Types of Exoplanets: Hot Jupiters, Super-Earths and Beyond

Exoplanets come in far more varieties than the planets of our solar system. Our neighborhood contains four rocky worlds, two gas giants, and two ice giants, but surveys of other stars have revealed planets with no solar system equivalent. Astronomers sort them mainly by radius and mass, because those two numbers together reveal whether a planet is rocky, gas-rich, or something in between.

Sorting planets by size and mass

Radius comes from transit depth, and mass comes from radial velocity or timing effects. Combining the two gives an average density. A planet with Earth's density is almost certainly rocky and iron-rich, while a planet with a lower density must hold a substantial envelope of hydrogen, helium, or water.

The class boundaries below are rough and shift as new data arrive. They are useful for organizing the catalog, not for drawing sharp lines through nature.

Class	Radius (Earth = 1)	Mass (Earth = 1)	Example
Earth-size rocky	0.8 to 1.25	0.1 to 2	TRAPPIST-1 b, about 1.1
Super-Earth	1.25 to 2	2 to 10	Many Kepler finds
Sub-Neptune	2 to 4	5 to 20	GJ 1214 b, about 2.7 radius and 6.5 mass
Neptune-like	4 to 6	10 to 30	Neptune itself, 3.9 radius and 17 mass
Hot Jupiter	About 11 and up	About 30 and up	51 Pegasi b, about half Jupiter's mass

Neptune and Jupiter are included as reference points from the solar system. Jupiter's radius is about 11 times Earth's, and its mass is about 318 times Earth's.

Hot Jupiters

Hot Jupiters are gas giants with orbits shorter than about 10 days, and they orbit closer to their stars than Mercury orbits the Sun. The first one confirmed around a Sun-like star, 51 Pegasi b, has an orbit of about 4.2 days. Its atmosphere is heated to roughly 1,000 degrees Celsius or more, far too hot for any liquid water.

Hot Jupiters were a surprise when they were first found. Models of planet formation suggested that giant planets should form far from their stars, where ice is abundant. The existence of hot Jupiters implied that giant planets can migrate inward through interactions with the disk of gas and dust that surrounded the young star. Their large size also made them easy to detect, which is why they dominated early catalogs.

Gas giants and ice giants

Gas giants like Jupiter have thick hydrogen and helium envelopes around smaller cores. Ice giants like Neptune and Uranus hold more heavy material, including water, ammonia, and methane, in their interiors. Many exoplanet surveys struggle to tell these two classes apart from radius and mass alone, because both can be several times Earth's size.

Cooler gas giants at wide orbits are harder to find. Transit surveys miss them because they rarely align with our line of sight, and radial velocity needs many years of data to detect their slow wobble. Direct imaging can see some of them if they are young and still glowing from formation heat.

Super-Earths and sub-Neptunes

Super-Earths and sub-Neptunes are the most common planets found so far in short-period orbits. Their radii range from about 1.25 to 4 Earth radii, which is a size range that has no close counterpart in our solar system. Between them lies a notable gap in the distribution of planet sizes, often called the radius valley, near 1.8 Earth radii. Few planets sit precisely in that range, which suggests that many planets lose or keep their gas envelopes depending on conditions after formation.

Sub-Neptunes may be rocky cores with hydrogen-rich atmospheres, or they may be water-rich worlds with dense steam or supercritical fluids. Their exact composition is still debated, and that uncertainty makes them interesting targets for atmosphere studies. GJ 1214 b, found in 2009, was an early example of a planet between Earth and Neptune in size, and its mass suggested a low overall density.

Earth-size rocky worlds

Earth-size rocky planets are the category most people hope to find, because they are the most similar to Earth. Their small size makes them hard to detect, and the transit signal from an Earth-size planet around a Sun-like star is only about 84 parts per million. Red dwarf stars make detection easier, because their habitable zones are closer, so transits come more often, and their stars are smaller, so an Earth-size planet blocks more of the light.

TRAPPIST-1 is a dim red dwarf with a system of several Earth-size planets, announced in 2017. Its inner planets have radii near that of Earth and orbit very close to their star, so their climates depend heavily on atmosphere. Proxima Centauri, the closest star to the Sun, hosts a planet with a minimum mass of about 1.1 Earth masses in its habitable zone, though its true mass and atmosphere remain unknown.

Unusual architectures

Some exoplanets defy simple classes. Circumbinary planets orbit two stars at once, and Kepler-16b, found in 2011, was one of the first confirmed examples. It circles a pair of stars that eclipse each other, so its orbit is shaped by two gravitational centers.

Lava worlds are rocky planets so close to their stars that their surfaces may melt, creating thin atmospheres of vaporized rock. Free-floating planets have no host star at all and drift through the galaxy alone, detected through microlensing or direct imaging. Each of these categories expands the range of what planets can be.

Why the boundaries blur

The labels above are convenient, but nature does not respect them. A super-Earth and a sub-Neptune may differ only in their envelope, and a planet's class can change if its atmosphere is stripped by starlight. Measurement uncertainty also blurs the boundaries, because radius and mass each carry error bars that can push a planet from one class into a neighbor.

Astronomers therefore treat the classes as a guide for comparing populations. The more useful question is how planets are distributed across the full range of size, mass, and orbit, because that distribution reveals how planets form and change over time.

Key takeaways

Exoplanets are sorted by size and mass, and the combination reveals density and likely composition. Gas giants, ice giants, sub-Neptunes, super-Earths, and rocky worlds each form a distinct population with its own formation story.

The most common planets found so far are not present in our solar system, which shows that our own planetary arrangement is only one of many possible outcomes. Hot Jupiters and sub-Neptunes in particular reshaped how astronomers think about planet formation.

Earth-size rocky worlds remain the hardest to detect and the most scientifically compelling. Each new measurement of their size, mass, and atmosphere brings the search for habitable environments a step closer to answering whether Earth is typical or unusual.