What’s Outside Our Solar System: Observing Exoplanets
Without all of the science, our solar system is essentially just a bunch of planets revolving around the Sun.
This solar system is a part of the Milky Way Galaxy, a huge collection of dust, stars, and their solar systems. What’s really interesting is that we’re not the only ones in this galaxy. Not that I’m saying that there’s extraterrestrial life (there might be), but there are a bunch of other star systems that surround us.
How much do we actually know about these star systems? A lot. Actually these star systems are one of the most researched topics in astronomy. Let me break it down.
The System of Stars
The universe is made up of several stars, either binary systems or multiple systems. A binary system is where two astronomical bodies are close enough that their gravitational attraction causes them to orbit around each other. While a multiple star system consists of three or more stars. Almost half of the stars in the universe are in binary systems!
The Hertzsprung-Russell Diagram
Stars can be classed by their spectra (the elements they absorb) and their temperature. There are seven main types of stars classifying in order of decreasing temperature; O, B, A, F, G, K, and M. The Sun for example, belongs to class G, a yellow dwarf, in the main sequence phase.
The Hertzsprung-Russell Diagram is a graph that plots the star colours (surface temperature or spectral type) compared to their luminosity (absolute magnitude). Astronomers use these diagrams to plot stars’ colour, temperature, luminosity, spectral type, and evolutionary stage.
Stars have specific nuclear properties according to their classes or groups. The high spectral resolution of stars allows for discovery of their physical and chemical properties. In particular, astronomers were able to detect exoplanets from characterizing stars.
What’s an Exoplanet?
Planets in our solar system orbit around the sun. Exoplanets orbit around other stars in our galaxy. The only problem is that they’re hard to see because they’re usually hidden by the bright glare of their host stars.
Some exoplanets discovered are found in binary star systems while others are in multiple star systems. Exoplanets revolve around a variety of stars but not around stars of the main sequence only.
The first confirmed exoplanet was found to orbit around a class K star which is rich in heavy elements. Currently, projects like Anglo-Australian Planet Search (AAPS) are working towards finding exoplanets that orbit around F, G, K, and M stars.
There’s several types of exoplants. The majority of exoplanets are discovered by researchers to be gaseous giants. Gas giants are mainly composed of hydrogen and helium. In our solar system, Jupiter and Saturn are classified as gas giants. None of these exoplanets are similar to Earth. Planets have been found to be ten times the mass of the Earth, called Jupiterian-like planets. For example, an exoplanet that is eight times the mass of Jupiter was detected around a star named KO I-13.
Other exoplanets are similar to planets like Mercury and Neptune. Exoplanets equivalent to the size of Earth have also been discovered. There’s are estimated one hundred billion exoplanets in the Milky Way Galaxy alone.
So how do we actually detect these exoplanets. Think about it, they’re light years away. Astronomers currently have two main methods of observation.
Radical Velocity Method
The radical velocity method is the first method to successfully detect exoplanets. It’s based on the observation of the oscillation or wavelength variation in the star’s spectral lines (a dark or bright line in an otherwise uniform spectrum, resulting from emission or absorption of light in a narrow range).
Most of our knowledge of exoplanets comes from the radical velocity method, It has an advantage to detecting exoplanets and determining the orbital period along with the eccentricity.
However, the mass of the planet detected by this method isn’t accurate. That’s because of the poor knowledge of the inclination angle between the orbit of the plant and the astronomer’s sight line.
The transit method is based on the observation of the exoplanet-star eclipses seen by the astronomer. The orbital inclination of the planet and its relative radius in relation to its parent star can be observed during transit.
The transit method can help reveal the mass, radius, and the density of the detected planet. The disadvantage is that the light flux of the planet is very weak compared to its host star.
Combining Detection Methods
Combining the data collected from the transit method and the radical velocity method, the masses and rays of the exoplanet can then be obtained. Other physical parameters can also be derived from the combination of the two methods.
For exoplanets in a simple star system, the radical velocity method is simpler to find the masses of the planets, whereas the transit method is more suitable for multiple star systems.
One of the most developed areas of astrophysics is the discovery of exoplanets. However, discovery is difficult because the light sent from an exoplanet is considerably weak compared to its host star.
Though exoplanet discovery is difficult, one of the most common detect methods is through the use of terrestrial telescopes.
In the AAPS project mentioned earlier, the Lick and Keck telescope can detect exoplanets around certain star varieties. Mostly F, G, K, stars and M stars which have magnitudes lower than 7.5.
Terrestrial telescopes play an important in the characterization of gaseous planets during transit. Today, terrestrial telescopes can detect satellites of Earth-size exoplanets that are in the habitable zone of their host star. In the future, large terrestrial telescopes will detect extrasolar planets that are in the habitable zone of the main sequence stars.
On Earth, it’s impossible to detect exoplanets which are smaller than Earth due to Earth’s atmosphere as ground based telescopes cannot obtain more astronomical object waves. However, spatial telescopes are effective in the detection and characterization of exoplanets.
Convection Rotation and Transits (COROT) is the first spatial mission using the transit observation method aiming to detect stellar seismology and exoplanets during their transits with high precision photmery.
The Kepler mission launched by NASA for studying astronomical objects by the transit method could pick up on photometric signals with high resolution. One of the objectives of the mission was to discover exoplanets orbiting in the habitable zone of their host stars.
Some spatial telescopes carry infrared instruments. The Hubble telescope can observe in the ultraviolet and optical wavelength. It allows the Hubble to provide astronomical data about the atmospheres giant gaseous exoplanets.
Instruments and Detections
There’s a bunch of noise that clouds the data collected by astronomers on exoplanets. Optical and interferometric technological devices are used to remove these obstacles that drive astronomers to false results during observations.
Observation of astronomical objects in the infrared wavelength is on the most efficient methods in the interstellar and interplanetary areas. Astronomers can draw some deduction about mineralogy composition of one planet in this domain of wavelength observation.
Instruments in the near future will be fit out with infrared, coronagraph and adaptive optics devices. JWST which is going to carry four infrared and spectrometric cameras, such as NIRCAM, NIRSPEC, MIRI and FGS, can study rocky Earth-like exoplanets and will work in the fields of infrared to detect more molecules in exoplanets.
Infrared cameras can detect stars in the binary system. New instruments have been invented to detect new exoplanets and to resolve observational issues such as luminosity between exoplanets and their host stars.
To understand the formation and evolution of planets, data from their host stars is helpful. The relationship between X-ray emission by a star’s corona, the age of the same star and the change of this relationship are requisite to reveal the following exoplanets’ history.
A strong interaction between planets and their host stars play an important role in the movement of each component as it synchronizes the rotation of the system. Analytical methods have been invented to calculate the force between astronomical objects that form a system in order to determine the planets and star velocities as well as positions with time.
Other characterizations such as orbital inclination of one planet and its radius relatively to the host star are obtained by means of photometry observation series during the corresponding planet transit phase.
Mass and radius of planet play important roles in the planetary internal structure and composition. These parameters can be obtained from stellar properties.
Spectral rays sent from astronomical objects hide their atmospheric compositions since the atmosphere characterization of an exoplanet is based on the studies of its host star too.
Spectroscopic observations of Jupiterian-like exoplanets have allowed astronomers to derive the thermal structure and molecules in their atmospheres. Astronomers are able to decipher atmospheric compositions of gaseous exoplanets from infrared data.
The characterization of the atmospheres of exoplanets is easier in the observation of spectra in the ultraviolet, optical and infra-red domains.
Atoms and molecules have been detected in exoplanets smaller than the size of Neptune! The composition, structure and evolution of planets that orbit a star depend on the star chemical compositions.
Scientists have gathered all the different molecules which exist in our solar system to determine the chemical components of exoplanet that similar to Earth.
Research for Extraterrestrial Life
What does all of this mean? Can we live on an exoplanet similar to Earth? Well, not exactly, but some really interesting findings are coming out of exoplanet discovery.
Research for habitable exoplanets is currently being done. Habitable exoplanets are equivalent to the search for planets having the same conditions as Earth and should orbit around a star similar to the Sun. It’s also proposed to observe exoplanetary satellites or exomoons because they may hold more habitable environment than giant planets.
The Future of Exoplanets
Water is still the main basis of life. Due to the presence of oxygen in the atmosphere of an exoplanet, it means life on such planets is not possible. The current priority is the detection of water and oxygen outside the solar system because of their roles as being the sources of life. Until then, we wait.
Hey, I’m Sabrina! I’m currently working on projects in Brain-Computer Interfaces and Connectomics. If you liked the article feel free to follow me on Medium, and connect with me on Linkedin!