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| Titel |
Identifying the upper atmosphere structure of the inflated hot sub-Neptune CoRoT-24b |
| VerfasserIn |
Ines Juvan, Helmut Lammer, Nikolai V. Erkaev, Luca Fossati, Patricio E. Cubillos, Eike Guenther, Petra Odert, Kristina G. Kislyakova, Monika Lendl |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250124724
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| Publikation (Nr.) |
 EGU/EGU2016-4202.pdf |
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| Zusammenfassung |
| The CoRoT satellite mission discovered two Neptune-type planets, CoRoT-24b and
CoRoT-24c, with observed transit radii of ≈3.7REarth and ≈4.9REarth and masses of
≤5.7MEarth and ≈28MEarth, respectively. From the deduced low mean densities it can be
expected that their planetary cores are most likely surrounded by H2 dominated
envelopes. While having very similar radii, the outer planet CoRoT-24c is at least 4.9
times more massive than its neighbour, indicating that their atmospheres can be
fundamentally different. Therefore, we have investigated the upper atmosphere
structure and escape rates of these two planets. We applied a hydrodynamic upper
atmosphere model including heating by absorption of stellar extreme ultraviolet and
X-ray (XUV) radiation, under the assumption that the observed transit radius RT is
produced by Rayleigh scattering and H2-H2 collision absorption in a pure hydrogen
atmosphere. This corresponds to a pressure level near 1 bar. We find an unsustainably high
hydrodynamic escape rate of 1.6 × 1011 g/s for the atmosphere of CoRoT-24b.
If real, such high atmospheric escape would lead to substantial mass loss from
the planetary atmosphere, shrinking it to ≈2.2REarth within ≈4 Myr, which is
inconsistent with the old age of the system. The solution to this discrepancy is that
the observed transit radius RT must be 30-60% larger than the actual planetary
radius at the 1 bar pressure level. We suggest that the observed transit radius RT is
produced by absorption through scattering processes due to high altitude clouds or
hazes. The Kepler satellite has discovered similar close-in low-density Neptune-type
planets. We propose that it is very likely that the observed transit radii for the vast
majority of these planets also differ from their actual planetary radii at the 1 bar
pressure level. This would introduce a systematic bias in the measured radii and has
dramatic implications in the determination of the mass-radius relation and for planet
synthesis studies. Our finding will become even more relevant in the near future
with the launch of space missions like CHEOPS, TESS, the JWST and PLATO. |
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