Exoplanets: A World Outside of Our Own

Over 6,000 exoplanets have been discovered in the three decades since the first planet was discovered outside our solar system. NASA’s definition of an exoplanet is “any planet outside of our solar system.” As our space knowledge increases and our telescopes improve, the number of discovered exoplanets will continue to rise.

The first discovery of an exoplanet came in 1992 at the Arecibo Observatory in Puerto Rico, when two exoplanets were discovered orbiting PSR B127+12, a pulsar 2,300 light years away from Earth: Poltergeist (PSR B1257+12 c), with a distance of 34 million miles from the pulsar, and Phoebeter (PSR B1257+12 d), with a distance of 43 million miles. In 1994, a third planet, Draugr, was discovered in the same system 17.4 million miles from the pulsar star. These first exoplanet discoveries were groundbreaking in the field of astronomy and led to the discoveries of other celestial objects, ultimately revolutionizing our understanding of our universe.

How do exoplanets take form? It all begins in a nebula, or a cloud of gas and dust, where once disturbed by a celestial object, the matter is pulled toward the center, which will later go on to form the host star. When the cloud begins to shrink, the matter starts spinning in a disk-like shape and flattens. Once this happens, the small particles of gas and dust run into each other and stick together, forming pebbles and eventually larger rocks. As the rocks crash into each other, their gravity increases, allowing them to collect other parts of debris. The rocks grow larger and larger as the host star forms. Once the rocks (now planetary embryos) become large enough, they can clear their own orbit. Once the clearing occurs, they are officially considered planets.

While there are many ways of categorizing exoplanets, the broader categories include terrestrial, neptunians, super-Earths, and gas giants. Terrestrial planets are small planets primarily composed of rock, metal, silicate, and carbon, with a solid or liquid surface. Some may have atmospheres of liquid water, such as LHS 1140 b, whose density leads us to believe it is an ocean world, as well as it being fortunately positioned within the goldilocks zone of its star. Characteristics similar to these make planets outside our solar system potentially habitable, which is why scientists tend to study them extensively. According to NASA’s Kepler Space Telescope, “20 to 50 percent of the stars in the sky are likely to have small, potentially rocky planets in their habitable zones.” Using Kepler Space Telescope data, we can estimate that 300 million planets have the potential to be habitable, the closest of which could be 20 light years from Earth.

The second category of exoplanets consists of super-Earths, which are rocky planets more massive than Earth yet lighter than ice giants such as Neptune and Uranus. Super-Earths can be up to 10 times the mass of Earth as well as twice the size. A Super-Earth’s category is based primarily on its size in comparison to terrestrial planets, which are categorized due to their composition. There can be many different types of super-Earths, from rocky planets to gaseous neptunes to ocean worlds—the compositions can range drastically.

The third broad classification of exoplanets is Neptunian planets, which (similar to super-Earths) are categorized based on size rather than composition. Neptunians, as their name suggests, are a similar size to Neptune or Uranus, yet smaller than gas giants like Saturn and Jupiter. While these planets have a broad range of characteristics, their atmospheres tend to be mainly composed of helium and hydrogen. However, their cores tend to be rocky with a composition of metals. The reason why Neptunians have a variety of different sub-classifications is because they are categorized mainly on size and their atmosphere, which creates sub categories such as mini-Neptunes, warm-Neptunes, and hot-Neptunes. Because planets like Uranus and Neptune contain the chemical compounds methane, ammonia, and water in a frozen form, they are often referred to as “ice giants.” Neptunian planets are more likely to form in the outer areas of planetary systems, where the isolation of the planet creates the icy exterior. While Neptunians are the least likely to be habitable, they provide information on the earlier ages of the Universe, such as how certain planetary systems formed and the gravitational shifting within orbits.

Lastly, there are gas giants, which are planets roughly the size of our Saturn and Jupiter. However, they can be much larger. Gas Giants’ atmospheres are composed mainly of hydrogen and helium, making them much larger in size than Earth. Their sub-categories include hot-Jupiters, warm-Jupiters, and super-Jupiters, which are all specified categories based on temperature or size. Instead of having a solid surface, they have a solid core and a surface made of a mixture of gases. Gas giants can range from very long orbital periods to extremely short orbital periods of just 18 hours to complete the trajectory around their host star. Unlike our own solar system, where Jupiter and Saturn are located just outside the asteroid belt, roughly 1.2 AU to 2.2 AU from the sun, gas giants have been discovered at very close and very far distances orbiting their host stars. It is celestial oddities such as these that make humans question the formation of the solar system. Do planets form farther from the star and migrate inwards? Or are they forming a short distance from the star and migrating further away? We are able to explore the possibilities of planet migration through gas giants because there is no necessary distance for the planet to be located.

These ideas lead us to the question: How can we detect these planets? There are five main methods of detection: astrometry, radial velocity, transit photometry, gravitational microlensing, and direct imaging. Astrometry tracks the star’s position and studies the “wobble” of the host star based on exoplanet pull. The wobble is then compared to the stars in the background, which allows us to estimate a planet’s mass as well as its orbital inclination. However, astrometry requires highly precise data, making it more susceptible to error. For that reason, only a small number of planets have been discovered through it. Radial velocity is very similar to astrometry in that it tracks the star’s “wobble” as well; however, it is different in that when viewing exoplanetary systems from the side, we use radial velocity, whereas astrometry is used when the system is viewed from above. This is because in radial velocity, we are essentially collecting the different wavelengths of the planet: the red and blue shift. When the planet moves towards us, we view the wavelength in blue shift, versus further away, which creates a red shift. By measuring the change in wavelength, we are able to calculate the velocity of the planet, and we can then collect information on the planet’s mass and its orbit. Transit photometry is used when a planet passes in front of its host star. When this occurs, there will be a slight change in the luminosity of the star or a “dip". This allows us to detect the planet. However, the only flaw to this method is that it takes the planet’s orbital period (however long that may be) to track it again. Gravitational micro-lensing involves the capturing of the bending of light of a different star as it passes behind a host star. When two stars align, the star behind the host star’s light curves around it due to Einstein’s theory of general relativity. If a planet is in the vicinity of the host star, the light will bend around the planet as well, allowing for its detection. While many planets have been discovered through gravitational microlensing, it is unrepeatable because the two stars will never align again, making it a one-time opportunity to discover it.. Finally, we have direct imaging, which involves using the light of the planet itself to identify it. Because the planet emits its own infrared signal, it can be captured by telescopes and separated from the star signal. However, direct imaging can be incredibly difficult to do because the light from the host star can oftentimes interfere with the detection.

When the idea of potential planetary systems outside of our own comes to mind, the question of habitability often arises. In order for a planet to have the potential for habitability, it must have a few specific characteristics, including living in the “goldilocks zone” or the orbital radius interval at which liquid water can exist. Not too close, yet not too far from the host star. Where the goldilocks zone is located is also dependent on the size and luminosity of the host star. The second characteristic would be the composition of a planet, which is very important in contributing to the sustainability of life. Using transmission spectroscopy on planets, we are able to determine the composition of the planets based on the elements they contain. The most important are biological elements such as nitrogen, carbon, and oxygen. We would also have to ensure the atmosphere is sustainable: that it has a sufficient balance of greenhouse gases to keep the climate controlled and that it has enough pressure to preserve the liquid water.

There are currently many new discoveries involving exoplanets; a few of the space missions have made incredible contributions. The Kepler Space Telescope, launched in 2009, was one of them (its first planet searching mission.) It surveyed planets and solar systems, as well as cataloging possible habitable zones. The mission went on for 9 years until October 30th, 2018, when it emptied its supply of hydrazine fuel in totality. The TESS, or the Transiting Exoplanet Surveying Satellite, was a mission run by MIT which launched in 2018. It searched for visible light wavelengths and completed that task so well its mission was extended through 2028. Lastly, the Nancy Grace Roman Space Telescope, which launched on August 30th, 2026, will investigate exoplanets and dark matter. It will use the methods of gravitational microlensing, direct imaging, and transit method in order to block starlight and locate exoplanets directly. Its field of view is 100x larger than that of the Hubble Space Telescope, allowing for more data capture.

Exoplanets are an important part of modern astronomy and allow us to have a better understanding of our Universe and planetary systems. By identifying different types of exoplanets, we are able to broaden understanding of the possibilities of extraterrestrial life. It’s possible to have a planet orbiting two host stars, it’s possible to have a planet orbit its star in less than 18 hours, and it’s possible to have a planet with a density of 0.093g/cm2. As space technology advances, so does our grasp on the limitations of, and potentials for, life on other planets. The universe is filled with trillions of worlds we’ve yet to discover, and we’ve only just begun to look.