Just 21.3 light-years away — close by cosmic standards — a quiet orange star hosts one of the most planet-rich systems we know of near our Sun. That star is HD 219134, and it has at least six worlds circling it. One of those worlds, HD 219134 g, has been measured carefully enough that scientists can start to describe what kind of place it might be.
A Star Close to Home
HD 219134 sits in the constellation Cassiopeia. At 21.3 light-years from Earth, it is one of the nearest stars known to host multiple planets. To give that distance some meaning: light travels faster than anything in the universe, yet even light would take more than 21 years to make the trip.
The star itself is a little smaller and cooler than our Sun. Its surface temperature is 4,913 K (K stands for Kelvin, a scale scientists use to measure heat — 4,913 K is very hot by everyday standards, but cooler than the Sun’s roughly 5,778 K). Stars with this temperature often look slightly orange rather than the pale yellow-white of our Sun. They are sometimes called K-type stars, or orange dwarfs.
Because HD 219134 is so close to us, astronomers have been able to study it in a lot of detail. That closeness is part of why so many planets have been found there. The nearer a star is, the easier it is to detect small signals that hint at orbiting worlds.
HD 219134 g: The Planet We Know Best in This Article
Of the six known planets in this system, HD 219134 g is the one with the most data available here. Scientists have measured two key things about it: its size and its mass.
In terms of size, HD 219134 g has a radius — that is, the distance from the center of the planet to its surface — of 3.28 times Earth’s. That makes it noticeably larger than our planet, but not a giant. In terms of mass, it weighs in at 10.8 times the mass of Earth. That is a significant amount of material packed into that world.
One orbit around its star takes 94.2 Earth days. So a full year on HD 219134 g is about three months long by our calendar. That orbit length tells scientists roughly how far the planet sits from its star, though the exact distance in kilometers or astronomical units has not been listed in the data we are working from here.
All six planets in the HD 219134 system were first detected starting in 2015, using a technique called the radial velocity method — more on that below.
What Kind of World Could It Be?
When scientists look at a planet’s radius and mass together, they can start to guess what it might be made of. HD 219134 g is about 3.28 times wider than Earth and 10.8 times more massive. That combination puts it in a category that researchers sometimes call a sub-Neptune or a super-Earth — worlds larger than Earth but smaller than Neptune.
Planets in this size range are actually the most common kind found around other stars so far. Yet we have none in our own Solar System. The gap between Earth (the largest rocky planet we have) and Neptune is empty in our neighborhood, which makes planets like HD 219134 g especially interesting to study.
Whether HD 219134 g is mostly rocky, or whether it has a thick atmosphere or envelope of gas and water, scientists are not yet sure. Its density — how tightly packed its material is — would help answer that, but getting a clear answer takes very precise measurements. What we can say is that it is likely not a small, purely rocky world like Earth or Mars, given its size and mass. It may have a significant envelope of lighter material surrounding a denser core, but that is still an open question.
As for whether it could support liquid water or life, scientists simply do not have enough information to say. Those questions remain unanswered for now.
The Full Family of Six
HD 219134 g is just one member of a busy planetary family. The system has at least six known planets in total. Some of them orbit very close to the star and complete a year in just a few Earth days. Others take longer, like HD 219134 g with its 94.2-day orbit.
Several planets in this system have been studied well enough that scientists know both their radius and their mass. Having both numbers is valuable because it lets researchers estimate a planet’s density. Some of the inner planets in the system appear to be rocky worlds, denser than HD 219134 g. This hints that the system may have a similar broad pattern to our Solar System — smaller, denser worlds closer in, and larger, lighter worlds farther out — though the details are quite different.
You can explore the arrangement of all six planets using the system explorer, which shows how they compare to one another and to planets in our Solar System.
How Scientists Found Them
The planets of HD 219134 were discovered using the radial velocity method. Here is how it works: a planet’s gravity tugs on its star as it orbits. That tug makes the star wobble slightly — first toward Earth, then away. When the star moves toward us, its light gets compressed into slightly shorter wavelengths. When it moves away, the light stretches into longer wavelengths. Scientists can detect these tiny shifts using sensitive instruments called spectrographs.
The size of the wobble tells scientists about the planet’s mass. The timing of the wobble tells them how long the orbit takes. This method has been used to find hundreds of exoplanets — planets orbiting stars other than our Sun — and it works especially well for planets that are fairly massive and orbit close to their star.
Some planets in the HD 219134 system were also detected by the transit method, where a planet passes in front of its star and causes a small dip in the star’s brightness. That extra method helped scientists nail down the radius of some of the planets.
Comparing This System to Ours
Our Solar System has eight planets spread over a wide range of distances from the Sun. The HD 219134 system is packed differently. Several of its planets orbit much closer to their star than Mercury orbits our Sun. The whole known family fits into a region that would be considered very close range by Solar System standards.
The star itself is cooler and smaller than the Sun, which affects how much heat and light reaches each planet. A planet at the same distance from HD 219134 as Earth is from the Sun would receive less warmth than Earth does.
At just 21.3 light-years away, HD 219134 is one of the closest multi-planet systems we have found so far. As telescopes improve and new missions launch, this system is likely to keep offering scientists a rich place to study how families of planets form, grow, and arrange themselves around a single star — which, in turn, helps us understand our own corner of the universe a little better.