Six worlds circle a single star 661 light-years away, each one moving in careful rhythm with its neighbors. The system is called K2-138, and the newest member found so far — a world called K2-138 g — gives scientists a richer picture of a family that was already fascinating. This system is a quiet corner of our galaxy worth getting to know.
Where Is K2-138?
K2-138 is a star in the constellation Aquarius. It sits 661 light-years from Earth. A light-year is the distance light travels in one year — about 9.5 trillion kilometers — so 661 light-years is an enormous distance, even by space standards.
The star itself is very similar to our Sun, but not identical. Its surface temperature is 5,356 K (that “K” stands for Kelvin, a scale scientists use to measure heat — it starts at absolute zero, the coldest anything can be). Our Sun’s surface is a little hotter, at around 5,778 K. So K2-138 is slightly cooler and a little dimmer than our Sun. That difference matters when you try to work out which of its planets might be warm enough for liquid water.
Because K2-138 is fairly similar to our Sun, scientists find it especially useful for comparisons. Systems like this one help us understand whether solar systems like ours are common, or whether our own arrangement of planets is unusual.
A Chain of Six
K2-138 has six known planets, labeled b, c, d, e, f, and g. That might seem like a strange naming system, but it follows a simple rule: “b” is always the first planet discovered around a star, and the rest follow in the order they were found.
What makes this family especially interesting is how evenly spaced the inner planets are. The first five planets — b through f — orbit in what scientists call a resonance chain. A resonance chain means the planets’ orbital periods (the time each one takes to travel around the star once) follow a neat mathematical pattern. Roughly every time the outer planet in a pair completes one orbit, the inner one has completed a whole-number of orbits. It is a little like cogs fitting together, or a musical rhythm where every beat lines up.
Resonance chains are not random. They usually mean the planets formed or settled into their orbits through a slow, gentle process rather than a violent one. K2-138’s inner chain is one of the longest resonance chains scientists have spotted around any star. You can explore systems like this one in the system explorer.
Meet K2-138 g
K2-138 g is the outermost known planet in the system. It was discovered in 2021 using the transit method — a technique explained in the next section. Compared to the rest of the family, g sits a little further out and does not fit as neatly into the resonance chain. Scientists are still studying exactly how it relates to its neighbors.
Here is what we know. K2-138 g has a radius of 3.01 times Earth’s radius — so it is about three times as wide. Its mass is 4.32 times Earth’s mass. Those two numbers together tell scientists something useful: the planet is not as dense as pure rock would be, which suggests it likely has a thick atmosphere or a layer of water or other lighter materials wrapped around a rocky or icy core. Planets in this size and mass range are often called sub-Neptunes — a term for worlds bigger than Earth but smaller than Neptune.
K2-138 g completes one orbit around its star every 42 days. That is its year — just 42 Earth days long. Its likely temperature has been estimated at 445 K, which works out to about 172 degrees Celsius. That is far too hot for liquid water on its surface, at least by what we understand today. Scientists haven’t yet measured many other details, so there is still a lot to learn about this world.
What Kind of Worlds Are These?
The six planets of K2-138 are not all the same. The inner planets — b, c, d, and e — are smaller, closer to Earth in size, and orbit very close to their star. The outer ones, f and g, are larger and fall into that sub-Neptune category.
When scientists measure both a planet’s size and its mass, they can estimate its density — how tightly packed the material inside it is. A very dense planet is likely made mostly of rock or metal, like Earth. A less dense planet probably has a lot of lighter stuff: thick gases, water, or ice. The inner planets of K2-138 seem to be rocky or close to rocky, while the outer ones appear to be less dense. This pattern — smaller rocky planets inside, larger fluffier planets outside — actually resembles our own solar system in a broad way, though the K2-138 planets are all packed much closer to their star than ours are.
The Habitable Zone Question
The habitable zone is the range of distances from a star where temperatures could allow liquid water to exist on a planet’s surface — not too hot, not too cold. Scientists sometimes call this the “Goldilocks zone.” Finding a planet in that zone does not mean it has water or life. It just means conditions might not be impossible.
For K2-138, the habitable zone’s exact location depends on the star’s brightness and temperature. Scientists think the outer planets, including K2-138 g, may be near or overlap with parts of that zone, but the planet’s estimated temperature of about 172 degrees Celsius makes liquid surface water seem unlikely. We also do not know enough about the planet’s atmosphere — if it has one — to say anything more certain. It is a question that remains open.
How Scientists Found Them
All six planets were found using the transit method. When a planet passes in front of its star from our point of view, it blocks a tiny fraction of the star’s light. A sensitive telescope can detect that small dip. By watching how often the dips repeat, scientists figure out the planet’s orbital period. By measuring how deep the dip is, they can estimate the planet’s size.
K2-138 has a special place in the history of exoplanet discovery because the first five planets were identified partly through a citizen science project — meaning ordinary members of the public helped search through telescope data alongside professional astronomers. K2-138 g was found later, in 2021, by research teams looking more deeply at the same system.
Systems like K2-138 remind us how much there still is to find. Six planets around one star, packed close together, moving in rhythms — and scientists are still working out what they are made of, whether any could hold water, and whether there might be more worlds yet to find further out.