What is a Terrestrial Planet?

The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute

In studying our Solar System over the course of many centuries, astronomers learned a great deal about the types of planets that exist in our universe. This knowledge has since expanded thanks to the discovery of extrasolar planets, many of which are similar to what we have observed here at home.

For example, while hundreds of gas giants of varying size have been detected (which are easier to detect because of their size), numerous planets have also been spotted that are similar to Earth – aka. “Earth-like”. These are what is known as terrestrial planets, a designation which says a lot about a planet how it came to be.


Also known as a telluric or rocky planet, a terrestrial planet is a celestial body that is composed primarily of silicate rocks or metals and has a solid surface. This distinguishes them from gas giants, which are primarily composed of gases like hydrogen and helium, water, and some heavier elements in various states.

The term terrestrial planet is derived from the Latin “Terra” (i.e. Earth). Terrestrial planets are therefore those that are “Earth-like”, meaning they are similar in structure and composition to planet Earth.

Earth-like planets. Image Credit: JPL
Artist’s concept for the range of Earth-like extrasolar planets that have been discovered in recent years. Credit: NASA/JPL

Composition and Characteristics:

All terrestrial planets have approximately the same type of structure: a central metallic core composed of mostly iron, with a surrounding silicate mantle. Such planets have common surface features, which include canyons, craters, mountains, volcanoes, and other similar structures, depending on the presence of water and tectonic activity.

Terrestrial planets also have secondary atmospheres, which are generated through volcanism or comet impacts. This also differentiates them from gas giants, where the planetary atmospheres are primary and were captured directly from the original solar nebula.

Terrestrial planets are also known for having few or no moons. Venus and Mercury have no moons, while Earth has only the one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more akin to large asteroids than actual moons. Unlike the gas giants, terrestrial planets also have no planetary ring systems.

The Earth's layers. Credit: discovermagazine.com
The Earth’s interior structure, shown here as consisting of multiple “layers”. Credit: discovermagazine.com

Solar Terrestrial Planets:

All those planets found within the Inner Solar System – Mercury, Venus, Earth and Mars – are examples of terrestrial planets. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core.

Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. In the case of the former, models of the moon’s composition suggest that the mantle is composed primarily of silicate rock and iron, which surrounds a core of iron and iron sulphide. Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water.

Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.

Extrasolar Terrestrial Planets:

Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since 2005, hundreds of potentially terrestrial extrasolar planets have been found – mainly by the Kepler space mission. Most of these have been what is known as “super-Earths” (i.e. planets with masses between Earth’s and Neptune’s).

Examples of extrasolar terrestrial planets include Gliese 876 d, a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese 876, which is located approximately 15 light years from Earth. The existence of three (or possibly four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the Gliese 581 system, another red dwarf roughly 20 light years from Earth.

The smallest of these, Gliese 581 e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581 c and Gliese 581 d, as well as a proposed fourth planet (Gliese 581 g) are more-massive super-Earths orbiting in or close to the habitable zone of the star. If true, this could mean that these worlds are potentially habitable Earth-like planets.

The first confirmed terrestrial exoplanet, Kepler-10b – a planet with between 3 and 4 Earth masses and located some 460 light years from Earth – was found in 2011 by the Kepler space mission. In that same year, the Kepler Space Observatory team released a list of 1235 extrasolar planet candidates, including six that were “Earth-size” or “super-Earth-size” (i.e. less than 2 Earth radii) and which were located within their stars’ habitable zones.

Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range. As of January, 2013, 2740 planet candidates have been discovered.


Scientists have proposed several categories for classifying terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the Solar System, which are composed primarily of a silicon-based rocky mantle and a metallic (iron) core.

Iron planets are a theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Planets of this type are believed to form in the high-temperature regions close to a star, and where the protoplanetary disk is rich in iron. Mercury is possible example, which formed close to our Sun and has a metallic core equal to 60–70% of its planetary mass.

Coreless planets are another theoretical type of terrestrial planet, one that consists of silicate rock but has no metallic core. In other words, coreless planets are the opposite of an iron planet. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common. Though the Solar System has no coreless planets, chondrite asteroids and meteorites are common.

And then there are Carbon planets (aka. “diamond planets”), a theoretical class of planets that are composed of a metal core surrounded by primarily carbon-based minerals. Again, the Solar System has no planets that fit this description, but has an abundance of carbonaceous asteroids.

Until recently, everything scientists knew about planets – which included how they form and the different types that exist – came from studying our own Solar System. But with the explosion that has taken place in exoplanet discovery in the past decade, what we know about planets has grown significantly.

For one, we have come to understand that the size and scale of planets is greater than previously thought. What’s more, we’ve seen for the first time that many planets similar to Earth (which could also include being habitable) do in fact exist in other Solar Systems.

Who knows what we will find once we have the option of sending probes and manned missions to other terrestrial planets?

Universe Today has articles on smallest terrestrial exoplanet and gas planets. For the latest information on confirmed extrasolar planets, be sure to check out the Kepler’s Planet Candidates.

For a full list of all confirmed and potential planets, consult the Extrasolar Planet Encyclopaedia.

Astronomy Cast has episodes on the terrestrial planets including Mars, and an interview with Darin Ragozzine, one of the Kepler Space Mission scientists.

Citizen Planet Hunters Find a Planet in a Four-Star System

A family portrait of the PH1 planetary system that was discovered in part due to crowdsourcing. Image Credit: Haven Giguere/Yale.

A family portrait of the PH1 planetary system: The newly discovered planet is depicted in this artist’s rendition transiting the larger of the two eclipsing stars it orbits. Off in the distance, well beyond the planet orbit, resides a second pair of stars bound to the planetary system. Image Credit: Haven Giguere/Yale.

A planet has been discovered orbiting in a four-star system — and no, that doesn’t mean the accommodations and conditions are excellent. It literally means four stars, where a planet is orbiting a binary star system that in turn is orbited by a second distant pair of stars. This is the first system like this that has ever been found, and its discovery demonstrates the power of citizen scientists, as it was found by a joint effort of amateurs participating on the Planet Hunters website under the guidance of professional astronomers.

This is might be an extremely rare planetary setup, astronomer Meg Schwamb from Yale says, as only six planets are currently known to orbit two stars, and none of these are orbited by other stellar companions. Astronomers are calling the newly found world a ‘circumbinary’ planet.

“Circumbinary planets are the extremes of planet formation,” said Schwamb, Planet Hunters scientist and lead author of a paper about the system presented Oct. 15 at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Reno, Nevada. “The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments.”

The planet is called PH1, for the first confirmed planet identified by the Planet Hunters citizen scientists, but it has the nickname of Tatooine, the planet in Star Wars that orbited two suns.

Planet Hunters uses data from the Kepler spacecraft, specially designed for looking for signs of planets.

The volunteers, Kian Jek of San Francisco and Robert Gagliano of Cottonwood, Arizona, spotted faint dips in light caused by the planet as it passed in front of its parent stars, a common method of finding extrasolar planets. Schwamb, a Yale postdoctoral researcher, led the team of professional astronomers that confirmed the discovery and characterized the planet, following observations from the Keck telescopes on Mauna Kea, Hawaii. PH1 is a gas giant with a radius about 6.2 times that of Earth, making it a bit bigger than Neptune.

“Planet Hunters is a symbiotic project, pairing the discovery power of the people with follow-up by a team of astronomers,” said Debra Fischer, a professor of astronomy at Yale and planet expert who helped launch Planet Hunters in 2010. “This unique system might have been entirely missed if not for the sharp eyes of the public.”

PH1 orbits outside the 20-day orbit of a pair of eclipsing stars that are 1.5 and 0.41 times the mass of the Sun. This planet is dense — it has perhaps about 170 times more mass than Earth — and is about half the diameter of Jupiter. It revolves around its host stars roughly every 138 days. Beyond the planet’s orbit at about 1000 AU (roughly 1000 times the distance between Earth and the Sun) is a second pair of stars orbiting the planetary system.

Gagliano, one of the two citizen scientists involved in the discovery, said he was “absolutely ecstatic to spot a small dip in the eclipsing binary star’s light curve from the Kepler telescope, the signature of a potential new circumbinary planet, ‘Tatooine,’ and it’s a great honor to be a Planet Hunter, citizen scientist, and work hand in hand with professional astronomers, making a real contribution to science.”

Jek expressed wonder at the possibility of the discovery: “It still continues to astonish me how we can detect, let alone glean so much information, about another planet thousands of light years away just by studying the light from its parent star.”

Read the paper here.

Source: Planet Hunters


Hubble's view showing a possible exoplanet Fomalhaut b (NASA/HST)

An exoplanet – or extrasolar planet – is a planet which orbits a star other than our own Sun.

After a bit of a false start – lasting many decades! – when a small number of detections of planets around other stars were reported but not confirmed, the first reliable, independently confirmed exoplanet was discovered – by Campbell, Walker, and Yang – in 1988 (though solid confirmation came only in 2003), around Gamma Cephei. Between 1988 and 2003, two planets were detected, and confirmed, orbiting a pulsar (which has the catchy name of PSR 1257+12) – in 1992 – and an exoplanet was discovered, and confirmed, around the ordinary (main sequence) star 51 Pegasi (in 1995). It was this discovery that started the modern exoplanet gold rush.

There are now nearly 400 exoplanets detected and confirmed (and a few more whose status is uncertain). The Extrasolar Planets Encyclopaedia is a website which keeps track of all announcements, confirmations, etc. It also has an excellent tutorial on the methods used to discover such planets.

The first multiple system – a star with more than one exoplanet – discovered was Upsilon Andromedae (this star is actually a binary, so the discovery was a first in two ways). The first planet was discovered in 1996, and the second (and third!) in 1999. In this case independent confirmation came quite quickly. Today more than 20 such multiple-planet systems are known.

Most exoplanets have been discovered by the radial velocity, or Doppler, method: the star’s apparent speed away from (or towards) us – as measured by sensitive spectrographs – varies in a regular way, due to the gravitational pull of the exoplanet (remember that two bodies in a stable gravitational system will orbit the center of mass). Almost all have been found by ground-based telescopes. This is likely to change in the next few years as dedicated space-based telescopes – such as NASA’s Kepler and the ESA’s COROT – continue to make new discoveries. As these use the transit method (detecting tiny changes in a star’s intensity, as an exoplanet goes between it and us), the Doppler method may soon lose its ‘most exoplanets discovered’ status.

There are literally dozens of Universe Today stories on exoplanets! Here are a few, covering many different aspects: Smallest Terrestrial Exoplanet Yet Detected, Astrometry Finds an Exoplanet, Exoplanet Has Oddball Orbit, New Technique Allows Astronomers to Discover Exoplanets in Old Hubble Images, Carbon Dioxide Detected on Exoplanet HD 189733 b, and Exoplanet Image Confirmed.

There’s also a great overview of this topic in the Astronomy Cast episode A Zoo of Extrasolar Planets, and the somewhat older episode Discovering Another Earth is excellent too.

Source: Wikipedia

Super Earths

An artist’s impression of Gliese 581d, an exoplanet about 20.3 light-years away from Earth, in the constellation Libra. Credit: NASA

The holy grail in the search for extrasolar planets will be the discovery of Earthlike planets orbiting other stars. With better telescopes and techniques, astronomers will eventually be able to even detect the atmospheres of extrasolar planets and determine if there’s life there. Although Earth-sized planets are impossible to detect with current observatories, astronomers are now finding super earths.

A super Earth is a terrestrial planet orbiting a distant star. But instead of having the mass of our own planet, it might have 2, 5, or even 10 times the mass of the Earth. Although that makes them large, very massive planets, they’re not as large or massive as gas giants.

And just because they’re called super Earths doesn’t mean they’re habitable, or even Earthlike in climate at all. Super Earths could be orbiting close to their parent star, or well outside the solar system’s habitable zone.

Scientists haven’t completely settled on a definition for super Earths. Some believe a planet should be considered a super Earth if it’s a terrestrial planet between 1 and 10 Earth masses, while others think it should be between 5 and 10 Earth masses.

The first super Earth ever discovered was found in 1991 orbiting a pulsar. Obviously that wouldn’t really be a very habitable place to live. The first super earth found orbiting a main sequence star was found in 2005, orbiting the star Gliese 876. It’s estimated to have 7.5 times the mass of the Earth, and orbits its parent star every 2 days. With such a short orbital period, you can expect that it’s orbiting very close to its parent star. Temperatures on the surface of the planet reach 650 kelvin.

The first super earth found within its star’ habitable zone was Gliese 581 c. It’s estimated to have 5 Earth masses, and orbits its parent star at a distance of 0.073 astronomical units (1 AU is the average distance from the Earth to the Sun). That’s pretty close to the star, and Gliese 581 c would probably have a runaway greenhouse effect, similar to Venus. But right beside that is Gliese 581 d, with a mass of 7.7 Earths and an orbit of 0.22 AU. This planet could very well have liquid water on its surface.

The smallest super Earth discovered so far is MOA-2007-BLG-192Lb, which has only 3.3 times the mass of the Earth, and was orbiting a brown dwarf star. But this record will probably be beaten by the time you read this, as planet hunters get better. It’s only a matter of time before a true Earthlike planet is discovered.

We have written many articles about super Earths. Here’s an article speculating on the kinds of atmospheres that super Earths might have, and another article about how similar super Earths really are to our own planet.

Here’s an artist’s impression of a super Earth features on NASA’s Astronomy Picture of the Day website, and here’s an article from NASA about super Earths.

We also recorded an episode of Astronomy Cast dealing with the different kinds of extrasolar planets you can find. Listen to it here. Episode 125: A Zoo of Extrasolar Planets.

Source: Wikipedia