The Astrophysical Journal, 881:114 (24pp), 2019 August 20

https://doi.org/10.3847/1538-4357/ab2a71© 2019. The American Astronomical Society. All rights reserved.

Impact of Stellar Superﬂares on Planetary Habitability
Yosuke A. Yamashiki1,2 , Hiroyuki Maehara3,4 , Vladimir Airapetian5,6 , Yuta Notsu7,8,9 , Tatsuhiko Sato10,
Shota Notsu9,11 , Ryusuke Kuroki1, Keiya Murashima12, Hiroaki Sato13, Kosuke Namekata9, Takanori Sasaki2,9 ,Thomas B. Scott14, Hina Bando12, Subaru Nashimoto12, Fuka Takagi15, Cassandra Ling1, Daisaku Nogami2,9, andKazunari Shibata2,16
1

Graduate School of Advanced Integrated Studies in Human Survivability, Kyoto University, Sakyo, Kyoto, Japan; yamashiki.yosuke.3u@kyoto-u.ac.jp
2
Unit of the Synergetic Studies for Space, Kyoto University, Sakyo, Kyoto, Japan
3
Okayama Branch Ofﬁce, Subaru Telescope, National Astronomical Observatory of Japan, NINS, Kamogata, Asakuchi, Okayama, Japan
4
Okayama Observatory, Kyoto University, Kamogata, Asakuchi, Okayama, Japan
5
NASA/GSFC/SEEC, Greenbelt, MD, USA
6
American University, Washington, DC, USA
7
Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA
8
National Solar Observatory, Boulder, CO, USA
9
Department of Astronomy, Kyoto University, Sakyo, Kyoto, Japan
10
Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki, Japan
11
Leiden Observatory, Leiden University, Leiden, The Netherlands
12
Faculty of Science, Kyoto University, Sakyo, Kyoto, Japan
13
Faculty of Engineering, Kyoto University, Sakyo, Kyoto, Japan
14
Interface Analysis Centre, University of Bristol, Bristol, UK
15
Faculty of Agriculture, Kyoto University, Sakyo, Kyoto, Japan
16
Astronomical Observatory, Kyoto University, Sakyo, Kyoto, Japan
Received 2019 April 17; revised 2019 June 12; accepted 2019 June 16; published 2019 August 20

Abstract
High-energy radiation caused by exoplanetary space weather events from planet-hosting stars can play a crucial rolein conditions promoting or destroying habitability in addition to the conventional factors. In this paper, we present theﬁrst quantitative impact evaluation system of stellar ﬂares on the habitability factors with an emphasis on the impact
of stellar proton events. We derive the maximum ﬂare energy from stellar star spot sizes and examine the impacts ofﬂare-associated ionizing radiation on CO2, H2, and N2+ O2-rich atmospheres of a number of well-characterized
terrestrial type exoplanets. Our simulations based on the Particle and Heavy Ion Transport code System suggest thatthe estimated ground-level dose for each planet in the case of terrestrial-level atmospheric pressure (1 bar) for each
exoplanet does not exceed the critical dose for complex (multicellular) life to persist, even for the planetary surface ofProxima Centauri b, Ross-128 b, and TRAPPIST-1 e. However, when we take into account the effects of the possible
maximum ﬂares from those host stars, the estimated dose reaches fatal levels at the terrestrial lowest atmosphericdepth on TRAPPIST-1 e and Ross-128 b. Large ﬂuxes of coronal X-ray and ultraviolet radiation from active stars
induce high atmospheric escape rates from close-in exoplanets, suggesting that the atmospheric depth can besubstantially smaller than that on Earth. In a scenario with the atmospheric thickness of one-tenth of Earth’s, theradiation dose from close-in planets including Proxima Centauri b and TRAPPIST-1 e reaches near fatal levels withannual frequency of ﬂare occurrence from their host stars.
Key words: planet–star interactions – solar–terrestrial relations – stars: ﬂare – Sun: coronal mass ejections (CMEs) –sunspots
Energetic stellar ﬂare events associated with CMEs frommagnetically active stars can contribute to the generation ofstellar transient XUV emission and form high-energy particles
accelerated in CME-driven shocks (Gopalswamy et al. 2017;
Kumari et al. 2017; Airapetian et al. 2019). These SEPs canpenetrate into exoplanetary atmospheres and cause chemicalchanges. These changes can be positive for the initiation ofprebiotic chemistry in the planetary atmospheres or detrimental
due to the destruction of a large fraction of ozone that transmits
UVC (1000–2800 Å) and UVB (2800–3150 Å) emission to the
exoplanetary surfaces (Segura et al. 2010; Airapetian et al.
2016, 2017b; Tilley et al. 2019).
Our own Sun is known to exhibit extreme ﬂare activity in thepast, including the so-called Carrington-class event (Townsend
et al. 2006). Recent observations by the Kepler space telescope
revealed that young solar-type stars generate a much higher
frequency of energetic ﬂares (superﬂares), which could have been
an important factor for habitability in the early history of our solar

1. Introduction
The deﬁnition of habitable zones for extrasolar planetarysystems is traditionally based on the conditions promoting thepresence of standing bodies of liquid surface water (determined
to be a conventional habitable zone [CHZ]), but other more
reﬁned boundaries may be considered (Kopparapu et al. 2013;
Ramirez et al. 2019). For example, the inner habitable boundary
may be deﬁned by critical ﬂuxes, which cause runaway/
moisture greenhouse effects (Kasting 1988), while the outerboundary may be constrained by the presence of carbon dioxidein the atmosphere in the gas phase, avoiding its condensation(Kasting et al. 1993). The exoplanets within CHZs around active
stars can be subject to high ionizing radiation ﬂuxes including
X-ray and extreme ultraviolet emission (referred as to XUV[1–1200 Å] emission), coronal mass ejections (CMEs), andassociated stellar energetic particles (SEP) events that can affectexoplanetary habitability conditions (Airapetian et al. 2017a;
Airapetian et al. 2019).
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Yamashiki et al.

Table 1
Basic Parameters of Target Planets and Their Host Stars, Including Their Projected Flare Energy
PlanetHost Star

Exoplanet Name

Radius(REarth)Size Class

Mass(MEarth)Spectra
Type

Teff
(K)

Radius(Re)

Prot(days)

Aspot,p (2pR)GJ 699 b
Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross-128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

1.37
1.82
3.19
1.07
1.10
1.09
1.06
0.77
0.92
1.05
1.13
0.76
1.00
0.53

super-Earth-size
super-Earth-size
Neptune-size
Earth-size
Earth-size
Earth-size
Earth-size
Earth-size
Earth-size
Earth-size
Earth-size
Earth-size
Earth-size
Mars-size

3.23
4.59
7.77
1.27
1.40
0.86
1.38
0.41
0.64
0.67
1.34
0.36
1.00
0.11

M4V
K5
G7
M5.5V
M4
M8
M8
M8
M8
M8
M8
M8
G2V
G2V

3278
4141
5637
3050
3192
2550
2550
2550
2550
2550
2550
2550
5778
5778

0.18
0.64
0.82
0.14
0.20
0.12
0.12
0.12
0.12
0.12
0.12
0.12
1.00
1.00

140.0
18.2
19.8
82.6
121.0
3.3
3.3
3.3
3.3
3.3
3.3
3.3
25.0
25.0

0.00030.00210.00660.00400.00020.00120.00120.00120.00120.00120.00120.00120.00300.0030

Flare Energy (erg)AnnualSpot Maximum

6.26E+31
4.93E+32
1.65E+33
9.7E+32
4.72E+31
2.70E+32
2.70E+32
2.70E+32
2.70E+32
2.70E+32
2.70E+32
2.70E+32
7.20E+32
7.2E+32

1.15E+32
2.13E+33
1.18E+34
5.55E+33
7.72E+31
9.09E+32
9.09E+32
9.09E+32
9.09E+32
9.09E+32
9.09E+32
9.09E+32
3.64E+33
3.64E+33

Here we present the ﬁrst comprehensive impact evaluation
system of expected ground-level radiation doses in close-interrestrial-type exoplanets around M dwarfs including Proxima
Centauri b (see Table 1) in response to severe solar proton
events (SPEs). This study represents a realistic model of thesurface dose evaluation for exoplanets with various possible
atmospheric pressures and compositions. Section 1 presents
the framework for evaluation of SPE particle ﬂuence at the topof exoplanetary atmospheres. In Section 2 we discuss theapplication of the Particle and Heavy Ion Transport code
System (PHITS) to a number of close-in exoplanets around M
dwarfs. Section 3 discusses the ground dose for various
exoplanetary systems and their consequences for the biologicalhabitability of complex life-forms. Section 4 describes theconclusions of the paper and future work.
Figure 1. Flare frequency vs. ﬂare energy for solar ﬂares. The fraction of ﬂarestars as a function of the rotation period. The solid line and dotted line represent
the estimated scaling low calculated using Equation (1) as different star spot
areas derived from Maehara et al. (2017).

2. Method
2.1. Outline of Fluence Estimation for Top of Atmosphere(TOA) on Each Planet from Stellar Proton Events andDeﬁnition of Maximum Flare Energy

system and/or most extrasolar systems (Maehara et al. 2012;
Shibayama et al. 2013; Takahashi et al. 2016; Notsu et al. 2019;
Airapetian et al. 2019). Extreme surges of 14C were detected in treerings (Miyake et al. 2012, 2013), which is considered strong
evidence of the occurrence of superﬂares more than one magnitude
stronger than the Carrington-class event (Usoskin et al. 2013).
Effects of stellar activity from host stars may also include periodic
sterilizing doses of radiation via stellar superﬂare activity (Lingam& Loeb 2017). While the frequency and maximum energy of solar
and stellar ﬂares from planet hosts have not been wellcharacterized, they may present a critical limiting factor on thedevelopment and persistence of life on terrestrial-type planets inour solar system (Jakosky et al. 2015; Schrijver et al. 2015; Kayet al. 2016) as well as on Earth-sized exoplanets (Atri 2017). Thus,
a consistent approach to determine the habitable zone accounting
for these factors is required. The characterization of these factors
can be made using a recently derived correlation between stellar
ﬂare frequency/intensity and star spot area, found from Kepler
data, which may overcome the difﬁculty in predicting a ﬂareimpacted system (Maehara et al. 2017).

Our analysis is based on the application of stellar ﬂare andstar spot data derived mostly from the Kepler mission (Notsu
et al. 2013, 2015a, 2015b, 2019; Maehara et al. 2015, 2017), in
the ExoKyoto exoplanetary database (Y. A. Yamashiki et al.
2019, in preparation). Our method utilizes star spot data
derived from optical light curves to be used in parametricstudies of the thickness of hypothetical exoplanetary atmospheres as the major attenuation factor of the incident radiation(see Table 1). These data are used as input for the PHITS (Satoet al. 2018) Monte Carlo simulation model that is used for
simulations of surface dose for terrestrial-type exoplanets.
The following equations derive an assumed stellar ﬂare
magnitude from observed stellar spot size data. For the estimationof spot size, we used the same method as Maehara et al. (2017).
Figures 1 and 2 illustrate ﬂare frequency versus ﬂare energy forsolar ﬂares. The solid line and dotted line represent the estimatedscaling low calculated using Equation (1) as a different star spot
area derived from Maehara et al. (2017).
Using the results of the above study, we derived the ﬂarefrequency distribution over its energy in the optical band as a2

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Yamashiki et al.

Figure 2. Flare frequency vs. ﬂare energy for stellar ﬂare. The fraction of ﬂarestars as a function of the rotation period. The solid line and dotted line represent
the estimated scaling low calculated using Equation (1) as different star spot
areas derived from Maehara et al. (2017).

Figure 3. Event-integrated spectra, of the GLE43 that occurred on the 1989
October 19 (solid line) and the Carrington ﬂare that occurred on the 1859
September 1 (dotted line) SPEs on Earth, based on parameters obtained fromreferences.

function of the stellar spot size as follows:

ﬂare as a proxy for SPE energy and (2) by applying the starspot area and/or rotational period correlation methodology.
The conversion equation is presented and discussed in the next
section. We use the above information to extract representative
star spot areas, which can be applied to the conversion
equations for ﬂare energy expressed in the following section.
Accordingly we obtain (a) annual maximum ﬂare (seeEquation (2)), spot maximum ﬂare (see Equation (3)) (Shibata
et al. 2013; Aschwanden et al. 2017), and (c) possible
maximum ﬂare, calculated assuming that the target star surface
is covered with star spots under the maximum percentage of
observed star spot area (set as 20% of half the spherical area).
Step 2. After the above procedure is completed for each starsystem, the possible quantitative exposures are assumed by thefollowing procedure: (4) estimating the ﬂuence of each stellar
proton event at the TOA using Equation (7).
As for the atmospheric compositions of exoplanets, three types
of atmospheres for typical extrasolar planets are considered(explained in detail in the following section). For those typicalatmospheric compositions, the potential doses for life on
extrasolar planets are determined through the following procedure.
Step 3. (5) Calculate the possible dose rate from the Monte
Carlo simulations using PHITS (Sato et al. 2018) for three
typical atmospheric compositions as extrasolar planetaryatmospheres, (6) normalize the dose by determining the Earthequivalent ratio, which was previously normalized by using(6a) the Carrington-class event, assuming that the event has
X45 class, or by (6b) the deepest observed ﬂare event GLE43,
which occurred in 1989, as X13 class (Xapsos et al. 2000).
(7) Calculate conversion coefﬁcients for each exoplanet bycomparing the values calculated in (4) and (6). (8) Convert thereference dose value calculated in (6) into each extrasolar
planet case using conversion coefﬁcients.

⎤1.05

⎡
Aspot
dN
⎥
= C0 [yr-1 erg-1] ⎢ -2.75
dE
A phot ⎦⎣ 10⎡ E flare ⎤-1.99, (1 )⎥⎢ 31⎣ 10 erg ⎦in which N is the ﬂare frequency (yr−1), Aspot is the total area ofstar spots, Aphot is the total visible area of the stellar surface,Eﬂare is the total expected stellar ﬂare energy (erg), and C0 is theﬂare frequency constant (1029.4).
Here we deﬁne E0=1031 erg and set N as 1; we then may
determine annual maximum ﬂare energy as follows:a- 1 ⎡ A ⎤1 + a
EAMF = C0 1 + a ⎢ 0 ⎥ E 01 + a ,
⎣A⎦
b

(2 )

in which EAMF is the annual maximum ﬂare energy, that is, thetotal expected stellar ﬂare energy per year (erg yr−1), a=−1.99, b=1.05.
The spot maximum ﬂare, that is, the maximum ﬂare energy
under a determined star spot area, can be illustrated as
ESMF

3 2
⎛ f ⎞ ⎛ B0 ⎞2 ⎡ Aspot (2pR2 ) ⎤
⎟⎜
⎟ ⎢
⎥ ,
⎝ 0.1 ⎠ ⎝ 10 3 G ⎠ ⎣0.001⎦= 7 ´ 10 32 (erg) ⎜(3 )in which f is the fraction of magnetic energy that can bereleased as ﬂare energy, B is the magnetic ﬁeld strength,
ESMF is the spot maximum ﬂare energy, that is, the theoreticalmaximum ﬂare energy with a determined star spot area (erg),
and Re is the solar radius (7×1010 cm).
Possible maximum ﬂare energy in this study was determined in
the following way: (1) Evaluate maximum star spot coverage ofthe star through observation of stellar light curves. In this study,
we observed a 20% coverage of star spot on Proxima
Centauri (Davenport 2016; Wargelin et al. 2017); accordingly
we determined the maximum star spot coverage as 20%. Then, (2)calculate maximum energy induced by the star spot area by
Shibata et al. (2013).
The outline of the estimation method is as follows:
Step 1. Derive the magnitude and frequency of stellar proton
events from each star (1) by using direct observation of a stellar

2.2. Monte Carlo Simulation for Air Shower Using PHITS
When high-energy SEPs precipitate into the planetaryatmosphere, they induce extensive air shower (EAS) by
producing various secondary particles, such as neutrons andmuons. We conducted a three-dimensional EAS simulation byusing the PHITS (Sato et al. 2018), which is a general purpose
Monte Carlo code for analyzing the propagation of radiation in
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Yamashiki et al.

Figure 4. Vertical proﬁle of radiation dose on Earth for normalized ﬂares, caused by hard proton spectrum (imitating GLE43) (a and b) and soft spectrum (imitating
Carrington by Townsend) (c and d) penetrating N2+ O2–rich (terrestrial type) atmosphere for Earth with 1030 erg (black triangle), 1032 erg (red circle), 1034 erg (bluesquare), and 1036 erg (red cross) in grays (a and c) and Sieverts (b and d). The vertical legend shows the following four typical atmospheric depth reference layers:
Martian surface atmospheric pressure equivalent to 9 g cm−2, terrestrial minimum atmospheric pressure, observed at the summit of the Himalayas, equivalent to
365 g cm−2 in this study, and (Earth’s) ground-level atmospheric pressure, equivalent to 1037 g cm−2. Possible exoplanetary surface was estimated as one-tenth ofterrestrial surface, equivalent to 103.7 g cm−2. Note that the value is not identical to the real observation data but is the nearest value employed in the Monte Carlonumerical simulation using PHITS.

any materials. PHITS version 2.88 with the recommended
setting for cosmic-ray transport simulation (Sato et al. 2014)
was used in this survey. In our simulations we assume the size
and the mass of the modeled planets to be the same as that of
the Earth.

has experienced severe atmospheric escape driven by strong
stellar ionizing radiation ﬂux. We also model a young Earthsized H2-rich atmosphere, because such an atmosphere isassumed for large super-Earth planets, whose gravitational pullmight be sufﬁciently large to retain substantial atmospheric H2.
Hydrogen-rich atmospheres of Earth-sized exoplanets can beformed due to capture of hydrogen from protoplanetaryatmospheres and/or during the accretion period (Elkins-Tanton& Seager 2008; Lammer et al. 2018). Thus, here we refer toyoung Earth-sized exoplanets.
The composition of the atmosphere for the above three
typical atmospheric types was set to 78% nitrogen, 21%
oxygen, and 1% argon for the Earth-like (N2+ O2) atmosphere,
100% carbon dioxide for the Martian/Venusian-type (CO2),
and 100% hydrogen for the young-Earth-type (H2). During theMonte Carlo simulation using PHITS, we assume thecomposition of the planet interior to be covered with sufﬁcientliquid water for the terrestrial type, while the same gas was

2.3. Chemical Composition of Exoplanetary Atmospheres
The impact of stellar proton events on a planet depends upon
its atmospheric composition. We consider three: Earth-like(N2+ O2 rich), Mars-like or Venus-like (CO2 rich), and ayoung Earth-sized or super-Earth planet with a primary H2-richatmosphere. We assume that the Earth-type atmosphere is thestandard land-ocean planetary atmosphere composed mostly of
nitrogen and oxygen (N2+ O2). A Venusian-like atmosphere isrepresented as a CO2-rich atmosphere resulting from the
runaway greenhouse effect and subsequent outgassing of CO2
from carbonates. The Mars-like atmosphere is an example of alow gravity–low pressure planetary CO2-rich atmosphere that
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Yamashiki et al.

Figure 5. Vertical proﬁle of radiation dose (in grays) on Earth and Mars for possible ﬂares on several different scales, caused by hard proton spectrum (imitating
GLE43) (a and c) and soft spectrum (imitating Carrington) (b and d) penetrating N2+ O2–rich (terrestrial type) atmosphere for Earth (a and b) and CO2-rich (Martiantype) atmosphere for Mars (c and d) with ﬂares every one-tenth of a year (36 days, black triangle), one year (red circle), spot maximum (green triangle), and possible
maximum (red cross). Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at the summit of theHimalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible exoplanetary surface is one-tenth of theterrestrial surface, equivalent to 103.7 g cm−2.

continuously ﬁlled in the planet interior for the other cases. In
our simulation of all model atmospheres (young Earth-type[H2], Earth-like [N2+ O2], Martian and Venusian-type [CO2])
cases we assume the exoplanet radius and mass to be 1 REarthand 1 MEarth, respectively. Numerical simulation of superEarths will be performed in the upcoming studies.

comparison with other solar ﬂares that produce GLEs. Thus,
the radiation dose at the ground level during the event is
expected to be not signiﬁcantly high, because only a small
fraction of the high-energy protons (with energies over 3 GeV
for an 1 bar atmosphere) and their secondary particles canpenetrate into the deep atmosphere.
To estimate the maximum impact on the ground level, wetherefore calculated the radiation dose during the solar ﬂare inassociation with a harder proton spectrum, GLE43, which isone of the most signiﬁcant GLEs that has occurred aftersatellite observations were started in the late 20th century. Itshould be noted that GLE43 was selected as a typical SPE
associated with a hard proton spectrum to estimate themaximum impact of SPE exposure at deeper locations in theatmosphere, although its ﬂare class was not extremely high(X13). GLE5 (1956 February 23) -type spectra were also
considered as a relevant event for the survey.
Figure 3 illustrates event-integrated spectra of the GLE43
and the Carrington ﬂare.

2.4. Event-integrated Spectra of Extreme SPEs
We assume that stellar accelerated protons are isotropically
distributed in space as they precipitate into the atmospheres ofthe modeled planets and have two different energy spectra
represented by the SPE spectra derived for the Carrington-classevent in 1859 (Townsend et al. 2006) and the 43rd groundlevel enhancement (GLE) in 1989 (Xapsos et al. 2000),
respectively.
The Carrington-class event is considered to be the largest
eruptive event recorded in modern human history. However,according to Smart et al. (2006), proton energy spectra
associated with the Carrington-class event were rather soft in
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Yamashiki et al.

Figure 6. Vertical proﬁle of radiation dose (in Sieverts) on Earth and Mars for possible ﬂares on several different scales, caused by hard proton spectrum (imitating
GLE43) (a and c) and soft spectrum (imitating Carrington) (b and d) penetrating N2+ O2–rich (terrestrial type) atmosphere for Earth (a and b) and CO2-rich (Martiantype) atmosphere for Mars (c and d) with ﬂares every one-tenth of a year (36 days, black triangle), one year (red circle), spot maximum (green triangle), and possible
maximum (red cross). Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at the summit of theHimalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible exoplanetary surface is one-tenth of theterrestrial surface, equivalent to 103.7 g cm−2.

2.5. The Inﬂuence of the Planetary Magnetic Fieldof the simulation procedures as well as their veriﬁcation results
for the solar energetic particle and galactic cosmic-ray
simulation in the terrestrial atmosphere were given in ourprevious papers (Sato 2015; Sato et al. 2018b).
The impacts of all components produced by cosmic-ray
interactions with the different atmospheric types in different
layers were also individually evaluated and ﬁnally integrated to
produce a ﬁnal ground-level dose value for each simulated
scenario. By examining all these different parameters together(atmospheric composition, geomagnetic ﬁeld strength, andsimulated cosmic-ray interactions), we have evaluated theatmospheric barrier needed for life on each of the target planets
to survive a stellar ﬂare event. This approach assumes that thepotential life is as similarly radiation tolerant as that present on
Earth.

We have simulated four scenarios of exoplanetary dipole
magnetic moments: (1) B=0 (unmagnetized planet), (2) 0.1×BEarth, (3) 1×BEarth (Earth-like magnetic moment), and (4)10×BEarth. The impact of the planetary magnetic ﬁeld on thesurface dose was modeled via the magnetospheric ﬁlter
functions for the above four different magnetic moments, 0,0.1, 1, and 10 BEarth, evaluated by Grießmeier et al. (2015).
The ﬂuence of protons, neutrons, positive and negative
muons, electrons, positrons, and photons was scored as afunction of the atmospheric depth. They were then converted tothe absorbed dose in grays and the effective dose in Sieverts(Sv) using the stopping power and the ﬂuence to the doseconversion coefﬁcients for the isotropic irradiation (International Commission on Radiological Protection, 2010), respectively. It should be mentioned that the effective dose is deﬁned
only for the purpose of radiological protection. However, weevaluated it for discussing the possible exposure effects onhuman-like life-forms because there is no alternative quantitythat can be used for this discussion. More detailed descriptions

2.6. The Maximum Stellar Flare Energy
Our goal is to study the effect of high ionizing particle ﬂuxes
caused by stellar activity on habitability of close-in Earth-sized
and super-Earth exoplanets located within habitable zones.
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Yamashiki et al.

Table 2Estimated Dose under Projected Flare Event—Annual Maximum Flare
ExoplanetName
GJ 699 b
Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross 128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

TOAa
(Gy)

TOAb
(Sv)

MSc
(Gy)

MSd
(Sv)

TMe
(Gy)

TMf
(Sv)

GLg
(Gy)

GLh
(Sv)

8.72E+01
9.64E+02
3.84E+02
9.37E+04
4.36E+03
4.97E+05
2.65E+05
1.33E+05
7.73E+04
4.46E+04
3.02E+04
1.55E+04
1.64E+02
7.05E+01

3.95E+00
4.36E+01
1.74E+01
4.24E+03
1.97E+02
2.25E+04
1.20E+04
6.04E+03
3.50E+03
2.02E+03
1.37E+03
7.00E+02
7.40E+00
3.19E+00

5.20E−01
5.75E+00
2.29E+00
5.60E+02
2.60E+01
2.97E+03
1.58E+03
7.97E+02
4.61E+02
2.66E+02
1.80E+02
9.22E+01
9.76E−01
4.21E−01

3.46E−01
3.83E+00
1.53E+00
3.72E+02
1.73E+01
1.97E+03
1.05E+03
5.30E+02
3.07E+02
1.77E+02
1.20E+02
6.14E+01
6.50E−01
2.80E−01

5.82E−05
6.44E−04
2.56E−04
6.26E−02
2.91E−03
3.32E−01
1.77E−01
8.91E−02
5.16E−02
2.98E−02
2.01E−02
1.03E−02
1.09E−04
4.71E−05

5.91E−04
6.54E−03
2.61E−03
6.36E−01
2.96E−02
3.37E+00
1.80E+00
9.06E−01
5.25E−01
3.02E−01
2.05E−01
1.05E−01
1.11E−03
4.78E−04

2.31E−08
2.56E−07
1.02E−07
2.49E−05
1.16E−06
1.32E−04
7.04E−05
3.54E−05
2.05E−05
1.18E−05
8.00E−06
4.10E−06
4.34E−08
1.87E−08

2.59E−07
2.86E−06
1.14E−06
2.79E−04
1.30E−05
1.48E−03
7.88E−04
3.97E−04
2.30E−04
1.32E−04
8.96E−05
4.59E−05
4.86E−07
2.09E−07

Notes. TOA ≈ 0 g cm−2. MS = Martian surface atmospheric pressure (9 g cm−2); TM = terrestrial minimum atmospheric pressure (365 g cm−2); GL = (Earth’s)
ground-level atmospheric pressure (1037 g cm−2).
aEstimated dose (Gy) by annual maximum ﬂare at TOA.
b
Estimated dose (Sv) by annual maximum ﬂare at TOA.
c
Estimated dose (Gy) by annual maximum ﬂare at MS.
d
Estimated dose (Sv) by annual maximum ﬂare at MS.
e
Estimated dose (Gy) by annual maximum ﬂare at TM.
f
Estimated dose (Sv) by annual maximum ﬂare at TM.
g
Estimated dose (Gy) by annual maximum ﬂare at GL.
h
Estimated dose (Sv) by annual maximum ﬂare at GL.
Table 3
Estimated Dose under Projected Flare Event—Spot Maximum Flare
ExoplanetName
GJ 699 b
Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross 128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

TOAa
(Gy)

TOAb
(Sv)

MSc
(Gy)

MSd
(Sv)

TMe
(Gy)

TMf
(Sv)

GLg
(Gy)

GLh
(Sv)

1.60E+02
4.16E+03
2.74E+03
5.36E+05
7.13E+03
1.67E+06
8.93E+05
4.49E+05
2.60E+05
1.50E+05
1.02E+05
5.20E+04
8.27E+02
3.56E+02

7.25E+00
1.89E+02
1.24E+02
2.43E+04
3.23E+02
7.58E+04
4.04E+04
2.03E+04
1.18E+04
6.79E+03
4.60E+03
2.36E+03
3.74E+01
1.61E+01

9.56E−01
2.49E+01
1.64E+01
3.20E+03
4.26E+01
9.99E+03
5.33E+03
2.68E+03
1.55E+03
8.96E+02
6.06E+02
3.11E+02
4.93E+00
2.13E+00

6.37E−01
1.65E+01
1.09E+01
2.13E+03
2.83E+01
6.65E+03
3.55E+03
1.79E+03
1.03E+03
5.96E+02
4.04E+02
2.07E+02
3.29E+00
1.42E+00

1.07E−04
2.78E−03
1.83E−03
3.58E−01
4.77E−03
1.12E+00
5.96E−01
3.00E−01
1.74E−01
1.00E−01
6.78E−02
3.48E−02
5.52E−04
2.38E−04

1.09E−03
2.83E−02
1.86E−02
3.64E+00
4.84E−02
1.14E+01
6.06E+00
3.05E+00
1.77E+00
1.02E+00
6.89E−01
3.53E−01
5.61E−03
2.42E−03

4.25E−08
1.11E−06
7.27E−07
1.42E−04
1.89E−06
4.44E−04
2.37E−04
1.19E−04
6.91E−05
3.98E−05
2.69E−05
1.38E−05
2.19E−07
9.45E−08

4.76E−07
1.24E−05
8.14E−06
1.59E−03
2.12E−05
4.97E−03
2.65E−03
1.34E−03
7.74E−04
4.46E−04
3.02E−04
1.55E−04
2.46E−06
1.06E−06

Notes. TOA ≈ 0 g cm−2. MS = Martian surface atmospheric pressure (9 g cm−2); TM = terrestrial minimum atmospheric pressure (365 g cm−2); GL = (Earth’s)
ground-level atmospheric pressure (1037 g cm−2).
aEstimated dose (Gy) by spot maximum ﬂare at TOA.
b
Estimated dose (Sv) by spot maximum ﬂare at TOA.
c
Estimated dose (Gy) by spot maximum ﬂare at MS.
d
Estimated dose (Sv) by spot maximum ﬂare at MS.
e
Estimated dose (Gy) by spot maximum ﬂare at TM.
f
Estimated dose (Sv) by spot maximum ﬂare at TM.
g
Estimated dose (Gy) by spot maximum ﬂare at GL.
h
Estimated dose (Sv) by spot maximum ﬂare at GL.

CHZs around low-luminosity M dwarfs are located within 0.05
au, which suggests that many of them orbit their host stars
within sub-Alfvenic distance and are subject to direct
irradiation via high particle ﬂuxes. To study the resulting
surface dose we selected four exoplanets around active

M dwarfs, one exoplanet around K dwarfs with detectedsuperﬂare, and one exoplanet around a G dwarf with higher
stellar activity than our Sun. We selected the target stars forthis survey according to the following procedure: (1) Selecthost star with exoplanet in habitable zone with direct superﬂare
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Figure 7. Vertical proﬁle of radiation dose (in grays) on Proxima Centauri b, Ross-128 b, TRAPPIST-Ie, and Kepler-283 c for possible ﬂares on several different
scales, caused by hard proton spectrum (imitating GLE43), penetrating N2 + O2–rich (terrestrial type) atmosphere for Earth with ﬂares every one-tenth of a year
(36 days, black triangle), one year (red circle), spot maximum (green triangle), and possible maximum (pink cross). Martian surface atmospheric pressure is equivalent
to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmosphericpressure is equivalent to 1037 g cm−2; possible exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

observation through Kepler observation (Kepler-283). (2)Select Kepler stars whose ﬂare frequency and magnitude canbe estimated from their activities (Kepler-1634). (3) Selectwell-documented host star for well-documented exoplanets(GJ699 (Barnards Star), Proxima Centauri, Ross-128, TRAPPIST-I). Stellar activities for all stars are estimated using theirlight curves.
Shibata et al. (2013) estimated the maximum value (upper
limit) of ﬂare energy, which is determined by the star spot areaand magnetic ﬁeld strength. We used this methodology to
calculate the theoretical maximum ﬂare energy for six host stars
using their star spot areas: 1.15×1032 erg for GJ 699 (Barnard’sstar), 2.13× 1033 erg for Kepler-283, 1.18×1034 erg for Kepler1634, 5.55× 1033 erg for Proxima Centauri, 7.72×1031 erg for
Ross 128, and 9.09×1032 erg for TRAPPIST-1.
With this method, the current observed star spot area in eachstar restricts the maximum ﬂare energy. However, it is unclear
whether the observed period represents the maximum or
minimum activity of the star. Accordingly, we also evaluatedthe potential maximum energy of the stellar ﬂare by thefollowing method.

For those stars whose stellar temperature is above 4000 K,
we estimated maximum ﬂare energy based on the relationshipbetween Kepler stars by comparing their maximum observed
energy and stellar temperature as well as their associated radii(H. Maehara 2019, private communication).
For those stars whose stellar temperature is below 4000 K,
we assumed, in the extreme situation, that 20% of the stellar
surface is covered by a star spot. Considering the extreme
condition, we calculated the maximum energy using Shibata
et al. (2013).
By introducing ﬂare energy as the input for considerable
maximum energy of the superﬂares for their planetary systems,
we may theoretically calculate the possible maximum dose for
their host planets.
3. Results
3.1. Validation for Normal Dose
Figure 4 shows the vertical proﬁle of radiation dose on Earthand Mars caused by SPEs with the hard proton spectrum(imitating GLE43) (a and b), and soft spectrum (imitating
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Figure 8. Vertical proﬁle of radiation dose (in Sieverts) on Proxima Centauri b, Ross-128 b TRAPPIST-Ie, and Kepler-283 c for possible ﬂares on several different
scales, caused by hard proton spectrum (imitating GLE43) (a and c) and soft spectrum (Townsend Carrington) (b and d) penetrating N2 + O2–rich (terrestrial type)atmosphere for Earth with ﬂares every one-tenth of a year (36 days, black triangle), one year (red circle), spot maximum (green triangle), and possible maximum (rosecross). Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at the summit of the Himalayas, isequivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible exoplanetary surface is one-tenth of the terrestrial
surface, equivalent to 103.7 g cm−2.

Carrington) (c and d), penetrating N2+ O2–rich (terrestrialtype) atmosphere Earth with 1030 erg (black triangle), 1032 erg(red circle), 1034 erg (blue square), and 1036 erg (red cross) ingrays (a and c) and Sieverts (b and d). This ﬁgure shows thatthe radiation dose at the tropopose (around 170 g cm−2
atmospheric depth) becomes 0.5 mSv, which mostly agrees
with the aerial observation, when the solar ﬂare energy isscaled to E0=1032. Note that this normalization has been
made for an idealized series of ﬂares, considering the horizontal
angle of the SPE injection as 90°; in other words, theprobability of reaching Earth is one in four.
Figures 5 and 6 show vertical proﬁle of radiation dose ingrays and Sieverts, respectively, on Earth and Mars for possible
ﬂares on several different scales, caused by hard proton
spectrum (imitating GLE43) (a and c) and soft spectrum(imitating Carrington reproduced by Townsend et al. 2006)(b and d) penetrating N2+ O2–rich (terrestrial-type) atmospherefor Earth (a and b) and CO2-rich (Martian type) atmosphere forMars (c and d), with ﬂares every one-tenth of a year (36 days,
corresponding to 7.2×1031 erg), one year (corresponding to

7.2×1032 erg), spot maximum ﬂare (corresponding to
3.6×1033 erg), and possible maximum ﬂare (corresponding to
1.6×1036 erg). In these scenarios, the spot maximum ﬂare is
the maximum possible ﬂare observed within decades in thetarget stellar system (in this case our solar system) estimatedbased on star spot area of the target star. According to thecalculation shown in these ﬁgures, the SPEs under the above
scenarios do not induce a critical dose at ground level when
we have sufﬁcient atmospheric depth such as Earth, even under
the possible maximum ﬂare (1.6×1036 erg) scenario, whereas
it becomes a nearly critical dose on the Martian surface withthinner atmospheric depth when the spot maximum ﬂare (3.6×1033 erg) event occurs.
3.2. Estimated Dose on Exoplanetary Surface under Different
Flare Scenarios
The estimated doses under the annual maximum ﬂare and under
the spot maximum ﬂare are shown in Tables 2 and 3, respectively.
The estimated doses at the TOA on GJ 699 b, Proxima Centauri b,
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Figure 9. Vertical proﬁle of radiation dose (in grays), caused by proton spectrum imitating GLE43 (a and b) and Carrington Flare (c and d) penetrating N2 + O2–rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy(a and c) and under spot maximum ﬂare energy (b and d), in grays. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

Ross-128 b, and TRAPPIST-1 e under the spot maximum ﬂare
(Shibata et al. 2013) become 1.60×102 Gy (7.25 Sv), 5.36×105 Gy (2.43×104 Sv), 7.13×103 Gy (3.23×102 Sv), and2.60×105 Gy (1.18×104 Sv), respectively.
Figures 7 and 8 illustrate the vertical radiation dose in grays
and Sieverts, respectively, for major documented planetarysystems, including Proxima Centauri b, Ross-128 b, TRAPPIST1 e, and Kepler-283 c (the only habitable planet in the Kepler
ﬁeld with observed ﬂares) for possible ﬂares on several different
scales, caused by hard proton spectrum (imitating GLE43)
penetrating N2+ O2–rich (terrestrial type) atmosphere for Earthwith ﬂares every one-tenth of a year (36 days), one year
(annual), spot maximum, and possible maximum ﬂare. In thesescenarios, the spot maximum ﬂare is the maximum possible ﬂareto be observed within decades in the target stellar system. On
these planets, the SPE does not reach critical levels with
sufﬁcient atmospheric depth (equal to that of the Earth) even forthe possible maximum ﬂare scenario, having 1.42× 10−4 Gy(1.6×10−3 Sv) for Proxima Centauri b and 3.70× 10−4 Gy(4.14×10−3 Sv) for Ross-128 b. Proxima Centauri b shows asmaller difference between the possible maximum ﬂare dose andspot maximum ﬂare dose, whereas a larger difference can befound on Ross-128 b, showing that Ross-128b is relatively calmin the range of the same temperature class.
However, when considering the possible maximum ﬂare,calculated assuming that the whole star is covered by the
maximum percentage of star spot (20%, observed from
Proxima Centauri’s light-curve survey), the estimated radiationdose at the terrestrial lowest atmospheric thickness measured at
the summit of Everest (at AD 365 g cm−2 set in this study)
applied to Proxima Centauri b, Ross-128 b, TRAPPIST-1 e,and Kepler-283 c, reaches a fatal dose of 0.36 Gy (3.64 Sv),
0.93 Gy (9.45 Sv), 3.03 Gy (30.8 Sv), and 0.68 Gy (6.89 Sv),
respectively.
We calculated the vertical proﬁle of the radiation dose, causedby the proton spectrum similar to the one reconstructed for the
GLE43 and Carrington-class events for each planet (GJ-699b,
Kepler-283 c, Kepler-1634 b, Proxima Centauri b, Proxima
Centauri b, Ross 128 b, TRAPPIST-1 e), with terrestrial-type
atmospheric compositions under annual maximum ﬂare and spot
maximum ﬂare events, by comparing that of Earth and Mars (seeFigures 9 and 10).
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Figure 10. Vertical proﬁle of radiation dose (in Sieverts), caused by proton spectrum imitating GLE43 (a and b) and Carrington ﬂare (c and d) penetrating N2 + O2–
rich (terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross128 b (red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂareenergy (a and c) and under spot maximum ﬂare energy (b and d), in Sieverts. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum
atmospheric pressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to1037 g cm−2; possible exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

(Barnard’s Star b) (0.267% of terrestrial atmospheric depth),3.27×102 g cm−2 for Proxima Centauri b (31.6% of terrestrial
atmospheric depth), 7.59×10 g cm−2 (7.31%) for Ross 128 b,
and 3.06×102 g cm−2 for TRAPPIST-1 e (29.5%) (Figures 9
and 10). We note that without sufﬁcient atmospheric depth, thesurface primitive life-forms on those planets suffer from severeradiation doses, even for relatively small-scale ﬂares.
We also performed calculations of the radiation doses for
CO2-rich and H2-rich atmospheres for each planet (seeFigures 11–14). The difference between each atmosphericcomposition does not become signiﬁcant especially whencompared with the N2+ O2– and CO2-rich types. However, it isevident that H2-rich atmosphere dissipates higher energetic
particles more signiﬁcantly.
The presence of a geomagnetic dipole shield around a planet
and its relative strength will inﬂuence its efﬁcacy for reducing
the irradiation effect of any solar ﬂares on surface life-forms.
First, we recalculated the surface dose (grays and Sieverts)
assuming that all exoplanets (GJ-699 b, Kepler-283 c, Kepler1634 b, Proxima Centauri b, Ross-128 b, TRAPPIST-1 e incomparison with Earth and Mars) have the same amount of

For the evaluation at different atmospheric depths we
employed the following four typical atmospheric depth
reference layers: TOA is equivalent to ≈0 g cm−2; Martiansurface atmospheric pressure (MS) is equivalent to 9 g cm−2;
terrestrial minimum atmospheric pressure, observed at thesummit of the Himalayas, is equivalent to 365 g cm−2 in thisstudy; and (Earth’s) ground-level atmospheric pressure isequivalent to 1037 g cm−2. Possible exoplanetary surface wasestimated as one-tenth of the terrestrial surface, equivalent to103.7 g cm−2. Note that the value is not identical to the real
observation data but to the nearest value employed in theMonte Carlo numerical simulation.
According to these calculations, we can specify the criticaldose for each planet, which is presumed in this study to be
10 Sv per annual event (see Section A.1. of the Appendix).
Using this threshold, we may determine the minimum
requirement of the atmospheric depth for terrestrial-type lifeform evolution. According to our analysis, the critical atmospheric depths required to secure terrestrial-type life-form
evolution on the surface of each modeled exoplanet exposed byannual severe ﬂare events are 2.77 g cm−2 for GJ 699 b
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Figure 11. Vertical proﬁle of radiation dose (in grays), caused by proton spectrum imitating GLE43 (a and b) and Carrington ﬂare (c and d) penetrating CO2-rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy(a and c) and under spot maximum ﬂare energy (b and d), in grays. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

magnetic shield as the Earth (B=BEarth) (see Figures 15
and 16). Then we evaluated the scenarios with the planetary
dipole magnetic ﬁeld (uniform over the whole planet surface)of (1) 0 (no magnetosphere), (2) 0.1×BEarth, (3) 1×BEarth(Earth level), and (4) 10×BEarth for four documented
exoplanets (Proxima Centauri b, Ross-128b, and TRAPPIST1 e, and Kepler-283 c) (see Figure 17).

from Proxima Centauri b, TRAPPIST-1 e, Ross-128b, andKepler-283 c are, respectively, 76.2, 53.5, 7.92, and 6.82 times
stronger than that of the Earth due to higher stellar XUV ﬂuxes
incident on the planetary atmospheres caused by closer
proximity to their respective host stars according to theequation proposed by Airapetian et al. (2017a).
In such a scenario, the radiation dose at Proxima Centauri b
and TRAPPIST-1 e reaches nearly fatal levels for complex lifeforms even through the modest annual ﬂares, reaching 1.32 Gy(8.09 Sv) and 1.09 Gy (6.68 Sv), respectively (see Figure 8).

3.3. Atmospheric Escape Induced by XUV Flux and AssociatedPossible Higher Radiation DoseXUV radiation caused by stellar superﬂares does not
signiﬁcantly increase the annual planetary surface UV ﬂux on
the exoplanets modeled unless high stellar magnetically driven
events (ﬂares and CMEs) severely damage their atmosphericthickness and impact their chemistry. However, since stellar
quiescent XUV radiation induces atmospheric escape from aterrestrial-type planet, a more critical situation may be
predicted, that is, the atmospheric depth of those exoplanets
can easily reach at least one-tenth of terrestrial atmosphericthickness. The atmospheric escape rate of O+/N+ ions

4. Discussion and Future Work4.1. Estimated Dose by Stellar Proton Event in Exoplanets
The stellar proton event impact onto different exoplanets has
been evaluated assuming three major types of atmosphere(N2 + O2, CO2, H2). In general, H2-rich atmosphere, which
may be present on either younger Earth-sized exoplanets or
super-Earths with relatively larger masses, has a maximum
absorption ratio compared with the other two atmospheres.

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Figure 12. Vertical proﬁle of radiation dose (in Sieverts), caused by proton spectrum imitating GLE43 (a and b) and Carrington ﬂare (c and d) penetrating CO2-rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy(a and c) and under spot maximum ﬂare energy (b and d), in Sieverts. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

surface, is mainly induced by the neutron particle, generated
as a secondary cosmic-ray when SEP reaches the atmosphere,
as shown in Figures 18 and 19.
Our conclusion is that, with relevant thickness of atmospheric depth for each type of atmosphere, there will be no
signiﬁcant damage to surface life-forms, except for somecritical planets located very close to their stars.
Projected stellar proton events with harder spectra using
GLE43 have a deeper penetration of intensive protons toward
the terrestrial atmospheric depth, whereas projected stellar
proton events with softer spectra using the Carrington ﬂare
have a sudden reduction of the dose at the mid altitude of theatmosphere (atmospheric depth between 101 and 102 g cm–2).
By taking a look at Figure 16, most of the critical dose only
applies for Proxima Centauri b and TRAPPIST-1 e when theatmospheric depth was lower than that of Martian surface when
all exoplanets have the same amount of magnetic shield as
Earth (B=BEarth), except for the scenario with spot maximum
ﬂare with proton spectrum imitating GLE43 (b). Each value ofthe magnetic ﬁeld may result in a signiﬁcant dose reduction at

This might be because of the lower molecular weight ofhydrogen, which enables larger molecular numbers under thesame atmospheric pressure. Under the other two major types ofatmosphere (N2 + O2, CO2), there are still signiﬁcant reduction
effects when atmospheric pressure is sufﬁcient as that of the
Earth. For TRAPPIST-1 e and Proxima Centauri b, even forannual maximum ﬂare events, the dose becomes relatively
large but not at the level that may affect complex life-forms.
However, with reduced atmospheric pressure to the level at the
top of the terrestrial surface (352 g cm−2), observed at thesummit of the Himalayas, the dose becomes signiﬁcantly high.
The dose with higher atmospheric depth than the Martiansurface, especially in units of Sieverts, has been evaluated to behigher than in previous papers calculated by different
approaches with hypothetical stellar ﬂare magnitude(Atri 2017). This may be induced by the difference indeﬁnitions of the effective doses, as well as precise numericalevaluation using PHITS (Sato et al. 2018b). This relatively
higher dose in Sieverts compared with the unit in grays,
calculated at the higher atmospheric depth than Martian13

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Figure 13. Vertical proﬁle of radiation dose (in Sieverts), caused by proton spectrum imitating GLE43 (a and b) and Carrington ﬂare (c and d) penetrating H2-rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy(a and c) and under spot maximum ﬂare energy (b and d), in grays. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

the TOA, from (1) 9.37×104 Gy (4.24×103 Sv) with nomagnetosphere to (3) 1.40×102 Gy (2.7×101 Sv) with
Earth-level magnetosphere (1×BEarth) almost 1/700 of thedose in grays (1/160 in Sv) that would be received with noprotective magnetic ﬁeld. Also at the ground level, the dose
was reduced from (1) 2.49×10−5 Gy (2.79×10−5 Sv) withno magnetosphere, to (3) 3.23×10−6 Gy (3.19×10−5 Sv)
with Earth-level magnetosphere (1×BEarth) on Proxima
Centauri b, almost one-tenth of the dose that would be received
with no protective magnetic ﬁeld. We may consider that, withthe presence of magnetic shields, those listed planets all
become habitable at least when the minimum amount of
atmospheric depth is present.

annual XUV dose values due to stellar ﬂares at the TOA of thetarget exoplanets are 105∼106 J m−2 (see Table 2). They are
all smaller than 0.001% of the terrestrial annual UV dose at the
TOA (∼4.3×109 J m−2).
In addition to estimating the ﬂare XUV values, we alsoroughly evaluate the quiescent component of XUV ﬂuxes fromstellar temperature (spectral class) and spot size (see Section
A.4. of the Appendix). As a result, the annual total ﬂux of thewhole XUV and UV wavelength range (10–4000 Å) at the
TOA of all our target exoplanets is smaller than the annual totalof the Earth. For example, in the most severe case, Kepler1634b shows 56% of the Earth values. The impulsive UV doses
from annual maximum ﬂares are not signiﬁcant whencompared with the annual dose from steady components. In
contrast, the XUV (1–1200 Å) ﬂuxes at the TOA of the target
exoplanets have much higher values compared with those at
Earth. For example, Proxima Centauri experiences ∼76 times
larger annual XUV ﬂux at its TOA compared with Earth’s
value, while TRAPPIST-1 e has ∼65 times larger ﬂux values.
This is because the XUV contribution in the overall UV

4.2. Impact of XUV Radiation
We also evaluated XUV ﬂux values from stellar ﬂares asfollows (see Section A.4. in the Appendix). Here we assume
that the UV energy portion is up to 10% of the total ﬂare energy
and estimate XUV dose by annual maximum ﬂare. As a result,

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Figure 14. Vertical proﬁle of radiation dose (in Sieverts), caused by proton spectrum imitating GLE43 (a and b) and Carrington ﬂare (c and d) penetrating H2-rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy(a and c) and under spot maximum ﬂare energy (b and d), in Sieverts. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of terrestrial surface, equivalent to 103.7 g cm−2.

emission from cool M dwarfs is larger than that for the Sun(Ribas et al. 2017).

for these planets is expected, the assumed atmospheric depths
especially for Proxima Centauri b and TRAPPIST-1 e may
reach 1/10 of the atmospheric pressure on Earth. In such a
scenario, the radiation dose on Proxima Centauri b and
TRAPPIST-1 e reach nearly fatal levels even through annual
ﬂares, reaching 1.32 Gy (8.09 Sv) and 1.09 Gy (6.68 Sv),
respectively (see Figures 7 and 8).

4.3. Estimation of Atmospheric Escape Rate due to
Photoionization
Since coronal XUV radiation induces atmospheric escape via
photoionization, we can estimate the atmospheric escape rate
on Proxima Centauri b, TRAPPIST-1 e, Ross-128b, andKepler-283 c using the proposed XUV ﬂux escape rate scalingby Airapetian et al. (2017a). In order to estimate these values,
we needed to obtain or synthesize the possible XUV ﬂux for
those hoststars. In this study, we calculated all XUV ﬂuxes
according to the method illustrated in the Appendix. Afterobtaining the XUV ﬂuxes, we compared them with theatmospheric escape rate from the Earth. The atmosphericescape rates via quiescent XUV emission from Proxima
Centauri b, TRAPPIST-1 e, Ross-128b, and Kepler-283 care, respectively, 76.2, 53.5, 7.92, and 6.82 times stronger thanthat of the Earth due to the higher XUV ﬂuxes incident on
close-in exoplanetary atmospheres. If not enough outgassing

4.4. Summary of XUV Studies
The following items are not well characterized in thepresented models: (1) the MUSCLES survey provides stellar
spectra ranging from XUV to IR based on observed (Chandra
and XMM and HST) and empirical estimates. However, the
MUSCLES study includes stars earlier than M4 dwarfs(Teff>3000 K), and thus there are no relevant data for coolerstars, such as TRAPPIST-1, whose Teff is almost 2500 K. Ourmodels based on these assumptions should be updated withnew observations to be performed in the near future(Airapetian et al. 2019). (2) The correlation between XUV
ﬂuxes and star spot sizes, assumed from previous solar

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Figure 15. Vertical proﬁle of radiation dose (in Sieverts), caused by proton spectrum imitating GLE43 (a and b) and Carrington Flare (c and d) penetrating H2-rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square) and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy (a andc) and under spot maximum ﬂare energy (b and d), in grays. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of terrestrial surface, equivalent to 103.7 g cm−2.

observations, is currently only at a hypothetical stage,
especially for cool M dwarfs. The speciﬁc relationship should
be investigated in detail in accordance with photospherictemperature and wavelength. In short, we have to investigate
each stellar temperature (spectral class) and planetary body inmore detail to clarify those relationships. (3) In general, there
are no sufﬁcient observational results about stellar activities
especially for active M dwarfs, which should be the focus of
future observations. (4) We need to deepen the survey for
active M dwarfs when collecting more UV-EUV data, as moststellar objects determined by the MUSCLES survey are notactive M dwarfs, except for Proxima Centauri. Moreover, as
for Proxima Centauri (Wargelin et al. 2017), the stellar
activity may change in accordance with the stellar cycle. (5)We made the ﬁrst assumption of the ratio for XUV and EUVin this survey, which should be supported and adopted
through sufﬁcient observational results. The Mega-MUSCLES project (Froning et al. 2018) will also focus on a survey
for TRAPPIST-1, with which we can check the validity of ourassumption.

5. Conclusion
Our results suggest that both SPE and XUV ﬂux doses are
signiﬁcantly higher at their TOA of close-in exoplanets aroundM dwarfs than those at Earth. For an exoplanet with a thickatmosphere (with an ozone layer for UV dose), the above
extreme ﬂuxes do not affect the dose of ionizing radiation at theplanetary surface. When a strong planetary magnetic ﬁeld is notpresent or a stellar driver (stellar wind or a CME) is strong
enough to perturb the global ﬁeld and induce strong ionospheric currents that can dissipate into the heat (Cohen et al.
2014; Airapetian et al. 2017b), large XUV ﬂuxes and massive
winds can thus affect the erosion of the atmosphere, reducing
its thickness on geological timescales (Airapetian et al. 2017b;
Garcia-Sage et al. 2017). Accordingly, if the ozone layer is
efﬁciently destroyed by SEP events and the exoplanetaryatmosphere is eroded via atmospheric escape (Segura et al.
2010; Airapetian et al. 2017a; Tilley et al. 2019), then stellar
XUV emission can penetrate into the planetary surface andprovide detrimental conditions to complex life-forms.

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Figure 16. Vertical proﬁle of radiation dose (in Sieverts), caused by proton spectrum imitating GLE43 (a and b) and Carrington ﬂare (c and d) penetrating H2-rich(terrestrial type) atmosphere on Proxima Centauri b (green square), TRAPPIST-1 e (blue circle), Kepler-283 c (brown square), Kepler-1634 b (blue cross), Ross-128 b(red square), and GJ-699 b (pink square) in comparison with the Earth (blue square) and Mars (red plus) in logarithmic scale under annual maximum ﬂare energy(a and c) and under spot maximum ﬂare energy (b and d), in Sieverts. Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmosphericpressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible
exoplanetary surface is one-tenth of the terrestrial surface, equivalent to 103.7 g cm−2.

density (N2+ O2, CO2, and H2) are, at their crustal surfaces,
protectively buffered from the radiation ﬂux arriving at the
TOA due to atmospheric attenuation of the incident radiation.
This supports the hypothesis of Proxima Centauri b as ahabitable planet once it was discovered, even though it islocated very close to its host star. In some cases, the incident
dose may be reduced sufﬁciently to support surface life, andthis is most notable for atmospheric systems containing
signiﬁcant H2, which would be more common for planets
larger than Earth.
In cases where atmospheric thickness alone is not sufﬁcient toattenuate the radiation from solar superﬂares to non-lifethreatening levels, the enhanced attenuation efﬁcacy presented
by oceanic bodies may prove critical in preserving marine but
not land-based life-forms. The critical atmospheric depth foreach planet to secure terrestrial-type life-form evolution on thesurface of each planet is 2.14×102 g cm−2 for the Proxima
Centauri b, 4.68×10 g cm−2 for the Ross 128 b, and 2.04×
102 g cm−2 for the TRAPPIST-1 e. If we set the critical dose as
1 mSv per year, the critical atmospheric depth becomes 1.05×

According to our scenario, if the atmospheric depth is <1/10of the terrestrial one, radiation doses become fatal for Proxima
Centauri b and TRAPPIST-1 e even under annual maximum
ﬂare. Our new proposed scenario also suggests that under highenergy and frequency of stellar ﬂares, we can expect that the
impact of ionizing radiation on terrestrial-type life-forms is notcritical for their evolution if the atmosphere is thick enough(≈1 bar). Further efforts should explore the conditions for
efﬁcient removal of the UV protection layer (the ozone layer)that reduces the annual UV dose. In this work, we developed auniversal estimation method of the stellar ﬂare frequency basedon the star spot area observed on each host star. The stellar ﬂarefrequency varies from annual maximum ﬂare, 10 yr ﬂare, andpossible maximum energy from the star. These estimations
were based on Maehara et al. (2017) and Shibata et al. (2013).
Our study indicates that for most “habitable” planets orbiting
M-class stars, the radiation dose at the TOA caused by periodic
solar superﬂare activity is far in excess of the critical thresholdfor terrestrial life to survive. However, the simulations also
indicate that planets with sufﬁcient atmospheric thickness and
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Figure 17. Vertical distribution of each radiation dose (in grays) under different magnetic shower caused by SPE air shower penetrating N2 + O2–rich (terrestrial type)on Proxima Centauri b (a and b) and on Ross 128 b (c and d) in logarithmic scale under annual maximum ﬂare energy (a and c) and under spot maximum ﬂare energy
calculated by Shibata et al. (2013) (b and d). Martian surface atmospheric pressure is equivalent to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at
the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-level atmospheric pressure is equivalent to 1037 g cm−2; possible exoplanetary surface is
one-tenth of terrestrial surface, equivalent to 103.7 g cm−2.

103 g cm−2 for the Proxima Centauri b, 7.8×102 g cm−2 for theRoss 128 b, and 1.04×103 g cm−2 for the TRAPPIST-1 e.
Do stellar ﬂares present a common life-limiting factor for thedevelopment of complex life-forms throughout the universe?
Our simulations certainly suggest this might be the case, butequally, for close-in exoplanets around M-type stars that haveno substantial atmosphere due to lack of degassing (fromvolcano-tectonic activity), the atmospheric depth might beinsufﬁcient to support terrestrial-type life in the ﬁrst place,
regardless of the periodic sterilizing effect of solar ﬂares.
Future galactic surveys of extrasolar planets aimed atdetermining habitable planets should characterize stellar
activity as part of the analysis process and couple this withassessments of atmospheric composition, the presence of liquidwater, and the rate of planetary rotation versus superﬂare
duration. Indications of liquid water on any exoplanet surface,especially oceanic bodies, will likely be a critical indicator for
habitability due to the effective protection it provides fromsolar radiation.

The authors express their sincere thanks to Astronomical
Observatory, Kyoto University, and NASA/GSFC for providing us relevant useful data sets and advice. The authors also
express their sincere thanks to Professor Takao Doi, Unit of the
Synergetic Studies for Space, Kyoto University, for his support
for ExoKyoto and observation projects, together with his
sincere encouragement for our survey. Special thanks to Ms.
Mayumi Tatsuda, secretary, and Ms. Nami Kimura, graduate
student, of Earth and Planetary Water Resources Assessment
Laboratory, Graduate School of Advanced Integrated Studies
in Human Survivability, for her support during the compilationof the whole survey. This work was also supported by JSPS
KAKENHI grant Nos. JP16J00320, JP16J06887, JP16H03955,
JP17H02865, JP17K05400, JP18J20048, and JP18H01569
and MEXT grant No. 26106006. Yuta Notsu and Shota Notsu
are supported by JSPS Overseas Research Fellowship
Program. Vladimir Airapetian was supported by NASA grant
80NSSC17K0463, TESS Cycle 1 grant 80NSSC19K0381, andNASA/GSFC ISFM SEEC grant.

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Figure 18. Vertical proﬁle of radiation dose in grays and Sieverts, caused by SPE air shower penetrating N2 + O2–rich (terrestrial type) atmosphere on Proxima
Centauri b with B=0 (a), (b) and with B= BEarth (c and d) under annual maximum ﬂare energy in grays (a and c) and Sieverts (b and d). Martian surface atmosphericpressure is equivalent to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at the summit of the Himalayas , is equivalent to 365 g cm−2; (Earth’s) groundlevel atmospheric pressure is equivalent to 1037 g cm−2; possible exoplanetary surface is one-tenth of terrestrial surface, equivalent to 103.7 g cm−2.

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Figure 19. Vertical proﬁle of radiation dose in grays and Sieverts, caused by SPE air shower penetrating N2 + O2–rich (terrestrial type) atmosphere on Ross-128 bwith B=0 (a and b) and with B=BEarth (c and d) under annual maximum ﬂare energy in grays (a and c) and Sieverts (b and d). Martian surface atmospheric pressureis equivalent to 9 g cm−2; terrestrial minimum atmospheric pressure, observed at the summit of the Himalayas, is equivalent to 365 g cm−2; (Earth’s) ground-levelatmospheric pressure is equivalent to 1037 g cm−2; possible exoplanetary surface is one-tenth of terrestrial surface, equivalent to 103.7 g cm−2.

the radiological sensitivities signiﬁcantly vary with species.
Thus, we decided to select the effective dose as an index for
discussing habitability because it is the most well-known
radiological protection quantity and set the critical dose to
10 Sv per annually occurring stellar proton event.

Appendix
MethodA.1. Radiation DoseAs mentioned above, two types of radiation doses, theabsorbed dose in grays and the effective dose in Sieverts, werededuced from the simulation. In general, the absorbed doses arehigher than the effective doses at the TOA because lower
energy protons, which have a small impact on the human bodydue to their shorter range, predominantly contribute to the doseat the location. In contrast, the relation is reversed at the groundlevel because of the contribution of neutrons, which have amore signiﬁcant impact on the human body and become very
important in such deeper locations.
In this study, we need to presume the critical dose fordiscussing the habitability of a planet. In general, mortality due
to radiation exposure is discussed with respect to the absorbed
dose throughout the whole body; for example, the whole-bodyabsorbed dose that is lethal for half of the exposed individuals,
LD50, is around 4 Gy for photon exposure (International
Commission on Radiological Protection, 2007). However,LD50 is expected to be different for cosmic-ray exposure due
to higher relative biological effectiveness. In addition, thewhole-body absorbed dose depends on the size of the creature;

A.2. Parameters and Equations
A single ﬂare event occurs on the stellar surface, and theenergy via electromagnetic wave radiates in all directions,whereas during a proton event, the energy has clear
directionality. For the SPEs, the release angle of the proton
may be limited within a certain angle from the solar equator.
We consider that the total area that may be affected by the SPEscan be expressed as
ASPE = 2pR e ´ TD ´ HSPE,(4 )

TD = 2R e sin q V,

(5 )

qH
,
360

(6 )

HSPE =

in which ASPE is the total ﬂare affected area in 1 au distance,
Re is Earth’s semimajor axis, HSPE is the ratio of SPE
horizontal release angle over the entire orbital circle, θV is the20The Astrophysical Journal, 881:114 (24pp), 2019 August 20

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Table 4
UV Energy from Annual Maximum Flare at TOA in Each Planet
Exoplanetflare aEUV

Name

(J m−2)

GJ 699 b
Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross-128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

8.74E+03
4.43E+02
4.60E+04
1.57E+04
8.75E+04
5.30E+04
2.82E+04
1.42E+04
8.24E+03
4.75E+03
3.22E+03
1.65E+03
3.70E−01
1.59E−01

Exoplanet

flare
EUV

b

flare
EUV,Earth

flare
EUV

c

flare d
EXUV

flux
EUV,Earth(%)(J m−2)

7.27E+04
3.69E+03
3.83E+05
1.31E+05
7.28E+05
4.41E+05
2.35E+05
1.18E+05
6.86E+04
3.96E+04
2.68E+04
1.37E+04
3.08E+00
1.33E+00

2.59E−06
1.31E−07
1.37E−05
4.66E−06
2.60E−05
1.57E−05
8.39E−06
4.22E−06
2.45E−06
1.41E−06
9.54E−07
4.89E−07
1.10E−10
4.72E−11

4.37E+03
2.22E+02
2.30E+04
7.86E+03
4.38E+04
2.65E+04
1.41E+04
7.12E+03
4.12E+03
2.38E+03
1.61E+03
8.24E+02
1.85E−01
7.96E−02

normal h
EUVnormal i
EVisible

normal j
EIRflare + quiescent k
EXUV

Name

(J m−2)(J m−2)(J m−2)(J m−2)

GJ 699 b
Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross-128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

1.65E+06
4.25E+08
1.79E+09
2.44E+07
9.25E+07
2.13E+08
1.14E+08
5.73E+07
3.32E+07
1.91E+07
1.30E+07
6.64E+06
3.37E+09
1.45E+09

8.68E+07
1.14E+10
1.14E+10
8.78E+08
5.01E+09
8.97E+09
4.79E+09
2.41E+09
1.40E+09
8.05E+08
5.44E+08
2.79E+08
1.93E+10
8.32E+09

7.79E+08
2.83E+10
1.33E+10
2.74E+10
5.81E+10
1.72E+11
9.19E+10
4.63E+10
2.68E+10
1.54E+10
1.05E+10
5.36E+09
2.04E+10
8.81E+09

2.79E+04
1.19E+06
6.90E+05
1.33E+07
1.50E+06
7.29E+07
3.89E+07
1.96E+07
1.13E+07
6.54E+06
4.42E+06
2.27E+06
1.74E+05
7.51E+04

flare
EXUV

e

flare
EXUV,Earth

flare
EXUV

f

flux
EXUV,Earthnormal g
EXUV(%)(J m−2)

7.27E+04
3.69E+03
3.83E+05
1.31E+05
7.28E+05
4.41E+05
2.35E+05
1.18E+05
6.86E+04
3.96E+04
2.68E+04
1.37E+04
3.08E+00
1.33E+00

2.51E−02
1.27E−03
1.32E−01
4.51E−02
2.51E−01
1.52E−01
8.11E−02
4.08E−02
2.37E−02
1.36E−02
9.23E−03
4.73E−03
1.06E−06
4.57E−07

2.35E+04
1.19E+06
6.67E+05
1.33E+07
1.38E+06
5.99E+07
3.20E+07
1.61E+07
9.32E+06
5.38E+06
3.64E+06
1.86E+06
1.74E+05
7.51E+04

flare + quiescent
EXUV
lflare + quiescent m
EUVflare + quiescentEUVnflare + quiescent
EXUV,Earth

0.16
6.82
3.96
76.21
8.61
418.36223.21112.34
65.07
37.52
25.39
13.01
1.00
0.43

(J m−2)l
1.66E+06
4.25E+08
1.79E+0.9
2.44E+07
9.25E+07
2.13E+08
1.14E+08
5.73E+07
3.32E+07
1.91E+07
1.30E+07
6.64E+06
3.37E+09
1.45E+09

flare + quiescent
EUV,Earth

0.00
0.13
0.53
0.01
0.03
0.06
0.03
0.02
0.01
0.01
0.00
0.00
1.00
0.43

Notes. TOA ≈ 0 g cm−2.
a
UV energy by annual maximum ﬂare at TOA.
b
Ratio to Earth’s annual maximum ﬂare.
c
Ratio to Earth’s annual UV ﬂux at TOA.
d
XUV energy by annual maximum ﬂare at TOA.
e
Ratio to Earth’s annual maximum ﬂare.
f
Ratio to Earth’s annual UV ﬂux at TOA.
g
Annual XUV energy by normal stellar radiation at TOA.
h
Annual UV energy by normal stellar radiation at TOA.
i
Annual visible ray energy by normal stellar radiation at TOA.
j
Annual IR energy by normal stellar radiation at TOA.
k
Annual total (ﬂare + quiescent) XUV energy at TOA.
l
Ratio to Earth/annual total (ﬂare + quiescent) XUV energy at TOA.
m
Annual total (ﬂare + quiescent) UV energy at TOA.
n
Ratio to Earth/annual total (ﬂare + quiescent) UV energy at TOA.

vertical release angle of SPEs, and θH is the horizontal release
angle of SPE (in this study we assume that θV and θH are equal
to 15° and 90°, respectively).
Thus, the expected SPE energy per unit area received at
Earth’s TOA during a determined period (1 yr) can be
expressed as
ESPEEarth =E flare ´ HP ´ RSPE,
ASPE

ﬂare energy per year. In this study, we employed 0.25
(Aschwanden et al. 2017).
From the above equation, we may calculate ﬂuence on any
exoplanet using the relative proportion to the ﬂuence at
Earth’s TOA.
A.3. Normalization(7 )In order to apply this comparison with stellar ﬂares, weemployed a normalized value based on the GOES X-ray classof the ﬂare events. We can safely assume that total energy
of ﬂares can be estimated from the GOES X-ray classin which HP=θH/180 is the horizontal exposure probability(we employ 0.5) and RSPE is the fraction of SPE energy in total
21

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Table 5UV Energy at TOA, with Synthesized Spectra Assuming 100% of the Impact Ratio of Star Spot in Equation (10)Exoplanetflare aEUV

Name

(J m−2)

Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross-128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

flare
EUV

b

flare
EUV

c

flare dEXUV

flare
EUV,Earth

flux
EUV,Earth(%)(J m−2)

8.24E+03
4.60E+04
1.57E+04
8.75E+04
5.30E+04
2.83E+04
1.42E+04
8.24E+03
4.75E+03
3.22E+03
1.65E+03
3.70E−01
1.60E−01

6.85E+04
3.83E+05
1.31E+05
7.28E+05
4.41E+05
2.35E+05
1.18E+05
6.86E+04
3.96E+04
2.68E+04
1.37E+04
3.08E+00
1.33E+00

2.44E−06
1.37E−05
4.66E−06
2.60E−05
1.57E−05
8.39E−06
4.22E−06
2.45E−06
1.41E−06
9.54E−07
4.89E−07
1.10E−10
4.72E−11

Exoplanet

normal h
EUVnormal i
EVisibleName(J m−2)

Kepler-283 c
Kepler-1634 b
Proxima Centauri b
Ross-128 b
TRAPPIST-1 b
TRAPPIST-1 c
TRAPPIST-1 d
TRAPPIST-1 e
TRAPPIST-1 f
TRAPPIST-1 g
TRAPPIST-1 h
Sol d (Earth)
Sol e (Mars)

1.96E+09
1.90E+09
2.44E+07
9.24E+07
2.53E+08
1.35E+08
6.79E+07
3.93E+07
2.27E+07
1.53E+07
7.86E+06
3.37E+09
1.45E+09

flare
EXUV

e

flare
EXUV

f

normal g
EXUV

flare
EXUV,Earthflux
EXUV,Earth(%)(J m−2)

4.12E+03
2.30E+04
7.86E+04
4.38E+04
2.65E+04
1.41E+04
7.12E+03
4.12E+03
2.38E+03
1.61E+03
8.24E+02
1.85E−01
7.96E−02

6.85E+04
3.83E+05
1.31E+05
7.28E+05
4.41E+05
2.35E+05
1.18E+05
6.86E+04
3.96E+04
2.68E+04
1.37E+04
3.08E+00
1.33E+00

2.36E−02
1.32E−01
4.51E−02
2.51E−01
1.52E−01
8.11E−02
4.08E−02
2.37E−02
1.36E−02
9.23E−03
4.73E−03
1.06E−06
4.57E−07

1.01E+05
7.07E+05
1.33E+07
1.38E+06
1.12E+08
5.99E+07
3.01E+07
1.75E+07
1.01E+07
6.81E+06
3.49E+06
1.74E+05
7.51E+04

normal j
EIRflare + quiescent k
EXUVflare + quiescent
EXUV
lflare + quiescent m
EUVflare + quiescentEUVn(J m−2)(J m−2)(J m−2)

1.12E+10
1.21E+10
8.78E+08
5.01E+09
8.97E+09
4.79E+09
2.41E+09
1.40E+09
8.04E+08
5.44E+08
2.79E+08
1.93E+10
8.32E+09

1.19E+10
1.41E+10
2.74E+10
5.81E+10
1.72E+11
9.19E+10
4.63E+10
2.68E+10
1.54E+10
1.05E+10
5.36E+09
2.04E+10
8.81E+09

1.05E+05
7.30E+03
1.33E+07
1.42E+06
1.12E+08
5.99E+07
3.01E+07
1.75E+07
1.01E+07
6.81E+06
3.49E+06
1.74E+05
750E+04

flare + quiescent
EXUV,Earth

0.60
4.19
76.21
8.17
644.10343.66172.95100.19
57.76
39.09
20.03
1.00
0.43

(J m−2)
1.96E+09
1.90E+0.9
2.44E+07
9.25E+07
2.53E+08
1.35E+08
6.79E+07
3.97E+07
2.27E+07
1.53E+07
7.86E+06
3.37E+09
1.45E+09

flare + quiescent
EUV,Earth

0.58
0.56
0.01
0.03
0.07
0.04
0.02
0.01
0.01
0.00
0.00
1.00
0.43

Notes. TOA ≈ 0 g cm−2.
a
UV energy by annual maximum ﬂare at TOA.
b
Ratio to Earth’s annual maximum ﬂare.
c
Ratio to Earth’s annual UV ﬂux at TOA.
d
XUV energy by annual maximum ﬂare at TOA.
e
Ratio to Earth’s annual maximum ﬂare.
f
Ratio to Earth’s annual UV ﬂux at TOA.
g
Annual XUV energy by normal stellar radiation at TOA.
h
Annual UV energy by normal stellar radiation at TOA.
i
Annual visible ray energy by normal stellar radiation at TOA.
j
Annual IR energy by normal stellar radiation at TOA.
k
Annual total (ﬂare + quiescent) XUV energy at TOA.
l
Ratio to Earth/annual total (ﬂare + quiescent) XUV energy at TOA.
m
Annual total (ﬂare + quiescent) UV energy at TOA.
n
Ratio to Earth of annual total (ﬂare + quiescent) UV energy at TOA.

(Namekata et al. 2017). Accordingly, we assumed that the totalenergy of the Carrington-class event (estimated as X45 class)was 4.5×1025 Joules and GLE43 (estimated as X13 class)was 1.3×1025 Joules.
To integrate the above reference of SPEs, applicable forstellar proton events, the following constants are determined as
E PE GLE43 =E GLE43 ´ RSPE
,
ASPE

(8 )

E PE Car =E Car ´ RSPE
,
ASPE

(9 )

in which EGLE43 is the total energy of GLE43 (estimated as
1.3×1032 [erg]=1. 3×1025 [Joule]) and ECar is the totalenergy of a Carrington-class event (estimated as 4.5×1032[erg]=1. 3×1025 [Joule]). Note that coefﬁcient HP has notbeen multiplied as it is obvious that those events released SEP
toward the Earth.
A.4. XUV Radiation
XUV radiation is considered one type of harmful radiation
released from a single stellar ﬂare event. The proportion of
XUV radiation in a single ﬂare event has not been fully
22

The Astrophysical Journal, 881:114 (24pp), 2019 August 20

Yamashiki et al.

determined. However, according to Aschwanden et al. (2017),
the energetic portion of the UV continuum (with the sum of
the ranges 200–228, 370–504, 504–912, 1464–1609, and
1600–1740 Å) is 3.96×1029 erg, equivalent to 1.8% of
estimated total ﬂare energy (X2.2 class event 2.2×1031 erg)
in a solar ﬂare. In this study, since there are no related studies
about the proportion of UV radiation as a function of stellar
temperature, diameter, and other factors, we use solar ﬂareresults (Aschwanden et al. 2017) as a reference and roughly
assume that the UV portion of 10% of total ﬂare energy is themost extreme case. To determine the portion of XUV in the
total UV dose of the ﬂare as one extreme case, we assume that
50% of the total UV energy emitted during stellar ﬂare is XUVenergy.
When determining normal UV irradiation for each of the
exoplanets, since there are no direct observations for most of thetarget stars, we synthesize the spectra using the publishedspectra curve from the MUSCLES project (France et al. 2016;
Loyd et al. 2016; Youngblood et al. 2016) (using dapt-const-ressed SED in version 2.2 data set) and applied a similar spectralon the basis of temperature, trying to create an extreme UV caseof a similar type to the star. We used the MUSCLES-observed
spectra for 13 stars (GJ1214, GJ551 [Proxima Cen], GJ876,
GJ436, GJ581, GJ667C, GJ176, GJ832, HD85512, HD40307,
HD97658, eps Eridanis, and the Sun) and synthesized thespectra for the target stars (TRAPPIST-1, Ross-128, Kepler-283,
Kepler-1634). In the synthesizing process we divide the whole
spectra into two, (1) one that is mostly related to thephotospheric temperature, applied for IR, VR, and UVA and
UVB, in particular for wavelengths longer than 1200 Å; and (2)one that is mostly related to the magnetic activity mainly in thechromosphere, applied for XUV, in particular for the wavelength from 1 to 1200 Å. For (2) we made the hypothesis thatthe intensity of XUV is in proportion to the star spot area, basedon the hypothesis that the star spot area represents the averagemagnetic ﬁeld strength of the target star, thus the chromosphericactivity level. In this study, because our aim is to evaluate
extreme cases under different star systems, we introduce a ratioto specify the weight of each effect. Currently, we set two
extreme cases when synthesizing the spectra of the XUV
portion, as (1) 100% of the total magnetic ﬂux-related(magnetic) term, generated from observed XUV (GJ551)
calculated using the relative portion of the star spot area, and(2) 60% of the magnetic term + 40% of the photospheretemperature-related (photospheric) term. We set 20% of themagnetic term + 80% of the photospheric term whensynthesizing the spectra of the EUV portion, while applying
the following equation to synthesized ﬂuxes in each wavelength:
FI = R AspotUV ´ FAspot + (1 - R AspotUV) FTeff ,GJ436 (3281 K). The spectra of Kepler-283 (4351 K) were
calculated using HD85512 (4305 K) and HD40307 (4783 K),
and the spectra of Kepler-1634 (5474 K) were synthesized withthat of eps Eridanis and our Sun. For the chromosphericcomponent (XUV), we synthesized spectra mainly from
Proxima Centauri and eps Eridanis, whose XUV components
were much higher than that of other stars.
Then we calculated the portion of UV (including XUV andothers) and set its value as the radiation boundary from the central
star. Irradiance at the top of the atmosphere of each planet can becalculated using normal radiation propagation. Estimating UVenergy from stellar ﬂares, we applied the statistical occurrence
probability and possible total energy of the ﬂare from each central
star. Applying this UV energy ratio and projected planetary
position, we calculated the energy at the TOA of each planet.
Using the above hypotheses, the most intensive UV and XUV at
the TOA induced by annual maximum ﬂares, assuming thisportion, were observed on Ross-128 b, reaching 8.75×105 J m−2
for total UV and 4.38×105 J m−2 for XUV (1–1200 Å),
followed by Kepler-1634 b with 4.60×104 J m−2, Proxima
Centauri b with 1.57×104 J m−2, and TRAPPIST-1 e with
8.24×103 J m−2, as shown in Table 4. Although all values arelargely compared with terrestrial impact values, they are all below
0.001% of the terrestrial annual UV dose at the TOA (estimated as
4.31×109 J m−2, shown in Table 4. Also, by comparing the total
dose during annual irradiation, the total value is far below theannual dose for the terrestrial case. The total annual UV(10–4000 Å) dose does not reach the terrestrial level for all of
the exoplanets, with the maximum value 0.53 in the terrestrial
case of Kepler-1634 b.
At the same time, limiting the dose of XUV (1–1200 Å),
most habitable exoplanets have a higher value compared withthat of Earth, reaching up to 76 times the value at Proxima
Centauri b, followed by TRAPPIST-1 e with 65 times in the(2) calculation, since the XUV portion in the normal irradiationof the Sun is smaller than the observed M dwarf and thus itreﬂects an annual dose. On TRAPPIST-1 b, c, and d, the XUV
portion reaches a signiﬁcant value of 418, 223, and 112,
respectively (as shown in Table 4). When applying the mostextreme cases (1), TRAPPIST-1 e becomes the highest,
reaching 100 times the value compared with Earth. On
TRAPPIST-1 b, c, and d for the (1) calculation, the XUV
portion reaches signiﬁcant values of 644, 344, and 173 times,
respectively (as shown in Table 5). They are, however,generally considered as nonhabitable planets in the system.
Having relevant atmospheric depth and an ion layer, equivalent
to that of the Earth—where most of XUV is absorbed by afactor of 10−15–10−22, the irradiation may be absorbed before
reaching the planetary surface. The ﬂare impact on the UV dose
only becomes important when superﬂares induce a signiﬁcantreduction of the ozone layer (Segura et al. 2010) or when they
accelerate signiﬁcant atmospheric escape (Airapetian et al.
2016). Accordingly the atmospheric protection becomes more
important for those planets due to higher ratio of normal
irradiation of XUV.

(10)

in which FI is the UV ﬂux for target stars, R AspotUV is the impact
ratio of star spot in UV (same for XUV), FAspot is the ﬂux
calculated from the relative size of the star spot area,
normalized by the observed GJ551 (Proxima Cen) star spot
area and its UV (XUV) spectra, and FTeff is the ﬂux calculated
from the photospheric temperature of the star.
For example, when synthesizing a portion of IR, VR, farFUV, middle-UV, and near-UV for TRAPPIST-1 (2550 K), for
the VR, IR region, we employed the spectra of GJ1214(2935 K) as a proxy for a lower temperature star. The spectra of
Ross-128 (3192 K) were computed with GJ876 (3062 K) and

ORCID iDs
Yosuke A. Yamashiki https://orcid.org/0000-00016241-4741
Hiroyuki Maehara https://orcid.org/0000-0003-0332-0811
Vladimir Airapetian https://orcid.org/0000-00034452-0588
23

The Astrophysical Journal, 881:114 (24pp), 2019 August 20

Yamashiki et al.

Yuta Notsu https://orcid.org/0000-0002-0412-0849
Shota Notsu https://orcid.org/0000-0003-2493-912X
Takanori Sasaki https://orcid.org/0000-0003-1242-7290

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