Research Paper On Exoplanets

One of the most extraordinary scientific discoveries in astronomy has been the findings of exoplanets- planets that orbit stars outside of our solar system. In 1992, Aleksander Wolszczan and Dale Frail announced their discovery of two exoplanets orbiting around a pulsar that was 1,000 light years away from Earth. Then, in 1995, Michel Mayor and Didier Queloz discovered the first exoplanet that orbited a star like our sun- 51 Pegasi b (Encrenaz, 2014). These discoveries led to a hunt for exoplanets- with thousands being discovered to date.

Exoplanets are difficult to discover. They’re several light years away and generally orbit a star that is bigger and brighter. Despite these obstacles, astronomers have developed several different methods

to help discover exoplanets. One of the first methods used was the radial velocity technique, which “consists of measuring the velocity of the star around the center of gravity of the star-planet system with high accuracy” (Encrenaz, 2014).

A common method used today is the Doppler Effect. An orbiting planet will cause the star to ‘wobble’ which causes a shift in the visible light emitted from the star (Sasselov, 2008). Gravitational lensing is another common technique used for discovering exoplanets. For this method to work, another star or another source of light needs to be behind the star being investigated. As the star is being observed, the background star’s light will be bent by the gravity of the observed star; and if there is an orbiting planet, this effect will be altered in a different way (Sasselov, 2008). The most successful technique in discovering exoplanets is the transiting method. In this method, the exoplanet passes directly in front of its host star, which causes a mini-eclipse. These mini-eclipses are detected by telescopes that can track the brightness of these stars (Heng & Winn, 2015). These

different techniques can reveal characteristics of exoplanets. The Doppler Effect can give the planet’s size, orbital period, and eccentricity. The transiting method reveals the planet’s radius and its distance from its host star. These results allow astronomers to perform a basic spectroscopy- measuring a planet’s spectrum of electromagnetic radiation- and thermal mapping of its atmosphere. In fact, astronomers, using these findings, have been able to learn about the temperature of exoplanet HD 189733 b- which is about 230°C (Sasselov, 2008).
In 2009, NASA launched the Kepler Spacecraft which sought Earth-like exoplanets orbiting a sun-like star in a habitable zone- where there’s a possibility of liquid surface water. Kepler uses the transiting method to detect exoplanets, but the transits must be identical and done at regular intervals. Sometimes, changes in brightness are caused by a binary star system which produces V-shaped light curves, not the U-shaped ones produced by planets. These false-positives are normally ruled out. Astronomers can calculate an exoplanet’s size from the magnitude drop in brightness- called the transit depth- and the size of the host star. Astronomers determine the orbital period by the number of repeated transits. With the orbital period and mass of the parent star, astronomers can use Kepler’s Third Law to calculate the exoplanet’s semi-major axis- its distance from its host star (Gould et al, 2015).
The Kepler Space Telescope is made primarily of a sensitive photometer that precisely measures a change in brightness. Kepler has a field of view of about 15°, which allows astronomers to observe 170,000 stars simultaneously- making it the largest, wide-field telescope. An Earth-size exoplanet causes in drop in brightness of less than 0.01%, so the photometer has extreme sensitivity. Detecting Earth-size exoplanet transits is the equivalent of “sensing the drop in brightness of a car’s headlights when a flea moves in front of it” (Gould et al, 2015). This precision cannot be measured from Earth’s surface because of events like sunrises, fog, clouds, and storms, so the space-based Kepler allows for the continuous observations of exoplanet transits. (Gould et al, 2015).
When launching Kepler, it was important for it to be positioned away from bright objects- the sun, moon, or planets that move along the ecliptic (the path of the sun). The field chosen was in the constellation Cygnus and Lyra, with many stars in the field. The Spacecraft travels in an Earth-trailing heliocentric orbit with a period of 372.5 days (Gould et al, 2015).
Within one month- March 2009- Kepler discovered five exoplanets- hot Jupiters with temperatures over 1500K. As of January 2015, Kepler has discovered 4175 possible exoplanets, with 1013 confirmed. Two new classes were founded- mini-Neptunes and super-Earths. Kepler’s primary mission concluded on May 11, 2013 because two of its four reaction wheels failed- three are necessary for precision (Gould et al, 2015).
In place of Kepler, NASA plans on launching the Characterizing Exoplanet Satellite- with a goal of measuring the transits of stars already known to host exoplanets- and the Transiting Exoplanet Survey Satellite- with a goal of surveying the whole sky for transiting exoplanets- in 2017 (Heng & Winn, 2015). In 2018, NASA plans to launch the James Webb Space Telescope, which will record exoplanets and stars in bright resolution (Encrenaz, 2014). The JWSP will be a successor to the Hubble Space Telescope, but will be optimized for infrared observations, instead of ultraviolet. The European Space Agency plans to launch the Planetary Transits and Oscillations of Stars in 2024 to build a catalog of true Earths (Heng & Winn, 2015).
The most powerful ground-based technology is the HARPS radial-velocity spectrograph in the Andes. HARPS can track stars’ gravitational wobbles with a high precision of one meter per second (“The First Potentially…, 2010). A new technology, direct imaging, allows astronomers to suppress a star’s light to create an image of the planets around it as dots. An aspheric mirror distorts light in such a way that enables astronomers to capture and record images of a star without airy rings- so all the light is effectively blocked. Direct imaging is the only direct way to observe the planets around a star (Belikov & Bendek, 2015).
These technologies have allowed astronomers to search for habitable, Earth-like planets. To be suitable for life, these planets would have to have: a diameter 0.5-1.5 times that of Earth (to enable a rocky surface), an orbital radius of 0.8-1.8 astronomical units (for liquid surface water), and the presence of atmospheric biomarkers- oxygen, methane, and water vapor are caused by life (Belikov & Bendek, 2015). Despite Kepler’s discoveries of possible Earth-like exoplanets, current methods are not well adapted to detect every potentially habitable planet even by the nearest stars. For example, Kepler’s use of the transiting method misses 199 out of every 200 Earth-like planets because a planet’s orbit must be inclined very little. The radial-velocity method probes close stars, but doesn’t have the sensitivity to explore sun-like stars. Consequently, astronomers have only detected planets around a small fraction of stars (Belikov & Bendek, 2015). The wide spread of exoplanets, along with their varied characteristics, illustrates the random nature of planet formation. Compared to other planets, Earth-like planets are small in mass radii, so they are harder to detect than other planets. (Seager, 2010).
The long-term goal of exoplanet-hunting is characterizing planets based on their atmospheres and searching for biomarkers. These efforts strive to observe a planet’s molecular atmosphere, cloud coverage, temperature, wind speed, and surface features. These observations can eventually determine if a planet is habitable. For example, molecular oxygen could be circumstantial for a planet’s habitability because ozone is produced by life. (Heng & Winn, 2015). Byproducts of metabolism are the most promising evidence. An example would be the presence of oxygen and reduced methane, because this causes a disequilibrium in the atmosphere (Fuji & Rein, 2014).
Astronomers have certified many detected exoplanets. In 2009, Corot-7b, which is five times the size of Earth, was discovered. It orbits 0.017 a.u. from its host star with a period of 20.5 hours, so the world is searing hot and is probably composed of rock and metals and volcanically active. Discovered in the same year, the super-Earth Gliese 1214b orbits 0.014 a.u. from its host star. It has a period of 38 hours and is cooler and dimmer than Corot-7b (Seager, 2010). On September 29, 2010, a team of astronomers announced a potentially habitable Super Earth- Gliese 581g. Gliese 581g is three times the size of Earth with a rocky composition and an orbit close to its parent star for moderate temperatures and a diameter 1.2-1.5 times the size of Earth’s. The planet orbits its star every 36.6 days, being close enough to be tidally locked and have a distinctive day and night cycle (“The First Potentially…, 2010). Kepler-22b orbits a sun-like star 600 light years away from Earth. It resides in its planet star’s habitable zone, so liquid water could exist (Billings, 2014). As technology increases, astronomers could closely observe these planets’ atmospheric chemical makeup, its surface environmental composition (whether the surface is rocky, icy, watery, or molten), temperature, and weather, which could be used to determine if a planet is habitable or not.
Many astronomers believe that exomoons could have similar compositions and atmospheres to Earth. Large moons in its parent star’s habitable zone could potentially hold liquid water- a gas giant’s gravitational pull could ripple tidal energy through its moon. These moons could be found through gravitational interactions with its planet- a moon and its planet orbit around a shared center of mass. When a moon passes in front of its host planet, astronomers could use the transiting method to observe a planet’s surface all from the light curve of a passing moon (Billings, 2014).
In our solar system, astronomers are look at Planet X- a possible, Neptune-size planet lurking in the dark reaches of our solar system. It was first dreamed up to explain perturbations in Uranus’ and Neptune’s orbits, but astronomers just don’t know that much about it. What they do know is that there are millions of icy bodies within the Kuiper Belt, and some are dwarf planets similar to Pluto. The Oort Cloud- the spherical swarm of a trillion comets at the end of our solar system- was observed by the Kepler Space Telescope in 2009, but nothing substantial was found. In 2003, Mike Brown and his team found a dwarf planet with an orbital range of 75-1000 AUs. In 2013, Scott Sheppard found a similar object, called 2012VP113. These elliptical orbits are cause when one celestial object is pushed around by another, but both of these objects are too far away from our gas giants to be influence (“Last Great Mysteries”, 2016).
The search for exoplanets and exomoons is providing astronomers with a glimpse of other worlds; possible even a glimpse into Earth-like planets.

In 1990, Carl Sagan prompted the Voyager 1 crew to take a photo of Earth. Sagan likened the picture to “a mote of dust suspended in a sunbeam.” (Belikov & Bendek, 2015). There are billions of stars in our Milky Way galaxy alone, so it’s possible for there to be other planets like our home. The findings of a planet like ours could revolutionize how life on our planet was formed. As technology increases, astronomers will be able to better observe a planet’s atmosphere and surface composition, which could find an Earth-like planet. The search for extraterrestrial worlds is one the most fundamental and noble pursuits of astronomy. The findings of these planets would launch us into our greatest astronomic achievement: the exploration of interstellar worlds beyond our home solar

system.