Here we examine how our knowledge of present day Venus can inform terrestrial exoplanetary science and how exoplanetary science can inform our study of Venus. In a superficial way the contrasts in ...knowledge appear stark. We have been looking at Venus for millennia and studying it via telescopic observations for centuries. Spacecraft observations began with Mariner 2 in 1962 when we confirmed that Venus was a hothouse planet, rather than the tropical paradise science fiction pictured. As long as our level of exploration and understanding of Venus remains far below that of Mars, major questions will endure. On the other hand, exoplanetary science has grown leaps and bounds since the discovery of Pegasus 51b in 1995, not too long after the golden years of Venus spacecraft missions came to an end with the Magellan Mission in 1994. Multi-million to billion dollar/euro exoplanet focused spacecraft missions such as JWST, and its successors will be flown in the coming decades. At the same time, excitement about Venus exploration is blooming again with a number of confirmed and proposed missions in the coming decades from India, Russia, Japan, the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). Here we review what is known and what we may discover tomorrow in complementary studies of Venus and its exoplanetary cousins.
In this work we discuss various selected mission concepts addressing Venus evolution through time. More specifically, we address investigations and payload instrument concepts supporting scientific ...goals and open questions presented in the companion articles of this volume. Also included are their related investigations (observations & modeling) and discussion of which measurements and future data products are needed to better constrain Venus’ atmosphere, climate, surface, interior and habitability evolution through time. A new fleet of Venus missions has been selected, and new mission concepts will continue to be considered for future selections. Missions under development include radar-equipped ESA-led EnVision M5 orbiter mission (European Space Agency
2021
), NASA-JPL’s VERITAS orbiter mission (Smrekar et al.
2022a
), NASA-GSFC’s DAVINCI entry probe/flyby mission (Garvin et al.
2022a
). The data acquired with the VERITAS, DAVINCI, and EnVision from the end of this decade will fundamentally improve our understanding of the planet’s long term history, current activity and evolutionary path. We further describe future mission concepts and measurements beyond the current framework of selected missions, as well as the synergies between these mission concepts, ground-based and space-based observatories and facilities, laboratory measurements, and future algorithmic or modeling activities that pave the way for the development of a Venus program that extends into the 2040s (Wilson et al.
2022
).
The transit method is biased toward short orbital period planets that are interior to their host star's Habitable Zone (HZ). These planets are particularly interesting from the perspective of ...exploring runaway greenhouse scenarios and the possibility of potential Venus analogs. Here, we conduct an analysis of predicted TESS planet yield estimates produced by Huang et al. (2018), as well as the TESS Object of Interest (TOI) list resulting from the observations of sectors 1 - 13 during Cycle 1 of the TESS primary mission. In our analysis we consider potential terrestrial planets that lie within their host star's Venus Zone (Kane et al. 2014). These requirements are then applied to a predicted planetary yield from the TESS primary mission (Huang et al. 2018) and the TOI list, which results in an estimated 259 Venus analogs by the end of the TESS primary mission, and 46 Venus analogs in the TOI list for sectors 1 - 13. We also calculate the estimated transmission spectroscopy signal-to-noise ratio (S/N) for Venus analogs from the predicted yield and TOI list if they were to be observed by the Near-Infrared Imager and Slitless Spectrograph (NIRISS) on the James Webb Space Telescope (JWST), as well as update the S/N cutoff values determined by Kempton et al. (2018). Our findings show that the best estimated Venus analogs and TOI Venus analogs with \(R_{p} < 1.5 \, R_\odot\) have an estimated transmission spectroscopy S/N \(> 40\) while planets with radii \(2 \, R_\oplus < R_p < 4 \, R_\oplus\) can achieve S/N \(> 100\)
The discovery of terrestrial exoplanets is uncovering increasingly diverse architectures. Of particular interest are those systems that contain exoplanets at a variety of star-planet separations, ...allowing direct comparison of exoplanet evolution (comparative planetology). The Kepler-1649 system contains two terrestrial planets similar both in size and insolation flux to Venus and Earth, although their eccentricities remain largely unconstrained. Here we present results of dynamical studies of the system and the potential effects on climate. The eccentricities of the Kepler-1649 system are poorly constrained, and we show that there are dynamically viable regions for further terrestrial planets in between the two known planets for a limited range of eccentricities. We investigate the effect of eccentricity of the outer planet on the dynamics of both planets and show that this results in high-frequency (1000-3000 year) eccentricity oscillations in long-term stable configurations. We calculate the resulting effect of these eccentricity variations on insolation flux and present the results of 3D climate simulations for the habitable zone planet. Our simulations demonstrate that, despite large eccentricity variations, the planet can maintain stable climates with relatively small temperature variations on the substellar hemisphere for a variety of initial climate configurations. Such systems thus provide key opportunities to explore alternative Venus/Earth climate evolution scenarios.
The success of the Transiting Exoplanet Survey Satellite (TESS) mission has led to the discovery of an abundance of Venus Zone (VZ) terrestrial planets that orbit relatively bright host stars. ...Atmospheric observations of these planets play a crucial role in understanding the evolutionary history of terrestrial planets, past habitable states, and the divergence of Venus and Earth climates. The transmission spectrum of a Venus-like exoplanet can be difficult to distinguish from that of an Earth-like exoplanet however, which could severely limit what can be learned from studying exoVenuses. In this work we further investigate differences in transmission between hypothetical exoEarths and exoVenuses, both with varying amounts of atmospheric carbon dioxide (CO\(_2\)). The exoEarths and exoVenuses were modelled assuming they orbit TRAPPIST-1 on the runaway greenhouse boundary. We simulated James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) PRISM transit observations of both sets of planets between 0.6-5.2 \(\mu\)m, and quantified the detectability of major absorption features in their transmission spectra. The exoEarth spectra include several large methane (CH\(_4\)) features that can be detected in as few as 6 transits. The CH\(_4\) feature at 3.4 \(\mu\)m is the optimal for feature for discerning an exoEarth from an exoVenus since it is easily detectable and does not overlap with CO\(_2\) features. The sulfur dioxide (SO\(_2\)) feature at 4.0 \(\mu\)m is the best indicator of an exoVenus, but it is detectable in atmospheres with reduced CO\(_2\) abundance.
The James Webb Space Telescope (JWST) has provided the first opportunity to study the atmospheres of terrestrial exoplanets and estimate their surface conditions. Earth-sized planets around Sun-like ...stars are currently inaccessible with JWST however, and will have to be observed using the next generation of telescopes with direct imaging capabilities. Detecting active volcanism on an Earth-like planet would be particularly valuable as it would provide insight into its interior, and provide context for the commonality of the interior states of Earth and Venus. In this work we used a climate model to simulate four exoEarths over eight years with ongoing large igneous province eruptions with outputs ranging from 1.8-60 Gt of sulfur dioxide. The atmospheric data from the simulations were used to model direct imaging observations between 0.2-2.0 \(\mu\)m, producing reflectance spectra for every month of each exoEarth simulation. We calculated the amount of observation time required to detect each of the major absorption features in the spectra, and identified the most prominent effects that volcanism had on the reflectance spectra. These effects include changes in the size of the O\(_3\), O\(_2\), and H\(_2\)O absorption features, and changes in the slope of the spectrum. Of these changes, we conclude that the most detectable and least ambiguous evidence of volcanism are changes in both O\(_3\) absorption and the slope of the spectrum.
The search for habitable planets has revealed many planets that can vary greatly from an Earth analog environment. These include highly eccentric orbits, giant planets, different bulk densities, ...relatively active stars, and evolved stars. This work catalogs all planets found to reside in the HZ and provides HZ boundaries, orbit characterization, and the potential for spectroscopic follow-up observations. Demographics of the HZ planets are compared with a full catalog of exoplanets. Extreme planets within the HZ are highlighted, and how their unique properties may affect their potential habitability. Kepler-296 f is the most eccentric <2 \(R_\oplus\) planet that spends 100% of its orbit in the HZ. HD 106270 b and HD 38529 c are the most massive planets (<13 \(M_J\)) that orbit within the HZ, and are ideal targets for determining the properties of potential hosts of HZ exomoons. These planets, along with the others highlighted, will serve as special edge-cases to the Earth-based scenario and observations of these targets will help test the resilience of habitability outside the standard model. The most promising observational HZ target that is known to transit is GJ 414 A b. Of the transiting, <2 \(R_\oplus\) HZ planets, LHS 1140 b, TRAPPIST-1 d and K2-3 d are the most favorable. Of the non-transiting HZ planets, HD 102365 b and 55 Cnc f are the most promising, and the best non-transiting candidates that are <2 \(R_\oplus\) are GJ 667 C c, Wolf 1061 c, Ross 508 b, Teegarden's Star b, and Proxima Cen b.
Understanding the physical characteristics of Venus, including its atmosphere, interior, and its evolutionary pathway with respect to Earth, remains a vital component for terrestrial planet evolution ...models and the emergence and/or decline of planetary habitability. A statistical strategy for evaluating the evolutionary pathways of terrestrial planets lies in the atmospheric characterization of exoplanets, where the sample size provides sufficient means for determining required runaway greenhouse conditions. Observations of potential exoVenuses can help confirm hypotheses about Venus' past, as well as the occurrence rate of Venus-like planets in other systems. Additionally, the data from future Venus missions, such as DAVINCI, EnVision, and VERITAS, will provide valuable information regarding Venus, and the study of exoVenuses will be complimentary to these missions. To facilitate studies of exoVenus candidates, we provide a catalog of all confirmed terrestrial planets in the Venus Zone, including transiting and non-transiting cases, and quantify their potential for follow-up observations. We examine the demographics of the exoVenus population with relation to stellar and planetary properties, such as the planetary radius gap. We highlight specific high-priority exoVenus targets for follow-up observations including: TOI-2285 b, LTT 1445 A c, TOI-1266 c, LHS 1140 c, and L98-59 d. We also discuss follow-up observations that may yield further insight into the Venus/Earth divergence in atmospheric properties.
