Types of planets – Exoplanet exploration: planets beyond our solar system

What is a telluric planet?

The basics

What is a telluric planet?

In our solar system, Earth, Mars, Mercury and Venus are terrestrial or rocky planets. For planets outside our solar system, those between half the size of Earth and twice its radius are considered terrestrial and others may be even smaller. Exoplanets twice the size of Earth and larger can also be rocky, but they are considered super-Earths.

Artist’s impression of how rocky, potentially habitable worlds elsewhere in our galaxy might appear. Data collected by telescopes in space and on the ground suggest that small, rocky planets are common. (Placing them so close together in a line is for illustrative purposes only.) Credit: NASA/JPL-Caltech/R. Hurt

Terrestrial planets (Earth-sized and smaller) are rocky worlds, composed of rock, silicate, water and/or carbon. To determine if any of these worlds have atmospheres, oceans, or other signs of habitability, more investigation is needed. Larger terrestrial exoplanets (those at least twice as massive as Earth) are classified as super-Earths.

Get to know some terrestrial planets

Get to know some terrestrial planets

TRAPPIST-1e, the fourth planet from the star, is in the habitable zone, meaning liquid water could exist on its surface.


The third planet from its star is shown here with a narrow band of water between the warm daytime side and the icy nightside of the planet.


In general, terrestrial planets have a bulk composition dominated by rock or iron, and a solid or liquid surface. These distant worlds may have gaseous atmospheres, but this is not a defining characteristic.

We have found rocky planets in the size range of Earth, the right distance from their parent stars to harbor liquid water (this is called the habitable zone). While these features don’t guarantee a habitable world — we can’t yet say whether these planets really have atmospheres or oceans — they can help point us in the right direction.


Future space telescopes will be able to scan the light from some of these planets, looking for water or a mixture of gases resembling our own atmosphere. We will have a better understanding of surface temperatures. As we continue to tick off items on the habitability list, we’ll get closer and closer to discovering a world bearing recognizable signs of life.

Seven terrestrial worlds of TRAPPIST-1

Seven terrestrial worlds of TRAPPIST-1

In 2017, NASA announced the discovery of most Earth-sized planets found within the habitable zone of a single star, called TRAPPIST-1. This system of seven rocky worlds – all with the potential for water on their surface – is an exciting discovery in the search for life on other worlds. Future study of this unique planetary system may reveal conditions suitable for life.

artists' concepts of the seven planets of TRAPPIST-1

This graphic shows, on the top row, the artist’s concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity compared to those of the earth. Credit: NASA/JPL-Caltech

In February 2018, a closer examination of the seven planets suggested that some could harbor much more water than Earth’s oceans, in the form of atmospheric water vapor for the planets closest to their star, water liquid for the others and ice for the furthest. a way. This research more accurately determined the density of each planet, making TRAPPIST-1 the best-known planetary system outside our own.

It’s impossible to know exactly what each planet looks like because they are so far away and so faint in relation to their host star. In our own solar system, the Moon and Mars have almost the same density, but their surfaces look totally different.

Artist's concept of the Trappist-1 planets

This artist’s concept shows what the TRAPPIST-1 planets might look like, based on available data on the planets’ diameters, masses, and distances from the host star, as of February 2018. Credit: NASA/JPL -Caltech

Based on the available data, here are scientists’ best guesses about what planets look like:

TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than Earth’s. TRAPPIST-1c also probably has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets – about 30% the mass of Earth. Scientists don’t know if it has a large atmosphere, an ocean, or a sheet of ice – all three would give the planet an “envelope” of volatiles, which would make sense for a planet of its density (less than half that of Earth).

Scientists were surprised that TRAPPIST-1e was the only planet in the system slightly denser than Earth, suggesting it may have a denser iron core than our home planet. Like TRAPPIST-1c, it doesn’t necessarily have a thick atmosphere, ocean, or ice sheet, making these two planets distinct in the system. It’s mysterious why TRAPPIST-1e has a much rockier composition than the rest of the planets. In terms of size, density, and the amount of radiation it receives from its star, it is the planet most similar to Earth.

TRAPPIST-1f, g, and h are far enough from the host star that any water they might possess could be frozen as ice on those surfaces. If they have thin atmospheres, they are unlikely to contain Earth’s heavy molecules, such as carbon dioxide.

How many terrestrial planets are there?

How many terrestrial planets are there?

The more we study, the more our understanding can change. An analysis of findings from NASA’s Kepler space telescope indicated that 20-50% of stars in the sky are likely to have potentially rocky small planets in their habitable zones. More recent data has shown the number is likely smaller – possibly between 2 and 12.

At first glance, this may seem disappointing: fewer potentially habitable rocky worlds among the thousands of exoplanets discovered so far. But that doesn’t change one of the most startling findings after more than 20 years of observation: planets in the habitable zone are common.

Much more data is needed, including a better understanding of the relationship between a planet’s size and its composition.

“We’re always trying to figure out how big a planet is while still being rocky,” said Jessie Dotson, an astrophysicist at NASA’s Ames Research Center in California’s Silicon Valley. She is also the project scientist for Kepler’s extended mission, known as K2. The spacecraft was retired in 2018, but discoveries are still being made using its data.

According to a study published in October 2020, about half of the stars with a temperature similar to our Sun could have a rocky planet capable of supporting liquid water on its surface.

Our galaxy contains about 300 million of these potentially habitable worlds, according to the results of a study using Kepler data. Some of these exoplanets could even be our interstellar neighbors, with four potentially within 30 light-years of our Sun and the closest likely to be around 20 light-years from us.

This research helps us understand the potential of these planets to have the elements necessary for life. It is an essential part of astrobiology, the study of the origins and future of life in our universe. “Kepler once told us there were billions of planets, but now we know a good chunk of those planets could be rocky and habitable,” said lead author Steve Bryson, a researcher at the Ames Research Center. of NASA in Silicon Valley, California.

A strange discrepancy in the size of the planets

A strange discrepancy in the size of the planets

Scientists have noted what appears to be a strange discrepancy in the size of planets. It was nicknamed the Fulton Gap, after Benjamin Fulton, lead author of a paper describing it. Kepler’s data shows that planets in a certain size range – those between 1.5 and 2 times the size of Earth – are rare. It is possible that this gap represents a critical split in planet formation: planets that reach the widest end of the radius gap rapidly draw in a thick atmosphere of hydrogen and helium gas and ascend into gaseous planets. , while planets smaller than the gap are not large enough. hold such a vibe and stay rocky above all else. On the other hand, the smaller planets that orbit close to their stars could be the cores of Neptune-like worlds whose atmospheres have been removed.

Explaining the Fulton gap will require a much better understanding of how solar systems form.

Infographic of types of exoplanets

An infographic shows a gas giant planet, a small telluric planet, a planet larger than Earth, but smaller than Newptune.

Exoplanets, planets beyond our solar system, whether orbiting other stars or free-floating between them, can make planets closer to home seem tame in comparison.

More to explore

  • Illustration of planet surface.

    Planet hop from TRAPPIST-1e

    The first discovery of seven Earth-sized planets around a single star offers a hopeful target in the search for life.

  • A downloadable version of a coloring page for TRAPPIST-1e.

    Coloring page TRAPPIST-1e

    What colors would appear under a red sun? Grab crayons, markers, paint or crayons and color in the hues of our TRAPPIST-1e coloring page based on our popular Exoplanet Travel Bureau poster.

  • Coloring in spanish

    TRAPPIST-1e Page Para Colorear

    ¿Qué matices aparecerían bajo un sol rojo?


Explore planet types: Gas Giant, Neptune-like, Super-Earth, and Terrestrial

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