A Planet Hunters Guide And Early Planetary Modeling Scenarios

A Planet Hunters Guide And Early Planetary Modeling Scenarios by Jennifer Connors

Introduction

The oldest natural science on our earth is Astronomy. The viewing of and studying stars, planets, moons and the motions of celestial objects originate in the roots of nearly every religion and culture on earth. Since first viewing the beauty of the heavens ~mankind's thoughts were filled with wonder, admiration and devotion to better understand our universe. Exoplanets are the planets that exist in other star systems. Most recently NASA’s planet hunting telescope Kepler returned enough data for us to know that exoplanets exist frequently throughout the universe. New generations of exoplanetary exploration missions are being built to continue the search and study of exoplanets. NASA’s recently launched TESS the Transiting Exoplanet Survey Satellite and the JWST the James Webb Telescope were designed to discover a diverse new array of exoplanets and peer deeply into the elemental building blocks of our universe. An intriguing variety of newly discovered star systems with exoplanets are beginning to change our understandings about what types of habitable worlds exist in the universe, how frequently certain types are found, and what the conditions of these many worlds might be like. What do we know about these exoplanetary star systems? And what information do we have to begin modeling and profiling the atmospheric conditions of habitable worlds?

The Search For Exoplanets

The NASA’s exoplanet archive contains 4306 confirmed exoplanets located across 3196 different star systems. More than 5649 exoplanet candidates await secondary telescope confirmations (Exoplanet Archive, 2020). In 2009 The Kepler Telescope Mission launched to study the starlight of a patch of the sky in the Lyra and Cygnus Star Constellations. The goal of this three year mission was to determine if exoplanets exist frequently in the universe. Planet hunters use the light data beamed down by Kepler to find new transiting exoplanets by transit detection method. When sudden drops appear in a stars light we find exoplanets transiting their sta. These drops in starlight are called transits. By using the transit method thousands of exoplanets have been detected and the study of star light teaches us many details about distant star systems; such as stellar and planetary parameters, orbital periods and orientation of orbiting objects, chemical compositions, and temperature. NASA’s Transiting Exoplanet Survey Satellite (TESS) launched in 2018 to search for exoplanets. Rotating every 27 days TESS plans to observe 85% of the night sky searching for exoplanets. All planet hunting missions so far have discovered 1350 gas giants, 1458 neptune-like planets, 1329 super-earths, 163 terrestrial-planets, and 6 unknowns. The orbital periods of exoplanets can range from ~0.1 days to many thousands of days. Categorically terrestrial planets are most similar to Venus, Earths and Mars have rocky cores and high abundancies of Iron. Terrestrial-planets are generally less than 4 times Earth’s radius. Ice giants are similar to our solar systems Neptune and Uranus, which are composed of rock and ice ranging between 4-8 times Earth’s radius. Gas giants resembling Jupiter and Saturn are composed of lighter gases ranging greater than 8 times Earth’s radius with stronger gravitational fields. Small earth-like planets are difficult to find because they create smaller dimmings on a stars light. Surveys searching for smaller earth-like planets require precision suppression of systematic effects that can be found in data. When you couple systematic effects with common noise reduction techniques used in data handling, the tiny decrease of light that an earth-like planets creates may be lost. Smaller numbers of earth-sized exoplanets found in the Kepler survey further defines the necessity for optimal detrending and removal of systematic effects (Petigura, 2015). Additional exoplanets may be found in a star system by detecting Transit Timing Variations (TTV's). Caused by local gravitational disturbances between orbiting objects variaitons alert the presence of other nearby exoplanets, moons or stars. Multi stars systems can be binary and trinary stars. Multi stars express enhanced gravitational relationships with each other creating larger TTV’s. Scientists have also detected exoplanets by radial velocity studies watching for wobbling stars. In radial velocity studies a planet orbiting a star may cause the star to wobble due to gravitational tugs created as two or more objects orbit the same center of mass. The stars wobbling makes light emitted from the star shift from blue to red lightwaves in the electro magnetic spectrum. Scientists discovered the very first exoplanets by radial velocity detections (Mayor & Maurice, 1985).

II. Prospects of Finding Exomoon/s

Ever greater numbers of exoplanets are being discovered and soon we will also detect numerous exomoons. More than 100 moons exist in our own solar system. Many newly discovered exoplanets are super earths and neptunes raising the probabilities of having hospitable exomoons (R. Heller, 2018). Beyond the basic orbital parameters of exomoons increased probabilities of being habitable are possible when a planet receives stellar flux reflection from other planets, has internal heating sources, has tidal heating or advanced atmospheric development in correlation with a magnetosphere. (R. Heller, 2018). Moons like planets can present with rings and rocky companions that can lead to detections if the moons orbital shadow is seen when the transiting host planet crosses a star. Probability of finding exomoons increase as our detection methods acquire precision. Future classifications of exomoons will redefine our conceptual ideas of potentially habitable worlds. Two examples of exomoon transit candidates are KID 6344983 and KID 4760478 (Kepler 1625-b-i). The possibility of Kepler 1625-b-i having exomoons exists but to date no exomoons have been clearly confirmed by a secondary observations. Leaving detections of exomoons nearly a blank page to be written upon.

III. Equilibrium Temperatures

Equilibrium temperatures indicate an exoplanets ability to have liquid water. If on average the energy coming in and going out from a star balance this is the planet's equilibrium temperature. A stars light or flux is energy received by a planet. Various stars emit different wavelengths of light throughout the electromagnetic spectrum. Larger planets like Jupitor, Saturn and Neptune exceed equilibrium temperatures indicating that these planets have internal heating sources. A planets albedo is determined by the materials that compose its surface and atmosphere. On earth the water that covers %71 of the surface absorbs most of the visible light our planet receives. Cloud and snow at the poles of earth reflect visible light while absorbing infrared light as heat. The terrain on earth varies of different types of sand and soil. When factors are combined to together earth has an albedo of ~30%. A planet like Mercury that has no atmosphere has a smaller albedo of ~6%. Higher reflectivity correlates with a higher planetary albedo. The adiabatic lapse rate of a planet correlates with how quickly the air in a planets atmosphere cools as elevation increases. To begin modeling exoplanetary conditions initial factors that can be considered are the planets albedo, adiabatic lapse rate, and equilibrium temperature. Study planetary thermal and chemical dynamics, pressures, compositions, core types, temperatures and energetic equilibriums will provide accurately balanced estimations.

IV. Habitable Zones

A Habitable Zone is the surrounding area near a star where temperatures allow liquid water to exist. The moist greenhouse or inner edge of a habitable zone is approximately ~340 Kelvins, here water vapour dramatically increases, and at ~373 Kelvins entire oceans will evaporate on earth like models. The opposite outer edge is the frost barrier, at which a planets temperature at approximately ~273 Kelvins water will easily freeze and the atmosphere will become opaque to stellar radiation. This outer edge can also be redefined as ~1.7 Astronomical Units away from a sunlike star. An Astronomical unit is the distance from earth to our sun. Additional factors that could extend habitability modes might be found in multi star systems. Two sources of energy received from stars or reflected light from nearby planets may enhance a planets equilibrium temperature. Added tidal heating, internal heat, advanced greenhouse ecologies, enhanced atmospheric dynamics with magnetospheres in correlation with the quantitative mass of an exoplanet may increase a planets ability to host liquid water. Increasing surface gravity may strengthen a planets magnetosphere aiding atmospheric structures that might make it harder for heat and atmospheric particle to escape into space.

V. Star Relations & Star Type

The greatest magnitude of known earth-like planets orbit Type M dwarf stars. These cooler dwarf stars can survive tens of billions of years longer than sun-like stars because of fuel conservation. M dwarfs are the most populous star type in the universe with a solar radius ranging up to ~0.6 times Sol and temperatures ranging up to ~3,900 Kelvins. M Dwarfs present much intrigue to scientific community researching life supporting worlds. Typically it is easier to detect a habitable world orbiting a M Dwarf with a closer habitable zone. The close proximity required for a planet to reside within a M Dwarf habitable zone presents multiple challenges to the existence and evolution of life. Habitable zone worlds in M Dwarf star systems experience increased X-Ray irradiances up to ~400 times stronger than earth. M-Dwarfs are known to have active flares and magnetic activity (R. Heller, 2018) that could sterilize a planet or prevent atmospheric developments.

Type K stars that are slightly cooler than our sun are the second most populous stars in the universe. They survive for tens of billions of years and present the perfect conditions for life and evolutionary processes. Type G stars like our sun having wider habitable zones are not as frequently found in the universe. Hotter end of Type G and F stars having habitable zones with orbital periods through the 500 - 700 day range are currently outside the limits of our detection methods currently as we need multiple transits found to confirm a planet. This gap in planetary detections leaves the greater majority of longer period planets in wide reaching habitable zones out of our early detections (Bryson, 2020).

Stars and planets very often show like qualities, smaller stars produce smaller planets (Fulton, 2018) and more massive stars produce more massive planets. To accurately estimate a planets radius and mass requires the ability to accurately estimate the radius of the host star. Stellar and planetary radius estimations are commonly derived from photometric estimations that have slight errors. With secondary reviews of a stars radius done by spectral analysis will improve %5 - %10 of fractional inaccuracies (Petigura, 2015) and further define precision star and planetary radius estimations.

VI. Early Spectral Analysis

Spectroposphy study of exoplanetary atmospheres notes the missing absorption lines of a planet and stars light spectrum gathered when the planet orbits a host star. Spectral lightwave studies gage how a stars light interacts with the components of an exoplanets atmosphere. Different molecules in atmospheres absorb different wavelengths of light, the missing absorption lines in a planets spectrum reveal which molecular compositions exist in the atmosphere. Oxygen and Ozone absorb wavelengths of light less than 300 nanometers while water absorbs most wavelengths of light over 700 nanometers. Increases of UV stellar radiations can decrease the accumulation of lighter gasses (Rugheimer, 2012). Atmospheres with dense heavy cloud cover and high reflection present increased observational difficulties (Crossfield, 2015). When Hawaii's Keck Telescope team performed spectral analysis of Kepler's habitable zone planets they found that the trace elements that support life here on earth also exist faraway on exoplanets throughout the universe. These distant planets were found to be abundant with hydrogen, oxygen, carbon, iron and the basic building blocks of Life. Carbon and oxygen were enriched in stars with planets, indicating that many exotic worlds are formed in carbon rich environments. The Kepler Space Observatory and Keck findings findings suggest that we could find more than two billion planets in our galaxy with life supporting capabilities. New technology like the Extremely Large Telescope (ELT) and the James Webb Telescope (JWST) will be capable of classifying atmospheric compositions of super earths, neptunes and gas giants with ease (Cowan, 2015).

Carbon dioxide, methane, chloromethane, ammonia, nitrous oxide, oxygen, ozone, water, sodium and potassium are detectable by spectral analysis. Atmospheres are made of condensates, hazes and clouds, probing higher layers of an atmosphere will reveal molecular opacities. Greenhouse emissions that are highly viable with the ability to capture and recycle energy in a planets atmosphere. Greenhouse gasses increase global atmospheric warmings and carbon dioxide also absorbs sunlight without releasing energy, this increases atmospheric warming profiles. The signs of life and greenhouse gases leave spectral fingerprints on a planets atmosphere for us to see (Kaltenegger, 2011). Earth has a magnetosphere and atmospheric fields that shield us from harmful radiations and helps protect the evolution of life on earth. Studying planetary atmospheric structure and make-ups better allow us to understand a planets habitability, formation and evolution in time. Planets with less than 1.5 Radius Earth are mostly composed of rocky cores (Fulton, 2018). Earthlike planets contain quantities of lighter gases composing a planets atmosphere. Smaller worlds rocky worlds high in can posses magnetic fields that aids atmospheric development. Colder planets can have water, oxygen, nitrogen, and carbon. These elements that form the building blocks of life are a thousand times less abundant than the lightest gases. Lighter elements and gasses in atmospheres add to a planets total size, mass, surface gravity and atmospheric structure. Planetary core characteristics of albedo, pressure, density, structure, chemical and thermodynamic equilibriums form our early understanding of atmospheric profiling. Given chemical and thermal equilibriums define which species may remain abundant under different types of pressures, temperatures and make-ups. Star systems evolve over a time and every planet may one day reach a point when its lighter gases and water vapor is stripped away by stellar radiation and winds.

IIV. Many New Worlds

Early findings indicate that ~15% of solar type stars—roughly one in six—have a 1–2 Earth radius planet. These early results are a reflection of the capabilities of the detection methods used (Petigura, 2015). Missing gaps found in the prevalence of earth sized planets of less than 1.3 times earth radius are attributed to technical detection limitations and the current precision of radius estimations for planets and stars (Fulton, 2017). Larger planets of short orbital periods are easily detected by transit observances. As our observation techniques improve we will find new varieties of earth-like and non earth-like worlds in many different star systems with diverse definitions of habitable zones. Kepler 22-b is the first planet of its star system. It orbits a larger Type G like our sun and is theorized to have a rocky core with oceans covering its surface. This planet is a super earth with a radius ~2.38 times earth with an orbital period of ~289 days, it exists perfectly within the stars habitable zone where liquid water can exist. Kepler 22-b was one of the first earth twins to be detected. Kepler 186-f is also a super earth orbiting a cooler M-Dwarf star that is roughly half the size of our sun every ~129.9 days. This planet is theorized to be a large rocky world ~1.71 times as massive as earth residing comfortable within an M Dwarf stars habitable zone.

Kepler 452-b orbits a Type G star every ~384.8 days mirroring an earth with double the surface gravity being ~1.7 times earths radius and ~3.2 times as massive. Kepler 1544-b is another super earth that orbits a cooler Type K star every ~168 days residing near the outer edge of the stars habitable zone. This planet is theorized as a rocky super earth ~1.8 times earths radius and ~3.8 times as massive, with an estimated equilibrium temperature of ~270 Kelvins. Kepler 1606-b is another super earth orbiting a Type G star every ~196 days residing at the inner edge of its G Type stars habitable zone. Being slightly larger at ~2 times earths radius and 5 times as massive as earth this planet has an estimated equilibrium temperature of ~324 Kelvins. Kepler 1625-b is a giant planet orbiting a cooler Type G star every ~287 days residing near the inner edge of the stars habitable zone. Kepler 1625-b has a host star that is nearing the end of its life cycle with an estimated age of ~8.7 billion years. This world is known to potentially have an exomoon with approximate equilibrium temperature of ~346 Kelvins. Kepler 1638-b is a super earth that orbits a Type G star every ~259 days within the stars habitable zone. This planet is ~1.9 times earths radius and ~ 4 times as massive with an estimated equilibrium temperature of ~281 Kelvins. Kepler 1653-b is also a super earth orbiting a slightly cooler Type K star roughly ~70% as massive as our sun every ~140 days. This planet is ~2 times earths radius and 5 times as massive residing within the stars habitable zone at an estimated equilibrium temperature of ~284 Kelvins. One last interesting find is HD 35512-b a rocky super earth orbiting a Type K Star every ~54 days residing at the inner edge of the stars habitable zone. This planet is theorized to have a rocky core that presents with land and sky similar to earth. Atmospheric modeling predicts that HD 35512-b has an atmosphere composed of water and carbon dioxide. This planet was discovered in by Swiss Astronomer Stephane Udry and the University of Geneva’s Astronomy Team by radial velocity detections (Kaltenegger, 2011).

IIIV. Conclusion

Recent results of exoplanet surveys suggest that exoplanetary star systems exist abundantly throughout our observable universe. With continued exoplanet surveys and further technological advances our catalog of known exoplanets will continue to grow allowing us to better understand the universe we live in. Questions of life's existence throughout other star systems in the universe will be answered when we better understand exoplanet conditions and the key to life’s beginnings. Our universe masterfully creates the conditions and buildings blocks needed for life to begin, the rest is determined by nature. Current and future planned exoplanet surveys will reveal great multitudes of potentially habitable worlds. Newer space survey equipment like the planned Extremely Large Telescope (ELT) and the James Webb Telescope (JWST) will aid the spectral analysis of these many new exoworlds.

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