The Different Kinds of Exoplanets You Meet in the Milky Way
At a glance:
Exoplanets are planets that orbit other stars. Some look like planets in our own solar system, while others are dramatically different.
Exoplanets seem to follow the same general rules as the planets in our solar system: Small planets are rocky, big planets are gassy, and the ones in between may be watery.
There are other rocky worlds orbiting stars at the right distance to allow liquid water to exist. We don’t know yet if any other world supports life.
There are more than 4,000 known exoplanets, bewildering in their variety. Some are familiar to us, worlds analogous to Venus or Neptune. Others are like nothing in our own solar system, so we describe them as hot Jupiters, carbon planets, and super-Earths. Their names are an alphabet soup, derived from star catalogs, telescopic sky surveys, and space telescopes: CoRoT-7b, Kepler-10c, and TRAPPIST-1a. What are these worlds that orbit other stars?
Learn why and how we study exoplanets, and how you can get involved.
Fifteen types of exoplanets are currently listed in the Unified Astronomy Thesaurus, and a few more names pop up in the literature. These types aren’t distinct; exoplanets can be described in more than one of these different ways. Five of the exoplanet types refer only to size (mass and diameter). In order, from smallest to biggest, they are rocky planets; super-Earths; mini-Neptunes; ice giants; and gas giants. Planets in the super-Earth and mini-Neptune size range may be ocean planets, having liquid-water oceans hundreds of kilometers deep. Some rocky planets in especially carbon-rich systems may be ultra-dry carbon planets, having rocks formed of compounds of silicon and carbon, different from our solar system’s silicon-and-oxygen rocks.
There are cold Jupiters and cold Neptunes as well, orbiting farther out, just like the planets they’re named after. Lava planets are superdense larger-than-Earth worlds in close, hot orbits; they don’t seem to occur farther out. Some of them, known as Cthonian planets, are likely the remnant cores of evaporated hot Jupiters. (The name, pronounced “THO-nee-an,” refers to the underworld.) Habitable planets are Earth-sized ones that orbit at a distance from their star where liquid water could be stable, meaning they could be habitable to life as we know it, under the right circumstances. Every star has a habitable zone, a range of distances in which surface liquid water could be stable. If stars are small and dim, habitable planets can have close orbits and be tidally locked.
Finally, any size of planet can be found in a few especially strange locations. There could be Trojan planets, smaller co-orbital companions to larger planets, though none have been certainly identified yet. (Our solar system has Trojan asteroids co-orbiting with many of the planets, but they are all far too small to be planets.) Pulsars—ultradense, rapidly spinning neutron stars with unimaginably intense magnetic fields—are such weird stellar objects that pulsar planets get their own moniker, too. And finally there are free-floating planets, also known as rogue planets, which have been ripped free from their stars through an unlucky gravitational encounter with a more massive object and journey alone through the galaxy. They are hard to find and we’ve found only one so far, but there are probably lots of them out there.
Now that you’re acquainted with the different types of exoplanets, let’s look at some of the better-known examples and see what kinds they are.
Emily Lakdawalla and Loren A. Roberts for The Planetary Society based on data from exoplanets.org
Different Sizes of Exoplanets
There is a relationship between a planet’s size (mass and diameter) and what type of planet it becomes. Small, low-mass planets tend to be rocky, while large, high-mass planets attract lighter-weight materials like water and hydrogen as they form, ending up like Neptune or Jupiter. Planets more massive than Saturn are all roughly the same size, because at that mass the planets are compressing themselves under their own weight. Above the masses on this diagram are the smallest stars, brown dwarfs.
The TRAPPIST-1 system contains a total of seven known Earth-sized planets. Three of them — TRAPPIST-1e, f and g — are located in the habitable zone of the star (shown in green in this artist’s impression), where temperatures are just right for liquid water to exist on the surface. Note that all 7 TRAPPIST-1 planets would easily fit within Mercury’s orbit; because TRAPPIST-1 is an ultra-cool dwarf star, its habitable zone is much closer than our Sun’s.
OGLE-2016-BLG-1195 Lb: The one that’s like Earth in a freezer
Some 12,800 light-years away, an Earth-sized world orbits in an Earth-distance orbit around a very tiny star. Because the star is so small and cold, the planet is probably super-cold and lacks an atmosphere. Like Triton or Pluto, materials that would be gases if they were warmer (like methane and carbon dioxide) are probably frozen to the surface. OGLE-2016-BLG-1195Lb was discovered by the microlensing technique using Spitzer and the Korea Microlensing Telescope Network.
NASA / JPL-Caltech
This artist's concept shows OGLE-2016-BLG-1195Lb, a planet that is about the same mass as Earth, and the same distance from its host star as our planet is from our Sun. But it orbits a star so small, scientists aren't sure if it's a star at all, and the planet must be very cold.
PSR B1257 + 12 system: The ones that orbit a pulsar
“The exoplanets PSR B1257+12 b, c and d were among the first discovered, and also happen to be three of the weirdest,” NASA says. Planets c and d were discovered in 1992 using the Arecibo radio telescope, orbiting a millisecond pulsar that was discovered in 1991 and is located about 1,000 light years from Earth. Arecibo detected tiny variations in the timing of the arrival of the pulsar’s radio pulses that betrayed the presence of planets. In 1994 a third, inner planet was found that is smaller than Mercury. The planets could not possibly have survived the supernova that created the pulsar, so they must have condensed around the star from a disk of material left behind by the explosion. What the surface of rocky worlds orbiting the remains of a star blasting out intense electromagnetic radiation look like is anybody’s guess, but the magnetic fields very likely create auroras on the planets.
NASA / JPL-Caltech
Planets at a pulsar: the PSR B1257 + 12 system
This artist's concept depicts the pulsar planet system discovered by Aleksander Wolszczan in 1992. Wolszczan used the Arecibo radio telescope in Puerto Rico to find 3 planets - the first of any kind ever found outside our solar system - circling a pulsar named PSR B1257+12. Pulsars are rapidly rotating neutron stars, which are the collapsed cores of exploded massive stars. They spin and pulse with radiation, much like a lighthouse beacon. Here, the pulsar's twisted magnetic fields are highlighted by the blue glow.
Lava Planets (Also Known As Hot Super-Earths or Super-Ios or Sometimes Cthonian Planets)
Kepler-10b: the one that’s like Earth if you ignore the fact that its rocks are melted
Kepler-10b was the first rocky planet that NASA’s Kepler Space Telescope found, using transit photometry, and it orbits a Sun-like star, albeit one much older than the Sun. However, Kepler-10b is much larger than Earth, with a diameter 1.4 times bigger and a mass 3.5 times bigger, and it orbits its star incredibly closely, taking only 20 hours to circle it at an altitude only about 1.5 times the diameter of the star itself. It must be tidally locked, so must be molten on its dayside, but its surface would be solid on its cold nightside.
NASA / Kepler Mission / Dana Berry
Planet Kepler-10b transiting its host star (artist's depiction)
Measuring 1.4 times the size of Earth, Kepler-10b was, at the time of its discovery in 2009, the smallest planet known outside our solar system. (Many smaller ones have since been discovered.) It is a rocky planet with a mass 4.6 times that of Earth and with an average density of 8.8 grams per cubic centimeter (which is significantly denser than Earth's 5.5).
CoRoT-7b: The Cthonian planet that’s probably an evaporated Jupiter
Celebrated as the first rocky exoplanet to be discovered (in 2009), CoRoT-7b orbits a Sun-like star. Apart from that, it is nothing like Earth. The planet circles its star so closely that its sun-facing surface is probably molten lava. It is likely tidally locked, making the dayside permanently molten and the nightside permanently frigid. Daytime temperatures on the planet may reach 2,000 degrees Celsius but may drop to -200 degrees Celsius on the nightside.
55 Cancri e: The Cthonian planet with sparkle skies
This artist's impression shows the super-Earth 55 Cancri e in front of its parent star. 55 Cancri e is about 40 light-years away and orbits a star slightly smaller, cooler and less-bright than our Sun. As the planet is so close to its parent star, one year lasts only 18 hours. It is tidally locked, and the temperature has been measured as 2,700 kelvins on the dayside and “only” 1,400 Kelvins on the nightside.
Mini-Neptunes or Super-Earths (Which Might Be Ocean Planets but It’s Hard to Tell Because They Have Clouds)
GJ 1214b is a "super-Earth" orbiting a small star located 40 light-years from our solar system. Observations suggest that its density is lower than that of Earth, so that it probably has a Ganymede-like composition (a metal and rock core overlain by a thick mantle of water in liquid or ice form). Unlike Ganymede, GJ 1214b is probably covered with a thick atmosphere of hydrogen and helium.
Wolf 1061c: the one that’s really close by and might be habitable
Wolf 1061 is a red dwarf about a third the mass of our Sun and only 13.8 light years away. It is orbited by at least 3 planets, the innermost of which (Wolf 1061b) is rocky; the other two are super-Earths or mini-Neptunes. Wolf 1061c, with a mass of 3.4 times Earth’s, orbits in the star’s habitable zone, close to its inner edge, while the outer, larger Wolf 1061d (about 8 times Earth mass, or half the mass of Neptune) travels just beyond the outer edge. Wolf 1061c is probably tidally locked, and although it’s in the habitable zone, what any habitable environments on it would look like is not very clear. We don’t know whether the high-pressure, high-temperature oceans on super-Earth ocean worlds would be conducive to biological chemistry.
Kepler-22b: Another super-Earth in the habitable zone of a Sun-like star
When it was discovered in 2011, Kepler-22b was the closest thing to another Earth then known: a smallish planet in the habitable zone of a Sun-like star, orbiting far enough away not to be tidally locked, in a 290-day orbit. However, it’s not small enough to be another Earth. At 2.4 times Earth’s diameter, it is a super-Earth or sub-Neptune, probably an ocean world.
NASA / Ames / JPL-Caltech
Kepler-22b: Closer to Finding an Earth
This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. It is the first planet that NASA's Kepler mission has confirmed to orbit in a star's habitable zone—the region around a star where liquid water, a requirement for life on Earth, could persist. Scientists do not yet know if the planet has a predominantly rocky, gaseous or liquid composition. It's possible that the world would have clouds in its atmosphere, as depicted here in the artist's interpretation.
Cold Jupiters and Neptunes
Kepler-16b: The one with two suns, so people call it “Tatooine” even though it’s Saturn-sized
Kepler-16b orbits a pair of stars that are smaller than the Sun, the first such circumbinary planet discovered. The two stars orbit each other with a period of 41 days, while the planet circles the both of them in a wider orbit taking 229 days. The planet is slightly smaller than Saturn and orbits within the system’s habitable zone. It would not have any solid surface, but any moons could. Like Saturn, it’s probably made mostly of metallic hydrogen.
Hot Jupiters/Hot Neptunes
Kepler 11: The one with lots of power in an itty bitty living space
The first planet to be discovered (in 1995) orbiting a Sun-like star, 51 Pegasi b is nothing like Earth. Its orbit takes only 4 days to complete, and the planet has a mass half that of Jupiter. We know its mass, but not its diameter, because it was discovered with the radial velocity method; however, a planet with so much mass must be a gas giant. 50 Pegasi b is the prototypical hot Jupiter.
ESO / M. Kornmesser / Nick Risinger (skysurvey.org)
51 Pegasi b
Artist’s impression of 51 Pegasi b, the first extrasolar planet discovered around a Sun-like star. It is a prototypical hot Jupiter, with an orbit that takes only 4 days to complete.
HD 189733b: The one where it rains glass sideways
One of the hottest of hot Jupiters, HD 189733b was discovered in 2005 by astronomers at the Haute-Provence Observatory in France. It is slightly larger in both mass and diameter than Jupiter, and orbits its star every 2.2 days at only 3 percent of Earth’s distance to the Sun. Followup observations with Spitzer showed strong temperature differences between the day and night sides, which should be common among hot Jupiters, since they orbit close enough to their host stars to be tidally locked. The temperature difference, combined with the gaseous composition of hot Jupiters, would naturally produce fierce winds blowing from dayside to nightside. Further observations with the Hubble Space Telescope’s imaging spectrograph revealed HD 189733b’s color to be a deep, dark blue, much bluer than Uranus or Neptune. The observation was later confirmed with SOFIA. The color probably arises from silicate particles—that is to say, droplets of glass—in the atmosphere.
KELT-11b: The one that’s as puffy as Styrofoam
KELT-11b is really feeling the heat. It’s in a 4.5-day orbit around a large, subgiant star. It’s less massive than Saturn, but has a bigger diameter than Jupiter, giving it an overall density less than one tenth that of water. Given its mass, that likely means it has a hugely inflated atmosphere, excited to dizzying heights by the heat of the nearby star, enveloping a much denser interior. The star is at a critical phase in its evolution, having exhausted the hydrogen in its core, and likely only tens of millions of years away from bloating into a giant that will engulf poor KELT-11b.
NASA, ESA, and P. Kalas
Fomalhaut b, a Directly Imaged Exoplanet
This false-color composite image, taken with the Hubble Space Telescope, reveals the orbital motion of the planet Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit. The Hubble images were taken with the Space Telescope Imaging Spectrograph in 2010 and 2012.
There are a few planets that are more difficult to describe but still worth mentioning because of peculiar characteristics. For instance, we’ve directly imaged a planet orbiting the star Fomalhaut, but we don’t know the planet’s mass or diameter. We know there’s a planet orbiting Proxima Centauri b that’s likely our closest neighboring planet, but we don’t know if it’s a rocky planet like Earth, a sub-Neptune ocean world, or even Neptune-sized; all we know is that its minimum mass is 1.3 times that of Earth. There’s a star called TOI-178 that might have two planets sharing the same orbit, one of them occupying a Trojan point of the other. And we know of at least one free-floating or rogue planet, PSO J318.5-22.
This page was written by Emily Lakdawalla and first published on 2 March 2020. Thanks to Emily Sandford for helpful review and comments. Planetary Society naming conventions for exoplanets are the same as for Sky & Telescope magazine: lowercase letter (b, c, etc.) solid with the star name unless the star name ends in a complete word/constellation, with star names as found at exoplanets.org. Examples: Kepler-20b, HD 40309g, 55 Cancri f, GJ 667 Cc (in the last example, C designates the stellar member of the star system, c is the second planet discovered around Gliese 667 C).