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ExoPlanet Topography, Biosignatures, and Artificial Mega-Structures

Seeing oceans, continents, quasi-static weather, and other surface features on exoplanets may allow us to detect and characterize life outside the solar system. The Proxima b planet resides within the stellar habitable zone allowing for liquid water on its surface, and it may be Earth-like. However, even the largest planned telescopes will not be able to resolve its surface features directly. In this paper, we demonstrate an inversion technique to image indirectly exoplanet surfaces using observed unresolved reflected light variations over the course of the exoplanets orbital and axial rotation: ExoPlanet Surface Imaging (EPSI). We show that the reflected light curve contains enough information to detect both longitudinal and latitudinal structures and to map exoplanet surface features. We demonstrate this using examples of Solar system planets and moons as well as simulated planets with Earth-like life and artificial megastructures.

We also describe how it is possible to infer the planet and orbit geometry from light curves. In particular, we show how albedo maps of Proxima b can be successfully reconstructed for tidally locked, resonance, and unlocked axial and orbital rotation. Such albedo maps obtained in different wavelength passbands can provide "photographic" views of distant exoplanets and spectra of surface features, such as vegetation, deserts, ice caps, oceans, etc. (see picture above). We estimate the signal-to-noise ratio necessary for successful inversions and analyse telescope and detector requirements necessary for the first surface images of Proxima b and other nearby exoplanets.

Berdyugina, S.V., Kuhn, J.R. 2017, ApJ, submitted, 23pp.

Space Weathering of Super-Earths

Rocky exoplanets are expected to be eroded by space weather in a similar way as in the solar system. In particular, Mercury is one of the dramatically eroded planets whose material continuously escapes into its exosphere and further into space. This escape is well traced by sodium atoms scattering sunlight. Due to solar wind impact, micrometeorite impacts, photo-stimulated desorption and thermal desorption, sodium atoms are released from surface regolith. Some of these released sodium atoms are escaping from Mercury's gravitational-sphere. They are dragged anti-Sun-ward and form a tail structure. We expect similar phenomena on exoplanets. The hot super-Earth 61 Vir b orbiting a G3V star at only 0.05 au may show a similar structure. Because of its small separation from the star, the sodium release mechanisms may be working more efficiently on hot super-Earths than on Mercury, although the strong gravitational force of Earth-sized or even more massive planets may be keeping sodium atoms from escaping from the planet.

We have simulated space weathering on Mercury (to verify our model) and on 61 Vir b as a representative super-Earth. We have found that sodium atoms can escape from this exoplanet due to stellar wind sputtering and micrometeorite impacts, to form a sodium tail. However, in contrast to Mercury, the tail on this hot super-Earth is strongly aligned with the anti-starward direction because of higher light pressure (see picture above). Our model suggests that 61 Vir b seems to have an exo-base atmosphere like that of Mercury.

Yoneda, M., Berdyugina, S.V., Kuhn, J.R. 2017, AJ, 154, id. 139, 10 pp.

First detection of a strong magnetic field on a Brown Dwarf

We have detected for the first time a strong magnetic field on a brown dwarf exhibiting transient non-thermal radio and optical emission bursts. LSR J1835+3259 is one the brightest representative of this class of objects and is at the border between low-mass stars and brown dwarfs, where the classical stellar chromospheric activity fades away and no strong magnetic fields are expected. We have measured near-infrared polarized and optical emission spectra of LSR J1835+3259 at several aspect angles during its two rotational periods of nearly 3h on two consecutive nights. During the first night, the magnetic field is found to be at least 5.2 kG and cover at least 11% of the dwarf visible hemisphere. This is first time that we can quantitatively associate brown dwarf non-thermal bursts with a few kG surface magnetic field and solve the puzzle of their driving mechanism. The emitting region topology recovered using spectral line profile inversions indicates the presence of hot plasma loops of at least 7000K with a vertical stratification of the sources producing both optical and radio emission. These loops rotate with the dwarf in and out of view causing the emission bursts (see plots).

The 5 kG magnetic field is detected at the base of the loops, while different frequency radio bursts are found to be associated with hydrogen optical emission sources at different heights above the surface where the magnetic field is weakens first to 2.5-2.8 kG at the H-beta height, then to 1.5-1.9 kG at the H-gamma height, and finally to 1.5-1.6 kG further above. On the second night, the emission in these loops faded together with the magnetic signal. During the last 20 min of observations on the second night a new, weaker emission region emerged in the opposite hemisphere with a similar vertical stratification of the emission along the loop and a marginal magnetic signal. It is feasible that these two regions at the opposite longitudes are associated with magnetic poles of the global field, but longer series of observations are needed to confirm this. We conclude that the activity on LSR J1835+3259 is probably driven by an interaction of a large scale magnetic field (active regions or magnetic poles) with small-scale, entangled, wide-spread and rapidly evolving magnetic fields. Their evolution leads to reconnections, flares, aurora-like emission, and radio-bursts modulated by the fast rotation. Our detection provides the first direct observational constraint for a magnetically driven non-thermal emission mechanism and for generation of magnetic fields in fully convective ultra-cool dwarfs. It also paves a path towards magnetic studies of hot Jupiters of similar temperatures. (Berdyugina et al. 2017).

In a follow up study, we confirm the magnetic field measurement from the analysis of polarization signal in the near-IR CrH bands. (Kuzmychov et al. 2017).

Berdyugina, S.V., et al. 2017, ApJ, 847, id. 61, 13 pp.
Kuzmychov, O., et al. 2017, ApJ, 847, id. 60, 16 pp. )

Quiet Sun magnetic fields as Markov chains

Quiet Sun magnetic fields demonstrate a very complex spatial and temporal behavior. Commonly used algorithms to identify and track features of small-scale fields suffer subjective interpretations. We have developed a new approach to study the complex evolution of these fields using statistical methods. It was applied to quiet Sun spectropolarimetric data obtained with the Imaging Magnetograph eXperiment (IMaX) instrument on board of the stratospheric balloon telescope SUNRISE with the angular resolution 0.15"-0.18" and the 33 s cadence. We have shown that the joint probability distribution functions (pdf) of the longitudinal and transverse components of the magnetic field, as well as of the magnetic pressure, satisfy the necessary and sufficient conditions for Markov chains. Thus, we have established that small-scale magnetic fields in the solar photosphere evolve like a memoryless temporal fluctuating quantity.

These plots show results of the Markov property tests. Columns correspond to the longitudinal and transverse components and the magnetic pressure. Rows correspond to two Markov property tests. The magnetic field quantities (abscissa) are shown in relative scale (dimensionless). Circles are chain occurrence probabilities p(b3,b2,b1), and red lines are pdfs. The good agreement between them indicates that both necessary and sufficient conditions for Markov chains are satisfied (Gorobets et al. 206). In a follow up paper, we have shown that small-scale magnetic fields can theoretically evolve to a maximum entropy limit which depends on the field complexity (Gorobets et al. 2017).

Gorobets, A.Y., Borrero, J.M, Berdyugina, S.V. 2016, ApJ Lett., 825, L18, 5pp
Gorobets, A.Y., Berdyugina, S.V., Riethmueller, T.L., et al. 2017, ApJSS, 233, id. 5, 10pp.

Solar corona vector magnetic field

Measuring the coronal vector magnetic field is still a major challenge in solar physics. This is due to the intrinsic weakness of the field (only a few Gauss) and the large thermal broadening of coronal emission lines. We have proposed using concurrent linear polarization measurements of near-infrared forbidden and permitted lines together with Hanle effect models to infer the coronal vector magnetic field. In the unsaturated Hanle regime both the direction and strength of the magnetic field affect the linear polarization, while in the saturated regime the polarization is insensitive to the strength of the field.

The relatively long radiative lifetimes of coronal forbidden atomic transitions implies that the emission lines are formed in the saturated Hanle regime and the linear polarization is insensitive to the strength of the field. By combining measurements of both forbidden and permitted lines, the direction and strength of the field can be obtained. We have developed an algorithm that combines linear polarization measurements of the SiX 1.4301 mcm forbidden line with linear polarization observations of the HeI 1.0830 mcm permitted coronal line to obtain the vector magnetic field. Further development and applications of this method will be a critical step towards interpreting the high spectral, spatial and temporal infrared spectro-polarimetric measurements that will be possible when the Daniel K. Inouye Solar Telescope (DKIST) is completed in 2019.

Dima, G., Kuhn, J.R., Berdyugina, S.V. 2016, Front. Astron. Space Sci., 3, art. 13

Discovery of the unique protoplanet leftover from the inner Solar system

In cooperation with an international team of astronomers, we have have found a unique object that appears to be made of inner Solar System material from the time of Earth's formation, which has been preserved in the Oort Cloud far from the Sun for billions of years. The tailless Manx comet from Oort Cloud C/2014 S3 (PANSTARRS) is the first object to be discovered on a long-period cometary orbit that has the characteristics of a pristine inner Solar System asteroid. It may provide important clues about how the Solar System formed.

This object formed in the inner Solar System at the same time as the Earth itself, but was ejected at a very early stage and preserved in the deep freeze of the Oort Cloud for billions of years. C/2014 S3 (PANSTARRS) was originally identified by the Pan-STARRS1 telescope as a weakly active comet a little over twice as far from the Sun as the Earth. Its current long orbital period (around 860 years) suggests that its source is in the Oort Cloud, and it was nudged comparatively recently into an orbit that brings it closer to the Sun. The team immediately noticed that C/2014 S3 (PANSTARRS) was unusual, as it does not have the characteristic tail that most long-period comets have when they approach so close to the Sun. As a result, it has been dubbed a Manx comet, after the tailless cat. Within weeks of its discovery, the team obtained spectra of the very faint object with ESO's Very Large Telescope in Chile.

Careful study of the light reflected by C/2014 S3 (PANSTARRS) indicates that it is typical of asteroids known as S-type, which are usually found in the inner asteroid main belt. It does not look like a typical comet, which are believed to form in the outer Solar System and are icy, rather than rocky. It appears that the material has undergone very little processing, indicating that it has been deep frozen for a very long time. The very weak comet-like activity associated with C/2014 S3 (PANSTARRS), which is consistent with the sublimation of water ice, is about a million times lower than active long-period comets at a similar distance from the Sun.

We conclude that this object is probably made of fresh inner Solar System material that has been stored in the Oort Cloud and is now making its way back into the inner Solar System. A number of theoretical models are able to reproduce much of the structure we see in the Solar System. An important difference between these models is what they predict about the objects that make up the Oort Cloud. Different models predict significantly different ratios of icy to rocky objects. This first discovery of a rocky object from the Oort Cloud is therefore an important test of the different predictions of the models. We estimate that observations of 50-100 of these Manx comets are needed to distinguish between the current models, opening up another rich vein in the study of the origins of the Solar System.

K.J. Meech, B. Yang, J. Kleyna, O.R. Hainaut, S. Berdyugina, J. V. Keane, M. Micheli, A. Morbidelli, R.J. Wainscoat: Inner solar system material discovered in the Oort cloud, Science Advances, 2, e1600038, 04/2016

Novel biosignatures for remote detection of life

The search for life on other planets is fascinating, challenging, and educative. If successful, it will teach us about ourselves, where we come from, and what our destiny is.

Our team in collaboration with the University of Hawaii (USA) and University of Aarhus (Denmark) have measured various biological photosynthetic pigments in the laboratory. They absorb almost all solar light of specific colors in the visible and convert it into chemical bonds to store energy. For example, chlorophyll pigments absorb blue to red light and reflect a small part of green in the visible, as seen in green plants. (Figure 1). All infrared light is reflected, and this is employed in agriculture to monitor water content in crops. Such biopigments are contained in plants, algae, bacteria, and even in human skin (carotenoids) and eyes (rhodopsin), creating the colored beauty of our world. They can also help find life on the surfaces of other planets.

We have found that the part of visible light reflected by various plants with vibrant colors oscillates in certain directions, while incident light oscillates in all directions (Figure 1). Thanks to this peculiarity, this reflected light can be detected remotely by using polarizing filters (similar to Polaroid sunglasses or 3D movie goggles) when viewed at specific angles even if the star outshines the planet by millions of times. We found that each biopigment has its own colored footprint in such polarized light.

Modeled spectra reflected off distant exo-Earth surfaces have demonstrated the advantage of using polarized light to distinguished photosynthetic biosignatures from minerals, ocean water and the atmosphere. The high contrast of the biosignatures in the polarized light is the key to finding them in the overwhelmingly bright stellar light that usually hides the exoplanetary signals.

This technique can be instrumental in searching for life in the planetary system nearest to the sun, Alpha Centauri, with existing telescopes. There are three stars in this system. While scientists are interested in finding life around all these stars, Alpha Centauri B, only 4.37 light years from Earth, seems optimal for life searches with current telescopes (Figure 2). In 2014, a small planet was discovered around the Alpha Centauri B . Unfortunately, this exoplanet is ten times closer to the star than Mercury is to the sun, so its surface is melting under the stellar heat, and it probably has no atmosphere. At a distance where planets like Earth with liquid water on their surface could exist (the "habitable zone"), no planets have been found as yet, but scientists are continuing to search for one. If such a planet is found, or even before that, it is possible to search for photosynthetic biosignatures in the Alpha Centauri B spectrum. Using the proposed polarization technique, this task becomes even more feasible. For more distant planetary systems telescopes larger than 30m, such as the Colossus, 75m telescope are needed.

Berdyugina S.V., Kuhn, J.R., Harrington, D.M., Santl-Temkiv, T., Messersmith, E.J.: Remote Sensing of Life: Polarimetric Signatures of Photosynthetic Pigments as Sensitive Biomarkers, International Journal for Astrobiology, (2016)

A safe approach to detect extraterrestrial civilizations

Earth-like civilizations generate heat from the energy that they utilize. The thermal radiation from this heat can be a thermodynamic marker for civilizations. We have modeled such planetary radiation on Earth-like planets and propose a strategy for detecting such an alien unintentional thermodynamic electromagnetic biomarker. We show that astronomical infrared (IR) civilization biomarkers may be detected within an interestingly large cosmic volume using a 70 m-class or larger telescope.

In particular, the Colossus telescope with achievable coronagraphic and adaptive optics performance may reveal Earth-like civilizations from visible and IR photometry timeseries taken during an exoplanetary orbit period. The detection of an alien heat signature will have far-ranging implications, but even a null result, given 70 m aperture sensitivity, could also have broad social implications.

Kuhn J.R., Berdyugina S.V.: Global warming as a detectable thermodynamic marker of Earth-like extrasolar civilizations: the case for a telescope like Colossus, International Journal of Astrobiology, vol. 14, pp. 401-410 (2015)