Exoplanets Exposed to the Core

Artist’s illustration of a scorched extrasolar planet HD 209458b in its orbit only 4 million miles from its yellow, Sun-like star. The planet is a type of extrasolar planet known as a "hot Jupiter." Credit: ESA, Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS, France) and NASA

An international team of scientists has found that giant exoplanets orbiting very close to their stars could lose a quarter of their mass during their lifetime. The team found that planets that orbit closer than 2% of an Astronomical Unit (AU), the distance between the Earth and the sun, may lose their atmospheres completely, leaving just their core. The team, led by Dr Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences, believe that the recently discovered CoRoT-7b “Super Earth”, with a diameter less than twice the diameter of Earth, could be the stripped core of a Neptune-sized planet.

(Editorial Note: The mass of planet COROT-7b is estimated to be between 5.6 to 11 Earth masses.)

The team used computer models to study the possible atmospheric mass loss over a stellar lifecycle for exoplanets at orbiting distances of less than 0.06 AU where the planetary and stellar parameters are very well known from observations. The 49 planets considered in the study included hot gas giants, planets with masses similar or greater than that of Saturn and Jupiter, and hot ice giants, planets comparable to Uranus or Neptune. All the exoplanets in the sample were discovered using the transit method, where the size and mass of the planet is deduced by observing how much its parent star dims as it the planet passes in front of it.

“If the transit data are accurate, these results have great relevance for planetary formation theories”, said Dr Lammer, who presented these results at the European Week of Astronomy and Space Science at the University of Hertfordshire.

“We found that the Jupiter-type gas giant WASP-12b may have lost around 20-25% of its mass over its lifetime, but that other exoplanets in our sample had negligible mass loss. Our model shows also that one major important effect is the balance between the pressure from the electrically charged layer of the planet’s atmosphere and the pressure from the stellar wind and coronal mass ejections (CMEs). At orbits closer than 0.02 AU, the CMEs — violent explosions from the star’s outer layers — overwhelm the exoplanet’s atmospheric pressure causing it to lose maybe several tens of percent of its initial mass during its lifetime.”

All the planets examined in the study were discovered using the transit method. As a planet passes in front of its parent star, the brightness of the star decreases. Credit: Hans Deeg

The team found that gas giants could evaporate down to their core size if they orbit closer than 0.015 AU. Lower-density ice giants could completely lose their hydrogen envelope at 0.045 AU. Gas giants orbiting at more than 0.02 AU lost about 5-7% of their mass. Other exoplanets lost less than 2%. Results suggest that CoRoT-7b could be an evaporated Neptune-like planet but not the core of a larger gas giant. Model simulations indicate that larger mass gas giants could not have been evaporated to the mass range determined for CoRoT-7b.

Gaining insight into how planets form and evolve around distant stars can help astrobiologists determine the best systems in which to search for habitable, Earth-like planets.