alx-ai/arxiv_papers · Datasets at Hugging Face

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shorter for hotter planets, but the temperature-dependance of the radiative timescale is stronger,
leading to decreased heat recirculation eﬃciency.
Subject headings: methods: data analysis — (stars:) planetary systems —
1.INTRODUCTION
Short-period exoplanets are expected to have atmo-
spheric compositions and dynamics that diﬀer signiﬁ-
cantly from Solar System giant planets3. These planets
orbit∼100×closer to their host stars than Jupiter does
from the Sun. As a result, they receive ∼104×more ﬂux
andexperiencetidalforces ∼106×strongerthanJupiter.
In contrast to Jupiter, which releases roughly as much
power in its interior as it receives from the Sun, short-
period exoplanets have power budgets dictated by the
ﬂux they receive from their host stars. Roughly speak-
ing, the stellar ﬂux incident on a planet does one of two
things: it is reﬂected back into space, or advected else-
where on the planet and re-radiated at diﬀerent wave-
lengths. The physical parameters that describe these
processes are the planet’s Bond albedo and redistribu-
tion eﬃciency.
1.1.Albedo
1CIERA Fellow, Northwestern University, 2131 Tech Dr,
Evanston, IL 60208
email: n-cowan@northwestern.edu
2Astronomy Department, University of Washington, Box
351580, Seattle, WA 98195
3For our purposes a “short period” exoplanet is one where the
periastron distance is less than 0 .1 AU, regardless of its actual
period, and regardless of mass, which may range from Neptune -
sized to Brown Dwarf. They are all Class IV and V extrasolar
giant planets in the scheme of Sudarsky et al. (2003).Giant planets in the Solar System have albedos greater
than 50%because ofthe presenceofcondensedmolecules
(H2O, CH 4, NH3, etc.) in their atmospheres. Planets
with eﬀective temperatures exceeding ∼400 K should be
cloud free, leading to albedos of 0.05–0.4 (Marley et al.
1999). If pressure-broadenedNa and K opacity is impor-
tant at optical wavelengths (as it is for brown dwarfs,
Burrows et al. 2000), then the Bond albedos of hot
Jupiters may be less than 10% (Sudarsky et al. 2000).
But the very hottest planets, the so-called class V extra-
solar giant planets ( Teﬀ>1500 K), might have very high
albedosdue to a high silicate cloud layer(Sudarsky et al.
2000). For a planet whose albedo is dominated by
clouds (as opposed to Rayleigh scattering) the albedo
depends on the composition and size of cloud particles
(Seager et al. 2000).
Earlyattempts to observe reﬂected light from exoplan-
ets (Charbonneau et al. 1999; Collier Cameron et al.
2002a; Leigh et al. 2003a,b; Rodler et al. 2008, 2010) in-
dicated that they might not be as reﬂective as Solar Sys-
tem gas giants (for a review, see Langford et al. 2010).
Measurements of HD 209458b taken with the Cana-
dian MOST satellite revealed a very low albedo ( <8%,
Rowe et al.2008), andit hassincebeentakenforgranted
that all short-period planets have albedos on par with
that of charcoal.
From the standpoint of the planet’s climate, the im-
portant factor is not the albedo at any one wavelength,2 Cowan & Agol
Aλ, but rather the integrated albedo, weighted by the in-
cident stellar spectrum, known as the Bond albedo and
denoted in this paper as AB. The relation between Aλ
and the planet’s Bond albedo is not trivial. If the albedo
is dominated by gray clouds, then the albedo at a sin-
gle wavelength can indeed be extrapolated to obtain AB.
For non-grayreﬂectance spectra, however, it is critical to
measureAλat the peak emitting wavelength of the host
startoobtainagoodestimateofthe planet’senergybud-
get. For example, as pointed out in Marley et al. (1999),
planets with identical albedo spectra, Aλ, mayhaveradi-
cally diﬀerent ABdepending on the spectraltype oftheir
host stars.
1.2.Redistribution Eﬃciency
The ﬁrst few measurements of hot Jupiter phase vari-
ations showed signs that these planets are not all cut
from the same cloth. Harrington et al. (2006) and
Knutson et al. (2007a) quoted very diﬀerent phase func-
tion amplitudes for the υAndromeda and HD 189733
systems. It was not clear whether the diﬀerences were
intrinsic to the planets, however, because the data
were taken with diﬀerent instruments, at diﬀerent wave-
lengths, and with very diﬀerent observation schemes (in
any case, subsequent re-analysis of the original data and
newly aquired Spitzerobservations of υAndromeda b
paint a completely diﬀerent picture of that system:
Crossﬁeld et al. 2010).
The uniform study presented in Cowan et al. (2007),
on the other hand, showed that HD 179949b and
HD209458bexhibit signiﬁcantlydiﬀerentdegreesofheat
recirculation, conﬁrming suspicions. But it was not clear
whether hot exoplanets were uni-modal or bi-modal in
redistribution: are HD 179949b and HD 209458b end-
members of a single distribution, or prototypes for two
fundamentally diﬀerent sorts of exoplanets?
The presence or lack of a stratospheric tempera-
ture inversion (Hubeny et al. 2003; Fortney et al. 2006;
Burrows et al. 2007, 2008; Zahnle et al. 2009) on the
day-sides of exoplanets has been invoked to explain
a purported bi-modality in recirculation eﬃciency on
hot Jupiters (Fortney et al. 2008). The argument, sim-
ply put, is that optical absorbers high in the atmo-

shorter for hotter planets, but the temperature-dependance of the radiative timescale is stronger,

leading to decreased heat recirculation eﬃciency.

Subject headings: methods: data analysis — (stars:) planetary systems —

1.INTRODUCTION

Short-period exoplanets are expected to have atmo-

spheric compositions and dynamics that diﬀer signiﬁ-

cantly from Solar System giant planets3. These planets

orbit∼100×closer to their host stars than Jupiter does

from the Sun. As a result, they receive ∼104×more ﬂux

andexperiencetidalforces ∼106×strongerthanJupiter.

In contrast to Jupiter, which releases roughly as much

power in its interior as it receives from the Sun, short-

period exoplanets have power budgets dictated by the

ﬂux they receive from their host stars. Roughly speak-

ing, the stellar ﬂux incident on a planet does one of two

things: it is reﬂected back into space, or advected else-

where on the planet and re-radiated at diﬀerent wave-

lengths. The physical parameters that describe these

processes are the planet’s Bond albedo and redistribu-

tion eﬃciency.

1.1.Albedo

1CIERA Fellow, Northwestern University, 2131 Tech Dr,

Evanston, IL 60208

email: n-cowan@northwestern.edu

2Astronomy Department, University of Washington, Box

351580, Seattle, WA 98195

3For our purposes a “short period” exoplanet is one where the

periastron distance is less than 0 .1 AU, regardless of its actual

period, and regardless of mass, which may range from Neptune -

sized to Brown Dwarf. They are all Class IV and V extrasolar

giant planets in the scheme of Sudarsky et al. (2003).Giant planets in the Solar System have albedos greater

than 50%because ofthe presenceofcondensedmolecules

(H2O, CH 4, NH3, etc.) in their atmospheres. Planets

with eﬀective temperatures exceeding ∼400 K should be

cloud free, leading to albedos of 0.05–0.4 (Marley et al.

1999). If pressure-broadenedNa and K opacity is impor-

tant at optical wavelengths (as it is for brown dwarfs,

Burrows et al. 2000), then the Bond albedos of hot

Jupiters may be less than 10% (Sudarsky et al. 2000).

But the very hottest planets, the so-called class V extra-

solar giant planets ( Teﬀ>1500 K), might have very high

albedosdue to a high silicate cloud layer(Sudarsky et al.

2000). For a planet whose albedo is dominated by

clouds (as opposed to Rayleigh scattering) the albedo

depends on the composition and size of cloud particles

(Seager et al. 2000).

Earlyattempts to observe reﬂected light from exoplan-

ets (Charbonneau et al. 1999; Collier Cameron et al.

2002a; Leigh et al. 2003a,b; Rodler et al. 2008, 2010) in-

dicated that they might not be as reﬂective as Solar Sys-

tem gas giants (for a review, see Langford et al. 2010).

Measurements of HD 209458b taken with the Cana-

dian MOST satellite revealed a very low albedo ( <8%,

Rowe et al.2008), andit hassincebeentakenforgranted

that all short-period planets have albedos on par with

that of charcoal.

From the standpoint of the planet’s climate, the im-

portant factor is not the albedo at any one wavelength,2 Cowan & Agol

Aλ, but rather the integrated albedo, weighted by the in-

cident stellar spectrum, known as the Bond albedo and

denoted in this paper as AB. The relation between Aλ

and the planet’s Bond albedo is not trivial. If the albedo

is dominated by gray clouds, then the albedo at a sin-

gle wavelength can indeed be extrapolated to obtain AB.

For non-grayreﬂectance spectra, however, it is critical to

measureAλat the peak emitting wavelength of the host

startoobtainagoodestimateofthe planet’senergybud-

get. For example, as pointed out in Marley et al. (1999),

planets with identical albedo spectra, Aλ, mayhaveradi-

cally diﬀerent ABdepending on the spectraltype oftheir

host stars.

1.2.Redistribution Eﬃciency

The ﬁrst few measurements of hot Jupiter phase vari-

ations showed signs that these planets are not all cut

from the same cloth. Harrington et al. (2006) and

Knutson et al. (2007a) quoted very diﬀerent phase func-

tion amplitudes for the υAndromeda and HD 189733

systems. It was not clear whether the diﬀerences were

intrinsic to the planets, however, because the data

were taken with diﬀerent instruments, at diﬀerent wave-

lengths, and with very diﬀerent observation schemes (in

any case, subsequent re-analysis of the original data and

newly aquired Spitzerobservations of υAndromeda b

paint a completely diﬀerent picture of that system:

Crossﬁeld et al. 2010).

The uniform study presented in Cowan et al. (2007),

on the other hand, showed that HD 179949b and

HD209458bexhibit signiﬁcantlydiﬀerentdegreesofheat

recirculation, conﬁrming suspicions. But it was not clear

whether hot exoplanets were uni-modal or bi-modal in

redistribution: are HD 179949b and HD 209458b end-

members of a single distribution, or prototypes for two

fundamentally diﬀerent sorts of exoplanets?

The presence or lack of a stratospheric tempera-

ture inversion (Hubeny et al. 2003; Fortney et al. 2006;

Burrows et al. 2007, 2008; Zahnle et al. 2009) on the

day-sides of exoplanets has been invoked to explain

a purported bi-modality in recirculation eﬃciency on

hot Jupiters (Fortney et al. 2008). The argument, sim-

ply put, is that optical absorbers high in the atmo-