An exoplanet, or an extrasolar planet, is defined as any planet found outside the Earth’s Solar System. The galaxy contains billions of stars that resemble the Sun; some of them have planets orbiting them, thus forming more or less similar systems to our own one. Such planets are usually smaller than the star they orbit and they do not emit light; thus, they cannot be viewed by traditional telescopes. The majority of these exoplanets have eccentric orbits rather than the nearly circular orbits of the Solar System. Exoplanets have been observed by means of special equipment, mathematical calculations, and by monitoring the movements of the stars. Movement exhibited by stars with planets orbiting them differs from that of stars without planets. This is due to the fact that planets usually have gravitational pull against their star. The pull of each planet may be weak, but the collective gravitational power applied by all the planets of a given star is rather significant; thus, such force affects the star’s movement (Tate, 2009).
In 1988, Campbell, Yang and Walker discovered an exoplanet around Gamma Cephei.This was the first reliable discovery of exoplanets that was independently confirmed. However, their discovery was met with skeptical response from astronomers since the observations were conducted under the limitations of the equipment employed. Nevertheless, the discovery received solid confirmation in 2003, when the team’s original results were confirmed by astronomical world; this conformation could be performed due to a significant advancement in technology. By October, 2012, 843 exoplanets had been discovered. This report focuses on exoplanets with the aim of discussing the discoveries being made in the field. In addition, the report highlights the future trend in the discoveries of exoplanets (Wilkinson, 2009).
Searching for Exoplanets
In the past, the exoplanets that were well-known to the scientists were only the giant ones, similar to Neptune and Jupiter in size; they are massive and, therefore, easy to observe. However, with the advancement in technology, relatively light in weight planets have been discovered, like Super-Earth. Super-Earth is defined as an exoplanet that is a few times bigger than the Earth and is likely to have earth-like environment (Seager, 2010). Scientists have intensified search for such exoplanets within the habitable zone, which is the region surrounding a star in which orbiting planets are more or less similar to the Earth (Figure 1). It is suspected that water can be found on the surface of these exoplanets; this fact raises a question whether life exists on such planets. This search for extraterrestrial life in habitable zones, which are believed to be neither too hot nor too cold, has contributed to the interest scientists have in this felid (Pilat-Lohinger, 2009).
The search for habitable planets is being conducted outside the Solar System. Different missions employ various techniques in the search for the stars. The proposed TESS mission, financed by NASA, will focus on exoplanet transits, which occur when a particular planet passes in front of its own star. Such a transit dims that particular star for a short period of time. TESS mission will embark on scanning the entire sky in order to find new exoplanets. The scan will focus on stars approximately 200-light years away from the Earth. Kepler mission focuses their search on a single region of the sky. The mission recently discovered two exoplanets which are about the size of the Earth (Seager, 2010).
Over time, indirect methods have been employed in detection and characterization of planets. Save for a few exceptional cases, these methods do not allow actual visualization of the planets as the bright light from the stars often overwhelms that of the planets’, hence making it hard to view the planets with ordinary telescopes. Doppler shift method, which relies on gravitational effect of the planets on the stars, has been applied widely. The method employs the use of telescope to measure the wavelength of oncoming light from a particular star over a given period of time: days, months, or even years. The variation in wavelength of light that comes from a certain star is used to infer for the presence of an exoplanet in its system (New World, 2010).
Spectroscopy involves the study of interaction that occurs between matter and light by carrying out analysis of light produced and absorbed by various substances. The different characteristic spectra produced by various substances act as their fingerprints. Therefore, it is possible to find out what the atmospheres of exoplanets are made of just by carrying out spectroscopic analysis. The presence of life on Earth affects the composition of the atmosphere. It is also believed that, if life were present on another planet, its atmosphere would be affected in a similar manner. Certain gases, like molecular oxygen, are known to be abundant in the atmosphere as a result of life present on Earth. This means that it is possible to determine whether or not a given planet is inhabited by searching for these gases. It is predicted that simple life, probably single celled organisms resembling bacteria, will be detected rather than complex life beings like plants and animals (New World, 2010).
Transit Spectroscopy to Determine Earth’s Chemistry
Spectroscopy is a tool routinely used by astronomers to identify the chemical compositions of objects. This is possible due to the fact that every atom and molecule have unique signature expressed in the form of light wavelengths, which can be either brighter or faint. When atoms are located in a hot transparent gas, the light emitted is usually brighter, thus producing emission lines. On the other hand, when light passes through a cool transparent gas from another hotter source, the light produced is fainter, hence resulting into absorption lines. The spectra of the produced lines, either emission or absorption, are characterized by different wavelengths, which are the signatures that reveal the available types of atoms as well as molecules (Heckert, 2007).
Transit spectroscopy has been utilized to analyze chemical composition of planets such as HD 209458b, which is a transiting planet. The planet usually passes in front of its star every 3.5 days (Heckert, 2007). When the planet passes behind the star, or when it is fully eclipsed by the star, the scientists measure the star’s emission without obstruction from the planet. When the planet is not eclipsed, then the spectra measured corresponds both to the planet and the star. The difference between the spectra found when the star is alone and that found when both the star and the planet emit light is important in determining the specific atoms and molecules present in their structure.
The use of Spitzer Space Telescope, as well as Hubble Space Telescope, in order to probe the atmosphere of hot gaseous exoplanets in transit has proved that it is quite possible to characterize the atmosphere of exoplanets. Hubble Space Telescope operates in the lower orbit of the Earth; it analyses short wavelength infrared and cosmos light. Spitzer views long wavelength infrared light; it includes 3 experimental devices: an infrared spectrograph, a multiband imaging photometer and an infrared array camera (Tessenyi et al., 2012).
Problems Associated with Super-Earth in Red Dwarf’s Habitable Zone
There has been much debate on possibility of having habitable planets in the region around red dwarf stars. This is due to the problem of tidal effects on the planets located around the red dwarf stars, which might lead to extra surface heating as well as tidal locking. Since the stars are active, it is believed that their stellar flares are capable of eroding planetary atmosphere much faster (Laura, 2012).
The James Webb Space Telescope (JWST) is a project that is yet to be launched in 2018; it is expected to complement and even extend discoveries that would have been done by Hubble Space Telescope. The JWST project uses the Near-Infra Spectrograph (NIRSpec) instrument, which is capable of viewing over a hundred sources simultaneously. On the other hand, Mid-Infrared Instrument (MIRI) has a sensitive camera and spectrograph which is capable of making observation far away in time as well as space, dating back to the period when the galaxies were still young (Hubble Site, 2008). Advanced Technology Large-Aperture Space (ATLAS) Telescope is a mission that will be able to characterize the atmosphere as well as the surface of an exoplanet of the same size as the Earth. This will occur in the long lived stars’ habitable zones at distances up to 45 pc, including the planets habitability, rotation rate, and climate (Postman, 2007).
An exoplanet is a planet that orbits a star outside our Solar System. The research on the presence of exoplanets has been on for decades. The methods of observation that have been used over time have been biased to detection of Jupiter sized planets. Currently, the hunt for exoplanets have intensified with advancement in technology and renewed interest in discovering life within the habitable zone. Characterization of planets has widely been performed by indirect methods such as Doppler Shift Method, which was largely limited. Scientists hope that utilizing transit spectroscopy, which determines the Earth’s chemistry, and spectroscopic analysis, which reveals the composition of the exoplanet’s atmosphere, will further the knowledge concerning exoplanets. The proposed satellites such as NIRSpec, MIRI, and ATLAST will enable a wide view of the space at once, thereby enabling more discoveries.