Two Earth-sized worlds created by planet-splitting red giant star

Norman Byrd's picture

Two Earth-sized planets circle a red giant star. By what is generally known about star evolution, they should not exist in such close proximity to their parent star. But a new theory suggests that they may have once been part of a massive gas giant that not only was ripped apart by the parent star but also helped the parent shed excess gas, allowing them to survive.

Some of the work astronomers do gravitates toward the theoretical, as when they attempt to explain certain anomalous conditions or the presence of phenomena. Such was the case when two Earth-sized planets were discovered circling a dying star. Although those who had first detected the two worlds -- using NASA's Kepler Telescope -- orbiting the red giant star KIC 05807616 had posited a theory as to how the planets had managed to survive so close to the massive stellar object, they had invited others to speculate as well. The first team to meet the challenge saw their work published in the Astrophysical Journal Letters, presenting the idea that the planets' positions weren't a simple migration or stellar gravitational grab but more likely the result of a planet-destroying process where a giant world had been torn asunder.

"Planets can still evolve, by disintegrating to several small bodies, or by being completely destroyed," Israel Institute of Technology researchers Ealeal Bear and Noam Soker, authors of the paper detailing the theory, wrote to by email.

According to the two scientists, when KIC 05807616 ballooned out into a red giant, it most likely destroyed any planets anywhere near it. However, one massive gas giant spiraled in but was saved by passing close enough to strip off excess gas as it was being ripped apart. Further studies have revealed what might be a third small body orbiting the star, a presence that could also be an indicator that the two near Earth-sized worlds were indeed once part of a much larger planet. Astronomers are searching for more orbiting objects.

The two planets, labeled KOI 55.01 and KOI 55.02, are closer to their parent star than Mercury is to the Sun. Although of the same general classification of planet as the Earth (terrestrial or rocky), they are much too close to the star to retain water.

Before the discovery of the two planets, the prevailing theory was that planets could not aid in the evolution of their parent stars. It was also believed that they would not survive the enormous reach of their red giant parent.

Stephane Charpinet of the University of Toulouse in France led the team that discovered the odd planetary system in December. It was originally posited that the two worlds were at one time massive gas giants that had had their atmospheres burned away by the ever-encroaching red giant star. Charpinet told in an email that Bear and Soker had proposed a "truly interesting alternative to our scenario."

The two Earth-sized planets are among the very few small extra-solar worlds that have have been discovered thus far. In fact, it was also in December (the day before Charpinet's revelation, which occurred on Dec. 22) that the two first near-Earth-sized planets were detected, including the first ever planet actually smaller than Earth.

Dr. Francois Fressin of the Harvard-Smithsonian Centre for Astrophysics led the study that found two terrestrial planets circling Kepler-20. Both planets orbit too close to their parent star to be in the habitable zone of their parent, however, which is a star not unlike the Sun. Designated Kepler-20e and -20f, the planets were estimated to be 0.87 Earth mass and 1.03 Earth mass, respectively.

Fressin's discovery also came from observations made with the Kepler Telescope.

Most extra-solar or exoplanets discovered to date have been of the gas giant or super-Earth varieties. The enormous distances from Earth make the extra-solar worlds extremely difficult to detect, increasing the odds that larger planets will be detected. Thus far, according to the Extrasolar Planets Encyclopedia, there have been 770 exoplanets discovered and confirmed.

(photo credit: NASA, Wikimedia Commons)

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