The discovery of two new earth-sized planets orbiting an older Sun-like star raises som fundamental questions about the evolution of stars; Could they really have survived being dipped into the stars atmosphere, when it blew up to be red giant?

Two Earth-sized planets have been discovered 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.

This discovery, published in the science journal Nature, may shed new light on the destiny of stellar and planetary systems, including our solar system.

When our sun nears the end of its life in about 5 billion years, it will swell up to what astronomers call a red giant, an inflated star that has used up most of its fuel. So large will the dying star grow that its fiery outer reaches will swallow the innermost planets of our solar system – Mercury, Venus, Earth and Mars.

Researchers believed that this unimaginable inferno would make short work of any planet caught in it – until now.

This report describes the first discovery of two planets – or remnants thereof – that evidently not only survived being engulfed by their parent star, but also may have helped to strip the star of most of its fiery envelope in the process. The team was led by Stephane Charpinet, an astronomer at the Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse-CNRS, in France.

"When our sun swells up to become a red giant, it will engulf the Earth," said Elizabeth ‘Betsy' Green, an associate astronomer at the University of Arizona's Steward Observatory, who participated in the research. "If a tiny planet like the Earth spends 1 billion years in an environment like that, it will just evaporate. Only planets with masses very much larger than the Earth, like Jupiter or Saturn, could possibly survive."

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. In the most plausible configuration, the two bodies would respectively have radii of 0.76 and 0.87 times the Earth radius, making them the smallest planets so far detected around an active star other than our sun.

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. They pointed out that the planetary system they observed offers a glimpse into the possible future of our own.

The discovery of the two planets came as a surprise because 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.

Sources:

Iowa State University: Planets that survived their star’s expansion
University of Arizona; Deep frieid planets
Université de Montreal: planets raises questions about the evolution of stars