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.
HotMol team has carried out polarimetric measurements of a mysterious star KIC 8462852 discovered by the Kepler mission to show extremely deep and irregular brightness dips. These events were attributed to asteroid-like eclipses or even to possible artificial large-scale space-constructions by an alien civilization. A large international team (including the HotMol team) has carried out photometric and polarimetric monitoring of this star and has detected the first brightness dips since the end of the Kepler space mission in 2013 May.
A sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. The team distinguished four main 1%–2.5% dips, named “Elsie,” “Celeste,” “Skara Brae,” and “Angkor,” which persisted on timescales from several days to weeks. The first funding are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips, (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. Therefore, the new data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale less than 1 μm. The data may also be consistent with models invoking variations intrinsic to the stellar photosphere.
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.
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).
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
GREGOR is a modern 1.5m solar telescope on Tenerife operated by KIS. It is also highly suitable for observing the planets of our solar system. We have built GREGOR Planet Polarimeter (GPP) new instrument to study properties of the planetary atmospheres. GPP allows for high-precision polarisation measurements of the light reflected by the planets. In order to reach a high spatial resolution, the instrument uses the adaptive-optics system (AO) of the telescope. For this purposes, the AO normally used for solar observations had to be extended with an additional wave-front sensor for observing the much fainter objects at night.
In November 2015, polarisation measurements of Uranus in different spectral ranges were recorded. The picture shows three measurements at 450, 550 and 650nm. The first row (I) shows the normal-intensity image of the planet. In the next two rows (Q/I and U/I), images of the linear polarized part of the light are shown. The black and white pattern indicates that part of the light is polarised parallel to the planet’s limb. The planet shows no circular polarised light which is evidenced in the images of the fourth row (V/I). The last row (P) shows the absolute value of the total polarised light. Here one can see that the degree of polarisation increases towards the limb to a maximum value of about 2-3% at these very small phase angles. Furthermore, an increase of the polarisation with shorter wavelengths is visible due to Rayleigh scattering.
These measurements provide important information for a better understanding of the physical processes in the planetary atmosphere and helps us to establish better atmospheric models and develop sensitive techniques for detecting and studying exoplanetary atmospheres.
Center-to-limb variation of intensity and polarization in continuum spectra of FGK stars for spherical atmospheres.
One of the necessary parameters needed for the interpretation of the light curves of transiting exoplanets or eclipsing binary stars, as well as interferometric measurements of a star or microlensing events is how the intensity and polarization of a light change from the center to the limb of a star. Scattering and absorption processes in stellar atmosphere affect both the center-to limb variation of intensity (CLVI) and polarization (CLVP).
The figure presents the center-to-limb variation of continuum polarization considering different contributions of scattering and absorption opacity for different spherical stellar model atmospheres with a range of effective temperatures (4000 – 7000 K), gravities (logg = 1.0 - 5.5) and at wavelength 4000Å. The color scale shows the logarithm of integrated continuum polarization within from 0 to 0.3.
From a sample of stellar model atmospheres we can select three groups with different behavior of maximum polarization depending on surface gravity and effective temperature:
- For sub-giant and dwarf stars (logg = 3.0-4.5), lower gravity and lower effective temperature of a star lead to higher limb polarization of the star.
- For giant and supergiant stars (logg = 1.0-2.5), the highest effective temperature yields the largest polarization. By decreasing of the effective temperature of a star down to 4500-5500K (depending on logg) the limb polarization decreases and reaches a local minimum. It increases again down to temperatures of 4000K.
- For the most compact dwarf stars (logg = 5.0-5.5) the limb polarization degree shows a maximum for models with effective temperatures in the range 4200-4600 K (depending on logg) and decreases toward higher and lower temperatures.
Jupiter’s sodium nebula, which originates from Io’s volcanic gas, shows variations in its brightness due to the volcanic activity on Io. Imaging observation of D-line brightness in the sodium nebula was performed from 2013 through 2015 in a conjunction with the HISAKI mission. The D-line brightness of the sodium nebula had been stably faint and dim until January 2015, but it showed a distinct enhancement from February through March, 2015. The brightness increased by three times during this enhancement
The ground-based observations of Jupiter’ sodium nebula from November 2013 through April 2015 in a conjunction with the HISAKI mission. Although the nebula had been faint and dim until January 2015, it showed a distinct enhancement from the end of January through the end of March, 2015. Year-to-year variations in Jupiter’s sodium nebula have been known by the past study. Moreover, this study captured the temporal scales of both increasing and decreasing phases in brightness of the nebula for the first time by means of high time resolution obser vations. On the other hand, we are not aware of any IR measurements within the same time range.
The event we found in the sodium nebula suggests that a volcanic enhancement occurred on Io, and caused certain changes in Io’s atmosphere and plasma torus, but we do not know how strongly they are correlated with the general mass budgets of S and O in the plasma torus. Na is only a minor constituent while S and O occupy the mass budget mostly in the plasma torus and atmosphere on Io. The detailed influ- ence on the torus or Jupiter’s magnetosphere by this event will be obtained when comparisons between these observations and the HISAKI data are made in future.
Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere.
Balmer line emission extracted from spectra detected with the Hale telescope. This modulation in the optical is correlated with the corresponding radio signal, and can be explained by a propagating electron beam striking the atmosphere, powered by auroral currents (Figure 1).
In an international cooperation Svetlana Berdyugina from the HotMol is involved in a campaign in which simultaneous radio and optical spectroscopic observations were performed of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.
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., Šantl-Temkiv, T., Messersmith, E.J.: Remote Sensing of Life: Polarimetric Signatures of Photosynthetic Pigments as Sensitive Biomarkers, International Journal for Astrobiology, (2015)
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)
Recently, polarimetry has become a promising method in the field of exoplanets, in particular with their detection and characterization. At KIS in the Hotmol group, we investigated how transiting exoplanets might induce some polarimetric effects by breaking the symmetry of the intensity integrated over the stellar disk. More specifically, this study produced models and focused on linear polarization. As these stars are also likely to have stellar spots able to break this symmetry, we took their presence into account in our models.
The figure presents the maximum polarization degree during transits for 88 exoplanetary systems as a function of host star surface gravity. The color scale refers to the effective temperature. The size of each circle designates the planet-to-star radii ratio (in the range from 0.1 to 0.17). The top, middle and bottom plots show the maximum polarization degree at wavelengths of 4000Å, 4500Å and 5000Å, respectively. The left plots were simulated assuming 200 data points per transit and the right plots 10 points per transit. The horizontal dashed line marks the lower limit (3σ) of achievable polarization sensitivity. The names on the plot refer to the most promising targets for polarimetric observations.
The solar photosphere has been intensely studied in the past years using atomic lines, but toshed light on the darkest and coolest parts of sunspots and starspots, the use of molecular lines is favorable because atomic lines receive a strong contribution from outside the spot umbra, whereas some molecular lines can only be formed in cool spots if the effective temperature of the stellar photosphere is high enough. The choice of the adequate molecule therefore represents the key to measuring magnetic fields unambiguously in unresolved spots wherein the molecules are formed. In addition, molecules exhibit high temperature, pressure, and magnetic field sensitivities and provide a unique tool to study physical properties of cool objects and particularly their magnetic fields through phenomena such as the Zeeman and Paschen-Back effects.
The importance of starspots, in addition to studying the underlying magnetic field and its evolution, lies in their significance for the exoplanetary research, where the presence of starspots can affect planetary parameters. Therefore, the properties of starspots – such as magnetic field, temperature, and size have to be monitored. In this analysis we investigate the temperature range in which the molecules TiO, FeH, MgH, and CaH can serve as indicators for magnetic fields on very active cool stars and we present synthetic Stokes profiles for the modeled spectral type. An overall comparison of the obtained signal shows the advantages of different molecules for various spectral type investigations: Maximum Stokes V signal (left panel) and Q signal (right panel) from the different molecular lines for different spectral types for a model star with v sin i = 10 km/s and 10% longitudinal or transverse magnetic field of 3 kG. In the plot, the applied spot temperatures are given, and the photosphere temperatures are implied by the indicated stellar spectral types. Clear differences can be seen in the usefulness of the analyzed molecules for the different spectral types and for different contrasts of the assumed photosphere and spot temperatures.
We also present and discuss a polarimetric effect caused by a planet transiting the stellar disk which breaks its symmetry and results in linear polarization of a partially eclipsed star. Our theoretical study of stellar limb polarization provides first realistic estimates of stellar intrinsic polarization during planet transits and also due to presence of cool spots on the stellar surface.
Kostogryz, N.M., & Berdyugina, S.V.: Center-to-limb polarization in continuum spectra of F, G, K stars, A&A 575, A89 (2015)
We have designed a new instrument BioPol and carried out laboratory spectropolarimetric measurements of a representative sample of plants containing various amounts of pigments such as chlorophyll, carotenoids, and anthocyanins. We have also measured a variety of non-biological samples (sands, rocks). Using our lab measurements, we have modeled intensity and polarized spectra of Earth-like planets having different surface coverage by photosynthetic organisms, deserted land, and ocean, as well as clouds. Our results demonstrate that polarized spectra provide very sensitive and unambiguous detection of photosynthetic organisms of various kinds. Our work paves the path towards analogous measurements of microorganisms and remote sensing of microbial ecology on the Earth and of extraterrestrial life on other planets and moons.
The results are submitted for publication to the International Journal of Astrobiology (Berdyugina et al.).