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Hot Earth or Young Venus? A nearby transiting rocky planet mystery

Venus and Earth provide astonishingly different views of the evolution of a rocky planet, raising the question of why these two rock y worlds evolv ed so differently. The recently disco v ered transiting Super-Earth LP 890-9c (TOI-4306c, SPECULOOS-2c) is a key to the question. It circles a nearby M6V star in 8.46 d. LP890-9c receives similar flux as modern Earth, which puts it very close to the inner edge of the Habitable Zone (HZ), where models differ strongly in their prediction of how long rocky planets can hold onto their water. We model the atmosphere of a hot LP890-9c at the inner edge of the HZ, where the planet could sustain several very different environments. The resulting transmission spectra differ considerably between a hot, wet exo-Earth, a steamy planet caught in a runaway greenhouse, and an exo-Venus. Distinguishing these scenarios from the planet’s spectra will provide critical new insights into the evolution of hot terrestrial planets into exo-Venus. Our model and spectra are available online as a tool to plan observations. They show that observing LP890-9c can provide key insights into the evolution of a rocky planet at the inner edge of the HZ as well as the long-term future of Earth.

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MNRAS 524, L10–L14 (2023) https://doi.org/10.1093/mnrasl/slad064
Hot Earth or Young Venus? A nearby transiting rocky planet mystery
L. Kaltenegger ,1,2‹
R. C. Payne,1,2
Z. Lin ,1,2,3
J. Kasting4
and L. Delrez 5
1Department of Astronomy, Cornell University, 311 Space Sciences Building, Ithaca, NY 14853, USA
2Carl Sagan Institute, Cornell University, 302 Space Sciences Building, Ithaca, NY 14853, USA
3Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139, USA
4Department of Geosciences, Penn State University, 435 K Deike Building, State College, PA 16801, USA
5Astrobiology Research Unit & STAR Institute, University Liege, Allee du 6 Aout 19C, B-4000 Liege, Belgium
Accepted 2022 May 24. Received 2022 May 24; in original form 2022 November 28
ABSTRACT
Venus and Earth provide astonishingly different views of the evolution of a rocky planet, raising the question of why these two
rocky worlds evolved so differently. The recently discovered transiting Super-Earth LP 890-9c (TOI-4306c, SPECULOOS-2c)
is a key to the question. It circles a nearby M6V star in 8.46 d. LP890-9c receives similar flux as modern Earth, which puts
it very close to the inner edge of the Habitable Zone (HZ), where models differ strongly in their prediction of how long rocky
planets can hold onto their water. We model the atmosphere of a hot LP890-9c at the inner edge of the HZ, where the planet
could sustain several very different environments. The resulting transmission spectra differ considerably between a hot, wet
exo-Earth, a steamy planet caught in a runaway greenhouse, and an exo-Venus. Distinguishing these scenarios from the planet’s
spectra will provide critical new insights into the evolution of hot terrestrial planets into exo-Venus. Our model and spectra are
available online as a tool to plan observations. They show that observing LP890-9c can provide key insights into the evolution
of a rocky planet at the inner edge of the HZ as well as the long-term future of Earth.
Key words: methods: numerical – techniques: spectroscopic – planets and satellites: individual: LP∼890-9∼c – planets and
satellites: atmospheres – planets and satellites: physical evolution – planets and satellites: terrestrial planets.
1 INTRODUCTION
Venus and Earth provide astonishingly different views of the evo-
lution of a rocky planet, raising the question of why these two
rocky worlds evolved so differently. While the development of Venus
and Earth are not accessible, rocky planets at the inner edge of
the Habitable Zone (HZ) provide rare insights into the possible
paths of climate evolution of hot rocky planets. Delrez et al. (2022)
recently announced the detection of the super-Earth LP 890-9c (also
SPECULOOS-2c) at the inner edge of the HZ (see e.g. Kasting 1988;
Kopparapu et al. 2013, 2017; Ramirez & Kaltenegger 2014; Luger
& Barnes 2015) transiting a low-activity nearby M6V star at 32 pc.
The planet is part of a two-planet system with the inner planet, LP
890-9b, first detected by TESS (Guerrero et al. 2021) (TOI-4306.01)
in a 2.73-d orbit, with incident flux of 4.09 ± 0.12 S⊕, inward of
the HZ. However, LP 890-9c, receives 0.906 ± 0.026 S⊕, placing it
within both the conservative and empirical HZ (Kaltenegger 2017;
Fujii et al. 2018; Schwieterman et al. 2018). This makes LP 890-9c
a key to understanding how Venus and Earth evolved.
To explore the range of possible atmospheres and assess whether
transmission spectra can illuminate that difference, we create seven
models for LP890-9c: two hot exo-Earth models based on modern
Earth with and without CO2, two moist greenhouse, one runaway
greenhouse steam, and one CO2-dominated exo-Venus model based
on modern Venus’ surface pressure. We chose those models to
 E-mail: lk433@cornell.edu, lkaltenegger@astro.cornell.edu
explore the possible evolutionary progress, these individual model
scenarios could represent steps in a terrestrial planet’s evolution
from a hot exo-Earth to an exo-Venus (Fig. 1) (also Walker, 1975;
Kasting, 1988; Yang, Cowan  Abbot 2013; Way et al. 2016;
Kopparapu et al. 2017; Way  Del Genio, 2020). In addition to
the models we created, we also include a modern Venus atmosphere
for comparison using the Venus International Reference Atmosphere
(VIRA; Moroz et al. 1985; Bierson  Zhang, 2020). That modern
Venus atmosphere has not been altered from the one in our own
Solar system and contains clouds at 70 km like Venus. We provide
our models (Fig. A1) and spectra (Fig. 2) online as a tool to plan
observations and assess the environment of this super-Earth at the
inner edge of the HZ. Although the masses of LP890-9c remain to
be measured (Mp  25 M⊕), the discovery team estimated a mass
of Mp 2.5 + 1.8 − 0.8 M⊕ based on the radius–mass relationship
(Chen  Kipping 2016), pointing out that LP890-9c system is the
second most favourable HZ terrestrial planet system for atmospheric
characterization in transmission, following the TRAPPIST-1 system.
While there are many possible atmospheric models for exoplanets
like LP890-9c (see e.g. Turbet et al. 2020; Fauchez et al. 2021 for
a review of models of TRAPPIST-1), here we explore the specific
question of how a rocky, Earth-like planet could evolve at LP890-
9c’s position and if the resulting spectra can be used to distinguish
these scenarios and deepen our understanding of the conditions at
the inner edge of the HZ. Our models show marked differences in
the resulting transmission spectra. Observations of LP890-9c with
JWST could illuminate terrestrial planet environments at the inner
© 2023 The Author(s)
Published by Oxford University Press on behalf of Royal Astronomical Society
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(a)
(b)
Figure 1. LP890-9c model scenarios from Hot Earths to Greenhouse
atmospheres: (a) Net outgoing infrared (FIR) and incident solar flux (FS)
and (b) planetary albedo (Ap) versus surface temperature (TS). Grey lines
indicate the models.
edge of the HZ, and also provide critical input for models to predict
Earth’s future.
2 METHODS
To assess whether different evolutionary stages of a hot rocky planet
generate different transmission spectra, we model seven scenarios for
LP890-9c: Hot Earth-analogues (Hot Earth 1 and 2), moist runaway
greenhouse (Runaway 1 and 2), a full runaway greenhouse/steam
atmosphere (Runaway 3), a CO2-dominated atmosphere (CO2-atm)
and we also include a modern Venus-analogue (modern Venus). Ta-
ble 1 presents relevant stellar and planetary, and Table 2 atmospheric
model parameters.
The model scenarios are motivated by the expected stages and/or
evolution of terrestrial planet at the inner edge of the HZ (Fig. 1): (i)
hot Earth, (ii) moist and full greenhouse state, (iii) CO2 dominated,
and (iv) Venus (see Kasting, 1988). We first set the surface tem-
perature (TS) of the planet, assume an isothermal troposphere, and
connect these to each other with a moist adiabat. Then, we do inverse
calculations that change TS to achieve flux balance (see Kasting,
Whitmire  Reynolds 1993). This method has been used to establish
the limits of the HZ because runaway greenhouse atmospheres are
highly unstable: higher TS increases water vapour concentrations,
which in turn increases the TS. We use this approach to simulate
the evolution of TS as a function of incident irradiation, identify
and model the runaway greenhouse stages. Fig. 1 shows radiation
fluxes, and their derived functions versus TS for the first five LP890-
9c models from hot Earths to greenhouse atmospheres: net flux (a)
outgoing infrared (FIR) and incident solar (FS), (b) planetary albedo
(Ap), the vertical lines indicate our models. FS and FIR are close for all
chosen models and thus may provide temporarily stable conditions.
The two hot Earth scenarios, ‘Hot Earth 1’ and ‘Hot Earth
2’, assume a modern Earth-like atmospheres (Kaltenegger, Lin 
Madden 2020a). We use Exo-Prime2, a well-known one-dimensional
rocky exoplanet atmosphere code (Madden  Kaltenegger, 2020)
that couples a climate–radiative–convective and a photochemistry
model, to a line-by-line radiative transfer code (e.g. Kaltenegger,
Traub  Jucks 2007) to compute the temperature, atmospheric
mixing ratio profiles, and Trident (MacDonald  Lewis 2022) to
generate the transmission spectra of all models. For the Hot Earth
1 scenario, we set the mixing ratios of O2 and CO2 to 0.21 and
3.55 × 10−4
(modern Earth values), respectively, and the surface
pressure to 0.97 bar, scaling the surface pressure with the planet’s
gravity. For the Hot Earth 2 scenario, we reduce atmospheric
CO2 to negligible concentrations, assuming it has been effectively
removed, as expected for hot rocky planet atmospheres with an active
carbonate–silicate cycle. We calculate both atmospheres to 45 km
height and isothermally extended beyond due to limitations of the
models in the hot upper atmosphere (see Kasting, Chen  Kopparapu
2015).
To model moist greenhouse conditions (Runaway 1 and 2), we
follow (Kopparapu et al. 2013) and model the atmospheres with N2,
H2O, and CO2 with the radiative–convective climate code only, where
we assume that the troposphere is fully saturated with water vapour.
Runaway 1 contains 20 per cent CO2 of the dry atmosphere, a similar
level to an Archean Earth. Runaway 2 only contains modern Earth
CO2 levels (10−6
), assuming that CO2 was effectively removed by the
increased water in the atmosphere. We chose the value of 20 per cent
of the dry atmosphere mixing ratio to explore the effect of CO2 on
the moist greenhouse atmospheres and the spectrum on planets at
the inner edge of the HZ. Note that there is no self-consistent limit
on how much CO2 could be in the atmosphere of a terrestrial planet
in a moist greenhouse stage. If all of Earth’s surface CO2, including
that stored in carbonate rocks, were in the atmosphere, the CO2
partial pressure would be ∼60 bar (Walker 1985). The Runaway 3
scenario assumes a steam atmosphere for a planet in a full runaway
greenhouse (see Ingersoll 1969; Rasool  De Bergh 1970), where all
water from the oceans has evaporated into the atmosphere, resulting
in a surface pressure of several hundred bars. The last two scenarios
are exo-Venus models. We model a CO2-dominated atmosphere,
CO2-atm, with water mixing ratio and surface pressure similar to
modern Venus using inverse calculations as described above. The
albedo of 0.53 for the CO2 atmosphere (Fig. 1) is much higher than
for water dominated/steam atmosphere (see also Kopparapu et al.
2013). In addition to our models, we also include a modern Venus
(VIRA) atmosphere for comparison, with modern Venus’ atmosphere
composition and known cloud height (Ignatiev et al. 2009; Pasachoff,
Schneider  Widemann 2011).
For all seven scenarios, we generate high-resolution transmission
spectra from 0.4 to 20 μm with Doppler- and pressure broadening
with several points per line width includes Rayleigh and Mie
scattering and the most spectroscopically relevant molecules from
the HITRAN 2016 line list (Gordon et al. 2017). Clouds that form
close to or on the terminator region can obscure spectral features
below the cloud layer (e.g. Kaltenegger  Traub, 2009; Robinson
et al. 2011; Macdonald  Cowan, 2019). Currently, for hot Earth-
like planets and runaway greenhouse scenarios, we do not have a
self-consistent way to model clouds (see e.g. Jansen et al. 2019).
While cloud formation research for hot rocky planets using global
circulation models (GCMs) is ongoing (e.g. Turbet et al. 2021;
Sergeev et al. 2022), we only indicate the highest cloud layer on
Earth at 12 km and on Venus at 70 km as a dotted line in Fig. 2 as a
guide.
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Figure 2. Model transmission spectra of LP890-9c: (top) Hot exo-Earths, (middle) moist and full runaway greenhouse, and (bottom) exo-Venus models
(CO2-atm and modern Venus) shown at a resolution of 1000 (solid) and 10 000 (light). The highest modern Earth clouds at 12 km and modern Venus clouds at
70 km are indicated as a dashed line.
Table 1. Stellar and planetary parameters.
Name: LP 890-9, TOI 4306, SPECULOOS-2
Spectroscopic and derived properties
Optical/NIR spectral type M6.0 ± 0.5
Teff (K) 2850 ± 75
M (M) 0.118 ± 0.002
R (R) 0.1556 ± 0.0086
L (10−3 L) 1.441 ± 0.038
Age (Gyr) 7.2 + 2.2 − 3.1
Planetary parameters
Orbital period (d) 8.46
Rp(R⊕) 1.367 + 0.055 − 0.039
Stellar irradiation (S⊕) 0.906 ± 0.026
Mp(M⊕) estimate 2.5 + 1.8 − 0.8
3 RESULTS: LP890-9C MODELS AND SPECTRA
Fig. A1 shows the atmospheric profiles and Fig. 2 the transmis-
sion spectra of the seven model scenarios for LP890-9c available
online (https://doi.org/10.5281/zenodo.7063160): (top) hot Earths,
(middle) moist and runaway greenhouse stages, and (bottom) exo-
Venus, summarized in Table 2.
Table 2. Atmosphere model parameters.
Model T (K) PSurf (bar) Surface mixing ratio
CO2 H2O
Hot Earth 1 303 0.97 3.600E−04 1.880E−02
Hot Earth 2 297 0.97 0.000E+00 1.350E−02
Runaway 1 405 3.86 1.780E−02 7.410E−01
Runaway 2 340 1.27 2.950E−04 2.140E−01
Runaway 3 1600 271.00 1.200E−06 9.960E−01
CO2-atm 593 93.00 9.999E−01 9.877E−06
Venus 735 92.10 9.650E−01 1.000E−05
The two Hot Earth scenarios (top) use modern Earth models for
modern and reduced CO2 (Hot Earth 1 and 2, respectively) at the
orbital distance of LP890-9c. Lower atmospheric CO2 results in
slightly lower TS. Major absorption features of H2O, CO2, O2, O3,
CH4, and N2O are labeled. Features of N2O, CH4, and H2O are
distributed throughout the spectrum. For O2 the strongest feature is
found in the visible, for O3 and CO2 in the IR. Assuming an effective
removal of CO2 in the ‘Hot Earth 2’ scenario CO2 absorption
features disappear from the transmission spectrum. Note that the
N2O absorption feature is among the most dominant features in the
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Hot Earths spectra (see also Segura, Kasting  Meadows 2005). The
surface flux of N2O was fixed in the model at modern Earth value; but
the minimal near-ultraviolet (near-UV) radiation from an inactive M
star effectively slows N2O photolysis enough that it can accumulate.
The buildup of N2O only has a minor impact on TS, especially when
compared to H2O and CO2 in a Hot Earth scenario. The two moist
runaway greenhouse scenarios, Runaway 1 and 2 (middle), show
H2O vapour increase with increasing TS, which in turn increases
the total surface pressure (PSurf) as expected for terrestrial planet
atmospheres near the inner edge of the HZ: leading to a PSurf of 1.27
and 3.46 bar for Runaway 2 (20 per cent CO2 of the dry atmosphere)
and Runaway 1 (modern Earth CO2), respectively. The stratospheric
temperature warms with increasing CO2, and stratospheric water
vapour increases with TS (see Leconte et al. 2013; Kasting et al. 2015;
Wolf  Toon, 2015). Runaway 3 shows a full runaway greenhouse
steam atmosphere, with a PSurf of 271 bar, and TS of 1600 K.
The transmission spectra (Fig. 2) show strong H2O features
throughout the wavelength range modelled, with the strongest CO2
feature in the IR. Increasing water vapour concentration in the
Runaway atmospheres increases the transit depth and the effec-
tive height of the individual absorption features, making the full
runaway greenhouse model, Runaway 3, the atmosphere with the
largest effective height and transit depth. Note that there is no
self-consistent modelling of how much CO2 should accumulate
in the atmosphere of a hot terrestrial planet. Thus, detecting the
CO2 features in combination with the H2O features can address
the question of whether CO2 will be effectively removed during
the runaway greenhouse stage. The two CO2-dominated atmosphere
models (bottom) show (i) a CO2-atmosphere (CO2-atm) based on
modern Venus (modelled with N2, H2O, and CO2) that produces a TS
of 599 K for a PSurf of 93 bar, like on modern Venus. We also added
(ii) the VIRA Venus atmosphere for observed Venus atmospheric
conditions (modern Venus) placed on LP890-9c for comparison with
92 bar PSurf and 735 K TS. The biggest difference in the spectra
of the CO2 atmospheres is due to the known cloud-layer at 70 km
for modern Venus, which strongly affects the depth of the spectral
features. The modern Venus spectrum is dominated by clouds as
well as Mie scattering, which flattens out the spectra compared to
the CO2-atm, where we did not include clouds or Mie scattering
due to the unknown height of any clouds that could develop. The
effective height of the planetary atmosphere ranges up to 80 km
for the hot Earths, up to 120 km for the moist greenhouse, up to
250 km for the runaway greenhouse, and up to 100 km for the CO2-
dominated Venus-like atmospheres spectra. Note that the modern
Venus atmosphere shows only a maximum effective height of the
absorption features of about 20 km due to the high cloud deck,
with points out how severely modern Venus-like cloud decks will
limit any characterization of modern Venus-analogues, compared to
atmospheres without cloud coverage at the terminator region. The
transit depth of the LP890-9c models are about 6 600 ppm; the largest
absorption feature depths are about 100 ppm deep. The models with
a higher effective height show bigger transit depth (Rp/Rs)2
.
4 DISCUSSION
How a hot Earth-like planet could evolve into a modern Venus is still
an area of active research with many open questions on critical issues,
such as how fast such a process could happen and what atmospheric
compositions and cloud properties of such evolutionary stages would
be. Our models show a variety of possible atmospheres on such a
path. If time evolution were to be included, assuming an Earth-like
water ocean, most of the water-rich atmospheres would likely have
lost their water due to hydrodynamic escape by this late in the parent
star’s history, based on current age estimates for LP890-9c. For an
Earth-analogue planet, we would expect it to either remain in a hot
Earth stage, where the planet would not lose its water to space or
to have already transitioned to a Venus-like state. Thus, we included
a modern Venus model in our comparison as well. But the initial
water inventory of LP890-9c is unknown. If it started its life as a
mini-Neptune, then water and other volatiles might still be present
despite large losses over time. We also do not know how much
extreme ultraviolet (EUV) energy is, or was, available to drive such
an escape. So, we could only speculate on the time-scale of water loss.
Thus, we have also included models for greenhouse stages because
the evolution through these stages has not yet been observed.
Recently, 3D global climate model (Turbet et al. 2021) simulated
beyond what solar irradiation surface water can no longer condense
on a Young Venus and a Young Earth (see also Leconte et al. 2013;
Kasting et al. 2014; Hamano, Kawahara  Abe 2015; Wolf  Toon
2015). Around a star like the Sun, the planet develops (or remains in)
a runaway greenhouse at incident stellar irradiation larger than 0.95
times modern Earth’s irradiation (S⊕) for a Young Venus (modelled
as a slow rotator of 5833 h) and 0.92 S⊕ for a Young Earth (modelled
as a faster rotator of 24 h). This means stellar irradiation greater
than that should have kept water from condensing on a young rocky
world, prohibiting oceans to form and initiating catastrophic water
loss. Intriguingly, the incident irradiation at LP890-9c’s position,
0.906 S⊕, is below but close to these limits. However, note that the
irradiation of a cooler red star is more effective in heating the surface
of a planet than the irradiation by a sun-analogue star (Kasting et al.
1993). Thus, the limits of ‘no water condensation’ should appear at
lower incident irradiation for cooler stars.
To explore the atmospheres of planets that are synchronously
locked orbiting low-mass stars, 3D GCMs will be required to assess
the effect of the dynamics and of clouds on the atmospheres, which is
not included in 1D models. Several GCM studies (e.g. Wordsworth
et al. 2010; Forget et al. 2013; Yang et al. 2013, 2019; Turbet et al
2021; Quirino et al. 2022) provide insightful results: stabilizing cloud
feedback could expand the inner edge of the HZ to about double the
stellar flux predicted by 1D climate models due to an increasing
planetary albedo due thick water clouds at the sub-stellar point (e.g.
Yang et al. 2013). Or clouds forming at increasing height could lead
to smaller stellar flux at the inner edge of the HZ (e.g. Leconte et al.
2013; Wolf  Toon 2015). These results highlight the importance of
3D GCMs in understanding the varying climate feedbacks for tidally
locked planets circling small host stars and the critical importance
of model intercomparison of GCMs because divergent results persist
(Yang et al. 2019), due to differences in cloud simulation, radiative
transfer, and atmospheric dynamics.
Surface UV levels on M star planets should span a wide range
(e.g. Scalo et al. 2007; Tarter et al. 2007; Shields, Ballard  Johnson
2016; O’Malley-James  Kaltenegger, 2019a), which could require
life to either shelter or evolve mitigation strategies for high UV
levels (e.g. Ramirez  Kaltenegger 2014, Luger  Barnes 2015;
O’Malley-James  Kaltenegger 2018, 2019a, b) influencing its
remote detectability.
5 CONCLUSIONS
The recently discovered transiting Super-Earth LP 890-9c is a key to
the question of how Venus and Earth developed so vastly differently.
LP890-9c receives about 0.9 times the flux of modern Earth, putting
it very close to the inner edge of the Habitable Zone, where models
differ strongly in their prediction of how long Earth-like planets can
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hold on to their water and what environment such planets could
sustain. To address this, we modelled seven scenarios, covering the
possible evolutionary stages of rocky planets at the inner edge of the
HZ for LP890-9c: Our models show marked differences in the climate
and resulting transmission spectra of chemicals that can reveal the
difference between hot exo-Earths, planets in runaway greenhouse
stages, a CO2-dominated, and a modern exo-Venus atmosphere.
Distinguishing these scenarios from the planet’s spectra can provide
critical new insights into how fast terrestrial planets lose their water,
the evolution of rocky planets at the inner edge of the HZ, and
the future evolution of our own planet. LP890-9c provides a rare
opportunity to explore the evolution of terrestrial planets at the inner
edge of the HZ and is a prime target for observations with telescopes
like JWST.
ACKNOWLEDGEMENTS
Special thanks to Jonathan Gomez Barrientos and Ryan MacDonald
for insightful comments. LK and RCP thank the Brinson Foundation.
LD is an F.R.S.-FNRS Postdoctoral Researcher.
DATA AVAILABILITY STATEMENT
The data underlying this article are available at https://doi.org/10.5
281/zenodo.7063160.
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Hot Earth or Young Venus? A nearby transiting rocky planet mystery

  • 1. MNRAS 524, L10–L14 (2023) https://doi.org/10.1093/mnrasl/slad064 Hot Earth or Young Venus? A nearby transiting rocky planet mystery L. Kaltenegger ,1,2‹ R. C. Payne,1,2 Z. Lin ,1,2,3 J. Kasting4 and L. Delrez 5 1Department of Astronomy, Cornell University, 311 Space Sciences Building, Ithaca, NY 14853, USA 2Carl Sagan Institute, Cornell University, 302 Space Sciences Building, Ithaca, NY 14853, USA 3Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139, USA 4Department of Geosciences, Penn State University, 435 K Deike Building, State College, PA 16801, USA 5Astrobiology Research Unit & STAR Institute, University Liege, Allee du 6 Aout 19C, B-4000 Liege, Belgium Accepted 2022 May 24. Received 2022 May 24; in original form 2022 November 28 ABSTRACT Venus and Earth provide astonishingly different views of the evolution of a rocky planet, raising the question of why these two rocky worlds evolved so differently. The recently discovered transiting Super-Earth LP 890-9c (TOI-4306c, SPECULOOS-2c) is a key to the question. It circles a nearby M6V star in 8.46 d. LP890-9c receives similar flux as modern Earth, which puts it very close to the inner edge of the Habitable Zone (HZ), where models differ strongly in their prediction of how long rocky planets can hold onto their water. We model the atmosphere of a hot LP890-9c at the inner edge of the HZ, where the planet could sustain several very different environments. The resulting transmission spectra differ considerably between a hot, wet exo-Earth, a steamy planet caught in a runaway greenhouse, and an exo-Venus. Distinguishing these scenarios from the planet’s spectra will provide critical new insights into the evolution of hot terrestrial planets into exo-Venus. Our model and spectra are available online as a tool to plan observations. They show that observing LP890-9c can provide key insights into the evolution of a rocky planet at the inner edge of the HZ as well as the long-term future of Earth. Key words: methods: numerical – techniques: spectroscopic – planets and satellites: individual: LP∼890-9∼c – planets and satellites: atmospheres – planets and satellites: physical evolution – planets and satellites: terrestrial planets. 1 INTRODUCTION Venus and Earth provide astonishingly different views of the evo- lution of a rocky planet, raising the question of why these two rocky worlds evolved so differently. While the development of Venus and Earth are not accessible, rocky planets at the inner edge of the Habitable Zone (HZ) provide rare insights into the possible paths of climate evolution of hot rocky planets. Delrez et al. (2022) recently announced the detection of the super-Earth LP 890-9c (also SPECULOOS-2c) at the inner edge of the HZ (see e.g. Kasting 1988; Kopparapu et al. 2013, 2017; Ramirez & Kaltenegger 2014; Luger & Barnes 2015) transiting a low-activity nearby M6V star at 32 pc. The planet is part of a two-planet system with the inner planet, LP 890-9b, first detected by TESS (Guerrero et al. 2021) (TOI-4306.01) in a 2.73-d orbit, with incident flux of 4.09 ± 0.12 S⊕, inward of the HZ. However, LP 890-9c, receives 0.906 ± 0.026 S⊕, placing it within both the conservative and empirical HZ (Kaltenegger 2017; Fujii et al. 2018; Schwieterman et al. 2018). This makes LP 890-9c a key to understanding how Venus and Earth evolved. To explore the range of possible atmospheres and assess whether transmission spectra can illuminate that difference, we create seven models for LP890-9c: two hot exo-Earth models based on modern Earth with and without CO2, two moist greenhouse, one runaway greenhouse steam, and one CO2-dominated exo-Venus model based on modern Venus’ surface pressure. We chose those models to E-mail: lk433@cornell.edu, lkaltenegger@astro.cornell.edu explore the possible evolutionary progress, these individual model scenarios could represent steps in a terrestrial planet’s evolution from a hot exo-Earth to an exo-Venus (Fig. 1) (also Walker, 1975; Kasting, 1988; Yang, Cowan Abbot 2013; Way et al. 2016; Kopparapu et al. 2017; Way Del Genio, 2020). In addition to the models we created, we also include a modern Venus atmosphere for comparison using the Venus International Reference Atmosphere (VIRA; Moroz et al. 1985; Bierson Zhang, 2020). That modern Venus atmosphere has not been altered from the one in our own Solar system and contains clouds at 70 km like Venus. We provide our models (Fig. A1) and spectra (Fig. 2) online as a tool to plan observations and assess the environment of this super-Earth at the inner edge of the HZ. Although the masses of LP890-9c remain to be measured (Mp 25 M⊕), the discovery team estimated a mass of Mp 2.5 + 1.8 − 0.8 M⊕ based on the radius–mass relationship (Chen Kipping 2016), pointing out that LP890-9c system is the second most favourable HZ terrestrial planet system for atmospheric characterization in transmission, following the TRAPPIST-1 system. While there are many possible atmospheric models for exoplanets like LP890-9c (see e.g. Turbet et al. 2020; Fauchez et al. 2021 for a review of models of TRAPPIST-1), here we explore the specific question of how a rocky, Earth-like planet could evolve at LP890- 9c’s position and if the resulting spectra can be used to distinguish these scenarios and deepen our understanding of the conditions at the inner edge of the HZ. Our models show marked differences in the resulting transmission spectra. Observations of LP890-9c with JWST could illuminate terrestrial planet environments at the inner © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article/524/1/L10/7198488 by guest on 22 June 2023
  • 2. Hot Earth or Young Venus? A rocky planet mystery L11 MNRASL 524, L10–L14 (2023) (a) (b) Figure 1. LP890-9c model scenarios from Hot Earths to Greenhouse atmospheres: (a) Net outgoing infrared (FIR) and incident solar flux (FS) and (b) planetary albedo (Ap) versus surface temperature (TS). Grey lines indicate the models. edge of the HZ, and also provide critical input for models to predict Earth’s future. 2 METHODS To assess whether different evolutionary stages of a hot rocky planet generate different transmission spectra, we model seven scenarios for LP890-9c: Hot Earth-analogues (Hot Earth 1 and 2), moist runaway greenhouse (Runaway 1 and 2), a full runaway greenhouse/steam atmosphere (Runaway 3), a CO2-dominated atmosphere (CO2-atm) and we also include a modern Venus-analogue (modern Venus). Ta- ble 1 presents relevant stellar and planetary, and Table 2 atmospheric model parameters. The model scenarios are motivated by the expected stages and/or evolution of terrestrial planet at the inner edge of the HZ (Fig. 1): (i) hot Earth, (ii) moist and full greenhouse state, (iii) CO2 dominated, and (iv) Venus (see Kasting, 1988). We first set the surface tem- perature (TS) of the planet, assume an isothermal troposphere, and connect these to each other with a moist adiabat. Then, we do inverse calculations that change TS to achieve flux balance (see Kasting, Whitmire Reynolds 1993). This method has been used to establish the limits of the HZ because runaway greenhouse atmospheres are highly unstable: higher TS increases water vapour concentrations, which in turn increases the TS. We use this approach to simulate the evolution of TS as a function of incident irradiation, identify and model the runaway greenhouse stages. Fig. 1 shows radiation fluxes, and their derived functions versus TS for the first five LP890- 9c models from hot Earths to greenhouse atmospheres: net flux (a) outgoing infrared (FIR) and incident solar (FS), (b) planetary albedo (Ap), the vertical lines indicate our models. FS and FIR are close for all chosen models and thus may provide temporarily stable conditions. The two hot Earth scenarios, ‘Hot Earth 1’ and ‘Hot Earth 2’, assume a modern Earth-like atmospheres (Kaltenegger, Lin Madden 2020a). We use Exo-Prime2, a well-known one-dimensional rocky exoplanet atmosphere code (Madden Kaltenegger, 2020) that couples a climate–radiative–convective and a photochemistry model, to a line-by-line radiative transfer code (e.g. Kaltenegger, Traub Jucks 2007) to compute the temperature, atmospheric mixing ratio profiles, and Trident (MacDonald Lewis 2022) to generate the transmission spectra of all models. For the Hot Earth 1 scenario, we set the mixing ratios of O2 and CO2 to 0.21 and 3.55 × 10−4 (modern Earth values), respectively, and the surface pressure to 0.97 bar, scaling the surface pressure with the planet’s gravity. For the Hot Earth 2 scenario, we reduce atmospheric CO2 to negligible concentrations, assuming it has been effectively removed, as expected for hot rocky planet atmospheres with an active carbonate–silicate cycle. We calculate both atmospheres to 45 km height and isothermally extended beyond due to limitations of the models in the hot upper atmosphere (see Kasting, Chen Kopparapu 2015). To model moist greenhouse conditions (Runaway 1 and 2), we follow (Kopparapu et al. 2013) and model the atmospheres with N2, H2O, and CO2 with the radiative–convective climate code only, where we assume that the troposphere is fully saturated with water vapour. Runaway 1 contains 20 per cent CO2 of the dry atmosphere, a similar level to an Archean Earth. Runaway 2 only contains modern Earth CO2 levels (10−6 ), assuming that CO2 was effectively removed by the increased water in the atmosphere. We chose the value of 20 per cent of the dry atmosphere mixing ratio to explore the effect of CO2 on the moist greenhouse atmospheres and the spectrum on planets at the inner edge of the HZ. Note that there is no self-consistent limit on how much CO2 could be in the atmosphere of a terrestrial planet in a moist greenhouse stage. If all of Earth’s surface CO2, including that stored in carbonate rocks, were in the atmosphere, the CO2 partial pressure would be ∼60 bar (Walker 1985). The Runaway 3 scenario assumes a steam atmosphere for a planet in a full runaway greenhouse (see Ingersoll 1969; Rasool De Bergh 1970), where all water from the oceans has evaporated into the atmosphere, resulting in a surface pressure of several hundred bars. The last two scenarios are exo-Venus models. We model a CO2-dominated atmosphere, CO2-atm, with water mixing ratio and surface pressure similar to modern Venus using inverse calculations as described above. The albedo of 0.53 for the CO2 atmosphere (Fig. 1) is much higher than for water dominated/steam atmosphere (see also Kopparapu et al. 2013). In addition to our models, we also include a modern Venus (VIRA) atmosphere for comparison, with modern Venus’ atmosphere composition and known cloud height (Ignatiev et al. 2009; Pasachoff, Schneider Widemann 2011). For all seven scenarios, we generate high-resolution transmission spectra from 0.4 to 20 μm with Doppler- and pressure broadening with several points per line width includes Rayleigh and Mie scattering and the most spectroscopically relevant molecules from the HITRAN 2016 line list (Gordon et al. 2017). Clouds that form close to or on the terminator region can obscure spectral features below the cloud layer (e.g. Kaltenegger Traub, 2009; Robinson et al. 2011; Macdonald Cowan, 2019). Currently, for hot Earth- like planets and runaway greenhouse scenarios, we do not have a self-consistent way to model clouds (see e.g. Jansen et al. 2019). While cloud formation research for hot rocky planets using global circulation models (GCMs) is ongoing (e.g. Turbet et al. 2021; Sergeev et al. 2022), we only indicate the highest cloud layer on Earth at 12 km and on Venus at 70 km as a dotted line in Fig. 2 as a guide. Downloaded from https://academic.oup.com/mnrasl/article/524/1/L10/7198488 by guest on 22 June 2023
  • 3. L12 L. Kaltenegger et al. MNRASL 524, L10–L14 (2023) Figure 2. Model transmission spectra of LP890-9c: (top) Hot exo-Earths, (middle) moist and full runaway greenhouse, and (bottom) exo-Venus models (CO2-atm and modern Venus) shown at a resolution of 1000 (solid) and 10 000 (light). The highest modern Earth clouds at 12 km and modern Venus clouds at 70 km are indicated as a dashed line. Table 1. Stellar and planetary parameters. Name: LP 890-9, TOI 4306, SPECULOOS-2 Spectroscopic and derived properties Optical/NIR spectral type M6.0 ± 0.5 Teff (K) 2850 ± 75 M (M) 0.118 ± 0.002 R (R) 0.1556 ± 0.0086 L (10−3 L) 1.441 ± 0.038 Age (Gyr) 7.2 + 2.2 − 3.1 Planetary parameters Orbital period (d) 8.46 Rp(R⊕) 1.367 + 0.055 − 0.039 Stellar irradiation (S⊕) 0.906 ± 0.026 Mp(M⊕) estimate 2.5 + 1.8 − 0.8 3 RESULTS: LP890-9C MODELS AND SPECTRA Fig. A1 shows the atmospheric profiles and Fig. 2 the transmis- sion spectra of the seven model scenarios for LP890-9c available online (https://doi.org/10.5281/zenodo.7063160): (top) hot Earths, (middle) moist and runaway greenhouse stages, and (bottom) exo- Venus, summarized in Table 2. Table 2. Atmosphere model parameters. Model T (K) PSurf (bar) Surface mixing ratio CO2 H2O Hot Earth 1 303 0.97 3.600E−04 1.880E−02 Hot Earth 2 297 0.97 0.000E+00 1.350E−02 Runaway 1 405 3.86 1.780E−02 7.410E−01 Runaway 2 340 1.27 2.950E−04 2.140E−01 Runaway 3 1600 271.00 1.200E−06 9.960E−01 CO2-atm 593 93.00 9.999E−01 9.877E−06 Venus 735 92.10 9.650E−01 1.000E−05 The two Hot Earth scenarios (top) use modern Earth models for modern and reduced CO2 (Hot Earth 1 and 2, respectively) at the orbital distance of LP890-9c. Lower atmospheric CO2 results in slightly lower TS. Major absorption features of H2O, CO2, O2, O3, CH4, and N2O are labeled. Features of N2O, CH4, and H2O are distributed throughout the spectrum. For O2 the strongest feature is found in the visible, for O3 and CO2 in the IR. Assuming an effective removal of CO2 in the ‘Hot Earth 2’ scenario CO2 absorption features disappear from the transmission spectrum. Note that the N2O absorption feature is among the most dominant features in the Downloaded from https://academic.oup.com/mnrasl/article/524/1/L10/7198488 by guest on 22 June 2023
  • 4. Hot Earth or Young Venus? A rocky planet mystery L13 MNRASL 524, L10–L14 (2023) Hot Earths spectra (see also Segura, Kasting Meadows 2005). The surface flux of N2O was fixed in the model at modern Earth value; but the minimal near-ultraviolet (near-UV) radiation from an inactive M star effectively slows N2O photolysis enough that it can accumulate. The buildup of N2O only has a minor impact on TS, especially when compared to H2O and CO2 in a Hot Earth scenario. The two moist runaway greenhouse scenarios, Runaway 1 and 2 (middle), show H2O vapour increase with increasing TS, which in turn increases the total surface pressure (PSurf) as expected for terrestrial planet atmospheres near the inner edge of the HZ: leading to a PSurf of 1.27 and 3.46 bar for Runaway 2 (20 per cent CO2 of the dry atmosphere) and Runaway 1 (modern Earth CO2), respectively. The stratospheric temperature warms with increasing CO2, and stratospheric water vapour increases with TS (see Leconte et al. 2013; Kasting et al. 2015; Wolf Toon, 2015). Runaway 3 shows a full runaway greenhouse steam atmosphere, with a PSurf of 271 bar, and TS of 1600 K. The transmission spectra (Fig. 2) show strong H2O features throughout the wavelength range modelled, with the strongest CO2 feature in the IR. Increasing water vapour concentration in the Runaway atmospheres increases the transit depth and the effec- tive height of the individual absorption features, making the full runaway greenhouse model, Runaway 3, the atmosphere with the largest effective height and transit depth. Note that there is no self-consistent modelling of how much CO2 should accumulate in the atmosphere of a hot terrestrial planet. Thus, detecting the CO2 features in combination with the H2O features can address the question of whether CO2 will be effectively removed during the runaway greenhouse stage. The two CO2-dominated atmosphere models (bottom) show (i) a CO2-atmosphere (CO2-atm) based on modern Venus (modelled with N2, H2O, and CO2) that produces a TS of 599 K for a PSurf of 93 bar, like on modern Venus. We also added (ii) the VIRA Venus atmosphere for observed Venus atmospheric conditions (modern Venus) placed on LP890-9c for comparison with 92 bar PSurf and 735 K TS. The biggest difference in the spectra of the CO2 atmospheres is due to the known cloud-layer at 70 km for modern Venus, which strongly affects the depth of the spectral features. The modern Venus spectrum is dominated by clouds as well as Mie scattering, which flattens out the spectra compared to the CO2-atm, where we did not include clouds or Mie scattering due to the unknown height of any clouds that could develop. The effective height of the planetary atmosphere ranges up to 80 km for the hot Earths, up to 120 km for the moist greenhouse, up to 250 km for the runaway greenhouse, and up to 100 km for the CO2- dominated Venus-like atmospheres spectra. Note that the modern Venus atmosphere shows only a maximum effective height of the absorption features of about 20 km due to the high cloud deck, with points out how severely modern Venus-like cloud decks will limit any characterization of modern Venus-analogues, compared to atmospheres without cloud coverage at the terminator region. The transit depth of the LP890-9c models are about 6 600 ppm; the largest absorption feature depths are about 100 ppm deep. The models with a higher effective height show bigger transit depth (Rp/Rs)2 . 4 DISCUSSION How a hot Earth-like planet could evolve into a modern Venus is still an area of active research with many open questions on critical issues, such as how fast such a process could happen and what atmospheric compositions and cloud properties of such evolutionary stages would be. Our models show a variety of possible atmospheres on such a path. If time evolution were to be included, assuming an Earth-like water ocean, most of the water-rich atmospheres would likely have lost their water due to hydrodynamic escape by this late in the parent star’s history, based on current age estimates for LP890-9c. For an Earth-analogue planet, we would expect it to either remain in a hot Earth stage, where the planet would not lose its water to space or to have already transitioned to a Venus-like state. Thus, we included a modern Venus model in our comparison as well. But the initial water inventory of LP890-9c is unknown. If it started its life as a mini-Neptune, then water and other volatiles might still be present despite large losses over time. We also do not know how much extreme ultraviolet (EUV) energy is, or was, available to drive such an escape. So, we could only speculate on the time-scale of water loss. Thus, we have also included models for greenhouse stages because the evolution through these stages has not yet been observed. Recently, 3D global climate model (Turbet et al. 2021) simulated beyond what solar irradiation surface water can no longer condense on a Young Venus and a Young Earth (see also Leconte et al. 2013; Kasting et al. 2014; Hamano, Kawahara Abe 2015; Wolf Toon 2015). Around a star like the Sun, the planet develops (or remains in) a runaway greenhouse at incident stellar irradiation larger than 0.95 times modern Earth’s irradiation (S⊕) for a Young Venus (modelled as a slow rotator of 5833 h) and 0.92 S⊕ for a Young Earth (modelled as a faster rotator of 24 h). This means stellar irradiation greater than that should have kept water from condensing on a young rocky world, prohibiting oceans to form and initiating catastrophic water loss. Intriguingly, the incident irradiation at LP890-9c’s position, 0.906 S⊕, is below but close to these limits. However, note that the irradiation of a cooler red star is more effective in heating the surface of a planet than the irradiation by a sun-analogue star (Kasting et al. 1993). Thus, the limits of ‘no water condensation’ should appear at lower incident irradiation for cooler stars. To explore the atmospheres of planets that are synchronously locked orbiting low-mass stars, 3D GCMs will be required to assess the effect of the dynamics and of clouds on the atmospheres, which is not included in 1D models. Several GCM studies (e.g. Wordsworth et al. 2010; Forget et al. 2013; Yang et al. 2013, 2019; Turbet et al 2021; Quirino et al. 2022) provide insightful results: stabilizing cloud feedback could expand the inner edge of the HZ to about double the stellar flux predicted by 1D climate models due to an increasing planetary albedo due thick water clouds at the sub-stellar point (e.g. Yang et al. 2013). Or clouds forming at increasing height could lead to smaller stellar flux at the inner edge of the HZ (e.g. Leconte et al. 2013; Wolf Toon 2015). These results highlight the importance of 3D GCMs in understanding the varying climate feedbacks for tidally locked planets circling small host stars and the critical importance of model intercomparison of GCMs because divergent results persist (Yang et al. 2019), due to differences in cloud simulation, radiative transfer, and atmospheric dynamics. Surface UV levels on M star planets should span a wide range (e.g. Scalo et al. 2007; Tarter et al. 2007; Shields, Ballard Johnson 2016; O’Malley-James Kaltenegger, 2019a), which could require life to either shelter or evolve mitigation strategies for high UV levels (e.g. Ramirez Kaltenegger 2014, Luger Barnes 2015; O’Malley-James Kaltenegger 2018, 2019a, b) influencing its remote detectability. 5 CONCLUSIONS The recently discovered transiting Super-Earth LP 890-9c is a key to the question of how Venus and Earth developed so vastly differently. LP890-9c receives about 0.9 times the flux of modern Earth, putting it very close to the inner edge of the Habitable Zone, where models differ strongly in their prediction of how long Earth-like planets can Downloaded from https://academic.oup.com/mnrasl/article/524/1/L10/7198488 by guest on 22 June 2023
  • 5. L14 L. Kaltenegger et al. MNRASL 524, L10–L14 (2023) hold on to their water and what environment such planets could sustain. To address this, we modelled seven scenarios, covering the possible evolutionary stages of rocky planets at the inner edge of the HZ for LP890-9c: Our models show marked differences in the climate and resulting transmission spectra of chemicals that can reveal the difference between hot exo-Earths, planets in runaway greenhouse stages, a CO2-dominated, and a modern exo-Venus atmosphere. Distinguishing these scenarios from the planet’s spectra can provide critical new insights into how fast terrestrial planets lose their water, the evolution of rocky planets at the inner edge of the HZ, and the future evolution of our own planet. LP890-9c provides a rare opportunity to explore the evolution of terrestrial planets at the inner edge of the HZ and is a prime target for observations with telescopes like JWST. 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