Transit least-squares survey -- III. A 1.9 R⊕1.9\,R_\oplus transit candidate in the habitable zone of Kepler-160 and a nontransiting planet characterized by transit-timing variations

The Sun-like star Kepler-160 (KOI-456) has been known to host two transiting planets, Kepler-160 b and c, of which planet c shows substantial transit-timing variations (TTVs). We used the archival Kepler photometry of Kepler-160 to search for additional transiting planets using a combination of our Wotan detrending algorithm and our transit least-squares (TLS) detection algorithm. We also used the Mercury N-body gravity code to study the orbital dynamics of the system. First, we recovered the known transit series of planets Kepler-160 b and c. Then we found a new transiting candidate with a radius of 1.91 (+0.17, -0.14) Earth radii (R_ear), an orbital period of 378.417 (+0.028, -0.025) d, and Earth-like insolation. The vespa software predicts that this signal has an astrophysical false-positive probability of FPP_3 = 1.8e-3 when the multiplicity of the system is taken into account. Kepler vetting diagnostics yield a multiple event statistic of MES = 10.7, which corresponds to an ~85 % reliability against false alarms due to instrumental artifacts such as rolling bands. We are also able to explain the observed TTVs of planet c with the presence of a previously unknown planet. The period and mass of this new planet, however, do not match the period and mass of the new transit candidate. Our Markov chain Monte Carlo simulations of the TTVs of Kepler-160 c can be conclusively explained by a new nontransiting planet with a mass between about 1 and 100 Earth masses and an orbital period between about 7 and 50 d. We conclude that Kepler-160 has at least three planets, one of which is the nontransiting planet Kepler-160 d. The expected stellar radial velocity amplitude caused by this new planet ranges between about 1 and 20 m/s. We also find the super-Earth-sized transiting planet candidate KOI-456.04 in the habitable zone of this system, which could be the fourth planet.Comment: published in A&A, 15 pages, 11 Figures (7 col, 4 b/w), 2 Table

Exoplanet Research with the Stratospheric Observatory for Infrared Astronomy (SOFIA)

When the Stratospheric Observatory for Infrared Astronomy (SOFIA) was conceived and its first science cases defined, exoplanets had not been detected. Later studies, however, showed that optical and near-infrared photometric and spectrophotometric follow-up observations during planetary transits and eclipses are feasible with SOFIA's instrumentation, in particular with the HIPO-FLITECAM and FPI+ optical and near infrared (NIR) instruments. Additionally, the airborne-based platform SOFIA has a number of unique advantages when compared to other ground- and space-based observatories in this field of research. Here we will outline these theoretical advantages, present some sample science cases and the results of two observations from SOFIA's first five observation cycles -- an observation of the Hot Jupiter HD 189733b with HIPO and an observation of the Super-Earth GJ 1214b with FLIPO and FPI+. Based on these early products available to this science case, we evaluate SOFIA's potential and future perspectives in the field of optical and infrared exoplanet spectrophotometry in the stratosphere.Comment: Invited review chapter, accepted for publication in "Handbook of Exoplanets" edited by H.J. Deeg and J.A. Belmonte, Springer Reference Work

Special cases : moons, rings, comets, trojans

Non-planetary bodies provide valuable insight into our current under- standing of planetary formation and evolution. Although these objects are challeng- ing to detect and characterize, the potential information to be drawn from them has motivated various searches through a number of techniques. Here, we briefly review the current status in the search of moons, rings, comets, and trojans in exoplanet systems and suggest what future discoveries may occur in the near future.Comment: Invited review (status August 2017

GJ 273: On the formation, dynamical evolution, and habitability of a planetary system hosted by an M dwarf at 3.75 parsec

Context. Planets orbiting low-mass stars such as M dwarfs are now considered a cornerstone in the search for life-harbouring planets. GJ 273 is a planetary system orbiting an M dwarf only 3.75 pc away, composed of two confirmed planets, GJ 273b and GJ 273c, and two promising candidates, GJ 273d and GJ 273e. Planet GJ 273b resides in the habitable zone. Currently, due to a lack of observed planetary transits, only the minimum masses of the planets are known: Mb sin ib=2.89 M⊕, Mc sin ic=1.18 M⊕, Md sin id=10.80 M⊕, and Me sin ie=9.30 M⊕. Despite being an interesting system, the GJ 273 planetary system is still poorly studied. Aims. We aim at precisely determine the physical parameters of the individual planets, in particular to break the mass–inclination degeneracy to accurately determine the mass of the planets. Moreover, we present thorough characterisation of planet GJ 273b in terms of its potential habitability. Methods. First, we explored the planetary formation and hydration phases of GJ 273 during the first 100 Myr. Secondly, we analysed the stability of the system by considering both the two- and four-planet configurations. We then performed a comparative analysis between GJ 273 and the Solar System, and searched for regions in GJ 273 which may harbour minor bodies in stable orbits, i.e. main asteroid belt and Kuiper belt analogues. Results. From our set of dynamical studies, we obtain that the four-planet configuration of the system allows us to break the mass– inclination degeneracy. From our modelling results, the masses of the planets are unveiled as: 2:89 ≤ Mb ≤ 3:03 M⊕, 1:18 ≤ Mc ≤ 1:24 M⊕, 10:80 ≤ Md ≤ 11:35 M⊕ and 9:30 ≤ Me ≤ 9:70 M⊕. These results point to a system likely composed of an Earth-mass planet, a super-Earth and two mini-Neptunes. From planetary formation models, we determine that GJ 273b was likely an efficient water captor while GJ 273c is probably a dry planet. We found that the system may have several stable regions where minor bodies might reside. Collectively, these results are used to comprehensively discuss the habitability of GJ 273bSpanish Ministry of Science and Education Ramón y Cajal programme ESP2017-87676-2-2 RYC-2012-09913CONICYT- FONDECYT/Chile Postdoctorado 3180405MIT’s Kavli Institut

Future Exoplanet Research: Science Questions and How to Address Them

Started approximately in the late 1980s, exoplanetology has up to now unveiled the main gross bulk characteristics of planets and planetary systems. In the future it will benefit from more and more large telescopes and advanced space missions. These instruments will dramatically improve their performance in terms of photometric precision, detection speed, multipixel imaging, high-resolution spectroscopy, allowing to go much deeper in the knowledge of planets. Here we outline some science questions which should go beyond these standard improvements and how to address them. Our prejudice is that one is never too speculative: experience shows that the speculative predictions initially not accepted by the community have been confirmed several years later (like spectrophotometry of transits or circumbinary planets).Comment: Invited review, accepte

Extrasolar enigmas: from disintegrating exoplanets to exoasteroids

Thousands of transiting exoplanets have been discovered to date, thanks in great part to the {\em Kepler} space mission. As in all populations, and certainly in the case of exoplanets, one finds unique objects with distinct characteristics. Here we will describe the properties and behaviour of a small group of `disintegrating' exoplanets discovered over the last few years (KIC 12557548b, K2-22b, and others). They evaporate, lose mass unraveling their naked cores, produce spectacular dusty comet-like tails, and feature highly variable asymmetric transits. Apart from these exoplanets, there is observational evidence for even smaller `exo-'objects orbiting other stars: exoasteroids and exocomets. Most probably, such objects are also behind the mystery of Boyajian's star. Ongoing and upcoming space missions such as {\em TESS} and PLATO will hopefully discover more objects of this kind, and a new era of the exploration of small extrasolar systems bodies will be upon us.Comment: Accepted for publication in the book "Reviews in Frontiers of Modern Astrophysics: From Space Debris to Cosmology" (eds Kabath, Jones and Skarka; publisher Springer Nature) funded by the European Union Erasmus+ Strategic Partnership grant "Per Aspera Ad Astra Simul" 2017-1-CZ01-KA203-03556

The TROY project: Searching for co-orbital bodies to known planets: I. Project goals and first results from archival radial velocity

The detection of Earth-like planets, exocomets or Kuiper belts show that the different components found in the solar system should also be present in other planetary systems. Trojans are one of these components and can be considered fossils of the first stages in the life of planetary systems. Their detection in extrasolar systems would open a new scientific window to investigate formation and migration processes. In this context, the main goal of the TROY project is to detect exotrojans for the first time and to measure their occurrence rate (eta-Trojan). In this first paper, we describe the goals and methodology of the project. Additionally, we used archival radial velocity data of 46 planetary systems to place upper limits on the mass of possible trojans and investigate the presence of co-orbital planets down to several tens of Earth masses. We used archival radial velocity data of 46 close-in (P<5 days) transiting planets (without detected companions) with information from high-precision radial velocity instruments. We took advantage of the time of mid-transit and secondary eclipses (when available) to constrain the possible presence of additional objects co-orbiting the star along with the planet. This, together with a good phase coverage, breaks the degeneracy between a trojan planet signature and signals coming from additional planets or underestimated eccentricity. We identify nine systems for which the archival data provide 1-sigma evidence for a mass imbalance between L4 and L5. Two of these systems provide 2-sigma detection, but no significant detection is found among our sample. We also report upper limits to the masses at L4/L5 in all studied systems and discuss the results in the context of previous findings.publishe

Signal preservation of exomoon transits during light curve folding

In the search for moons around extrasolar planets (exomoons), astronomers are confronted with a stunning observation. Although 3400 of the 4500 exoplanets were discovered with the transit method and although there are well over 25 times as many moons than planets known in the Solar System (two of which are larger than Mercury), no exomoon has been discovered to date. In the search for exoplanet transits, stellar light curves are usually phase-folded over a range of trial epochs and periods. This approach, however, is not applicable in a straightforward manner to exomoons. Planet-moon transits either have to be modeled in great detail (including their orbital dynamics, mutual eclipses, etc.), which is computationally expensive, or key simplifications have to be assumed in the modeling. One such simplification is to search for moon transits outside of the planetary transits. The question we address in this report is how much in-transit data of an exomoon remains uncontaminated by the near-simultaneous transits of its host planet. We develop an analytical framework based on the probability density of the sky-projected apparent position of an exomoon relative to its planet and test our results with a numerical planet-moon transit simulator. For exomoons with planet-moon orbital separations similar to the Galilean moons, we find that only a small fraction of their in-transit data is uncontaminated by planetary transits: 14% for Io, 20% for Europa, 42% for Ganymede, and 73% for Callisto. The signal-to-noise ratio (S/N) of an out-of-planetary-transit folding technique is reduced compared to a full photodynamical model to about 38% (Io), 45% (Europa), 65% (Ganymede), and 85% (Callisto), respectively. For the Earth’s Moon, we find an uncontaminated data fraction of typically just 18% and a resulting S/N reduction to 42%. These values are astonishingly small and suggest that the gain in speed for any exomoon transit search algorithm that ignores the planetary in-transit data comes at the heavy price of losing a substantial fraction of what is supposedly a tiny signal in the first place. We conclude that photodynamical modeling of the entire light curve has substantial, and possibly essential, advantages over folding techniques of exomoon transits outside the planetary transits, in particular for small exomoons comparable to those of the Solar System

Deceleration of high-velocity interstellar photon sails into bound orbits at α Centauri

At a distance of about 4.22 lightyears, it would take about 100,000 years for humans to visit our closest stellar neighbor Proxima Centauri using modern chemical thrusters. New technologies are now being developed that involve high-power lasers firing at 1 gram solar sails in near-Earth orbits, accelerating them to 20% the speed of light (c) within minutes. Although such an interstellar probe could reach Proxima 20 years after launch, without propellant to slow it down it would traverse the system within hours. Here we demonstrate how the stellar photon pressures of the stellar triple $\alpha$ Cen A, B, and C (Proxima) can be used together with gravity assists to decelerate incoming solar sails from Earth. The maximum injection speed at $\alpha$ Cen A to park a sail with a mass-to-surface ratio ($\sigma$) similar to graphene (7.6e-4 gram/m$^{-2}$) in orbit around Proxima is about 13,800 km/s (4.6% c), implying travel times from Earth to $\alpha$ Cen A and B of about 95 years and another 46 years (with a residual velocity of 1280 km/s) to Proxima. The size of such a low-$\sigma$ sail required to carry a payload of 10 grams is about 10$^5$ m$^2$ = (316 m)$^2$. Such a sail could use solar photons instead of an expensive laser system to gain interstellar velocities at departure. Photogravitational assists allow visits of three stellar systems and an Earth-sized potentially habitable planet in one shot, promising extremely high scientific yields.Comment: 9 pages, 5 figures (4 colored

Wotan: Comprehensive time-series de-trending in Python

The detection of transiting exoplanets in time-series photometry requires the removal or modeling of instrumental and stellar noise. While instrumental systematics can be reduced using methods such as pixel level decorrelation, removing stellar trends while preserving transit signals proves challenging. As a result of vast archives of light curves from recent transit surveys, there is a strong need for accurate automatic detrending, without human intervention. A large variety of detrending algorithms are in active use, but their comparative performance for transit discovery is unexplored. We benchmark all commonly used detrending methods against hundreds of Kepler, K2, and TESS planets, selected to represent the most difficult cases for systems with small planet-to-star radius ratios. The full parameter range is explored for each method to determine the best choices for planet discovery. We conclude that the ideal method is a time-windowed slider with an iterative robust location estimator based on Tukey's biweight. This method recovers 99% and 94% of the shallowest Kepler and K2 planets, respectively. We include an additional analysis for young stars with extreme variability and conclude they are best treated using a spline-based method with a robust Huber estimator. All stellar detrending methods explored are available for public use in Wōtan, an open-source Python package on GitHub (https://github.com/hippke/wotan)