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Exoplanet Exploration: Planets Beyond our Solar System

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What is a terrestrial planet?

The basics

What is a terrestrial planet?

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

Artist's concept of how rocky, potentially habitable worlds elsewhere in our galaxy might appear. Data gathered 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 some of these worlds have atmospheres, oceans or other signs of habitability, it takes more investigation. Larger terrestrial exoplanets (those at least twice as massive as Earth) are classified as super-Earths.

Get to know some terrestrial planets

In general, terrestrial planets have a bulk composition that is dominated by rock or iron, and a solid or liquid surface. These faraway worlds may have gaseous atmospheres, but that's not a defining feature.

We've found rocky planets in Earth's size range, at the right distance from their parent stars to harbor liquid water (this is known as the habitable zone). While these characteristics don't guarantee a habitable world - we can't quite tell yet if these planets really do possess atmospheres or oceans - they can help point us in the right direction.

Future space telescopes will be able to analyze the light from some of these planets, searching for water or a mixture of gases that resembles our own atmosphere. We will gain a better understanding of temperatures on the surface. As we continue checking off items on the habitability list, we'll draw closer and closer to finding 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 the most Earth-sized planets found in the habitable zone of a single star, called TRAPPIST-1. This system of seven rocky worlds - all of them 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 could reveal conditions suitable for life.

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

In February 2018, a closer look at the seven planets suggested that some could harbor far more water than the oceans of Earth, in the form of atmospheric water vapor for the planets closest to their star, liquid water for others, and ice for those farthest away. That research pinned down the density of each planet more precisely, making TRAPPIST-1 the most thoroughly known planetary system apart from our own.

It's impossible to know exactly how each planet looks, because they are so far away and so faint compared to their host star. In our own solar system, the Moon and Mars have nearly the same density, yet their surfaces appear entirely different.

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

Based on available data, here are scientists' best guesses about the appearances of the planets:

TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than Earth's. TRAPPIST-1c also likely has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets - about 30 percent the mass of Earth. Scientists are uncertain whether it has a large atmosphere, an ocean or an ice layer - all three of these would give the planet an "envelope" of volatile substances, which would make sense for a planet of its density (less than half that of Earth).

Scientists were surprised that TRAPPIST-1e is 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 does not necessarily have a thick atmosphere, ocean or ice layer - making these two planets distinct in the system. It is 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, this is the most similar planet 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 across these surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules of Earth, such as carbon dioxide.

How many terrestrial planets are out there?

How many terrestrial planets are out there?

The more we study, the more our understanding can change. One analysis of discoveries by NASA's Kepler Space Telescope indicated that 20 to 50 percent of the stars in the sky are likely to have small, potentially rocky planets in their habitable zones. Newer data showed that the number is likely smaller - possibly between 2 and 12.

At first glance, that might seem disappointing: fewer rocky, potentially habitable worlds among the thousands of exoplanets found so far. But that doesn't change one of the most astonishing conclusions after more than 20 years of observation: Planets in the habitable zone are common.

Much more data are needed, including a better understanding of how a planet's size relates to its composition.

"We're still trying to figure out how big a planet can be and still be 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 research released in October 2020, about half the stars similar in temperature to our Sun could have a rocky planet capable of supporting liquid water on its surface.

Our galaxy holds an estimated 300 million of these potentially habitable worlds, based on results in 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 about 20 light-years from us.

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

A strange gap in planet sizes

A strange gap in planet sizes

Scientists have noted what seems to be a strange gap in planet sizes. It's been dubbed the Fulton gap, after Benjamin Fulton, lead author on a paper describing it. The Kepler data show that planets of a certain size-range- those between 1.5 and 2 times the size of Earth - are rare. It's possible that this gap represents a critical divide in planet formation: Planets that reach the larger end of the radius gap quickly attract a thick atmosphere of hydrogen and helium gas and balloon up into gaseous planets, while planets smaller than the gap are not large enough to hold such an atmosphere and remain primarily rocky. On the other hand, the smaller planets that orbit close to their stars could be the cores of Neptune-like worlds that had their atmospheres stripped away.

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

Exoplanet Types Infographic

More to Explore

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

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

¿Qué matices aparecerían bajo un sol rojo?

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Explore the planet types: Gas Giant, Neptune-like, Super-Earth and Terrestrial
Or move on to the building blocks of galaxies: stars!

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national aeronautics and space administration

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Terrestrial | Planet Types - Exoplanet Exploration: Planets Beyond our Solar System

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In our solar system, Earth, Mars, Mercury and Venus are terrestrial, or rocky, planets. Exoplanets between half of Earth's size to twice its radius are considered terrestrial and others may be even smaller.

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Exoplanet Exploration: Planets Beyond our Solar System

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What is a gas giant?

For most of human history our understanding of how planets form and evolve was based on the eight (or nine) planets in our solar system. But over the last 25 years, the discovery of more than 4,000 exoplanets, or planets outside our solar system, changed all that.

Gas giants, like Jupiter or Saturn in our solar system, are composed mostly of helium and/or hydrogen. Gas giants nearer to their stars are often called "hot Jupiters." More variety is hidden within these broad categories. Hot Jupiters, for instance, were among the first exoplanet types found - gas giants like Jupiter, yes, but orbiting so close to their stars that their temperatures soar into the thousands of degrees (Fahrenheit or Celsius). These large planets make such tight orbits that they cause a pronounced "wobble" in their stars, tugging their stellar hosts this way and that, and causing a measurable shift in the spectrum of light from the stars. That made hot Jupiters easier to detect in the early days of planet hunting using the radial velocity method.

Get to know some gas giants

Similar in size to Jupiter, these gas-dominated planets orbit extremely close to their parent stars, circling them in as few as 18 hours. We have nothing like them in our own solar system, where the closest planets to the Sun are rocky and orbiting much farther away. The questions about hot Jupiters are as big as the planets themselves: Do they form close to their stars or farther away before migrating inward? And if these giants do migrate, what would that reveal about the history of the planets in our own solar system?

To answer those questions, scientists will need to observe many of these hot giants very early in their formation. The detection of the exoplanet HIP 67522 b, thought to be the youngest hot Jupiter ever found (in June 2020), could expand our understanding. It orbits a well-studied star that is about 17 million years old, meaning the hot Jupiter is likely only a few million years younger, whereas most known hot Jupiters are more than a billion years old. The planet takes about seven days to orbit its star, which has a mass similar to the Sun's. Located only about 490 light-years from Earth, HIP 67522 b is about 10 times the diameter of Earth, or close to that of Jupiter. Its size strongly indicates that it is a gas-dominated planet.

The discovery offers hope for finding more young hot Jupiters and learning more about how planets form throughout the universe.

Migrating giants?

Migrating giants?

There are three main hypotheses for how hot Jupiters get so close to their parent stars. One is that they simply form there and stay put. But it's hard to imagine planets forming in such an intense environment. Not only would the scorching heat vaporize most materials, but young stars frequently erupt with massive explosions and stellar winds, potentially dispersing emerging planets.

It could be more likely that gas giants develop farther from their parent star, past a boundary called the snow line, where it's cool enough for ice and other solid materials to form. Jupiter-like planets are composed almost entirely of gas, but they contain solid cores. It would be easier for those cores to form past the snow line, where frozen materials could cling together like a growing snowball.

The other two hypotheses assume this is the case, and that hot Jupiters then wander closer to their stars. But what would be the cause and timing of the migration?

One idea posits that hot Jupiters begin their journey early in the planetary system's history while the star is still surrounded by the disk of gas and dust from which both it and the planet formed. In this scenario, the gravity of the disk interacting with the mass of the planet could interrupt the gas giant's orbit and cause it to migrate inward.

The third hypothesis maintains that hot Jupiters get close to their star later, when the gravity of other planets around the star can drive the migration. The fact that HIP 67522 b is already so close to its star so early after its formation indicates that this third hypothesis probably doesn't apply in this case. But one young hot Jupiter isn't enough to settle the debate on how they all form.

Llamaradas de Furia

More to Explore

Located less than 32 light-years from Earth, AU Microscopii is among the youngest planetary systems ever observed by astronomers, and its star throws vicious temper tantrums!

This far-off blue planet may look like a friendly haven - but don't be deceived! Weather here is deadly. The cobalt color comes from a hazy, blow-torched atmosphere containing clouds laced with glass.

Este planeta lejano puede parecer un refugio amigable... ¡pero no te dejes engañar! ¡El clima en este mundo es mortal! ¡El color azul cobalto del planeta proviene de una atmósfera brumosa e incendiaria que contiene nubes de cristales de vidrio!

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Developing with the stars

Developing with the stars

In 2007, astronomers using NASA's Spitzer Space Telescope found evidence showing that gas-giant planets form quickly, within the first 10 million years of a Sun-like star's life.

Gas giants could get their start in the gas-rich debris disk that surrounds a young star. A core produced by collisions among asteroids and comets provides a seed, and when this core reaches sufficient mass, its gravitational pull rapidly attracts gas from the disk to form the planet.

Scientists using Spitzer and ground-based telescopes searched for traces of gas around 15 different Sun-like stars, most with ages ranging from 3 million to 30 million years. With the help of Spitzer's infrared spectrometer instrument, they were able to search for relatively warm gas in the inner regions of these star systems, an area comparable to the zone between Earth and Jupiter in our own solar system. They also used ground-based radio telescopes to search for cooler gas in the outer regions of these systems, an area comparable to the zone around Saturn and beyond.

All of the stars in the study, including those as young as a few million years, have less than 10 percent of Jupiter's mass in gas swirling around them. This indicates that gas giant planets like Jupiter and Saturn have already formed in these young planetary systems, or they never will.

Explore the planet types: Gas Giant, Neptune-like, Super-Earth and Terrestrial
Or move on to the building blocks of galaxies: stars!

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national aeronautics and space administration

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Gas Giant | Planet Types - Exoplanet Exploration: Planets Beyond our Solar System

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What is a gas giant? These Extremely large planets can be far from their stars like Jupiter in our solar system, or scorchingly hot and extremely close to their suns.

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What types of planets can have debris and rings surrounding them?