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Remains of gas giants, swallowed by red giant star, now orbit as small planets

Astronomers find a star system where a red giant appears to have swallowed two …

Two small planets orbit the core of what was formerly a red giant.
Two small planets orbit the core of what was formerly a red giant.
Photograph by S. Charpinet

There may come a day when exoplanet discoveries start to bore me, but we're not there yet. A day after the Kepler team announced the identification of Earth-sized planets orbiting a distant star, they're back with the description of a truly bizarre planetary system. In its past, KIC 05807616 expanded out to red giant size, swallowing two gas giants in the process. The cores of these planets continued to orbit, reemerging once the star shrunk again. Meanwhile, their impact on the star may have caused it to become an unusually hot form of dwarf star.

The star in question, KIC 05807616, has a rather interesting description: "a seemingly isolated pulsating hot B subdwarf." These have a somewhat unusual history. Normally, stars near the mass of our sun expand out as red giants, but then contract as they switch from fusing hydrogen to fusing the helium that has built up at their cores. Hot B dwarfs occur when something happens during the red giant phase that removes all the star's hydrogen, leaving nothing but a helium fusing core behind. KIC 05807616 has been in this stage for less than 20 million years, and in addition to its high temperatures, undergoes regular fluctuations, hence the "pulsating" part of that description.

Generally, the removal of hydrogen from a red giant is assumed to be done by having a companion star strip it off. But KIC 05807616 doesn't have a companion, which is why the researchers term it "isolated."  That small, odd-sounding phrase actually said quite a lot.

The pulsating it's doing brought it to the attention of the Kepler team, as the telescope is designed to pick up variations in the output of distant stars, not all of which will come from orbiting planets. In addition to the sorts of variability we expect for a star of this type, there were two low-frequency shifts in the star's light that caught the researchers' attention.

The additional variability was still very short, just below six and above eight hours, so it was possible to obtain a lot of observations of this variability, and confirm that it wasn't part of the star's normal pulsing. But it also didn't look like the signal you'd expect if the star had planets orbiting in the appropriate plane to block some of the light reaching Earth.

The authors offer two possible explanations, both of which involve planets orbiting in a different plane. One is that we're seeing additional starlight reflected to us by the planet's surface; the other is that the planets are so close to the star (and thus so hot), we're seeing the difference between the hot "day" side and the cooler "night" side of the planets.

Given these assumptions and what we know about the star and the planets' orbital frequency, it's possible to make some reasonable estimates about what the planets should look like. Although it's possible to get these observations out of a system with gas giants, most of the solutions had very small planets, possibly smaller than the Earth.

A few things argue against gas giants. One is the fact that these things orbit so close to the host star, which means that they would have been well within the outer envelope during its red giant phase. That means that any gas giants at this location would have had to have formed and migrated inwards in less than 20 million years. They're also so close to the star that they probably wouldn't be stable, as high temperatures and radiation should drive the gas off.

Instead, the authors suggest that the planets are all that remains of former gas giants. As the star expanded into a red giant, the planets were engulfed, and interactions with the star dragged them into ever closer orbits, while blasting away all but the gas giants' rocky cores. When the red giant phase ended, the cores re-emerged as small, rocky planets. Because of the drag caused by plowing through the interior of the star, their orbits had decayed to the very short ones detected in the Kepler data. The authors note that the orbits display a 3:2 resonance, which may have helped keep them from spiraling all the way into the core of the star.

The authors also suggest that the presence of planets within the red giant's envelope may explain how the star lost its hydrogen in the first place, since it's easy to imagine that they would have been quite disruptive. And, if that's correct, it could explain why we see a number of hot B subdwarfs without obvious companion stars to strip them of their hydrogen. It certainly gives the astrophysical modelers something to look into.

Nature, 2011. DOI: 10.1038/nature10631  (About DOIs).

Listing image by Photograph by S. Charpinet

Channel Ars Technica