Astronomers Use Spectroscopy to Find Earth-like Planets

Title: Theoretical Reflectance Spectra of Earth-Like Planets through Their Evolutions:

Astronomers: Yui Kawashima and Sarah Rugheimer

Date: April 3, 2019

Alexis Ifill

April 10, 2019

David Kipping

Impact of Clouds on the Detectability of Oxygen, Water, and Methane with Future Direct Imaging Mission LUVOIR

https://arxiv.org/pdf/1904.01019.pdf

As the search to find life elsewhere in the universe continues, scientists want to know more and more what the atmospheres of Earth-like planets (5 parsecs away) are like. “Earth-like” meaning a similar mass, radius, and orbital period to Earth’s. One way of identifying the molecular make-up of these atmospheres is reflectance spectroscopy. The astronomers in this study have plans to use the visible light and near-infrared telescope LUVOIR, which is intended to be used by 2021. The parameters set for this study are specific to the LUVOIR telescope, which doesn’t exist yet, so the data are relative and averaged in order to assume the overall general effects of cloud properties,  namely altitude and coverage (see Table 2). These cloud properties greatly affect the detectability of important molecules on the reflectance spectra of Earth-like planets. The astronomers did this by creating a model of Earth at different geological epochs. Depending on the altitude of clouds and how much of the planet the clouds cover, they can hinder the detectability of molecules that are vital signs of potential life like O2, H2O, and CH4. These molecules are found in abundance throughout Earth’s history because they are the building blocks of oxygenic photosynthesis. However, this study was focused particularly on the abundance of O2 in Earth’s atmosphere, as it is a promising sign that these Earth-like planets are capable of harvesting life..

(Table 2)

The three points in geological history at which the Earth was examined were: after the Great Oxidation Event (GOE), after the Neoproterozoic Oxidation Event (NOE), and the present day. These points in time were chosen because these are times in which Earth’s atmosphere was considered particularly active: during the GOE, O2 started to build up in the atmosphere that eventually lead to the NOE, which marked the beginning of multicellular life. This is why O2 is considered a target molecule for this study. In other words, the astronomers are looking at Earth during different periods of its evolution in order to have something to compare Earth-like planets to with atmospheres that have different cloud properties.

Questions:

These astronomers tend to answer the question: How long would it take LUVOIR to detect O2 in a GOE atmosphere, an NOE atmosphere, and a present-day atmosphere?

Expectations:

  • We expect to find different atmospheric compositions than that of Earth as we have already detected the existence of hot Jupiters and mini-Neptunes.
  • We expect to find planets with atmospheres composed of no more than about 25% O2. Anything more would increase chances of widespread fires on the planet.
  • We expect to find planets with seemingly featureless surfaces, which would indicate clouds and hazes that can absorb and scatter any light illuminating atmospheric composition.

Methods:

  • The astronomers used a line-by-line radiative transfer model to calculate altitudes of the clouds based on temperature and how disperse they are.
  • The astronomers also considered the impact of noise on the detection of spectra of Earth-like planets. They did this using a coronagraph noise simulator modified to LUVOIR’s parameters.

Results: DETECTABILITY OF O2, H2O, AND CH4 WITH LUVOIR:

CH4: Earth-like planets with GEO-like atmospheres: 10 hours

         Earth-like planets with NEO-like atmospheres: 30 hours

Earth-like planets with present day Earth-like atmospheres: > 6000 hours due to a low abundance in the present-day atmosphere, which is because of O2 destroying most of the methane.

H2O: The astronomers discovered that, generally, LUVOIR can detect H2O within 3-10 hours. Water for GEO and NEO epochs were less abundant because of the high surface temperature of the Earth, which caused evaporation.

CO2: Earth-like planets with GEO-like atmospheres: Up to 600 hours

         Earth-like planets with NEO-like atmospheres: Up to 300 hours

         Earth-like planets with present day Earth-like atmospheres: Up to 100 hours

It seems that when carbon dioxide is more abundant in the atmosphere, it takes longer to fully detect.

Results: INFLUENCE OF CLOUDS ON SPECTRA OF EARTH-LIKE PLANETS

At short wavelengths (more than 9 μm), the atmosphere is seemingly thick with mostly water, causing clouds to sink and have a lower altitude, thus decreasing their albedo. At long wavelengths (less than 9 μm), the atmosphere is thinner and thus a higher altitude, thus increasing their albedo. Despite this however, both wavelengths highlight a similar spectra, thus the astronomers concluded that altitude of clouds negligibly affected how well LUVOIR can detect molecules in the atmospheres.

The black line represents a clear sky atmosphere in present-day Earth. The red, green, and blue lines are different altitudes. We can see the the output of flux at any given wavelength for all of the altitudes are fairly similar.

The astronomers confirmed that detectability does vary when cloud coverage is concerned, but not by that much.

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Gone with the Wind: examining the dynamics and survival of fractal clouds in galactic winds in varying regimes of turbulence.

Title: On the dynamics and survival of fractal clouds in galactic winds

Authors: W. E. Banda-Barragán, F. J. Zertuche, C. Federrath, J. García Del Valle, M. Bruggen, and A. Y. Wagner

First Author’s Institutions: Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany and Facultad de Ingeniería Civil y Mecánica, Universidad Técnica de Ambato, Av. Los Chasquis y Río Payamino S/N, Ambato 180206, Ecuador

Publication Status: arXiv.org e-Print archive, 11th April 2019 open access, accepted for publication in MNRAS

Galactic Winds: an overview

Galactic winds are streams of high velocity, highly charged particles mixed with varying amounts of hot and cool gas that are observed blowing out of galaxies at speeds of between 300 and 3000 km/s. Galactic winds are phenomena that originate from starbursts from newly formed massive stars supernova explosions. The other potential source of energy for these winds is active galactic nuclei, otherwise known as supermassive black holes from the centre of most galaxies. Multi-wavelength observations of star-forming galaxies reveal that galactic winds, driven by stellar feedback are large-scale, multi-phase outflows comprised of several gases, das and cosmic ray components. Within the gas component, galactic winds have a hot ionised phase that typically moves at speeds of 500-1500 km/s with temperatures of around 10⁷ Kelvin (K). They also have a cold atomic/molecular phase that is significantly colder around 10² – 10⁴ K and moves significantly more slowly at 50-300 km/s. These galactic winds are thought to have played several important roles in the evolution of galaxies, including but not limited to redistributing metals throughout the galaxy, regulating start formation and influencing the structure of galactic disks.

Towards a new parameter space

However, galactic winds are still poorly understood and have commanded a wide field of research around them recently. At the moment, the main problem in the study of galactic winds is understanding how dense gas in the cold phase and survives in the hot outflow and how it reaches 100-1500 pc above and below galactic planes. There are two main theories in the study of galactic winds right now. The first proposes that there is a momentum-drive acceleration that acts as a mechanism to transport clouds from low to high latitudes. Clouds near the galactic plane advance from low to high latitudes by either thermal-gas ram pressure, radiation pressure or cosmic ray pressure. The main problem with this theory, however, is that there is not yet an effective explanation of how dense clouds survive the disruptive effects of pressure gradients and dynamical instabilities to be become entrained in the wind. The second proposes that thermal  instabilities act as the trigger for the in situ formation of clouds at high latitudes

This paper mainly deals with the former theory by studying clouds that are being ram-pressure accelerated by supersonic winds using hydrodynamical simulations. Uniquely this study uses  Uniquely this study uses turbulent fractal density profiles are considered for the initial cloud set ups – a parameter space which has been unexplored by a number of other recent studies on fractal clouds in galactic winds. Moreover, this parameter in galactic wind-fractal cloud models with self-consistent magnetic fields and turbulent clouds is much less explored by its peers in the field. Once both magnetic fields and turbulence have been included it has been shown to significantly affect morphology, dynamics and survival of windswept clouds.  

Thus, by varying the initial density in a probability density function (PDF) between two extremes of turbulence from solenoidal to compressive, this paper aims to show the effects of this variation on turbulent fractal clouds. Through the use of this methodology the study, the paper isolated the impact of changes to the initial statistical parameters of the density field on morphology, dynamics and survival of fractal clouds in galactic winds.

Research Methodology

In order to run the simulations necessary for the research, the researches used PLUTO code for Astrophysical GasDynamics to created 2D and 3D models in the Cartesian coordinate system. Using the Harten-Lax-van Leer-Contact Riemann Solver of Toro with the Courant-Friedrichs-Lewy number of Ca  = 0.3, mass, momentum and energy conservation laws of ideal hydrodynamics can be solved for as follows:

  1. ∂ ρ/∂t + ∇ · [ρv] = 0
  2. ∂ [ρv] /∂t + ∇ · [ρvv + IP] = 0
  3. ∂E /∂t + ∇ · [(E + P) v] = 0
  4. ∂/[ρC] ∂t + ∇ · [ρCv] = 0

Where  ρ = mass density,  v = velocity, P = (γ − 1) ρɛ = gas thermal pressure, E = ρɛ + 0.5 ρv² = total energy density, ɛ = specific internal energy, C =Lagrangian scalar

The simulations are designed as scale-free wind-cloud models and instead of including radiative cooling, the effects of energy losses in the gas has been approximated by a soft adiabatic index of γ = 1.1. The initial conditions for the 2D and 3D model simulations that were run are indicated in the table below.

Screen Shot 2019-04-15 at 15.03.07.png

Table 1: Initial conditions for the 2D and 3D. Source: arXiv:1901.06924v2, pg 5

Results and Conclusions

The results of the study found that the disruption process of quasi-isothermal clouds immersed in supersonic winds occurs in the four stages in both uniform and fractal cloud models. However, the resulting morphology, dynamics and destruction time-scales of clouds differ depending on their initial density distributions as demonstrated by the figure below:

Screen Shot 2019-04-15 at 14.10.50.png 

Figure 1: Figure 1 shows 2D slices at X3 = 0 of the cloud density, ρCcloud, normalised with respect to the wind density, ρwind, in three 3D models, at six different times in the range 0 ≤ t/tcc ≤ 2.5. Panel 2a of this figure shows the evolution of the uniform cloud model, panel 2b shows the solenoidal cloud model, and panel 2c shows the evolution of the compressive cloud model.

In the first stage, the initial impact of the wind triggers both reflected and refracted shocks. This is because the turbulent density fields in fractal clouds have a more intricate substructure than uniform density fields, which favours shock splitting. In the next stage, the cloud expands as a result of internal shock heating and pressure-gradient forces which cause the winds to accelerate and stretch downstream. This shock-driven expansion facilitates the wind to cloud momentum transfer and increases the area of the clouds. In the third stage, the acceleration continues and the cloud loses mass due to stripping by short wavelength instabilities. A long-standing turbulent filamentary tail forms on the rear side of the cloud due to vortical motions which remove gas from the cloud and moves it downstream. By the time we reach the fourth and final stage, the cloud has accelerated enough for long-wavelength instabilities to grow at the edge and thus the cloud breaks up into smaller cloudlets causing it to further expand. In the uniform and fractal models, this break up differs; in fractal cloud models the break up is a more steady process of disruption whereas in the uniform model it is more abrupt.

In conclusion, the study found that fractal clouds mix, accelerate and are disrupted earlier than uniform clouds due to their porosity which allows internal refracted shocks to be propagated more easily through the clouds. The research also found that within fractal clouds, solenoidal clouds are rapidly and steadily disrupted while also being less confined, more accelerated and having higher velocity dispersions than their compressive cloud counterparts. Lastly, cloud survival depends on how pressure gradient forces and dynamical instabilities affect the cloud. Compressive clouds are less prone to these instabilities than their solenoidal counterparts because of their higher density nuclei and lower acceleration. As a result, compressive clouds develop less turbulence and mix with ambient medium later than solenoidal counterparts

A love story between two stars in a dense space

Title:Enlarging habitable zones around binary stars in hostile environments

https://arxiv.org/pdf/1903.01995.pdf

Authors:Bethany A. Wootton and Richard J. Parker

Authors Institution:The University of Sheffield, Department of Astronomy

Status:Royal Astronomical Society, open access on arXiv. Published on March 7, 2019

A Partnership between Stars

“Find yourself the perfect partner that is right for you, and then even in a complicated place, you guys will create endless possibility together.” 

Sounds like some typical human relationship advice, right? 

Actually, this quote could also very well relate to a topic on Astronomy: The relationship of binary stars. Recent findings by astronomers Bethany Wooton and Richard Parker at the University of Sheffield, have shown that a bond between binary stars, despite orbiting in tough environments, can lead to creations of enlarged habitable zones, which increases the chances of finding life in outer space. 

Let’s break down this relationship advice using the eyes of an astronomer. 

Research shows that a roughly estimated 50 percent of stars are part of binary, or multi-star systems, meaning that they have companion stars to orbit with. These binary stars have a beautiful relationship, mutually orbiting around their common center of gravity as shown in figure 1.

Figure 1: Binary Star System

Not only do they call for a lovely relationship, binary stars have been vital to the advancement of astrophysics as they allow us to accurately determine the masses of these stars. Once we find their masses, we can measure an array of crucial information such as their: size, luminosity, lifespan, temperature, and more. This information opens doors to absolutely new terrains of research. 

However, dense star-forming regions such as the Orion Nebula Cluster, analyzed by the authors of this research project, can be a harsh and violent environment which alters binary star systems due to dynamical interactions. These clusters are filled with young stars that can modify the semi major axis and eccentricity of a nearby binary stars. The authors found that there are cases when the bond between the binary stars is strong enough that they manage to find just the right orbit that actually allows them to merge and enlarge their ‘habitable zone’ for neighboring planets.

Get in the Zone (Habitable Zone)

So, what is a habitable zone and what does it mean to have a larger one?

As humans continue to ask vast questions about life beyond earth, astronomers will likely inform you to stick with looking at habitable zones, also known as the Goldilocks Zone, where things are not too hot, not too cold, but “juuuust right” as shown in figure 2. The habitable zone is commonly measured for by the ability for liquid water to exist on a planet. 

Figure 2: Habitable Zones

Planets in the habitable zone are more likely to be able to sustain life due to temperature, and other factors such as atmosphere and reflectivity.

This paper finds that in a representative stellar nursery in the Orion Nebula Cluster, comprising of 352 binaries, 18 binaries would have their orbits embraced closer together over time. These rearranged orbits would result in an expansion of their habitable zones. Furthermore, one or two of these binaries would find their habitable zones not only enlarging, but also overlapping. Figure 3 below shows the merging and enlargement of the blue-shaded habitable zones around two stars in a binary system as a result dynamical interaction in a dense, star-forming region (Orion Nebula Cluster). Panel (a) shows the binary relationship at 0 Million Years. Panel (b) shows the same binary relationship 10 Million Years later after the orbits have been altered and the stars have been pulled closer. 

The result created by two bonded stars that remain connected together through millions of years despite being in a dense area, is a merged and enlarged habitable zone (shaded in blue). This enlarged habitable zone causes an increased likelihood of one of mankind greatest mysteries: A planet with potential life forms outside of earth.

Figure 3: Increased Habitable Zones for Binary Stars in the Orion Nebula Cluster
The empirical habitable zone is shown by the cyan shading. The orbit of the binary system is shown by the solid ellipse. 
In panel (a) we show the binary at 0Myr, where the semimajor axis is 6.4 au, eccentricity e = 0.32 and each star has its own distinct habitable zone. In panel (b) we show the system at 10Myr after it has undergone a hardening interaction to a = 5.4 au and e = 0.59 au. The habitable zones are now enlarged (especially that of the secondary star) and have merged.

Satellites and Ice Giants: What Can Uranus’ and Neptune’s Satellite Systems Tell Us About Exomoons?

Title: In Situ Formation of Icy Moons of Uranus and Neptune

Authors: Judit Szulagyi, Marco Cilibrasi, Lucio Mayer

First Author’s Institution: University of Zurich

Status: Received August 10, 2018; Revised XX, 2018; Accepted XX, 2018


Introduction: Satellites in our Solar System

Although our Moon has played a paramount role in global politics and astronomical discovery, as a satellite of a rocky planet, it is a minority in our solar system. 170 out of the total of 173 satellites belong to the solar system’s ice and gas giants: Jupiter, Saturn, Neptune, and Uranus. Because of the drastically different environments of the planets in our solar system, satellite formation around each type of planet is different. While our Moon was formed by impact, gas giants’ satellites around Jupiter and Saturn are thought to have been formed from circumplanetary disks – or CPDs – that emerged as a side effect of planet formation. While the theory of CPD satellite formation has been tested for gas giants, the question of whether these disks could form around smaller outer planets, specifically ice giants, remained unanswered. However in August of 2018, a study from the Center for Theoretical Astrophysics and Cosmology in Zurich revealed computational evidence supporting the formation of CPDs – and consequently satellites – around ice giants.

CPDs – What are they and when do they form?

Before embarking on how Neptune and Uranus’ moons were formed, it is essential to clarify what a CPD is. Circumplanetary disks are gaseous and dust-filled disks that form around proto-planets as a result of planet formation. Astronomers hypothesize that the formation of satellite systems arise from these disks. Yet the concept of a CPD is relatively new, and therefore little is known about the precise circumstances under which they form. What is certain, however, is that simulating CPDs around Jupiter using a computer modeling system called radiative hydrodynamic simulation yields results that align with Jupiter’s observable satellite system. Radiative hydrodynamical simulations specifically mimic how planets radiate away heat from the time of their formation, up until full formation of the planet. Because this technology yielded accurate results in studying Jupiter’s satellites, the scientists testing for ice giants’ moon formations employed the same modeling system alongside the numerical simulation technique population synthesis. Using these modeling systems, scientists discovered that the parameters affecting whether a CPD forms or not are planetary mass, and temperature. Then, after plugging in planetary data of Uranus and Neptune, these technologies provided evidence supporting the formations of satellite forming CPDs around ice giants after planetary cooling (Figure 1).

Figure 1. This figure shows the circumplanetary disks (CPD) around Uranus (left panel) and Neptune (right panel). The different rows account for the different temperatures of each planet throughout the process of planetary cooling. These CPDs show the disks from which Uranus’ and Neptune’s satellite systems formed.
Figure 1. This figure shows the circumplanetary disks (CPD) around Uranus (left panel) and Neptune (right panel). The different rows account for the different temperatures of each planet throughout the process of planetary cooling. These CPDs show the disks from which Uranus’ and Neptune’s satellite systems formed.

Uranus’ Satellite Formation

Using the technologies discussed above, scientists in this experiment gathered data from 25,000 individual calculations. Taking into account the practical amounts of dust and gas that would be present in a CPD formed after Uranus’ formation, this experiment proved that there would have been enough mass to create Uranus’ current satellite system from a CPD. The data suggests that a CPD formed once Uranus reached 500° K and led to the creation of a satellite system in just ~500,000 years.

Neptune’s Satellite Formation: Not as Expected

When scientists tested for the satellites that formed around Neptune, they were met with surprising results. Neptune is known for Triton – a massive moon most likely originating from the Kuiper Belt that possesses 99% of the mass within Neptune’s entire satellite system. While this moon is the most well known, the data from this experiment suggests that Neptune originally had a satellite system comparable to Uranus’. This data aligns with another study from 2018 which posited that in order for Triton to have been captured, Neptune must have had a similar satellite system to Uranus. Therefore, the discovery of Neptune’s CPD satellite formation provides the necessary link among current astronomical research.

Exomoons: Unpacking the Implications of in situ Icy Moon Formation

While the discoveries from this experiment provide invaluable findings about our own solar system, the implications of this study reach even further. Neptune and Uranus – the ice giants –  are representative of a highly common type of exoplanet. Therefore, if Neptune and Uranus are able to form exoplanets as a result of their own planetary formation, there must be a much greater number of exomoons than previously hypothesized. Furthermore, icy moons are strong contenders for extraterrestrial life, so if there are more icy exomoons than we had expected, the chance of finding extraterrestrial life is increased among the Universe. Thus, while we may be grateful for the opportunity to learn more about the planets and moons in our solar system, the ramifications of this experiment prove to be profound in the larger astronomy community of discovery.

Faster Water Escape on High Obliquity Planets


Title: Faster Water Escape on High Obliquity Planets

Author: Wanying Kang

Status: Published in Astronomy & Astrophysics; open access

First Author’s Institution: School of Engineering and Applied Sciences Harvard University

Cambridge, MA 02138, USA

Introduction

Human civilization appears to be an infinitesimal spec when faced with the thousands of exoplanets discovered throughout recent history. Exoplanets invite newfound potential for habitability beyond Earth, with the sheer number of possibilities alone. Wanying Kang’s paper “Faster Water Escape on High Obliquity Planets” explores the effects of high obliquity on stratospheric humidity, which could lead to habitability. Earth-like planet stratospheres contain little water vapor which is why most clouds are found in the troposphere. High-obliquity affects stratospheric humidity through seasonal variation, getting moisture from high altitudes, and the nature of overall warmer surface temperature caused by high-obliquity. Kang’s experiments use a 3D general circulation model to investigate the moist stratosphere that exists in high-obliquity planets. Are these humid stratospheres a key to a habitable planet or another Venus fate by a runaway greenhouse state? 

Methods

The 3D general circulation model used presents a more comprehensive understanding of the process of water escape on high-obliquity planets. The paper refers back to previous research by 1D models that neglected to consider factors such as seasonal and spatial variation. The experiment uses the increased spectral resolution to better depict radiation. H20 is considered the only greenhouse gas for simplicity and CO2 absorption is not considered. Atmosphere circulation was simulated by using a finite-volume dynamic core. The experiments used increasing isolation: one experiment set at zero obliquity, and the other at 80. These two experiments present an opportunity to better understand the cause of humid stratospheres, and if they can lead to habitable planets! 

The 3D general circulation model allows for a comprehensive analysis of seasonal variation, which is present on high obliquity planets due to more intense seasons which cause warmer surface temperatures during polar days. Second, it considers moisture entering the stratosphere from higher altitudes, allowing it to escape the cold trap and make it into the upper atmosphere. Lastly, it accounts for the all-around high surface-temperatures of high obliquity planets. 

Results 

Upper atmospheres are significantly moister in the high-obliquity insulated experiment, and the lower obliquity experiment almost triggers runaway greenhouse affects but ultimately crashes. The high-obliquity experiment has higher water escape rates, making water vapor more visible in these models. However, both experiments prove to be inhabitable. The low obliquity experiment is inhabitable due to its potential for the runaway gas effect, whereas the high obliquity experiment also proves to be inhabitable due to water escape. 

While high-obliquity planets don’t fall victim to the runaway greenhouse state, water escape speeds up and leaves planets uninhabitable. However, these experiments only account for insulation and obliquity. The results of the experiment claim that considering the rotation rate for Earth-like planets could have an effect on the cold trap and thus water escape. These experiments also don’t account for atmospheric composition. In the end, high-obliquity humid stratospheres may not prove to be habitable yet, but it does reveal that these are the exoplanets are more likely to be discovered due to surface evaporation caused by the high obliquity. 

The First Exoplanet Detected by Optical Interferometry

Title: First direct detection of an exoplanet by optical interferometry-Astrometry and K-band spectroscopy of HR 8799 e

Authors: S. Lacour, M. Nowak, J. Wang, O. Pfuhl, F. Eisenhauer, R. Abuter, A. Amorim, N. Anugu, M. Benisty, J. P. Berger, H. Beust, N. Blind, M. Bonnefoy, H. Bonnet, P. Bourget, W. Brandner, A. Buron, C. Collin, B. Charnay, F. Chapron, Y. Clénet, V. Coudé du Foresto, P. T. de Zeeuw, C. Deen, R. Dembet, J. Dexter, G. Duvert, A. Eckart, N. M. Förster Schreiber, P. Fédou, P. Garcia, R. Garcia Lopez, F. Gao, E. Gendron, R. Genzel, S. Gillessen, P. Gordo, A. Greenbaum, M. Habibi, X. Haubois, F. Haußmann, Th. Henning, S. Hippler, M. Horrobin, Z. Hubert, A. Jimenez Rosales, L. Jocou, S. Kendrew, P. Kervella, J. Kolb, A.-M. Lagrange, V. Lapeyrère, J.-B. Le Bouquin, P. Léna, M. Lippa, R. Lenzen, A.-L. Maire, P. Mollière, T. Ott, T. Paumard, K. Perraut, G. Perrin, L. Pueyo, S. Rabien, A. Ramírez, C. Rau, G. Rodríguez-Coira, G. Rousset, J. Sanchez-Bermudez, S. Scheithauer, N. Schuhler, O. Straub, C. Straubmeier, E. Sturm, L. J. Tacconi, F. Vincent, E. F. van Dishoeck, S. von Fellenberg, I. Wank, I. Waisberg, F. Widmann, E. Wieprecht, M. Wiest, E. Wiezorrek, J. Woillez, S. Yazici, D. Ziegler and G. Zins.

First Author’s Institution: LESIA Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France and the Max Plank Institute for extraterrestrial Physics, Giessenbachstrabe 1,85748 Garching, Germany

Status: Published in Astronomy & Astrophysics; open access

            Since the golden age of exoplanet exploration began nearly 30 years ago, we have successfully catalogued over 4000 of these incredibly diverse and mysterious worlds. Even though this list continues to grow, we still have little understanding beyond the vague details of their orbit, size and basic composition. Our methods for direct observations of exoplanets is still extremely limited, as we struggle to discriminate the spectral signals of planets from the overshadowing brightness of their stars. While directly observing exoplanets might seem like the most straightforward method for exoplanet detection, it has only found 0.3% of all known exoplanets.

            To tackle this problem, a team led by Sylvestre Lacour at the Paris Observatory in France pioneered a new technique that uses optical interferometry to directly observe planets at close angular distances from their stars. Their technique not only is a revolutionary tool for understanding the thermal and gravitational make-up of exoplanets, but also it allows the distance between a planet and its star to be calculated with ten times more accuracy. Their study shows the power of optical interferometry by measuring the atmosphere of the young and distant super-Jupiter exoplanet HR8799e, located in the Pegasus constellation.

What is Optical Interferometry?

            This technique combines two or more telescopes to create detailed images of objects that would otherwise be unobservable by an individual telescope. When light beams reflected by an object reach each telescope, they are combined at a screen, where they create a pattern of light and dark areas called interference fringes. These delicate fringes can then be translated into distances and create very detailed images of objects many light years away.

            Lacour’s team used four large telescopes in Chile’s Atacama Desert to observe the distant HR8799 star system, 29 light years away from our solar system. These 8.2-meter diameter telescopes are called Unit Telescopes (UTs), and they use the Very Large Telescope Interferometer (VLTI) operated by the European Southern Observatory to create images of small objects.  The light coming in the UTs are combined in an instrument called GRAVITY, which creates direct images using near infrared radiation.

            Exoplanets are commonly observed in infrared because their spectra can be compared to that of brown dwarfs, objects that emit infrared radiation and who’s size range between a Jupiter-sized planet and a small star. The L-T transition is an especially important section of the infrared spectra, as it can be used as a basis for understanding exoplanet atmospheres as a function of temperature. However, infrared interferometry can be tricky in that it requires incoming light beams to be coherent, meaning that they must have the same frequency and constant phase periods. Prior to Lacour et al. ‘s study, it was hard to detect these waves from such great astronomical distances and very precise optics like were needed.

Determining the Orbits of Exoplanets  

            The position of HR8799e relative to its star was calculated by creating a baseline to calibrate the fluxes coming from the five exoplanets. They found that the star is approximately ten times brighter than its planets, so the detection integration time (DIT) was decreased to 1 second to reduce the noise coming from the star relative to the total incoming signal. The exoplanet was located northeast of the star, as shown in the box in the upper right corner (Figure 1). This was calculated by GRAVITY’s observations of the amplitude of the periodic light beams emitted by the exoplanet over its trajectory around the star (Figure 1). Lacour et al.’s method brilliantly increases these measures by a degree of magnitude when compared to measurements made by direct imaging– a stepping stone for astrometry.

Figure 1 The visibility of HR8799e as it travels around its star. Visibility measures the different in waves emitted from the planet and from the star. The visibility results lie along the bottom black dotted line because this line corresponds to the light emitted solely by the planet, without the interference of the star. The boxed graph on the top right illustrates the north east position of the planet relative to its star using the visibility measures shown in the graph.

             The incredible accuracy of this technique can be applied not only to determine the orbit of a distant planet relative to its star, but also to determine the orbital constraints of multiple planets within a system. By comparing the position of HR8799e in GRAVITY measurements and in previous studies, Lacour and his colleagues demonstrated that the planets in the HR8799 system do not orbit the same plane. The high accuracy of their technique allowed them to reach this conclusion by observing just one point in HR8799e’s orbit. They determined a semi major axis of 16.4 +2.1 −1.1 AU, an eccentricity of 0.15 ± 0.08, and an inclination of 25◦ ± 8 ◦, and then fit these points in a Kepelerian orbit.

Determining Exoplanet Atmospheres

            Optical interferometry also proved incredibly successful in calculating temperature and surface gravity of the mysterious super-Jupiter planet. To determine the temperature of the exoplanet, a best fit line was used to compare the GRAVITY observations to data from previous studies (Figure 4). The GRAVITY spectrum was calculated by multiplying the amplitude of the light emitted by the exoplanet through its orbit to the theoretical amount light coming from the star. It was discovered that the exoplanet has similar temperatures to a very hot brown dwarf, at around 1400 Kelvins. They also showed that the surface gravity in this planet is extremely high, at 104 cm s2, which is around ten times that of Jupiter.

Figure 2 The best fit line for the near infrared spectra of HR8799e is shown in orange.

            The temperature calculations were used to determine that L-T transitions of exoplanet is lower than that of brown dwarfs because iron and silicate clouds effect the pressure of these objects (Figure 3). These findings continue to push the boundaries for exoplanet research and better characterize these mysterious worlds beyond vague astrometric measurements.

Figure 3 The steep increase temperature relative to surface gravity shows that the L-T Transition of exoplanets is lower than that of Brown Dwarfs. The black lines represents exoplanets with different ratios, relative to Jupiter’s radius.
This article written by Nina Castro, an undergraduate student at Barnard College.

3rd Rock from the Star: Examining Stars to Find Habitable Exoplanets

Title: Stellar Characterization Necessary to Define Holistic Planetary Habitability

Authors: Natalie Hinkel, Irina Kitiashvili, Patrick Young, Allison Youngblood

First Author’s Institution: Southwest Research Institute

Status: submitted in response to the solicitation of feedback for the Decadal Survey on Astronomy and Astrophysics (Astro 2020) by the National Academy of Sciences, open access on arXiv.

When searching for a habitable exoplanet, the focus has been on whether or not that planet has liquid water. Liquid water is essential to Earth’s biosphere and in maintaining an environment capable of sustaining life. Generally, if a planet is within a star’s habitable zone, it is assumed that there is sufficient atmospheric pressure and radiant energy from the star to support liquid water. The habitable zone focuses on the proximity of a planet to its host star. Already linking the quest for a habitable exoplanet to its star. The authors of today’s paper delve into this relationship further, proposing that the astrophysics and astrobiology community focus on exoplanets’ host stars. By doing so, a more holistic definition of planet habitability, beyond whether a planet lies in the habitable zone, can be achieved.

What do stars tell us?

Stars and their planets are formed at the same time from the same materials from the same molecular cloud. Therefore, the authors propose that the composition of host stars can be used as proxies for the interior content of planets. The authors thus claim that if a star possess rock-forming elements, the planet will contain the same or related elements.

Beyond their birth, stars tell us about even more about planetary habitability. The absolute flux and spectral properties of the star’s UV and x-ray emissions impact exoplanet photochemistry and atmosphere. Thus, it is important to monitor a star’s UV and x-ray emissions. Currently, photometry from the Kepler and TESS in the optical can detect flares, but this does not mean that UV and x-ray are not present on the star. The activity of the host star directly affect the planet’s surface.

Furthermore, stellar age are essential for determining how long a planet will dwell within the habitable zone. Current technology for determining stellar age (gyrochronology, age-activity relations, lithium depletion, surface gravity features, metallicity, and kinematics) is most effective with stars younger than 1 Gy, but is less effective for determining the ages of older and low mass stars.

Updating stellar models

Until recently, only 1-dimensional stellar models based off of mixing-length theory have been used to determine a star’s age (often from stellar comparison). The authors of this paper propose that 3-dimensional time-based models that also consider the EOS, stellar interior structure, composition, and effects of radiation, magnetic fields, and turbulence. These 3d models of stellar convection would be able to reproduce surface turbulent dynamics and radiation properties of stars. These models can compute the initial conditions using a stellar evolution code to fit the observed stellar physical properties.

In conclusion

The stellar host and orbiting planet are intrinsically related. Stars can tell us the composition of exoplanets, which data is fundamental to determining whether a planet can have active geochemical cycle, feeding new elements to different regions of the planet, including the atmosphere. This data provides us with information beyond if a planet has liquid water. It could potentially tell us if there is surface water on the planet, how the water is maintained on the planet, if there are necessary elements for life on the planet, if life on the planet could survive star activity, and more!

Stars are fundamental to detecting habitable exoplanets, but they could be fundamental to predicting sustained habitability of a planet.