KEPLER'S FIRST ROCKY PLANET: KEPLER-10b

The frequency of Earth-size planets in the habitable zone (HZ) of Sun-like stars, hereafter η ⊕, is a key parameter to evaluate the yield of nearby Earth analogs that can be detected and characterized by future missions. Yet, this value is poorly constrained as there are no reliable exoplanet candidates in the HZ of Sun-like stars in the Kepler field. Here, we show that extrapolations relying on the population of small (<1.8 R ⊕), short-period (<25 days) planets bias η ⊕ to large values. As the radius distribution at short orbital periods is strongly affected by atmospheric loss, we reevaluate η ⊕ using exoplanets at larger separations. We find that η ⊕ drops considerably, to values of only ∼5%–10%. Observations of young (<100 Myr) clusters can probe short-period sub-Neptunes that still retain most of their envelope mass. As such, they can be used to quantify the contamination of sub-Neptunes to the population of Kepler short-period small planets and aid in more reliable estimates of η ⊕.

The ESA PLATO mission will provide unprecedented data to study the diversity of planets orbiting up to the habitable zone of bright Sun-like stars. PLATO will detect and characterise exoplanets using the transit method combined with ground-based radial velocity measurements, and study the host stars with asteroseismology. PLATO’s core observing sample consists of Sun-like stars of V < 11. For statistical studies, PLATO will also monitor a large sample of Sun-like stars with V < 13 and cool late-type dwarfs with V < 16. To benefit from PLATO’s advanced photometric capabilities, the general community will be invited to submit proposals on complementary science topics in the framework of a guest observer’s programme. The PLATO payload consists of four groups of six cameras each that overlap covering a total field of about 2150 deg2 with four different sensitivities. Two additional cameras will observe the brightest stars (V < 8.5) in two-colours, and will be used as fine guidance sensor. PLATO is the third medium-class mission in ESA’s Cosmic Vision programme, with a planned launch date in 2026. The satellite will operate in an orbit around the second Lagrange point, L2. We will present the status of the mission science definition and performance, and of the satellite and ground-segment developments. 

The Kepler Mission is a space observatory launched in 2009 by NASA to monitor 170 000 stars over a period of four years to determine the frequency of Earth-size and larger planets in and near the habitable zone of Sun-like stars, the size and orbital distributions of these planets, and the types of stars they orbit.Kepler is the tenth in the series of NASA Discovery Program missions that are competitively-selected, PI-directed, medium-cost missions. The Mission concept and various instrument prototypes were developed at the Ames Research Center over a period of 18 years starting in 1983. The development of techniques to do the 10 ppm photometry required for Mission success took years of experimentation, several workshops, and the exploration of many ‘blind alleys’ before the construction of the flight instrument. Beginning in 1992 at the start of the NASA Discovery Program, the Kepler Mission concept was proposed five times before its acceptance for mission development in 2001. During that period, the concept evolved from a photometer in an L2 orbit that monitored 6000 stars in a 50 sq deg field-of-view (FOV) to one that was in a heliocentric orbit that simultaneously monitored 170 000 stars with a 105 sq deg FOV.Analysis of the data to date has detected over 4600 planetary candidates which include several hundred Earth-size planetary candidates, over a thousand confirmed planets, and Earth-size planets in the habitable zone (HZ). These discoveries provide the information required for estimates of the frequency of planets in our galaxy. The Mission results show that most stars have planets, many of these planets are similar in size to the Earth, and that systems with several planets are common. Although planets in the HZ are common, many are substantially larger than Earth.

Detecting and confirming terrestrial planets is incredibly difficult due to their tiny size and mass relative to Sun-like host stars. However, recent instrumental advancements are making the detection of Earth-like exoplanets technologically feasible. For example, Kepler and TESS photometric precision means we can identify Earth-sized candidates (and PLATO in the future will add many long-period candidates to the list), while spectrographs such as ESPRESSO and EXPRES (with an aimed radial velocity precision [RV] near 10 cm s − 1 ) mean we will soon reach the instrumental precision required to confirm Earth-mass planets in the habitable zones of Sun-like stars. However, many astrophysical phenomena on the surfaces of these host stars can imprint signatures on the stellar absorption lines used to detect the Doppler wobble induced by planetary companions. The result is stellar-induced spurious RV shifts that can mask or mimic planet signals. This review provides a brief overview of how stellar surface magnetoconvection and oscillations can impact low-mass planet confirmation and the best-tested strategies to overcome this astrophysical noise. These noise reduction strategies originate from a combination of empirical motivation and a theoretical understanding of the underlying physics. The most recent predications indicate that stellar oscillations for Sun-like stars may be averaged out with tailored exposure times, while granulation may need to be disentangled by inspecting its imprint on the stellar line profile shapes. Overall, the literature suggests that Earth-analog detection should be possible, with the correct observing strategy and sufficient data collection.

We present the detection of five planets--Kepler-62b, c, d, e, and f--of size 1.31, 0.54, 1.95, 1.61 and 1.41 Earth radii (R⊕), orbiting a K2V star at periods of 5.7, 12.4, 18.2, 122.4, and 267.3 days, respectively. The outermost planets, Kepler-62e and -62f, are super-Earth-size (1.25 R⊕ < planet radius ≤ 2.0 R⊕) planets in the habitable zone of their host star, respectively receiving 1.2 ± 0.2 times and 0.41 ± 0.05 times the solar flux at Earth's orbit. Theoretical models of Kepler-62e and -62f for a stellar age of ~7 billion years suggest that both planets could be solid, either with a rocky composition or composed of mostly solid water in their bulk.

The Kepler Mission was designed to measure the frequency of Earth-size planets in the habitable zone of Sun-like stars. A crucial component for recovering the underlying planet population from a sample of detected planets is understanding the completeness of that sample - what fraction of the planets that could have been discovered in a given data set were actually detected. Here we outline the information required to determine the sample completeness, and describe an experiment to address a specific aspect of that question, which is the issue of transit signal recovery. We investigate the extent to which the Kepler pipeline preserves individual transit signals by injecting simulated transits into the pixel-level data, processing the modified pixels through the pipeline, and comparing the measured transit signal-to-noise ratio (SNR) to that expected without perturbation by the pipeline. We inject simulated transit signals across the full focal plane for a set of observations of length 89 days. On average, we find that the SNR of the injected signal is recovered at MS = 0.9973(+/-0.0012)xBS-0.0151(+/-0.0049), where MS is the measured SNR and BS is the baseline, or expected, SNR. The 1{\sigma} width of the distribution around this correlation is +/-2.64%. We discuss the pipeline processes that cause the measured SNR to deviate significantly from the baseline SNR; these are primarily the handling of data adjacent to spacecraft re-pointings and the removal of harmonics prior to the measurement of the SNR. Finally we outline the further work required to characterise the completeness of the Kepler pipeline.

Context. Identifying and characterizing habitable and potentially inhabited worlds is one of the main goals of future exoplanet direct-imaging missions. The number of planets within the habitable zone (HZ) that are accessible to such missions is a key metric to quantify their scientific potential, and it can drive the mission and instrument design. Aims. While previous studies have shown a strong preference for a future mid-infrared nulling interferometer space mission, such as LIFE, to detect planets within the HZ around M dwarfs, we here focus on a more conservative approach toward the concept of habitability and present yield estimates for two stellar samples consisting of nearby (d &lt; 20 pc) Sun-like stars (4800 K ≤ Teff ≤ 6300 K) and nearby FGK-type stars (3940 K ≤ Teff ≤ 7220 K) accessible to such a mission. Methods. Our yield estimates are based on recently derived occurrence rates of rocky planets from the Kepler mission and our LIFE exoplanet observation simulation tool LIFEsim, which includes all main astrophysical noise sources, but no instrumental noise sources as yet. In a Monte Carlo-like approach, we marginalized over 1000 synthetic planet populations simulated around single and wide binary stars from our two samples. We use new occurrence rates for rocky planets that cover the entire HZ around FGK-type stars, marginalize over the uncertainties in the underlying occurrence rate model, present a parameter study investigating the dependence of the planet yield on different instrumental and astrophysical parameters, and estimate the number of detectable HZ planets that might indeed harbor liquid surface water. Results. Depending on a pessimistic or optimistic extrapolation of the Kepler results, we find that during a 2.5-yr search phase, LIFE could detect between ~10–16 (average) or ~5–34 (including 1σ uncertainties) rocky planets (0.5 R⊕ ≤ Rp ≤ 1.5 R⊕) within the optimistic HZ of Sun-like stars and between ~4–6 (average) or ~1–13 (including 1σ uncertainties) exo-Earth candidates (EECs) assuming four collector spacecraft equipped with 2 m mirrors and a conservative instrument throughput of 5%. The error bars are dominated by uncertainties in the underlying planet occurrence rates and the extrapolation of the Kepler results. With D = 3.5 m or 1 m mirrors, the yield Y changes strongly, following approximately Y ∝ D3/2. With the larger sample of FGK-type stars, the yield increases to ~ 16–22 (average) rocky planets within the optimistic HZ and ~5–8 (average) EECs, which corresponds to ~50% of the yield predicted for M dwarfs in LIFE paper I. Furthermore, we find that in addition to the mirror diameter, the yield depends strongly on the total throughput, but only weakly on the exozodiacal dust level and the accessible wavelength range of the mission. Conclusions. When the focus lies entirely on Sun-like stars, larger mirrors (~3 m with 5% total throughput) or a better total throughput (~20% with 2 m mirrors) are required to detect a statistically relevant sample of ~30 rocky planets within the optimistic HZ. When the scope is extended to FGK-type stars, and especially when M dwarfs are included, a significant increase in the number of detectable rocky HZ planets is obtained, which relaxes the requirements on mirror size and total throughput. Observational insight into the habitability of planets orbiting M dwarfs, for example, from the James Webb Space Telescope, is crucial for guiding the target selection and observing sequence optimization for a mission such as LIFE.

The Kepler Mission was designed to measure the frequency of Earth-like planets in the habitable zone of Sun-like stars. A requirement for determining the underlying planet population from a sample of detected planets is understanding the completeness of that sample—what fraction of the planets that could have been discovered in a given data set were actually detected. Here we describe an experiment designed to address a specific aspect of that question, which is the issue of signal throughput efficiency. We investigate the extent to which the Kepler pipeline preserves transit signals by injecting simulated transit signals into the pixel-level data, processing the modified pixels through the pipeline, and measuring their detection statistics. For the single channel that we examine initially, we inject simulated transit signal trains into the pixel time series of each of the 1801 targets for the 89 days that constitute Quarter 3. For the 1680 that behave as expected in the pipeline, on average we find the strength of the injected signal is recovered at 99.6% of the strength of the original signal. Finally we outline the further work required to characterise the completeness of the Kepler pipeline.

Kepler is a discovery-class mission designed to determine the frequency of Earth-radius planets in and near the habitable zone of solar-type stars. A habitable zone of a star is defined as a range of orbits within which a rocky planet can support liquid water on its surface. The most intriguing question driving the search for habitable planets is whether they host life. The aim of this paper is to study the motion of a “test particle” inside the exoplanetary system Kepler-22. This system consists of a sun-like star, Kepler-22, and a terrestrial exoplanet, Kepler-22b. This exoplanet is situated in the habitable zone of its star. Kepler-22b is located about 180 pc from Earth in the constellation of Cygnus. It was discovered by NASA’s Kepler Space Telescope in December 2011 and the planet is about 2.4 times the radius of Earth. Scientists don't yet know if Kepler-22b has a rocky, gaseous or liquid composition. In this study, let’s derive Lagrange points and perform several numerical tests to discover different possible orbits around the star Kepler-22. From many numerical tests performed, it is also possible to found two tadpole orbits around the Lagrange points L4 and L5 and a tadpole orbit around the exoplanet Kepler-22b, which encircles the two Lagrange points L1, and L2. Some of these orbits are found in the habitable zone and others outside. We have also examined the possibility of the existence of an exomoon around the terrestrial exoplanet Kepler-22b. In this case we have considered the mass of this exomoon. The Circular Restricted Three-Body Problem is used in this study. If it is further assumed that the third body (for example a planet, satellite, an asteroid or just a “test particle”) travels in the same plane as the two larger bodies, then there is the Planar Circular Restricted Three-Body Problem

We characterize the occurrence rate of planets, ranging in size from 0.5 to 16 R ⊕, orbiting FGK stars with orbital periods from 0.5 to 500 days. Our analysis is based on results from the “DR25” catalog of planet candidates produced by NASA’s Kepler mission and stellar radii from Gaia “DR2.” We incorporate additional Kepler data products to accurately characterize the efficiency of planets being recognized as “threshold crossing events” by Kepler’s Transiting Planet Search pipeline and labeled as planet candidates by the robovetter. Using a hierarchical Bayesian model, we derive planet occurrence rates for a wide range of planet sizes and orbital periods. For planets with sizes 0.75–1.5 R ⊕ and orbital periods of 237–500 days, we find a rate of planets per FGK star of &lt;0.27 (84.13th percentile). While the true rate of such planets could be lower by a factor of ∼2 (primarily due to potential contamination of planet candidates by false alarms), the upper limits on the occurrence rate of such planets are robust to ∼10%. We recommend that mission concepts aiming to characterize potentially rocky planets in or near the habitable zone of Sun-like stars prepare compelling science programs that would be robust for a true rate in the range f R,P = 0.03–0.40 for 0.75–1.5 R ⊕ planets with orbital periods in 237–500 days, or a differential rate of 0.06–0.76.

The habitable zone (HZ) around a star is typically defined as the region where a rocky planet can maintain liquid water on its surface. That definition is appropriate, because this allows for the possibility that carbon-based, photosynthetic life exists on the planet in sufficient abundance to modify the planet's atmosphere in a way that might be remotely detected. Exactly what conditions are needed, however, to maintain liquid water remains a topic for debate. In the past, modelers have restricted themselves to water-rich planets with CO2 and H2O as the only important greenhouse gases. More recently, some researchers have suggested broadening the definition to include arid, "Dune" planets on the inner edge and planets with captured H2 atmospheres on the outer edge, thereby greatly increasing the HZ width. Such planets could exist, but we demonstrate that an inner edge limit of 0.59 AU or less is physically unrealistic. We further argue that conservative HZ definitions should be used for designing future space-based telescopes, but that optimistic definitions may be useful in interpreting the data from such missions. In terms of effective solar flux, S(eff), the recently recalculated HZ boundaries are: recent Venus--1.78; runaway greenhouse--1.04; moist greenhouse--1.01; maximum greenhouse--0.35; and early Mars--0.32. Based on a combination of different HZ definitions, the frequency of potentially Earth-like planets around late K and M stars observed by Kepler is in the range of 0.4-0.5.

The discovery of many low-mass exoplanets, including several planets within the habitable zone of their host stars, has led to the question of which kind of atmosphere surrounds them. Recent exoplanet detections have revealed the existence of a large population of low-mass planets (<3 M ⊕) with H2-dominated atmospheres that must have been accreted from the protoplanetary disk. As the gas disk usually has an ~10% fraction of helium, we model the possible enrichment of the primordial He fraction in the atmosphere of planets with mass between 0.75 M ⊕ and 3.0 M ⊕ that orbit in the classical habitable zone of Sun-like stars. Depending on the mass accreted by the planet during the gas disk phase and the stellar high-energy flux between ~10 and 120 nm, we find that Earth-like planets with masses between ~0.95 M ⊕ and 1.25 M ⊕ inside the habitable zone of Sun-like stars can end up with He-dominated primordial atmospheres. This finding has important implications for the evolution of Earth-like habitats, as these thick helium-enriched primordial atmospheres can inhibit the habitability of these planets. The upcoming generation of giant telescopes, such as the Extremely Large Telescope, may enable us to observe and explore these atmospheres.

Context. Magnetic activity and surface flows at different scales pertub radial velocity measurements. This affects the detectability of low-mass exoplanets. Aims. In these flows, the effect of supergranulation is not as well characterized as the other flows, and we wish to estimate its effect on the detection of Earth-like planets in the habitable zone of Sun-like stars. Methods. We produced time series of radial velocities due to oscillations, granulation, and supergranulation, and estimated the detection limit for a G2 star and a period of 300 days. We also studied in detail the behavior of the power when the signal of a 1 MEarth planet was superposed on the signal from the stellar flows. Results. We find that the detection rate does not reach 100% except for the supergranulation level we assume, which is still optimistic, and for an excellent sampling. Conclusions. We conclude that with current knowledge, it is a very challenging task to find Earth twins around Sun-like stars with our current capabilities.

&amp;lt;p&amp;gt;The atmospheric evolution of rocky planets is shaped by a range of astrophysical, geophysical, and geochemical processes. Interpreting observations of potentially habitable exoplanets will require an improved understanding of how these competing influences interact on long timescales. In particular, the interpretation of biosignature gases, such as oxygen, is contingent upon understanding the probable redox evolution of lifeless worlds. Here, we develop a generalized model of terrestrial planet atmospheric evolution to anticipate and interpret future observations of habitable worlds. The model connects early magma ocean evolution to subsequent, temperate geochemical cycling. The thermal evolution of the interior, cycling of carbon-hydrogen-oxygen bearing volatiles, surface climate, crustal production, and atmospheric escape are explicitly coupled throughout this evolution. The redox evolution of the atmosphere is controlled by net planetary oxidation via the escape of hydrogen to space, the loss of atmospheric oxygen to the magma ocean, and oxygen consumption via crustal sinks such as outgassing of reduced species, serpentinization reactions, and direct &amp;amp;#8220;dry&amp;amp;#8221; oxidation of fresh crust.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The model can successfully reproduce the atmospheric evolution of a lifeless Earth: it consistently predicts an anoxic atmosphere and temperate surface conditions after 4.5 Gyrs of evolution. This result is insensitive to model uncertainties such as the details of atmospheric escape, mantle convection parameterizations, initial radiogenic inventories, mantle redox, the efficiency of crustal oxygen sinks, and unknown carbon cycle and deep-water cycle parameters. This suggests abundant oxygen is a reliable biosignature for literal Earth twins, defined as Earth-sized planets at 1 AU around sunlike stars with 1-10 Earth oceans and less initial carbon dioxide than water.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;However, if initial volatile inventories are permitted to vary outside these &amp;amp;#8220;Earth-like&amp;amp;#8221; ranges, then dramatically different redox evolution trajectories are permitted. We identify three scenarios whereby Earth-sized planets in the habitable zones of sunlike stars could accumulate oxygen rich atmospheres (0.01 - 10 bar) in the absence of life. Specifically, (i) high initial CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;:H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O endowments, (ii), &amp;gt;50 Earth ocean water inventories, or (iii) extremely volatile poor initial inventories, could all result in oxygen-rich atmospheres after 4.5 Gyrs of evolution. These false positives arise despite the assumption that there is always sufficient non-condensible atmospheric gases, N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, to maintain an effective cold trap. Fortunately, all three oxygen false positive scenarios could potentially be identified by thorough characterization of the planetary context, such as from using time resolved photometry to constrain surface water inventories.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The model also sheds light on the atmospheric evolution of Venus and Venus-like exoplanets. We can successfully recover the modern state of Venus&amp;amp;#8217; atmosphere, including a dense CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-dominated atmosphere with negligible water vapor and molecular oxygen. Moreover, there is a clear dichotomy in the evolutionary scenarios that recover modern Venus conditions, one in which Venus was never habitable and perpetually in runaway greenhouse since formation, and another whereby Venus experienced ~1-2 Gyr of surface habitability with a ~100 m deep ocean. We explore the likelihood of each scenario and suggest future in situ observations that could help discriminate between these two alternative histories.&amp;lt;/p&amp;gt;

Recent observations and analysis of low mass (<10 M ⊕ ), exoplanets have found that rocky planets only have radii up to 1.5-2 R ⊕ . Two general hypotheses exist for the cause of the dichotomy between rocky and gas-enveloped planets (or possible water worlds): either low mass planets do not necessarily form thick atmospheres of a few wt. %, or the thick atmospheres on these planets easily escape driven by x-ray and extreme ultraviolet (XUV) emissions from young parent stars. Here we show that a cutoff between rocky and gas-enveloped planets due to hydrodynamic escape is most likely to occur at a mean radius of 1.76±0.38 (2σ) R ⊕ around Sun-like stars. We examine the limit in rocky planet radii predicted by hydrodynamic escape across a wide range of possible model inputs using 10,000 parameter combinations drawn randomly from plausible parameter ranges. We find a cutoff between rocky and gas-enveloped planets that agrees with the observed cutoff. The large cross-section available for XUV absorption in the extremely distended primitive atmospheres of low mass planets results in complete loss of atmospheres during the ~100 Myr phase of stellar XUV saturation. In contrast, more massive planets have less distended atmospheres and less escape, and so retain thick atmospheres through XUV saturation and then indefinitely as the XUV and escape fluxes drop over time. The agreement between our model and exoplanet data leads us to conclude that hydrodynamic escape plausibly explains the observed upper limit on rocky planet size and few planets (a "valley") in the 1.5-2 R ⊕ range.