The KOBE experiment: K-dwarfs Orbited By habitable Exoplanets

We study the Roman sensitivity to exoplanets in the Habitable Zone (HZ). The Roman efficiency for detecting habitable planets is maximized for three classes of planetary microlensing events with close caustic topologies. (a) The events with the lens distances of Dl ≳ 7 kpc, the host lens masses of $M_{\rm h}\gtrsim 0.6\, {\rm M}_{\odot }$. By assuming Jupiter-mass planets in the HZs, these events have q ≲ 0.001 and d ≳ 0.17 (q is their mass ratio and d is the projected planet-host distance on the sky plane normalized to the Einstein radius). The events with primary lenses, $M_{\rm h} \lesssim 0.1 \, {\rm M}_{\odot }$, while their lens systems are either (b) close to the observer with Dl ≲ 1 kpc or (c) close to the Galactic bulge, Dl ≳ 7 kpc. For Jupiter-mass planets in the HZs of the primary lenses, the events in these two classes have q ≳ 0.01, d ≲ 0.04. The events in the class (a) make larger caustics. By simulating planetary microlensing events detectable by Roman, we conclude that the Roman efficiencies for detecting Earth- and Jupiter-mass planets in the Optimistic HZs (OHZs, which is the region between [0.5, 2] au around a Sun-like star) are 0.01 and $5{{\ \rm per\ cent}}$, respectively. If we assume that one exoplanet orbits each microlens in microlensing events detectable by Roman (i.e. ∼27 000), this telescope has the potential to detect 35 exoplanets with the projected planet-host distances in the OHZs with only one having a mass ≲10M⊕. According to the simulation, 27 of these exoplanets are actually in the OHZs.

‘The definitive version is available at www3.interscience.wiley.com Copyright Royal Astronomical Society

With the discovery of rocky planets in the temperate habitable zone (HZ) of the close-by cool star TRAPPIST-1, the question of whether such planets could harbour life arises. Habitable planets around red dwarf stars can orbit in radiation environments that can be life-sterilizing. Ultraviolet (UV) flares from these stars are more frequent and intense than solar flares. Additionally, their temperate HZs are closer to the star. Here we present UV surface environment models for TRAPPIST-1’s HZ planets and explore the implications for life. TRAPPIST-1 has high X-ray/extreme-ultraviolet activity, placing planetary atmospheres at risk from erosion. If a dense Earth-like atmosphere with a protective ozone layer existed on planets in the HZ of TRAPPIST-1, UV surface environments would be similar to the present-day Earth. However, an eroded or an anoxic atmosphere would allow more UV to reach the surface, making surface environments hostile even to highly UV tolerant terrestrial extremophiles. If future observations detect ozone in the atmospheres of any of the planets in the HZ of TRAPPIST-1, these would be interesting targets for the search for surface life. We anticipate our assay to be a starting point for in-depth exploration of stellar and atmospheric observations of the TRAPPIST-1 planets to constrain their UV surface habitability.

Recent studies have revealed that, in addition to insolation, the CO2 degassing rate is an important limiting factor on the resulting climate of Earth-like The CO2 degassing rate should change with time owing to the thermal evolution of planetary interiors. Here we use a mantle degassing model coupled with a parameterized convection model to estimate the climatic evolution of Earth-like planets with different masses and distances from their central stars. The CO2 degassing rate decreases monotonically with time owing to the decreasing mantle temperature. The CO2 degassing rate for large planets is higher than that for small planets because the interiors of large planets are hotter than those of small However, the effects arising from the difference in CO2 degassing rate could be compensated by the difference in the silicate weathering rate derived from the difference in the surface area among planets with different size and with a similar land/sea ratio as the Earth. These results indicate that most of the Earth-like planets in the outer regions of the habitable zones (HZs) of old planetary systems are in the climate mode (i.e., oscillating between and warm states), irrespective of planetary mass. Because the state lasts much longer than the warm climate state, Earth-like planets in HZs around old stars would statistically be observed as snowball planets.

M dwarfs are prime targets in the search for exoplanets due to their prevalence and the enhanced radial velocity (RV) detectability of low-mass planets. Recent advancements in instrumentation have extended RV observations from the optical to the near-infrared (nIR) domain, enabling more effective study of M dwarfs, which frequently host rocky planets. The nIR range offers increased RV sensitivity and potentially reduced stellar activity signals, making it particularly well suited for these stars. We analyzed spectropolarimetric data of the M4.5V star GJ,4274, acquired using the nIR spectropolarimeter SPIRou, to characterize its stellar activity and to search for planetary signals. We additionally examined TESS photometric data to search for transits corresponding to the potential signals in the RV data. In this study, we employed a line-by-line framework to derive and analyze the RV signal of GJ,4274 acquired with SPIRou. From the SPIRou spectra, we also derived and analyzed the longitudinal large scale magnetic field, B_ and the surface temperature variations, dTemp. We combined the SPIRou RV data with RV measurements from CARMENES, to model possible planetary signals with both circular and eccentric orbits. We modeled the activity with a quasiperiodic Gaussian process. We also performed injection-recovery tests on the photometric TESS data to assess transit detectability. We report the discovery of two exoplanets around the nearby M dwarf star GJ,4274. Both planets are in a circular orbit with respective periods of P_b = 1.6339 ± 0.0001 and P_c = 69.6^ d, the latter located beyond the habitable zone. From their RV semi-amplitude signals (K_b = 5.10^ m/s and K_c = 4.11±0.64,m/s), we derive minimum masses of m_b i M_⊕ and m_c i = 8.4 ± 1.3, M_⊕, respectively. Photometric observations from TESS rule out most transit scenarios, including for the inner planet, yielding a maximum inclination of i_b łeq 83^ ̧irc for planet b. We do not detect RV variations related to stellar magnetic activity in the SPIRou time series, despite a prominent large-scale magnetic field with an amplitude of up to ±206^ G. From the activity indicators, we derive a rotation period for the star of $4.600^ $,d. The two reported planets belong to the sub-Neptune class. GJ 4274 b has one of the shortest orbital periods among planets orbiting < 0.2 M_⊙ M dwarfs within 15 pc, while GJ 4274 c lies in a sparsely populated region of the mass–period diagram. Their discovery adds to the census of nearby planetary systems and provides promising targets for future surveys.

NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds ), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows, and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimized for planets near quadrature, but HabWorlds ’ coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet’s orbital inclination and/or the coronagraph’s inner working angle (IWA). We use a recently created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ∼20○ − 60○ would be accessible with HabWorlds for a planet with an Earth equivalent instellation in ∼46 systems, while the ocean glint signature at phase angles of ∼130○ − 170○ would be accessible in ∼16 systems, assuming an IWA = 62 mas (3λ/D). Improving the IWA = 41 mas (2λ/D) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability.

Deriving orbital parameters and masses of extrasolar planets by means of measuring the variation of the host star positions requires an astrometric accuracy of 10 microarcsec. To achieve this goal, a consortium with partners from Germany, the Netherlands, and Switzerland, in agreement with ESO, will enhance the PRIMA facility at the VLTI with Differential Delay Lines (DDLs). We give an overview of the PRIMADDL project, which consists of developing hardware, astrometric operation tools, and data reduction software, and outline the anticipated astrometric planet search program to be carried out with this facility. 1 Narrow-angle Astrometry with the VLTI PRIMA, the instrument for Phase Referenced Imaging and Micro-arcsecond Astrometry is currently being developed at ESO. PRIMA will implement the dual-feed capability at the VLTI for both UTs and ATs to enable simultaneous interferometric observations of two objects that are separated by up to 1 arcmin, without requiring a large continuous field of view. PRIMA will be composed of four major sub-systems: Star Separators, Differential Delay Lines (DDLs), a laser metrology system, and Fringe Sensor Units (FSU). The system is designed to perform high-accuracy (10μas) narrow-angle differential astrometry in K-band with two FSUs and, with one FSU in combination with AMBER or MIDI, phase-referenced aperture synthesis imaging. The purpose of the DDLs in differential astrometry is to increase the astrometric accuracy by separating the large OPD correction terms which are common for the two stars from the small differential terms, and to increase the sensitivity by stabilizing the fringe pattern (in a closed loop with the laser metrology) and thus allow for longer integrations. 2 DDLs and Astrometric Software for PRIMA In order to speed up the full implementation of the 10μas astrometric capability of the VLTI and to carry out a large astrometric planet search program, a consortium lead by the Observatoire de Geneve (Switzerland), the Max Planck Institute for Astronomy in Heidelberg (Germany), and the University of Leiden/NOVA (The Netherlands) agreed with ESO to build and deliver the Differential Delay Lines for PRIMA (see Fig. 1) and to provide all necessary operation 266 Launhardt et al. and software tools to perform narrow-angle astrometry at the 10μas level. This includes developing and building all the DDL hardware, the construction and analysis of an astrometric error budget, the establishment of an operations and calibration strategy, and the development of observation preparation and data reduction software: Fig. 1. The PRIMA-DDL project: breakdown of hard and software components 3 The Astrometric Planet Search Program When completed in 2007, we will use the upgraded PRIMA facility to detect and characterize extra-solar planets through the reflex motions of their host stars in the plane of the sky. Two core programs are planned to be carried out over a duration of at least three years: 1) Observe all stars with known radial-velocity planets that are in reach of the VLTI and have a suitable phase reference star. We will resolve the sin i uncertainty of the planet masses and thus constrain the uncertain upper end of the planetary mass function. For stars with multiple planetary systems we will derive the relative inclination of the orbits. We will follow up long-term radial velocity trends and search for new planets in longer-period orbits for which astrometry is more sensitive than the radial velocity method. 2) Planet search around stars of different mass and evolutionary status without known planets. The search for planets by the radial-velocity technique is restricted to stars with narrow and stable spectral lines, thus excluding A and most F stars with their broad spectral lines as well as pre-main sequence stars. Our astrometric planet search program will explicitly include such stars. For nearby (< 20 pc) late-type (F-M) main sequence stars, the primary new discovery space opened by such an astrometric facility would be Saturn down to Uranus-mass planets with orbital periods of a few years (a = 1–5AU).

The <i>habitable zone</i> of a star is the range of distances from it where temperatures allow the existence of water in its liquid state on the surface of a planet with sufficient atmospheric pressure. This chapter is an overview of a study meant to identify potentially habitable exoplanets using actual data available online from NASA’s Kepler Mission.

Formation of terrestrial planets in Habitable Zones around M stars

Kepler 186f: First Earth-Sized Planet in Habitable Zone

Silicon immersion gratings (SIGs) offer several advantages over the commercial echelle gratings for high resolution infrared (IR) spectroscopy: 3.4 times the gain in dispersion or ~10 times the reduction in the instrument volume, a multiplex gain for a large continuous wavelength coverage and low cost. We present results from lab characterization of a large format SIG of astronomical observation quality. This SIG, with a 54.74 degree blaze angle (R1.4), 16.1 l/mm groove density, and 50x86 mm² grating area, was developed for high resolution IR spectroscopy (R~70,000) in the near IR (1.1-2.5 &mu;m). Its entrance surface was coated with a single layer of silicon nitride antireflection (AR) coating and its grating surface was coated with a thin layer of gold to increase its throughput at 1.1-2.5 m. The lab measurements have shown that the SIG delivered a spectral resolution of R=114,000 at 1.55 m with a lab testing spectrograph with a 20 mm diameter pupil. The measured peak grating efficiency is 72% at 1.55 m, which is consistent with the measurements in the optical wavelengths from the grating surface at the air side. This SIG is being implemented in a new generation cryogenic IR spectrograph, called the Florida IR Silicon immersion grating spectrometer (FIRST), to offer broad-band high resolution IR spectroscopy with R=72,000 at 1.4-1.8 um under a typical seeing condition in a single exposure with a 2kx2k H2RG IR array at the robotically controlled Tennessee State University 2-meter Automatic Spectroscopic Telescope (AST) at Fairborn Observatory in Arizona. FIRST is designed to provide high precision Doppler measurements (~4 m/s) for the identification and characterization of extrasolar planets, especially rocky planets in habitable zones, orbiting low mass M dwarf stars. It will also be used for other high resolution IR spectroscopic observations of such as young stars, brown dwarfs, magnetic fields, star formation and interstellar mediums. An optimally designed SIG of the similar size can be used in the Silicon Immersion Grating Spectrometer (SIGS) to fill the need for high resolution spectroscopy at mid IR to far IR (~25-300 &mu;m) for the NASA SOFIA airborne mission in the future.

The current exoplanet database includes 5454 confirmed and candidate planets observed with the Kepler mission. We find 932 planet pairs from which we extract distance and orbital period ratios. While earlier studies used a logarithmic spacing, which lacks a physical model, we employ here the theory of harmonic orbit resonances, which contains quantized ratios instead, to explain the observed planet distance ratios and to predict undetected exoplanets. We find that the most prevailing harmonic ratios are (2:1), (3:2), and (5:3) in 73% of the cases, while alternative harmonic ratios of (5:4), (4:3), (5:2), and (3:1) occur in the other 27% of the cases. Our orbital predictions include 171 exoplanets, 2 Jupiter moons, 1 Saturn moon, 3 Uranus moons, and 4 Neptune moons. The accuracy of the predicted planet distances amounts to a few percent, which fits the data significantly better than the logarithmic spacing. This information may be useful for targeted exoplanet searches with Kepler data and to estimate the number of live-carrying planets in habitable zones.

Any biosphere emerges, lives, and grows producing entropy. Entropy production is a thermodynamic function crucial in the framework of non-equilibrium thermodynamics as it is directly related to the dynamical behaviour of far-from equilibrium systems. The extent of entropy production is proportional to the ability of such systems to dissipate free energy and thus to ‘live’, to evolve, to grow in complexity. Generally, a certain threshold of entropy production must be exceeded for the emergence of complex self-organizing structures. Thus, the entropy production can be considered as the thermodynamic thrust that drives life emergence and evolution. In this perspective, we propose that the value of the planetary entropy production (PEP) can provide a first order estimate of the thermodynamic potential of planetary environment to sustain a complex biosphere. Here we use a simplified approach to evaluate the upper limit to the PEP and to the corresponding free energy as function of stellar temperature and orbital parameters of the planet. We found that only Earth-like planets in the circumstellar habitable zone (CHZ) of G and F stars can have a PEP value higher than the Earth value. Further significant thermodynamic differences exist between the inner and outer edge of the CHZ, with the inner edge being thermodynamically more advantageous for the development of complex biospheres. Interestingly, among the recently proposed habitable exoplanets, the ones belonging to the Hycean planets appear the thermodynamically best candidates.

Exoplanet habitability is traditionally assessed by comparing a planet's semimajor axis to the location of its host star's "habitable zone," the shell around a star for which Earth-like planets can possess liquid surface water. The Kepler space telescope has discovered numerous planet candidates near the habitable zone, and many more are expected from missions such as K2, TESS, and PLATO. These candidates often require significant follow-up observations for validation, so prioritizing planets for habitability from transit data has become an important aspect of the search for life in the universe. We propose a method to compare transiting planets for their potential to support life based on transit data, stellar properties and previously reported limits on planetary emitted flux. For a planet in radiative equilibrium, the emitted flux increases with eccentricity, but decreases with albedo. As these parameters are often unconstrained, there is an "eccentricity-albedo degeneracy" for the habitability of transiting exoplanets. Our method mitigates this degeneracy, includes a penalty for large-radius planets, uses terrestrial mass–radius relationships, and, when available, constraints on eccentricity to compute a number we call the "habitability index for transiting exoplanets" that represents the relative probability that an exoplanet could support liquid surface water. We calculate it for Kepler objects of interest and find that planets that receive between 60% and 90% of the Earth's incident radiation, assuming circular orbits, are most likely to be habitable. Finally, we make predictions for the upcoming TESS and James Webb Space Telescope missions.

Laser frequency combs are an ideal calibration source for precision astronomical spectrographs. We report on the demonstrated long term operation of a laser frequency comb that we designed and built as the primary calibrator for the Habitable Zone Planet Finder (HPF). The core technology of the comb is based on robust, polarization maintaining fiber coupled electro-optic modulators and broadband supercontinuum generation spanning 700-1600 nm in an efficient silicon nitride waveguide. The comb is continuously maintained on and ready to use, and since May 2018 the laser frequency comb has had a total uptime of 97%.