Super-Earths

The planets of our own Solar System come in two kinds: small rocky worlds up to the size of Earth, and giants from Neptune upward. Nothing in between. When astronomers began counting planets around other stars, that gap filled at once. The most common planets in the Milky Way turned out to be a size our Solar System does not have: larger than Earth, smaller than Neptune. They are called super-Earths.

Between Earth and Neptune

The name describes size, not character. A super-Earth is simply bigger and heavier than Earth but smaller and lighter than Neptune; NASA puts the upper limit at about ten Earth masses. Some may be rocky worlds with thin air, some may be covered in deep oceans, and some may wrap a rocky core in a thick envelope of gas. “Super-Earth” says nothing about whether any of them is like Earth.

Gliese 876 d, found in 2005 around a red dwarf, has at least about six times Earth’s mass. In 2009 CoRoT-7b became the first super-Earth whose radius was measured as well as its mass.

The most common planets

The Kepler space telescope settled how common they are. By 2014 more than three-quarters of its planet candidates lay between Earth and Neptune in size. A 2013 study found that the number of planets rises steadily from Neptune’s size down to about twice Earth’s width and then stays level, and another estimated that about 11% of Sun-like stars have a planet one to two times Earth’s width receiving one to four times as much sunlight as Earth does.

A gap in the sizes

In 2017 Benjamin Fulton and his colleagues measured the sizes of about two thousand Kepler planets precisely enough to see a pattern: there are less than half as many planets between 1.5 and 2 times Earth’s width as on either side. Small planets come in two groups, rocky cores up to about 1.5 Earth radii, and planets about 2 to 3 Earth radii that kept a layer of gas.

The usual explanation is that many were born with gas envelopes and some lost them. In one model the star’s light boiled the gas away in the first hundred million years; in another, the heat left over from the planet’s own formation did it. A third view, proposed for planets around red dwarfs, is that the divide is between rocky and water-rich planets rather than a gap in size. The question is not settled.

Why we have none

Nobody knows why the Solar System has no super-Earth. One hypothesis, from Konstantin Batygin and Gregory Laughlin in 2015, is that young Jupiter moved inward towards the Sun and back out, and the debris it stirred up drove any early close-in planets into the Sun, leaving room only for small rocky planets to form later.

What Webb is seeing

The James Webb Space Telescope is now studying their atmospheres. For 55 Cancri e, a super-Earth whose dayside may be molten, a 2024 study ruled out bare lava and found signs of an atmosphere probably rich in carbon dioxide or carbon monoxide, although its brightness varies in ways not yet understood. For LHS 1140 b, which orbits in its star’s habitable zone, Webb has ruled out a thick hydrogen atmosphere; a nitrogen atmosphere or a water world are possible but unconfirmed.