Dusty outflows in planetary atmospheres: Understanding “super-puffs” and transmission spectra of sub-Neptunes. (arXiv:1902.04188v1 [astro-ph.EP])

Dusty outflows in planetary atmospheres: Understanding “super-puffs” and transmission spectra of sub-Neptunes. (arXiv:1902.04188v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Wang_L/0/1/0/all/0/1">Lile Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dai_F/0/1/0/all/0/1">Fei Dai</a>

`Super-puffs’ are planets with anomalously low mean densities ($lesssim
10^{-1}~{rm g cm}^{-3}$). With a low surface gravity, the extended atmosphere
is susceptible to extreme hydrodynamic mass loss (`boil off’) on a timescale
much shorter than the system’s age. Even more puzzling, super-puffs are
estimated to have a scale height of $sim 3000~{rm km}$, yet recent
observations revealed completely flat transmission spectra for Kepler 51b and
51d. We investigate a new scenario that explains both observations: non-static
outflowing ($dot{M}gtrsim 10^{-10}~M_oplus~{rm yr}^{-1}$) atmospheres that
carry very small dust grains ($sim 10~{rm A}$ in size, $sim 10^{-2}$ in mass
fraction) to high altitudes ($lesssim 10^{-6}~{rm bar}$). Dust at high
altitudes inflates the observed transit radius of the planet while flattens the
transmission spectra.Previous static atmospheric models struggles to achieve
cloud elevation and production of photochemical haze at such high altitudes. We
propose to test this scenario by extending the wavelength coverage of
transmission spectra. If true, dusty atmospheric outflows may affect many young
($lesssim 10^9~{rm yr}$), low mass ($lesssim 10~M_oplus$) exoplanets,
thereby limit our ability to study the atmospheric composition in transmission,
and inflate the observed transit radius of a planet hence obscure the
underlying mass-radius relationship.

`Super-puffs’ are planets with anomalously low mean densities ($lesssim
10^{-1}~{rm g cm}^{-3}$). With a low surface gravity, the extended atmosphere
is susceptible to extreme hydrodynamic mass loss (`boil off’) on a timescale
much shorter than the system’s age. Even more puzzling, super-puffs are
estimated to have a scale height of $sim 3000~{rm km}$, yet recent
observations revealed completely flat transmission spectra for Kepler 51b and
51d. We investigate a new scenario that explains both observations: non-static
outflowing ($dot{M}gtrsim 10^{-10}~M_oplus~{rm yr}^{-1}$) atmospheres that
carry very small dust grains ($sim 10~{rm A}$ in size, $sim 10^{-2}$ in mass
fraction) to high altitudes ($lesssim 10^{-6}~{rm bar}$). Dust at high
altitudes inflates the observed transit radius of the planet while flattens the
transmission spectra.Previous static atmospheric models struggles to achieve
cloud elevation and production of photochemical haze at such high altitudes. We
propose to test this scenario by extending the wavelength coverage of
transmission spectra. If true, dusty atmospheric outflows may affect many young
($lesssim 10^9~{rm yr}$), low mass ($lesssim 10~M_oplus$) exoplanets,
thereby limit our ability to study the atmospheric composition in transmission,
and inflate the observed transit radius of a planet hence obscure the
underlying mass-radius relationship.

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