kepler discovers new system of hot planets

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The discovery of a compact system of scorched planets orbiting a former red giant has important implications in theories about the evolution of stars.
new planetary system

An artist's impression of hot planets orbiting close to a former red giant core.
Credit: S. Charpinet

SYDNEY: The discovery of a compact system of scorched planets orbiting a former red giant has important implications in theories about the evolution of stars.

The new planets, named KOI 55.01 and KOI 55.02, are the first to be found using a new technique based on tiny fluctuations in brightness. They were discovered orbiting closely around a hot subdwarf B star called KIC 05807616, located 3,850 light years away. Once a red giant, this star has been partly stripped by the nearby planets, which have in turn been transformed from gas giants to small, iron-rich remnants.

The discovery was made using data collected by NASA’s Kepler spacecraft, the planet hunter that has already discovered 170 planetary systems and 708 extrasolar planets (which are planets that lie outside the Solar System). Just yesterday, the discovery of a different pair of extrasolar planets was reported.

“This discovery should have interesting implications in our understanding of the evolution of stellar systems,” said co-author Stéphane Charpinet, an astrophysicist at the University of Toulouse in France, of the study published in Nature today.

“It suggests that planets may play a significant role in the evolution of stars, especially in the late stages. This system is indeed the first documented case of planets influencing a star’s evolution.”

An unexpected find

According to Charpinet, when his team started to look at the periodic variations in light emitted from star KIC 05897616, they weren’t looking for planets at all, and ended up finding them by chance.

“Stars can be shaken by vibrations and these oscillations can tell us about the structure of the star internal layers that we cannot see otherwise,” Charpinet said. “However, for KIC 05807616, in addition to the clearly detected vibrations [that we were initially interested in], we found two additional, very weak variations in brightness, less than 0.005% of the mean brightness of the star, that could not be attributed to causes associated to the star itself.”

The researchers proposed the presence of two planets in close proximity to the star as the most likely cause of the tiny fluctuations. As the planets orbit KIC 05807616, the ‘dayside’ of each planet facing the star is illuminated. As the orientation of the planets relative to the star and us, the observers, changes over its orbit, the amount of light directed towards us fluctuates. It’s the same phenomena that has us observing variations in the Moon’s brightness.

A first for planetary discovery

The close proximity and illumination of half of the surface of each planet also produces extreme heating of their exposed surface, around 8,000 to 9,000 degrees Kelvin. The high temperatures cause the planets to radiate light, as well as reflect it. Both mechanisms produce the tiny fluctuations in brightness observed in the study.

The planets are the first to be discovered using this method. “The brightness changes [typically observed] are so minuscule that this effect has mostly been used to characterise known planets rather than to discover new ones,” said Michael Ireland, an astrophysicist at Macquarie University in Sydney who was not involved in the study. “This research shows that with precise monitoring of the combined star and planet brightness, the contrast of the surface of roasted planets between their day and night is enough to discover new populations of planets.”

A new mechanism for stellar evolution?

The planets’ orbits around KIC 05807616 have been calculated as 50 times closer than Mercury is to Earth, the shortest orbits ever observed around an active star. The findings point to an intriguing history of the star-planet system.

“The most consistent explanation for this system is that the two detected planets were originally giant gaseous planets that have been engulfed by the star when it became a red giant,” said Charpinet. “They were dragged deep into the red giant to their current location.”

As the original red giant transformed into the star that is observed today, it would have shed its outer hydrogen rich layers, losing mass as a result. However, typically encountered rates of mass loss aren’t enough to trigger the formation of the hot subdwarf B star that Kepler observed. Charpinet and his colleagues propose the newly discovered planets provided the extra “pull” of mass required to evolve the star into its present state.

“[In the same process, the outermost layers of the] giant planets were evaporated and stripped of their most volatile layers, leaving only small bodies, which are presumably the dense iron cores of the former giant planets,” he said. The remaining cores are known as Chthonian planets.

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