The most interesting thing about the worlds known as KOI 55.01 and KOI 55.02 is not just the fact that they are — if current thinking holds — the smallest planets yet detected around an active star other than our Sun, but that they are evidently survivors of the most extreme kind of experience. KOI 55, their host star, is of subdwarf B class, the exposed core of a red giant that has lost most of its gaseous envelope. The two planets that circle it are in such tight orbits that they would have been engulfed when the central star went through its red giant expansion.

What a scenario, one we’ve often contemplated in these pages as we look toward the future of our own Sun. We tend to think in terms of planets that survive the red giant phase by orbiting far enough from the primary not to be swallowed up in it — smaller worlds like Mercury, Venus and the Earth would not survive the experience. But KOI 55.01 and KOI 55.02 evidently were swallowed, and probably represent the remains of gas giants that underwent the plunge. At present, they are thought to have radii 0.76 and 0.87 times the Earth’s radius.

Elizabeth Green (University of Arizona) explains:

“When our sun swells up to become a red giant, it will engulf the Earth. 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.”

Green participated in this work as a member of a team led by Stephane Charpinet (Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse-CNRS). The team’s findings make an intriguing case for the proposition that planets may directly influence the development of their host stars in this late phase of stellar evolution. Here they seem to have done it by contributing to the mass loss that is necessary for a subdwarf B star to form.

Here’s Green again on how this would work:

“As the star puffs up and engulfs the planet, the planet has to plow through the star’s hot atmosphere and that causes friction, sending it spiraling toward the star. As it’s doing that, it helps strip atmosphere off the star. At the same time, the friction with the star’s envelope also strips the gaseous and liquid layers off the planet, leaving behind only some part of the solid core, scorched but still there.”

Finding remnant planets around a stellar core, planets that have passed through the maelstrom of red dwarf expansion, is surprising enough, but the work also came from an unexpected direction. The team’s objective had been to use Kepler data to study astroseismology, examining the rhythmic expansions and contractions that pressure and gravitational forces produce inside a star as it burns. It’s useful stuff because with enough data, astronomers can estimate the star’s mass, temperature and size, as well as learning something about its internal structure.

Astroseismology takes time as researchers accumulate information about the star’s variations in brightness and compare these to theoretical models of stellar interiors. Green had already been studying hot subdwarf stars in the galactic plane and had accumulated spectra of KOI 55 with instruments on Kitt Peak before the Kepler mission was launched. Kepler was able to show the star’s pulsational modes with great clarity and it was in the midst of examining the Kepler data that periodic modulations occurring every 5.76 and 8.23 hours began to turn up. The researchers were able to show that the modulations could not have been caused by internal pulsations.

Two planets were the best explanation, each orbiting closer to their star than Mercury is to the Sun. Conditions on these worlds today must be hellish, as KOI 55 is much hotter than the Sun, but their tight orbits tell us that things were once worse still, when the planets would have been engulfed during the star’s expansion.

“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.”

The paper is Charpinet, “A compact system of small planets around a former red-giant star,” Nature 480, 496–499 (22 December 2011). Abstract available. This University of Arizona news release offers further details.