Discovery of a planetary system around the compact core of an old red giant star

An international team led by a CNRS researcher from the Institut de Recherche en Astrophysique et Planétologie (IRAP, CNRS – Université Toulouse III – Paul Sabatier) has discovered the ruins of a planetary system, consisting of the cores of two former giant planets stripped of their gaseous envelopes, orbiting around the remnants of the core of a red giant. These two exoplanets are the smallest, hottest and closest to their parent star ever discovered. This finding, published in the 22 December 2011 issue of the journal Nature, could shed new light on the fate of planetary systems.

Artist’s rendering of the two alien planet candidates KOI 55.01 and KOI 55.02, which apparently survived their star’s red-giant phase. They now circle very close to the star’s core. (Image: © S. Charpinet)

While analyzing data from NASA’s Kepler spacecraft on pulsations in the star KIC 05807616, a team of astrophysicists led by French researcher Stéphane Charpinet, at the Institut de Recherche en Astrophysique et Planétologie (IRAP), noticed an intriguing phenomenon: two tiny variations of a mere 0.005% in the star’s brightness, with a 5.76 and 8.23 hour-periodicity. The astrophysicists inferred the presence of two bodies in orbit around the star, as such variations could not be put down to the oscillations of the planet or to any other cause.

The changes observed are due to the reflection of the star’s light on the illuminated surface of the bodies and to the difference in thermal emission between the illuminated (warmer) and unilluminated (colder) hemispheres. The researchers’ calculations show that, in order to produce such small variations in brightness, the size of the orbiting bodies must be comparable to that of the Earth, probably 0.76 and 0.87 Earth radii, which makes them the smallest planets ever detected around a still active star, other than the Sun1.

The planets have also broken another record: at distances of only 897,000 km and 1,137,000 km, they are closer to their star than any other exoplanet ever observed. These two bodies are quite foolhardy compared with Mercury, the planet closest to our Sun, which orbits at around 58 million kilometers! Since star KIC 05807616 is particularly hot, with a surface temperature of 27,400 °C, the temperature on the illuminated hemispheres of the two planets could reach 7700-8700 °C, breaking yet another record. In comparison, the surface temperature on Venus, the hottest planet in our solar system, is a ‘mere’ 470 °C!

This raises a number of questions about the nature of such bodies, capable of surviving such hellish conditions so close to their parent star. According to the astrophysicists, the most plausible explanation is that they are remains of former gas-giant planets. The bodies were once ‘hot Jupiters’ orbiting close to their star, which was still burning hydrogen in its core. Towards the end of its life, KIC 05807616 expanded and became a red giant. The planets orbiting around it were probably engulfed in the star’s outer layers and their envelopes were stripped away as a result. All that remains today from this extreme event are the ruins of the planetary system. These include the planets, reduced to their dense cores alone (mainly made up of iron and other heavy elements), and KIC 05807616, the helium-fusing core of the former red giant, surrounded by a thin layer of hydrogen. To form the star as it is today, the red giant must have ejected most of its envelope through a mechanism that increased mass loss, and it is possible that the two planets discovered orbiting around the star triggered this process.

Notes

1Smaller exoplanets have been detected, but they are in orbit around pulsars, neutron stars that are the inert remains of supernovae. Such planets probably formed after the explosion of the star, from material ejected in the explosion.
2The solid cores of former gas giants whose envelopes have been stripped away are called chthonian planets.

Further Resources

IRAP Contacts

  • Dr. Stéphane Charpinet (CNRS), Institut de Recherche en Astrophysique et Planétologie, Observatoire Midi-Pyrénées , Toulouse & KITP, University of California Santa Barbara. Courriel: stephane.charpinet@irap.omp.eu, Tél: +1 (805) 893-6358 (avant le 21 Décembre; 9 heures de décalage horaire), 06 28 25 31 94 (à partir du 23 Décembre)
  • Dr Valérie Van Grootel, Université de Liège, Liège, Belgique, Courriel: valerie.vangrootel@ulg.ac.be, Tel: +32 4 366 9730

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