Exoplanets |Definition, Properties, and Facts

This article has been just updated: January 6th, 2020

What is an Exoplanet

An exoplanet is also known as an Extrasolar planet i.e both Extrasolar planet and exoplanet have the same definitions. However, In this article we will cover “what is an exoplanet?”, its definition, detections, properties of exoplanets, how many exoplanets are there? etc. In simple terms, we can say that an exosolar or exoplanet is a planetary body which is basically outside the solar system. As a matter of fact, now more than 3000 exoplanets are already known and still more that than 1000 may be found in the near future. As well as that, first exosolar or exoplanet was found and discovered in 1992 i.e in this time the search of the first exoplanet was confirmed. We know that these exoplanets are the planet that is beyond our solar system. In this article, we will also cover Earth-like planets.

Earth-like planets

Without a doubt, most of the exoplanets or extrasolar planets are found by NASA’s Kepler Space Telescope. It is worlds orbiting other stars. Many exoplanets are already found through the ground and space-based observatories. Furthermore, the exoplanets or Earth-like planets are basically searched, and still search is going on so that the scientist, and astronauts can be sure that they are planets. It is estimated by the scientist and astronauts that there can be trillions of planets available around other stars. The huge number of exoplanets are already found, and remaining are being searched i.e awaiting confirmations.

Facts about Exoplanets

Facts about » Exoplanets |Definition, Properties, and Facts
  • As a matter of fact, there are many facts related to exoplanets or, Earth-like planets such as – astronauts are trying to search, find, and measure the gases present in exoplanets, it allows the astronauts to understand the gases exist in gaseous envelopes.
  • Moreover, one of the interesting facts is that astronauts can measure various important factors such as – surface temperature, orbits, colors of Earth-like planets, and also magnetic fields. Finally, scientists will able to find more information as the technology, and detections method develops or improves.
  • Apart from this, there are many zones in exoplanets where life can be supported, the surface is called the habitable zone where water can exist, and these zones are also considered as a prime candidates zone.
  • As a matter of fact, there are many sun-like stars whose size is equal to earth. these vary important places where life can be possible.
  • As we know that there was a mission launched to search for distant worlds. The name of the mission was – Kepler’s mission. Furthermore, the Kepler mission still continues today. The main aim of this Kepler mission was to search for distant worlds which include Hubble space, and telescope. The main and important part in the search of exoplanets and Earth-like planets is to continue ground-based observation to search for distant worlds.
  • The first exoplanets or Earth-like planets which is detected, and confirmed was in 1992 which was the detection of two planets that exist outside the solar system.
  • As per the reports of 2018, till now 3,578 Earth-like planets, or exoplanets have been confirmed.
  • As we already mentioned, the search for exoplanets is done by NASA with there Kepler mission. According to NASA till now they have detected 7000 planets using their space telescope.

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Facts about Exoplanets 1 » Exoplanets |Definition, Properties, and Facts

In our own solar system, the planets come with exoplanets which are various sizes and compositions from small rocky planets to huge gas giants. Exoplanets differ from each other. Over the past 20 years or so years, astronomers have confirmed the existence of thousands of exoplanets or planets around the other stars. Exoplanets vary enormously in size, mass, composition, and nearly every conceivable parameter. Whenever it comes to exoplanets astronomers know the properties of the planets they discover as well as they know the properties of the stars being orbited. Astronomers group types of exoplanets are as follows Earth-size, Earth-like, Super-Jupiters, Gas giants, Rocky worlds the size of the Earth, Rocky-giants, Super-Earths, mini-Neptunes and gas dwarfs.



Super-Earths are the rocky planets of between 1 and about 10 Earth masses. In addition, these are Earth-like planets with a low-bulk density, Super-Earths are mainly composed of Hydrogen and Helium; with higher densities are water-rich and silicate-rich. Neither super means super habitable nor says about the surface conditions of the exoplanet, it merely means larger than our Earth. However, Super-Earths might be more suitable for life than our Earth but there are no Super-Earths in our solar system.


Rockey Planets are also known as terrestrial planets. These exoplanets generally composed of heavier elements like silicate rocks or metals. Their solid planetary surface makes them suitable for harboring complex life.


The exoplanets are similar to Saturn and Jupiter are Gas Giants. Gas Giant planets composed of hydrogen and helium. Gas Giants are also known as failed stars. In addition, the basic elements of star and Gas Giants are similar. The planets exceeding 10 Earth masses are all called as Gas Giants. Gas Giants probably have a small rocky core. In or solar system there are four Gas Giants Jupiter, Saturn, Uranus, Neptune (though Neptune and Uranus are known as ice-giants due to a high amount of water and Ammonia content. The first exoplanets were discovered as Gas-Giants.


A Hot Jupiter is a high-temperature variant of a Gas-Giant. However, Hot Jupiter orbits its host star in a very close orbit(0.015-0.5AU). In addition, hot stars are very common in our galaxy which is not what astronomers expected as gas giants orbit in cold regions in the solar system. Hot Jupiters can be detected via the radial-velocity method easily. It has been proposed that red giants at a distance similar to Jupiter could be hot Jupiters.


Rogue planets are the exoplanets dark and very difficult to detect as there is no star around. These are called orphan planets. are free-floating planets and believed that these isolated worlds were ejected from developing systems. However, few of them have been detected which are very common in our galaxy. The number of Rogue planets in our Galaxy exceeds the count of stars therefore 200 to 400 billion rogue planets are floating through the Milky Way. Especially these are small rocky planets to huge giant gas which are very difficult to detect. They can be detected via microlensing. Microlensing causes a background star to appear a little bit brighter when a massive object passes in front of it.


These exoplanets are rapidly rotating neutron stars. These exoplanets are discovered to detect anomalies in the pulsation period. After the supernova explosion, these super-dense rapidly spinning stars are the remains of the large stars. These exoplanets are formed after the supernova explosion and now orbit around the dead star. These planets are the first type of planet that was discovered beyond the solar system and it shocked the astronomical world.


These exoplanets are entirely covered in water. These exoplanets originally formed of objects made of ice and rock far from their parent star. As they came close to the star, started melting the ice and formed oceans. The entire exoplanets are now densely covered with oceans like the consistency of ice. Water worlds possess thousands of times more water than the Earth may be more common than Earth-like planets( Rocky planets).


These exoplanets are gas giants but migrated far too close to their parent star. Their atmosphere has been roasted away leaving only a rocky and metallic core. It may be possible that their surface may be covered by molten lava. Some Super-Earths may actually be chthonian planets due to similarity in terrestrial planets.


These exoplanets are having the same mass, radius, composition, and atmosphere to Earth. These exoplanets are called Earth-like planets. The exoplanets exist in the habitable zone of the star. Although several Earth-sized planets have been discovered. Although several Earth-size planets have been discovered orbiting in the habitual zone around the stars.


Facts about Exoplanets 2 » Exoplanets |Definition, Properties, and Facts

With the currently available telescope, it is not possible to detect exoplanets orbiting other stars that are too far away. Direct images of exoplanets are currently exceptions and in any case, we will not be able to see much more than a dot light. Nothing that will reveal any surface details of the planet. Since the planets cannot be observed directly, the planet hunters decided instead to observe stars and look for the minute effects that orbiting planets can have upon them.


The radial velocity method is also known as Doppler spectroscopy. Essentially the radial velocity method consists not of looking for signs of planets themselves. But in observing a star for signs of movement. Up until the launch of the planet-hunting spacecraft Kepler in 2009, radial velocity was the most effective method for locating extrasolar planets. The vast majority of exoplanets detected from Earth were discovered by this method. To be precise; a planet doesn’t orbit around a star.

The reality is that both the star and the planet orbit around their common center of mass. Since the star is much more massive than the planet this center is very close to the star or even inside its volume – but it is never the center of the star itself. A star with planets around it is never completely motionless; it is orbiting around the common center of mass. This can be observed from Earth with the help of the Doppler Effect. The effect works not only for sound waves but also for electromagnetic radiation.


This method detects distant planets by measuring the minute dimming of a star as an orbiting planet passes between it and Earth. The passage of a planet between the star and the Earth is called “transit.” If such a dimming is detected at regular intervals and last a fixed length of time, then it is very probable that a planet is orbiting the star and passing in front of it once every orbital period. The size of the host star can be known with considerable accuracy from its spectrum and photometry, therefore, it gives astronomers a good estimate of the orbiting planet’s size but not its mass. This makes photometry an excellent complement to the spectroscopic method, which provides an estimate of a planet’s mass, but not its size. Using both methods combining mass and size scientists can calculate the planet’s density an important step towards assessing its composition.


Microlensing is the only known method capable of discovering planets at a truely great distance from the Earth. Whereas radial velocity searches look for planets in our immediate galactic neighborhood up to 100 light-years from the Earth, and transit photometry can potentially detect planets at a distance of 100 light-years, Microlensing can find planets orbiting stars near the center of the galaxy, 1000 of light-years away. This method is unique in its capability. To rapidly survey the population of cold planets with sensitivity to planetary mass that goes down to just below the mass of the Earth. The population of stars that it surveys are low mass stars, typically M dwarfs between here and center of the Galaxy.


Facts about Exoplanets 3 » Exoplanets |Definition, Properties, and Facts

As of 1st August 2019, there are 4,031 confirmed exoplanets. Most of the planets were discovered by the Kepler space telescope. In addition to the confirmed exoplanets, there are 2,420 potential exoplanets from its first mission that is yet to be confirmed, and 534 from its Second Light mission.

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