Exoplanets – Knowing The Unknown (Types of Exoplanets)

Exoplanets – Knowing The Unknown (Types of Exoplanets)

(Last Updated On: August 23, 2019)


So, the first question, what are exoplanets? well, as the name suggests these are the planets which are outside our solar system and most of these have not been discovered by us. According to the latest data of 2019, 4,001 exoplanets have been discovered out of which 18 are earth shaped. the more we discover more interesting it becomes. Our own Milky Way galaxy is expected to contain trillions of planets.

Types of exoplanets

(From cannonballs to dwarfs)

There are various ways of classifying exoplanets. This can be according to size, by the type of host star, how close the planet is to the star, and the composition of the planet. The terms used to refer to exoplanets often describe multiple characteristics at the same time.

Exoplanets by mass

One of the simplest ways to classify and understand exoplanets is to describe its mass. This is typically done with concerning three planets in our solar system, the Earth, Jupiter and Neptune. A sub-Earth would have much less mass than the Earth. A planet such as Mercury or Mars would be described as a sub-Earth. A super-Earth has a mass that is between five and ten times that of the Earth. If the body in question is a rocky planet more than ten times the mass of the Earth, then it is known as a mega Earth.

A mini Neptune is a transitional planet, that is somewhere between a rocky planet such as Earth, and a gas giant such as Jupiter. These planets typically have thick atmospheres, along with layers of rocks and liquids. Brown Dwarfs are about just as massive as a planet can get. Like super Jupiters, brown dwarfs are much more massive than Jupiter but are only marginally larger. Even more massive bodies become the smallest and lightest red dwarf stars and are massive enough to sustain nuclear fission.

The predicted sizes of various kinds of planets - exoplanets
The predicted sizes of various kinds of planets

Ocean planet

Miller’s Planet in Interstellar, Kamino in Star Wars and 2181 Despoina in Mass Effect 3 are all examples of ocean planets. These kinds of planets are believed to have much more water than all the oceans of the Earth. Up to 5 percent of the masses of some of the Trappist- 1 planets are believed to be because of water. By comparison, all the oceans on the Earth make up only 0.05 percent of its mass. These planets are of particular interest to astrobiologists because they are the planets most likely to be habitable to life.


The oceans on the extra-solar planets could be so deep, that the water would convert to exotic forms of ice just due to pressure, even under high temperatures. This poses a challenge to the habitability of the planet. For the life forms to receive nutrients, it is important to have access to the silicon and rocky “ground” beneath the oceans. An ice shell can isolate the oceans from the rock, making it hard for the life forms to survive. Many of the mega Earths and the super-Earths are expected to be ocean planets.

Kepler-62 f - exoplanet
An artist’s concept of Kepler-62 f – A super-Earth and ocean planet

Ice planet

Ocean planets are essentially larger versions of ice worlds in our solar system, such as Ganymede, Enceladus or Europa. All of these moons have global subsurface oceans locked beneath a permanent shell of water ice. Ice planets should not be confused with ice giants, which are essentially planets such as Neptune or Uranus. There is some overlap between ocean planets and ice planets. Ocean planets with a frozen outer ice shell are also considered ice planets.  ever, not all ice planets are ocean planets, as the ice can be frozen material of various kinds. This can include methane, ammonia, or carbon compounds. Ice planets typically have high reflectivity.

The extreme cold would pose serious challenges for life to exist on the surface. The subsurface oceans could potentially support life though. The habitability of ice worlds depends on a source of energy. This could be heated through tidal forces, geothermal vents, or internal heat from the core of the planet. Ice worlds could also have pools of liquid on the surface, and can potentially support exotic forms of life, or life forms not based on carbon. OGLE-2016-BLG-1195L b is an exoplanet that is could be an ice planet and has a surface temperature of minus 220 degrees Celsius.

OGLE-2016-BLG-1195L b - exoplanet
An artist illustration of OGLE-2016-BLG-1195L b planet


Lava planet

The Pyronite homeworld in Ben 10, Excalbia from Star Trek, and Mustafar from Star Wars are all examples of lava planets. These kinds of planets have significant lava flows on their surfaces, peppered with several active volcanoes. Essentially they would be larger versions of Jupiter’s moon, Io. The Earth was also a lava planet, soon after it was formed and before it cooled down. Lava planets would have to be in orbit close to the host star, receiving a large amount of heat. The heat should be intense enough to keep the crust in a molten state.

Tidal forces would also contribute to 20 Types of exoplanets keeping the planet geologically active. COROT-7b is a planet that is twenty-three times closer to the host star than Mercury is to the sun. The surface temperatures reach close to 2,327 degrees Celsius. Like the Sun creates a water cycle on the Earth, COROT-7 creates a rock cycle on the planet, where the rock vaporizes, and rains back down on oceans or lakes of lava. Kepler 78 b is even closer to the host star, 40 times closer than Mercury is to the Sun. A single year on the planet lasts only 8.5 Earth days. The surface is permanently molten, with temperatures reaching 2,760 degrees Celsius.

COROT-7b artist’s impression

Desert planet

Arrakis in Dune, Tatooine or Geonosis in Star Wars, and the alien world in Forbidden Planet. These planets are essentially rocky, terrestrial planets, with deserts covering most of the surface. Venus was thought to be a desert planet about 1 billion years now, and Mars is such a planet now. The Earth is expected to become a desert planet in the distant future. Studies have shown that desert planets may be more habitable to life than ocean planets.

This is because the water in ocean planets eventually trap so much heat, that it leads to a runaway greenhouse effect, that strips the planet of all the water. By comparison, the small amounts of water can remain on desert planets for a longer duration of time. The thermal characteristics of desert planets mean that liquid water can exist on the surface at farther distances than the host star, than an ocean world. At the same time, desert planets are more resistant to global cooling. This is because the rocky surface absorbs more heat, and less heat is reflected off by water or ice. While the name makes it look like a bleak and hot place because of our perceptions, desert planets may be more hospitable to life than the water worlds.

Coreless planets

An iron planet has no outer silicate mantle or crust. A coreless planet is an opposite. The entire planet could be a solid object of the only rocky mantle, without a metallic core in the middle. A coreless planet can form in stellar discs that are rich in water. A high amount of water  in the mantle would mean that the iron would form large quantities of iron oxide, and remain locked in the mantle. The iron trapped in this way will not fall towards the center of the planet, forming the metallic core. The metallic core within the Earth is what gives us a magnetic field. This magnetic field acts as a shield around the Earth and protects the inhabitants on the surface from the ravage  of cosmic rays. Cosmic rays bombarding life forms are not good for their genetic integrity and are likely to lead to mutations.

As such, a magnetic field is essential for life as we know it on Earth to exist on another exoplanet. Without a metallic core, there is no magnetic field. However, this does not mean that coreless planets cannot sustain exotic forms of life. A coreless planet is a theoretical variety of exoplanets, and no such body has been identified so far, within our solar system or elsewhere. Coreless planets are expected to have similar radii and masses as terrestrial planets with metallic cores, making them extremely difficult to identify from a distance.

Helium planet

A gas giant typically has an atmosphere rich in hydrogen. A helium planet would have an atmosphere rich primarily in helium. The process that leads to the formation of helium planets is similar to the process that leads to the formation of chthonian (pronounced as chi-tonian)  planets. Hydrogen is lighter than helium and is likely to get stripped away from the atmosphere first. This means that a hot Jupiter or a hot Neptune would have most of the hydrogen stripped away, but retain an atmosphere rich in helium.

The carbon in the atmosphere would not have any hydrogen to bond with, to form methane, which is typically seen in gas giants. Instead, the carbon would form carbon monoxide and carbon dioxide. The presence of these compounds can be detected at interstellar distances. Because of the atmospheric composition, a helium planet would appear grey or white to the naked eye. Gliese 436 b is a strong candidate for being a helium planet. It is grayish-white in appearance. There is also much more carbon monoxide and much less methane in the atmosphere, as compared to what is expected in a typical gas giant. The atmospheric stripping has also swollen up the atmosphere, giving the planet an unusually large exosphere.

Eccentric planets

Eccentric Jupiters are gas giants that have an eccentric, elongated orbit. Instead of being in a more or less circular orbit, the eccentric Jupiters have orbits that resemble the orbits of comets. The closest approach of a planet to the host star is known as the perihelion, and the farthest point of the orbit is known as aphelion. At the perihelion, an eccentric Jupiter may be much closer to the host star than Mercury is to the Sun. At the aphelion, the eccentric  Jupiter moves well into the outer solar system, more than twice the distance that the Earth is from the Sun. Such eccentric Jupiters are extremely common types of exoplanets, even more so than hot Jupiters.

About half the systems identified with planets have an eccentric Jupiter. Such planets pose a serious threat to habitability on any rocky planets in the habitable zone of the host star.  because of the immense gravity wells of the eccentric Jupiters. As they traverse the zones typically occupied by rocky planets, they are likely to disturb the orbits of the planets, removing them from the habitable zones. Such a planet would sweep the inner system clean, leaving behind only asteroid sized objects. It is unknown what process leads to the formation of eccentric Jupiters

Carbon planets

The protoplanetary discs that led to the formation of the solar system were rich in oxygen, which is why the terrestrial planets of Venus, Earth, and Mercury have substantial amounts of oxygen and silicon compounds. At the planetary core of such terrestrial planets, is a metal-rich core. In systems that formed where there was less oxygen and more carbon, some of the planets would have a metal-rich core, surrounded by molten carbines. Above that could be layers of graphite or even diamond. When stars die, they release tremendous amounts of carbon.

A subsequent generation of stars and planets that form from the ejected material, are more likely to be carbon-rich. As the universe gets older and more stars to die, there is expected to be an increasing number of carbon planets. There are no confirmed carbon planets because from a distance they would be hard to distinguish from terrestrial, water-rich planets. The exoplanet PSR J1719-1438 b, in orbit around a millisecond pulsar, 4,000 light-years away towards the constellation of Serpens maybe such a carbon planet.

HD76920 b
The orbit of HD76920 b, if it would be in our Solar System. The orbit of Phaeton, an asteroid, is shown for comparison.

So these were all the planets.

Here’s a bonus

In June 2018, ISRO announced the first confirmed exoplanet discovery by a team led by Professor Abhijit Chakraborty of Physical Research Laboratory (PRL) in Ahmedabad. The hot Neptune gas giant is about 27 times as massive as the Earth. Initially, the body was identified as an exoplanet candidate. The team used the PRL Advance Radial-velocity Abusky Search (PARAS) spectrograph to 4 Introduction Hot Neptune Introduction 5 confirm that the gas giant 600 light-years away was indeed an exoplanet. The observation campaign lasted for about one and a half years. With the discovery, India became one of the few countries in the world to confirm an exoplanet.

Read Also: Life of Stars – Birth To Death

Vidhu sarwal

He is a student of class X. He is having an interest in electronics and astronomy. His mind is always engrossed in one or the other circuit. He is fond of electrical gadgets. He loves to know about the functioning of gadgets.

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