The sizes nobody has next door
The solar system skips a whole range of planet sizes. Earth is 1 Earth radius, and the next planet up is Neptune at 3.9 times that. Nothing sits in between, yet that missing middle is where the archive finds most of its planets: 3,654 of the 6,401 with a listed radius fall between 1.25 and 6 Earth radii. The Kepler telescope survey revealed how common they are. See the planet sizes compared page for the solar system's own line-up.
Astronomers call the 1.25 to 2 group super-Earths and the 2 to 6 group mini-Neptunes. The names describe size only. A super-Earth may be rock, rock with a thick atmosphere or something wetter. Mass helps, and 2,436 planets in the archive have both a radius and a mass or minimum mass. 55 Cancri e is a good example. It was found by its wobble in 2004 with a minimum mass near 8 Earth masses, and later transit measurements gave it a radius of 1.88 Earth radii. Radius and mass together give a planet's density, and density hints at what it is made of.
How we sort them
These cut-offs use radius in Earth radii and follow common practice, but astronomers draw the lines in slightly different places and nature doesn't. For comparison, using NASA's planetary fact sheet: Mercury is 0.38 Earth radii across, Mars 0.53, Neptune 3.9 and Jupiter 11.2.
| Class | Radius (Earth radii) | Planets | Share | Median orbit (days) | Median distance (light years) |
|---|---|---|---|---|---|
| Smaller than Earth | under 0.8 | 87 | 1.4% | 3.5 | 816 |
| Earth-sized | 0.8 to 1.25 | 496 | 7.7% | 4.8 | 1,290 |
| Super-Earths | 1.25 to 2 | 1,212 | 18.9% | 6.5 | 1,716 |
| Neptune-sized and mini-Neptunes | 2 to 6 | 2,442 | 38.2% | 14.1 | 1,556 |
| Jupiter-sized | 6 to 15 | 1,923 | 30.0% | 85.3 | 604 |
| Larger than Jupiter-sized | over 15 | 241 | 3.8% | 3.3 | 1,204 |
What the groups mean
Super-Earths and Earth-sized planets are the right size to be rocky. Studies of planets with measured masses find that around 1.6 Earth radii it's roughly even odds whether a planet is mostly rock, and between about 1.5 and 2 Earth radii planets shift from rocky to carrying thick gas layers.
Mini-Neptunes and Neptune-sized planets are bigger than that, and most are probably wrapped in gas. Uranus (4.0 Earth radii) and Neptune (3.9) belong in this group, but the typical member is smaller, with a median of 2.7 Earth radii. It's the most common size we have found.
Jupiter-sized planets are giant gas worlds. The ones that orbit very close to their star are called hot Jupiters. We count a hot Jupiter as a planet with at least 0.25 Jupiter masses and an orbit of 10 days or less, which gives 703 in the archive. Other astronomers use slightly different cut-offs.
The gap in the middle
In 2017 the California-Kepler Survey, using precise Keck measurements of the host stars, showed that planets between about 1.5 and 2 Earth radii are scarce. The dip is called the radius gap or the Fulton gap, after Benjamin Fulton, who led the work. It separates small rocky planets from planets with thick envelopes of gas.
Two explanations compete. In photoevaporation, ultraviolet and X-ray light from the young star blows the gas off planets that start out only slightly gassy, leaving bare rock. In core-powered mass loss, heat left over from the planet's formation does the stripping. Both predict a gap, and astronomers are still testing which one dominates.
The archive's pooled radii don't show the gap cleanly. The two quarter-Earth-radius steps from 1.5 to 2 hold 433 and 349 planets, against 430 just below and 457 further up. The signal blurs because the archive mixes surveys with different precision, and the gap moves with orbital period and star type. The pooled table also lists radii for 1,587 planets that have never been seen to transit, so those values can't be transit measurements. It takes careful samples to see the gap.
Hot Jupiters weren't supposed to exist
When 51 Pegasi b appeared in 1995 it was a giant planet circling its star in about four days. HD 209458 b, found in 1999, soon became the first planet seen crossing its star. The archive gives it a radius of 1.39 times Jupiter's, with only about 0.73 of Jupiter's mass. Many hot Jupiters are swollen like that. Proposed causes include heat from the star and heat deposited deep inside the planet, and no single answer has won.
These planets are conspicuous because they're easy to detect, and the archive shows 703 that meet our hot-Jupiter rule. They aren't typical. Among the Sun-like stars Kepler watched, small planets were far more common. The star type page shows which stars host them.
Bigger than Jupiter
241 planets have a listed radius above 15 Earth radii, about 1.3 times Jupiter's. The typical planet in this group circles its star in just 3.3 days. Heat from the star is one explanation for why such planets are swollen. Some others are young giants that may still be contracting, and a few have radii that are uncertain. Treat the largest values with caution.
How a size is measured
A transit gives a ratio. The dip in starlight is the planet's area divided by the star's area, so the planet's radius is only as good as the radius of its star. If the star is 10% bigger than assumed, so is the planet. That's why better stellar measurements, as in the California-Kepler Survey, were what revealed the gap. In the archive, 4,815 planets are flagged as transiting, and their sizes come from the transit depth plus a model of the star, so they're derived, not read off directly. Planets found by wobble alone have a minimum mass and no transit radius, so any size listed for them is an estimate from other sources.
A selection effect to keep in mind
Big planets are easier to detect, so they show up more than their true share. The counts here describe the planets we have found, which leans toward large planets on short orbits, and not the whole galaxy.