Target Planet Research Articles

Influence of Orbit and Mass Constraints on Reflected Light Characterization of Directly Imaged Rocky Exoplanets

Survey strategies for upcoming exoplanet direct imaging missions have considered varying assumptions of prior knowledge. Precursor radial velocity surveys could have detected nearby exo-Earths and provided prior orbit and mass constraints. Alternatively, a direct imaging mission performing astrometry could yield constraints on the orbit and phase angle of target planets. Understanding the impact of prior mass and orbit information on planetary characterization is crucial for efficiently recognizing habitable exoplanets. To address this question, we use a reflected-light retrieval tool to infer the atmospheric and bulk properties of directly imaged Earth-analogs while considering varying levels of prior information and signal-to-noise ratio (S/N). Because of the strong correlation between the orbit-related parameters and the planetary radius, prior information on the orbital distance and planetary phase angle yield much tighter constraints on the planetary radius: from Rp=2.95−1.95+2.69R⊕ without prior knowledge, to Rp=1.01−0.19+0.33R⊕ with prior determination of the orbit for S/N = 20 in the visible/near-infrared spectral range, thus allowing size determination from reflected light observations. However, additional knowledge of planet mass does not notably enhance radius ( Rp=0.98−0.14+0.17R⊕ ) or atmospheric characterization. Also, prior knowledge of the mass alone does not yield a tight radius constraint ( Rp=1.64−0.80+1.29R⊕ ) nor improves atmospheric composition inference. By contrast, because of its sensitivity to gas column abundance, detecting a Rayleigh scattering slope or bounding Rayleigh opacity helps to refine gas mixing ratio inferences without requiring prior mass knowledge. Overall, apart from radius determination, increasing the S/N is more beneficial than additional prior observations.

Optimization of Solar-Sail Control When a Vehicle Moves along Cyclic Heliocentric Trajectories

This paper considers the problem of constructing time-optimal trajectories for a spacecraft (SC) with a solar sail. The trajectories under consideration consist of repeated cycles of spacecraft movement to the target heliocentric orbit and back to the initial one. A model of a perfectly reflecting sail is used, which allows using the programs for optimal control of the sail angle, obtained on the basis of the Pontryagin maximum principle. The heliocentric motion is modeled in a flat polar coordinate system, and the spacecraft itself makes cyclic flights between two terrestrial planets along a closed trajectory. A boundary-value problem is formulated, in the solution of which the approach of the spacecraft to the target planet with the equalization of velocities is ensured (the encounter problem). Simulations of four cycles of Earth–Mercury–Earth and Earth–Mars–Earth motion with a characteristic acceleration of the solar sail of 0.25 mm/s2 have been carried out, for which the duration of one cycle is on average 2000 and 2341 days, respectively. Optimal sail-orientation control programs are obtained for a wide range of launch dates, and methods of searching for and choosing the initial values of conjugate variables are shown. The obtained results demonstrate the ability of a spacecraft with a solar sail to implement controlled motion along closed trajectories with a minimum duration of individual Earth–destination planet–Earth flights.

Imaging rotating and orbiting exoplanets with the solar gravitational lens

ABSTRACT We utilize the well-established properties of the solar gravitational lens (SGL) to consider more realistic observational scenarios. Actual exoplanets, which may be the target of an SGL observational campaign, are not stationary. Their appearance also changes in a variety of ways, including changes due to their diurnal rotation and varying illumination due to their orbital motion around their host star. The nature of the SGL is such that imaging with one telescope is accomplished with a cadence of one pixel at a time, with substantial per-pixel integration times. Therefore, capturing a single snapshot of the target planet with a realistically sized telescope is not possible. Instead, the planetary surface must be reconstructed by inverting the combined effect of the SGL’s point-spread function and temporal changes induced by the planetary dynamics. Using the Earth as a stand-in, we demonstrate practical feasibility of this approach, by simulating a dynamical system and then recovering topographic images of acceptable quality. The dynamics-induced temporal variability of the exoplanet represents an added challenge, but even in the presence of such dynamics, use of the SGL for exoplanet imaging remains feasible.

Optimization of a Low-Thrust Heliocentric Trajectory between the Collinear Libration Points of Different Planets

The aim of this study is to optimize a low-thrust interplanetary trajectory using collinear libration points L1 and L2 as the junction points of the geocentric or planetocentric segments of the trajectory with the heliocentric segment. The problem of optimizing the heliocentric segment of perturbed low-thrust interplanetary transfer is considered in the four-body ephemeris model, which includes the Sun, Earth, target planet, and spacecraft. To optimize the trajectories, an indirect approach is used based on Pontryagin’s maximum principles and the continuation method. The possibility of reducing the characteristic velocity in comparison with the estimates obtained through the method of zero sphere of influence is shown.

Featureless transmission spectra of 12 giant exoplanets observed by GTC/OSIRIS

Context. Exoplanet atmospheres are the key to understanding the nature of exoplanets. To this end, transit spectrophotometry provides us opportunities to investigate the physical properties and chemical compositions of exoplanet atmospheres. Aims. We aim to detect potential atmospheric signatures in 12 gaseous giant exoplanets using transit spectrophotometry and we try to constrain their atmospheric properties. Methods. The targets of interest were observed using transit spectrophotometry with the GTC OSIRIS instrument. We estimated the transit parameters and obtained the optical transmission spectra of the target planets using a Bayesian framework. We analyzed the spectral features in the transmission spectra based on atmospheric retrievals. Results. Most of the observed transmission spectra were found to be featureless, with only the spectrum of CoRoT-1b showing strong evidence for atmospheric features. However, in combination with the previously published near-infrared transmission spectrum, we found multiple interpretations for the atmosphere of CoRoT-1b due to the lack of decisive evidence for alkali metals or optical absorbers. Conclusions. Featureless spectra are not necessarily indicative of cloudy atmospheres if they poorly constrain the altitudes of cloud decks. Precise constraints on the models of hazes and clouds strongly depend on the significance of the observed spectral features. Further investigations on these exoplanets, especially CoRoT-1b, are required to confirm the properties of their atmospheres.

Generalization of the Rutherford Formula and Synthesis of Trajectories with Gravity Assist Maneuvers

This article studies the development of new effective methods for designing spacecraft (SC) trajectories for missions using the generalization of the Rutherford formula for the scattering of charged particles proposed by the authors for the case of gravitational scattering. For controlled gravity assist maneuvers of SC, a rule is formulated that makes it possible to purposefully create beams of trajectories with specified properties as a result of a gravity assist maneuver. Modifications of this rule are presented that make it possible to implement efficient and reliable beam recurrent procedures for searching for the ballistic scenarios of interplanetary flights. Of particular importance is the synthesis of sequences of gravity assist maneuvers that provide the given change in the asymptotic velocity of the SC relative to the target planet.

L 363-38 b: A planet newly discovered with ESPRESSO orbiting a nearby M dwarf star

Context. Planets around stars in the solar neighbourhood will be prime targets for characterisation with upcoming large space- and ground-based facilities. Since large-scale exoplanet searches will not be feasible with such telescopes, it is crucial to use currently available data and instruments to find possible target planets before next-generation facilities come online. Aims. We aim to detect new extrasolar planets around stars in the solar neighbourhood via blind radial velocity (RV) searching with ESPRESSO. Our target sample consists of nearby stars (d < 11 pc) with few (<10) or no previous RV measurements. Methods. We used 31 radial velocity measurements obtained with ESPRESSO at the VLT between December 2020 and February 2022 of the nearby M dwarf star (M★ = 0.21 M⊙, d = 10.23 pc) L 363-38 to derive the orbital parameters of the newly discovered planet. In addition, we used TESS photometry and archival VLT/NaCo high-contrast imaging data to put further constraints on the orbit inclination and the possible planetary system architecture around L 363-38. Results. We present the detection of a new extrasolar planet orbiting the nearby M dwarf star L 363-38. L 363-38 b is a planet with a minimum mass of mp sin(i) = 4.67 ± 0.43 M⊕ orbiting its star with a period of P = 8.781 ± 0.007 days, corresponding to a semi-major axis of a = 0.048 ± 0.006 AU, which is smaller than the inner edge of the habitable zone. We further estimate a minimum radius of rp sin(i) ≈ 1.55–2.75 R⊕ and an equilibrium temperature of Teq ≈ 330 K. Conclusions. With this study, we further demonstrate the potential of the state-of-the-art spectrograph ESPRESSO in detecting and investigating planetary systems around nearby M dwarf stars, which were inaccessible to previous instruments such HARPS.

Commentary: Astronomy from space after the JWST

Commentary: Astronomy from space after the <i>JWST</i>

Directed Panspermia Using Interstellar Comets.

It may be that habitable planets are common but life is rare. If future advances in telescopes increasingly suggest this is so, humankind might feel motivated to seed lifeless planets with resilient terrestrial organisms or synthetic forms designed to thrive on the target planet. A useful mechanism for achieving this goal at a relatively low cost is to use interstellar comets transiting the Solar System to convey microbial cargoes toward nearby planetary systems, where they could disseminate the inoculum via their dust trails. Conversely, it is conceivable that terrestrial life was deliberately seeded in this matter, a hypothesis that could be tested if we found evidence for life on other Solar System bodies that displayed common basic biochemical signatures. Our scenario raises a number of ethical and technological challenges that need to be addressed.

Cassini Bistatic Radar Experiments: Preliminary Results on Titan’s Polar Regions

In bistatic radar observations, reflected echoes from the surface of a target planet can be analyzed to infer its surface statistics and near-surface constituents. In this work, a preliminary inspection of two X-band bistatic radar observations gathered by the Cassini spacecraft about Titan’s polar regions is presented. Profiles of relative dielectric constant and root-mean-square (rms) surface slope are provided as outputs of the analysis, discussed, and compared with the present knowledge of Titan geomorphology. For the assessment of the rms slope, proportional to the spectral broadening of reflected echoes, a basic fitting procedure was applied to the received spectra using a Gaussian template, to later evaluate the full-width half-maximum of the fitting curve. The dielectric constant was computed from the power ratio between orthogonally circularly polarized components of signal reflections from Titan. Dielectric constant estimates are, on average, consistent with the expected materials covering the dry surfaces of the planet, while slightly low values were found over the seas. The rms slopes are generally low compared to past bistatic observations of other targets. Titan’s north polar seas are revealed to feature an unprecedented smoothness, with 0.01^circ of slope as an upper bound. Similar values were inferred for isolated spots in the southern pole, hinting at the possible presence of basins filled with liquid hydrocarbons. The main issues with the analysis are emphasized throughout the document, and some ideas for future work are presented in the conclusions.

A Coronagraph with a Sub-λ/D Inner Working Angle and a Moderate Spectral Bandwidth

Future high-contrast imaging spectroscopy with a large segmented telescope will be able to detect atmospheric molecules of Earth-like planets around G- or K-type main-sequence stars. Increasing the number of target planets will require a coronagraph with a small inner working angle (IWA), and wide spectral bandwidth is required if we enhance a variety of detectable atmospheric molecules. To satisfy these requirements, in this paper, we present a coronagraphic system that provides an IWA less than 1λ 0/D over a moderate wavelength band, where λ 0 is the design-center wavelength and D denotes the full width of the rectangular aperture included in the telescope aperture. A performance simulation shows that the proposed system approximately achieves a contrast below 10−10 at 1λ 0/D over the wavelengths of 650–750 nm. In addition, this system has a core throughput ≥10% at input separation angles of ∼0.7–1.4λ 0/D; to reduce telescope time, we need prior information on the target’s orbit by other observational methods to a precision higher than the width of the field of view. For some types of aberration including tilt aberration, the proposed system has a sensitivity less than ever-proposed coronagraphs that have IWAs of approximately 1λ 0/D. In future observations of Earth-like planets, the proposed coronagraphic system may serve as a supplementary coronagraphic system dedicated to achieving an extremely small IWA.

Different perspectives on the b-plane: perturbation effects and use for resonant flyby design

Orbital resonances can be leveraged in the mission design phase to target planets at different energy levels. On the other side, precise models are needed to predict possible threatening returns of natural and artificial objects closely approaching a target planet. To this aim, we propose a semi-analytic extension of the b-plane resonance model to account for perturbing effects inside the planet’s sphere of influence. We compute the actual values of the perturbing coefficients by means of precise numerical simulations, whereas their expression stems from the properties of hyperbolic trajectories and asymptotic planetocentric velocity vectors. We apply the proposed b-plane model to design ballistic resonant flybys by solving a multilevel mixed-integer nonlinear optimization problem.

Radiation tolerance of GaAs1-xSbx solar cells

High radiation tolerance of GaAs1-xSbx based solar cells is demonstrated for the low-intensity-low-temperature (LILT) conditions of the target planets Saturn, Jupiter, and Mars. The GaAs1-xSbx-based cells are irradiated with high energy electrons to assess the effect of harsh radiation environment on the solar cell and the response of the cell is then investigated in terms of its photovoltaic operation. This system shows significant radiation resistance to the high energy electron environment for the conditions of the planets of interest. An unusual increase of the short circuit current after irradiation is observed at low temperature, which is supported by a simultaneous increase in the external quantum efficiency of the cell under the same conditions. The open circuit voltage and fill factor of the cell are especially tolerant to irradiation, which is also reflected in unchanged dark current-voltage characteristics of the solar cell upon irradiation particularly at LILT.

Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus

Abstract During the late stage of terrestrial planet formation, hit-and-run collisions are about as common as accretionary mergers, for expected velocities and angles of giant impacts. Average hit-and-runs leave two major remnants plus debris: the target and impactor, somewhat modified through erosion, escaping at lower relative velocity. Here we continue our study of the dynamical effects of such collisions. We compare the dynamical fates of intact runners that start from hit-and-runs with proto-Venus at 0.7 au and proto-Earth at 1.0 au. We follow the orbital evolutions of the runners, including the other terrestrial planets, Jupiter, and Saturn, in an N-body code. We find that the accretion of these runners can take ≳10 Myr (depending on the egress velocity of the first collision) and can involve successive collisions with the original target planet or with other planets. We treat successive collisions that the runner experiences using surrogate models from machine learning, as in previous work, and evolve subsequent hit-and-runs in a similar fashion. We identify asymmetries in the capture, loss, and interchange of runners in the growth of Venus and Earth. Hit-and-run is a more probable outcome at proto-Venus, being smaller and faster orbiting than proto-Earth. But Venus acts as a sink, eventually accreting most of its runners, assuming typical events, whereas proto-Earth loses about half, many of those continuing to Venus. This leads to a disparity in the style of late-stage accretion that could have led to significant differences in geology, composition, and satellite formation at Earth and Venus.

Life’s Origins and the Search for Life on Rocky Exoplanets

The study of the origin(s) of life on Earth and the search for life on other planets are closely linked. Prebiotic chemical scenarios can help priori-tize target planets for the search for life (as we know it) and can provide informative prior probabilities to help us assess the likelihood that particular spectroscopic features are evidence of life. The prerequisites for origins scenarios themselves predict characteristic spectral signatures. The interplay between origins research and the search for extraterrestrial life starts with laboratory work to guide exploration within our own Solar System, which will then inform future exoplanet observations and laboratory research. Exoplanet research will, in turn, provide statistical context to conclusions about the nature and origins of life.

Directed Panspermia, Wild Animal Suffering, and the Ethics of World‐Creation

ABSTRACTDirected panspermia is the deliberate seeding of lifeless planets with microbes, in the hopes that, over evolutionary timescales, they will give rise to a complex self‐sustaining biosphere on the target planet. Due to the immense distances and timescales involved, human beings are unlikely ever to see the fruits of their labours. Such missions must therefore be justified by appeal to values independent of human wellbeing. In this article, I investigate the values that a directed panspermia mission might promote. Paying special attention to the outcome in which sentient animals evolve, I argue that we have strong reasons to believe the value of a mission would be negative. Research on wild animal suffering has shown that there is a huge amount of suffering among wild animals on Earth. I argue that there are structural features of evolution by natural selection which explain the prevalence of suffering on Earth and make it predictable that suffering would prevail on the target planet too. Finally, using insights from procreative ethics I argue on non‐consequentialist grounds that creators have duties to their sentient creations which cannot be met in directed panspermia missions.

Automatic Scheduling Tool for Balloon-Borne Planetary Optical Remote Sensing

The balloon-borne Planetary Atmosphere Spectroscopic Telescope (PAST), China’s first planetary optical remote-sensing project, will be launched for testing and conducting scientific flights during 2021 and 2022. Images of the planetary atmosphere and plasma in ultraviolet and visible wavelengths will be used to investigate the diversity of the planetary space environment in the solar system and their different drivers. Because simultaneous observation of multiple target planets in the solar system is possible, effective observation scheduling is critical to acquire high scientific merit spectroscopic imaging data. Herein, we demonstrate an automatic scheduling tool (AST) to aid the planning of observation schedules. The AST is primarily based on a planetary ephemeris and is realized on the basis of the geometrical information and optical requirements of the telescope. The temporal variations of the planetary reference frames can also be obtained to assist in the positioning and data processing of the telescope. As a part of the Chinese deep-space exploration plan, several ground-based planetary optical telescopes will be constructed in China in the future. With the use of the proposed AST, such telescopes can achieve maximum efficiency.

Exoplanet imaging performance envelopes for starshade-based missions

We lay out the capabilities and limitations of starshade-based missions aiming to measure the reflected light spectra of temperate planets from an imaging perspective. We use the Starshade Imaging Simulation Toolkit for Exoplanet Reconnaissance to conduct high fidelity end-to-end optical simulations, taking a step forward from simplified analytical equations, exploring and quantifying the impact of an array of observational conditions, including natural parameters such as target star types, planet types, distances, planet phases, and exo-zodiacal dust, and starshade perturbations such as tilt, shift off line of sight, edge errors, and glint. We find that signal-to-noise ratio (SNR) requirements used for establishing detection and spectral characterization, is not suitable under realistic observation conditions for a wide range of targets. We show that even if we assume that the spatially distributed, time-varying background noise could be known and calibrated to a level of 1%, each target star will need its own SNR requirement based on its unique observation conditions, nearly always resulting in a higher threshold SNR, with values as high as X5 from currently established requirement, and in some cases impossible to detect. We conduct statistical analysis using end-to-end optical simulations, taking into account observationally based priors and update previously established completeness values for an array of target stars and mission configurations, accounting for starshade perturbations, and background knowledge at a level of one percent and find that completeness values are negatively impacted and reduced by up to 50% across targets even at ranges shorter than 10 pc. Finally, we utilize information from over hundreds of thousands of detailed imaging simulations to map accessible target stars for both optimistic and pessimistic scenarios, reassessing the expected capabilities of starshade-based high contrast direct imaging missions.

The optimized approach to get the fundamental property of the gravity assists maneuvers from the Jacobi integral

The design of interplanetary trajectories using a series of gravity assists maneuvers begins with the ballistic mission design. It is reasonable to construct the corresponding initial approximation using the patched conics method within the model of the circular restricted three-body problem. Such a construction requires the calculation of the “transfer parameter” Vinf - the dimensionless asymptotic velocity of the spacecraft relative the target planet, when switching from heliocentric arcs to planetocentric segments and vice versa. In the circular restricted three-body problem model, Vinf can be calculated using the Jacobi integral J (or using it’s analogue - the Tisserand parameter TTiss and the basic property of the Jacobi integral for the gravity assists maneuvers within the framework of the circular restricted three body problem: const J ≈ TTISS ≈ 3 – Vinf Vinf . According to this property, the J value does not change during the multiple gravity assists maneuvers that preserve the Jacobi integral constant are performed. This fact is known in astrodynamics but it is classically derived in a rather cumbersome method. In this study, a shorter method for its obtaining is proposed. The modifications of the representation of the Jacobi integral in the circular restricted three-body problem for the various configurations of three bodies are presented.

Occurrence rates of planets orbiting M Stars: applying ABC to Kepler DR25, Gaia DR2, and 2MASS data

ABSTRACT We present robust planet occurrence rates for Kepler planet candidates around M stars for planet radii Rp = 0.5–4 R⊕ and orbital periods P = 0.5–256 d using the approximate Bayesian computation technique. This work incorporates the final Kepler DR25 planet candidate catalogue and data products and augments them with updated stellar properties using Gaia DR2 and 2MASS point source catalogue. We apply a set of selection criteria to select a sample of 1746 Kepler M dwarf targets that host 89 associated planet candidates. These early-type M dwarfs and late K dwarfs were selected from cross-referenced targets using several photometric quality flags from Gaia DR2 and colour–magnitude cuts using 2MASS magnitudes. We estimate a habitable zone occurrence rate of $f_{\textrm {M,HZ}} = 0.33^{+0.10}_{-0.12}$ for planets with 0.75–1.5 R⊕ size. We caution that occurrence rate estimates for Kepler M stars are sensitive to the choice of prior due to the small sample of target stars and planet candidates. For example, we find an occurrence rate of $4.2^{+0.6}_{-0.6}$ or $8.4^{+1.2}_{-1.1}$ planets per M dwarf (integrating over Rp = 0.5–4 R⊕ and P = 0.5–256 d) for our two choices of prior. These occurrence rates are greater than those for FGK dwarf target when compared at the same range of orbital periods, but similar to occurrence rates when computed as a function of equivalent stellar insolation. Combining our result with recent studies of exoplanet architectures indicates that most, and potentially all, early-type M dwarfs harbour planetary systems.