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Evidence for Two More Earth-Sized Planets ...Orbiting Former Red Giant Star
12.26.2011

[Note: The "modulations in the pulsations" in the star's brightness, referred to in the article, are not transit signals, and these are not confirmed planets by the criteria and established methods of the Kepler Science Team.]

Article excerpts from Universe Today: ...two planets, KOI 55.01 and KOI 55.02, orbit the star KOI 55, a subdwarf B star, which is the leftover core of a red giant star. …According to lead author Stephane Charpinet, “Having migrated so close, they probably plunged deep into the star’s envelope during the red giant phase, but survived” …(albeit “deep-fried”). They are estimated to have radii of 0.76 and 0.87 that of Earth, the smallest ... so far orbiting an active star. …the star had already been the subject of study using the telescopes at Kitt Peak National Observatory, part of a project to examine pulsating stars. …the team used data from the orbiting Kepler space telescope which is free of interfering atmospheric effects. …Two tiny modulations in the pulsations of the star were found, which further analysis indicated could only come from planets passing in front of the star (from our viewpoint) every 5.76 and 8.23 hours.

See the abstract of the paper on the Nature website. Downloading the full article requires a single-article payment of $32.00 US or a subscription to Nature.

Artist's rendition of two hot Earth-sized planets orbiting a subdwarf B star, KOI 55. (Illustration: S. Charpinet)
Artist's rendition of two hot Earth-sized planets orbiting a subdwarf B star, KOI 55. (Illustration: S. Charpinet; sources: UANews and Universe Today.)

See also University of Arizona press release of 2011 December 21, by Daniel Stolte, Excerpts from press release: …Two Earth-sized planets ... circling a dying star that has passed the red giant stage. Because of their close orbits, the planets must have been engulfed by their star while it swelled up to many times its original size. …Researchers believed that this unimaginable inferno would make short work of any planet caught in it – until now. …The team was led by Stephane Charpinet, an astronomer at the Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse-CNRS, in France.

…The two planets, named KOI 55.01 and KOI 55.02, circle their host star in extremely tight orbits. Having migrated so close, they probably plunged deep into the star's envelope during the red giant phase, but survived....

The host star, KOI 55, is what astronomers call a subdwarf B star: It consists of the exposed core of a red giant that has lost nearly its entire envelope. In fact, the authors write, the planets may have contributed to the increased mass loss necessary for the formation of this type of star.

The authors concluded that planetary systems may therefore influence the evolution of their parent stars.

…the research team had not set out to find new planets far away from our solar system, but to study pulsating stars. Caused by rhythmic expansions and contractions brought about by pressure and gravitational forces that go along with the thermonuclear fusion process inside the star, such pulsations are a defining feature of many stars.

By studying the pulsations of a star, astronomers can deduce the object's mass, temperature, size and sometimes even its interior structure. This is called asteroseismology.

"Those pulsation frequency patterns are almost like a finger print of a star," Green said. "It's very much like seismology, where one uses earthquake data to learn about the inner composition of the Earth." ... the team used data obtained from NASA's Kepler Space Telescope for this study.

...While analyzing KOI 55's pulsations, the team noticed the intriguing presence of two tiny periodic modulations occurring every 5.76 and 8.23 hours that caused the star to flicker ever so slightly, at one five thousandth percent of its overall brightness. They showed that these two frequencies could not have been produced by the star's own internal pulsations.

The only explanation came from the existence two small planets passing in front of the star every 5.76 and 8.23 hours. To complete their orbits so rapidly, KOI 55.01 and KOI 55.02 have to be extremely close to the star, much closer than Mercury is to our sun. On top of that, the sun is a cool star compared to KOI 55, which burns at about 28,000 Kelvin, or 50,000 degrees Fahrenheit.

…"We think this is the first documented case of planets influencing a star's evolution," Charpinet said. ..."I find it incredibly fascinating that after hundreds of years of being able to only look at the outsides of stars, now we can finally investigate the interiors of a few stars – even if only in these special types of pulsators – and compare that with how we thought stars evolved," Green said. "We thought we had a pretty good understanding of what solar systems were like as long as we only knew one – ours. Now we are discovering a huge variety of solar systems that are nothing like ours, including, for the first time, remnant planets around a stellar core like this one."

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