What is an Exoplanet?
Outer space has intrigued astronomers and scientists for decades. However, a subject that has particularly garnered attention is exoplanets. Exoplanets can be defined as any planet beyond our solar system. These include both planets that orbit other stars, as well as other free-floating exoplanets, called rogue planets (read our guide to rogue planets), that orbit a galactic center and are not tethered to any star.
This exoplanet definition encompasses planets of varying compositions, shapes and sizes. While some exoplanets are icy, some others can be rocky.
In the debate of planet vs exoplanet, the key differentiating factor is the star around which these bodies orbit. We can find analogies between planets and exoplanets, however, the central star and the celestial system pivotal to these bodies distinguishes them.
The Discovery of Exoplanets
The first exoplanet discovered by researchers was in the early 1990s, which set off interest in them and led to advancements in the study and discovery of more of these worlds beyond our solar system. On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail spoke about the two exoplanets discovered by them, orbiting the pulsar PSR 1257+12. This discovery, considered to be the first definitive detection of exoplanets, was later confirmed.
Exoplanets can rarely be discovered using the conventional direct imaging where astronomers look for exoplanets, like other planets through their telescopes. Only the young gas giant planets orbiting very far from their stars can be discovered using this technique. Over the last few decades, there have been improvements made to the number and the observing power of the telescopes launched into space.
Most of the other exoplanets are now discovered using indirect methods. These include measuring the dimming of a star that happens to have a planet pass in front of it, called the transit method, or monitoring the spectrum of a star for the indicative signs of a planet pulling on its star and causing its light to subtly Doppler shift.
A lot of exoplanets have been discovered by observing “transits,” i.e. the slight dimming of light from a star when a tiny planet passes between it and our telescopes. Some other observation methods include gravitational lensing, all called the “wobble method.”
These methods, when used in a combination, can help researchers study the statistics of a planet system without imaging the planets themselves. One such planet system discovered using a combination of methods is TRAPPIST-1 which is around 40 light years away and has seven roughly Earth-sized planets orbiting a small, red star. This planetary system has been studied using both ground and space telescopes and researchers have learnt about the diameters and the mass of the planets using the gravitational pull between the planets.
In the last few decades, most of the thousands of exoplanets discovered have been found using NASA’s Kepler Space Telescope.
Exoplanets Discovered using NASA’s Kepler Space Telescope
Most exoplanets discovered can be credited to the Kepler Space Telescope, an observatory that began its work in 2009. As of mid-March 2018, Kepler had confirmed 2,342 exoplanet discoveries and had also revealed the existence of ~2,200 others. A large fraction of the exoplanets discovered so far, lie in a relatively small region i.e. within thousands of light-years of our solar system of our galaxy, the Milky Way.
Exoplanets are very distant from Earth – even the closest known exoplanet to Earth, Proxima Centauri b, is still about 4 light-years away. Proxima Centauri b, discovered in 2016, is a super Earth exoplanet that orbits an M-type star.
The Kepler mission was specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size planets and smaller planets in or near the habitable zone (“Goldilocks zone”) i.e. the area around a star where rocky planets could have liquid water on the surface. The Kepler telescope began its extended mission, K2, to stare at many different parts of the sky for 80 days at a time, after the second of Kepler’s four gyroscope-like wheels failed in 2013. Although the spacecraft was retired in 2018, Kepler data is still being used to find exoplanets.
One of the major learnings from data from NASA’s Kepler spacecraft is that exoplanets of a certain size-range are rare – those between 1.5 and 2 times the diameter of Earth. This would place them among the super-Earths.
One of other telescopes that has aided the discovery and study of exoplanets is NASA’s Spitzer Space Telescope (2013-2020). Although it was not designed to search for exoplanets, its infrared instruments aided.in the notable discovery of the TRAPPIST-1 system.
Moreover, in 2018, the Transiting Exoplanet Survey Satellite (TESS) was launched as a successor to the Kepler telescope to discover exoplanets in orbit around the brightest dwarf stars, the most common star type in our galaxy. Some other notable telescopes that hold great promise for exoplanet discovery are NASA’s James Webb Space Telescope and the Nancy Grace Roman Space Telescope.
Process of Discovering an Exoplanet
There are five common methods scientists commonly use to discover exoplanets, as shown in the image below: Radial Velocity, Transit, Direct Imaging, Gravitational Microlensing and Astrometry. Of these, the two most common techniques are the transit and radial velocity methods.
To describe it further, the transit method relies on the instance when a planet passes directly between an observer and the star it orbits, and in this process it blocks some of that starlight. For this brief period of time, the star’s light gets dimmer or the planet casts a shadow. Although a tiny change, this is enough to indicate to the astronomers about the presence of an exoplanet around a distant star.
The other prominent method looks for wobble in the stars caused by orbiting planets. This changes the color of light seen by astronomers while observing a star.This wobble is caused by the gravitational tug caused on stars by their orbiting planets. This affects the star’s light spectrum. The star appears blue. If it is moving away from the observer, if the star moves in the direction of the observer and appears red when it moves away from the observer. This is the radial velocity method.
After the exoplanet has been discovered, the next step is to name the exoplanet. The first part of an exoplanet name is commonly termed after the telescope or survey that discovered it. The number that follows it, is the order in which the star was cataloged by position. The lowercase letter in the name of the exoplanet represents the order in which the planet was found. The first planet found is always named b, with following planets named c, d, e, f and so on.
Types of Exoplanets
Researchers have categorized exoplanets discovered so far into the following categories: Gas giant, Neptunian, super-Earth and terrestrial.
A gas giant is a large planet often composed of helium and/or hydrogen. Like Jupiter and Saturn in our solar system, they don’t have hard surfaces. Instead they are composed of swirling gasses above a solid core. They can typically be much larger than Jupiter, and much closer to their stars than anything found in our solar system. Some of the largest planets in the exoplanet list are from this category. The largest exoplanet discovered so far is the HD 100546 b.
Neptunian exoplanets have sizes analogous to Neptune or Uranus in our solar system. They typically have hydrogen and helium-dominated atmospheres with cores of rock and heavier metals. Super-Earths are more massive than Earth yet lighter than ice giants like Neptune and Uranus. They can be made of gas, rock or a combination of both. They are unlike any other planets in our solar system and are between twice the size of Earth and up to 10 times its mass. Some of the prominent and largest exoplanets in this category are HAT-P-26b and GJ 436 b.
Terrestrial exoplanets are half of Earth’s size to twice its radius. Some others may be even smaller. Earth, Mercury, Venus or Mars in our solar system are rocky terrestrial planets. These exoplanets are rocky worlds, normally composed of rock, silicate, water and/or carbon.
Planets vs Exoplanets
While comparing planets vs exoplanets, it is key to note that planets that orbit around other stars are called exoplanets. Going by the definition of planets, we only have 8 planets. However, there are thousands of exoplanets discovered by researchers around the world. Moreover, there are various factors differentiating the habitability and the composition of exoplanets and planets. The lists of planets we know are limited. However, the exoplanet list is growing constantly and is continuously being studied further.
Our Knowlege of Exoplanets is Evolving
In March 2022, NASA confirmed that over 5000 exoplanets have been discovered marking the 30-year journey of discovery led by NASA space telescopes. In March last year, a list of 65 exoplanets were added to the NASA Exoplanet Archive, thus further extending the exoplanet list. While the exoplanet list is constantly growing, this field of study is evolving and opening up newer advancements in the study of outer space and astronomy.