J/MNRAS/466/1868 Neptune-like planets low-density overabundance (Cubillos+ 2017)
An overabundance of low-density Neptune-like planets.
Cubillos P., Erkaev N.V., Juvan I., Fossati L., Johnstone C.P., Lammer H.,
Lendl M., Odert P., Kislyakova K.G.
<Mon. Not. R. Astron. Soc., 466, 1868-1879 (2017)>
=2017MNRAS.466.1868C 2017MNRAS.466.1868C (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Exoplanets ; Morphology ; Mass loss
Keywords: hydrodynamics - planets and satellites: atmospheres -
planets and satellites: fundamental parameters
Abstract:
We present a uniform analysis of the atmospheric escape rate of
Neptune-like planets with estimated radius and mass (restricted to
Mp<30M⊕). For each planet, we compute the restricted
Jeans escape parameter, Λ, for a hydrogen atom evaluated at the
planetary mass, radius, and equilibrium temperature. Values of
Λ≤20 suggest extremely high mass-loss rates. We identify
27 planets (out of 167) that are simultaneously consistent with
hydrogen-dominated atmospheres and are expected to exhibit extreme
mass-loss rates. We further estimate the mass-loss rates (Lhy) of
these planets with tailored atmospheric hydrodynamic models. We
compare Lhy to the energy-limited (maximum-possible high-energy
driven) mass-loss rates. We confirm that 25 planets (15 per cent of
the sample) exhibit extremely high mass-loss rates
(Lhy>0.1M⊕/Gyr), well in excess of the energy-limited
mass-loss rates. This constitutes a contradiction, since the hydrogen
envelopes cannot be retained given the high mass-loss rates. We
hypothesize that these planets are not truly under such high mass-loss
rates. Instead, either hydrodynamic models overestimate the mass-loss
rates, transit-timing-variation measurements underestimate the
planetary masses, optical transit observations overestimate the
planetary radii (due to high-altitude clouds), or Neptunes have
consistently higher albedos than Jupiter planets. We conclude that at
least one of these established estimations/techniques is consistently
producing biased values for Neptune planets. Such an important
fraction of exoplanets with misinterpreted parameters can
significantly bias our view of populations studies, like the observed
mass-radius distribution of exoplanets for example.
Description:
We compiled our sample by collecting and cross-checking the lists of
exoplanets from the NASA Exoplanet Archive
(http://exoplanetarchive.ipac.caltech.edu), the Exoplanets Data
Explorer (http://exoplanets.org, Han et al. 2014PASP..126..827H 2014PASP..126..827H), and
The Extrasolar Planets Encyclopaedia (http://exoplanet.eu, Schneider
et al. 2012, ASP Conf. Ser. 461, 447). We selected the targets with
measured planetary radii and masses, whose mass is less than
∼2 Neptune masses (Mp<30M-{Eeath}_). We adopted stellar rotational
angular velocity ({Omage}rot) from McQuillan, Mazeh & Aigrain (2013,
Cat. J/ApJ/775/L11) and ages from Morton et al. (2016, Cat.
J/ApJ/822/86). Our final sample consists of 167 planets (Table A1).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 131 167 *Observed and derived parameters for the
sub-Neptune planet sample.
refs.dat 67 45 References
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Note on tablea1.dat: This table only includes planets with estimated mass (Mp)
and transit radius (Rp), with Mp<30M⊕.
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 15 A15 --- Name Name
17- 21 F5.2 Mgeo Mp Planetary mass
23- 27 F5.2 Mgeo E_Mp Error on Mp (upper value)
29- 33 F5.2 Mgeo e_Mp Error on Mp (lower value)
35- 38 F4.2 Rgeo Rp Planetary radius
40- 43 F4.2 Rgeo E_Rp Error on Rp (upper value)
45- 48 F4.2 Rgeo e_Rp Error on Rp (lower value)
50- 54 I5 K Teq Equilibrium temperature (1)
56- 60 F5.1 --- Lambda Restricted Jeans escape parameter
(comes from equation (1))
62- 66 F5.2 g/cm+3 rhop Planetary mean density (2)
68- 72 F5.3 au a Orbital semimajor axis
74- 77 F4.2 Msun Ms Stellar mass
79- 82 F4.1 Gyr Age ?=- Age
84- 86 F3.1 Sun Omegarot ?=- Rotational angular velocity, in solar unit
88- 95 F8.1 mW/m2 FXUV Stellar XUV-flux received by the planet for
the given orbital semimajor axis (a),
stellar mass (Ms), age, and
rotational angular velocity (Omegarot)
97-103 E7.2 s-1 Lhy ?=- Hydrodynamic mass-loss rate (3)
105-111 E7.2 s-1 Len ?=- Energy-limited XUV-driven mass-loss
rate (3)
113-120 F8.1 --- Lhy/Len ?=- Ratio for hydrodynamic to energy-limited
XUV-driven mass-loss rates
122-126 A5 --- Ref References for Planetary Masses and Radii,
in refs.dat file
129-131 A3 --- Note [R,T ] R and T indicate RV- and TTV-estimated
mass, respectively
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Note (1): The equilibrium temperature assumes zero Bond albedo and efficient
day-night energy redistribution.
Note (2): The planetary mean density assumes the observed mass and transit
radius.
Note (3): Lhy and Len (Section 3) are the hydrodynamic and energy-limited
XUV-driven (Len) mass-loss rates of the selected hydrodynamic-modelled
planets (mostly low-density planets with Lambda<20).
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Byte-by-byte Description of file: refs.dat
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Bytes Format Units Label Explanations
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1- 5 A5 --- Ref Reference code
7- 25 A19 --- BibCode BibCode
27- 47 A21 --- Aut Author's name
49- 67 A19 --- Com Comments
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 15-Oct-2019