TOI-1105 b is a world about three times wider than Earth, orbiting a star 259 light-years away. It sits in a category scientists call mini-Neptunes — planets that are larger than Earth but smaller than the ice giants in our own solar system. Studying worlds like this one helps us understand just how many different kinds of planets exist across the galaxy.
Where TOI-1105 b Lives
TOI-1105 b orbits a star called TOI-1105. That star sits about 259 light-years from Earth. A light-year is the distance light travels in one full year — nearly 9.5 trillion kilometers — so 259 light-years is an enormous distance by everyday standards, yet fairly close in the scale of our galaxy.
The star TOI-1105 is cooler than our Sun. Its surface temperature is about 4,678 K (K stands for Kelvin, a temperature scale scientists use; 0 K is as cold as anything can get). For comparison, our Sun’s surface sits around 5,778 K. A star this cool tends to look orange rather than yellow-white, and it belongs to a type astronomers call a K-type star. As far as we know, TOI-1105 b is the only planet confirmed in this system so far.
TOI-1105 b circles its star very quickly. One full orbit — what we would call a year on that world — takes only 10.9 Earth days. That is a tight, fast path around the star.
How Big and Heavy Is TOI-1105 b?
Size and mass are two of the most useful things scientists can measure about a planet. They don’t always tell the same story.
TOI-1105 b has a radius — that’s the distance from the center of a planet to its surface — of about 3.06 times Earth’s radius. So if you could line up copies of Earth side by side, you would need just over three of them to stretch across this planet. That puts it clearly in mini-Neptune territory: bigger than Earth, but much smaller than Neptune, which is about four times Earth’s width.
Its mass is about 9.58 times Earth’s mass. Mass measures how much matter is packed into an object. So TOI-1105 b is both wider and heavier than Earth — but in ways that tell an interesting story when you look at them together. You can compare the sizes of planets like this one to get a feel for how they stack up visually.
What Mini-Neptunes Are Made Of
Mini-Neptunes are one of the most common types of planet that astronomers find around other stars. Our own solar system doesn’t have one, which makes them especially interesting to study.
Scientists think most mini-Neptunes have a rocky or icy core at the center. Wrapped around that core is a thick layer of gas — mostly hydrogen and helium, the two lightest elements in the universe. Some mini-Neptunes may also hold a lot of water deep inside, though not necessarily as liquid water we would recognize. At high pressures, water can exist in strange, dense forms.
Because we can’t visit TOI-1105 b or send a probe through it, scientists piece together what it’s made of by using its size and mass together. The recipe for figuring this out is called the planet’s density — how much mass is packed into each unit of volume. A dense planet is heavy for its size, like a iron cannonball. A less dense planet is light for its size, like a beach ball.
What Density Tells Us About the Inside
When you combine TOI-1105 b’s radius of 3.06 Earths with its mass of 9.58 Earths, you get a planet that is noticeably less dense than a rocky world like Earth. A pure rock planet of the same size would need to be much heavier. Because TOI-1105 b is lighter than that, scientists think it almost certainly has a thick envelope of gas or lighter materials surrounding a denser core.
This is exactly what we expect from a mini-Neptune. The bulk of the planet’s volume is probably made up of lighter stuff — gas, water in unusual forms, or both — sitting on top of a rocky or icy interior. Scientists describe this kind of structure as a volatile envelope, where volatile means materials that are light and can form gases or fluids easily.
Nailing down exactly what the layers look like inside takes more data than we have right now. Scientists haven’t measured the planet’s composition directly, so the picture above is a reasonable scientific estimate, not a certain fact. The mass-radius diagram is a useful tool for seeing how a planet’s mass and size together hint at its inner makeup.
A Hot World in a Tight Orbit
Because TOI-1105 b orbits so close to its star, it receives a lot of energy. Scientists estimate its equilibrium temperature — the temperature a planet would reach if it absorbed and released starlight perfectly — at about 594 K, which is roughly 321 degrees Celsius. That is far hotter than anywhere on Earth’s surface.
At those temperatures, any water at the surface would exist as steam, not liquid. This means TOI-1105 b almost certainly does not have conditions friendly to life as we know it. Scientists don’t describe it as a habitable world.
The tight orbit also raises a question that planetary scientists find fascinating: how does a thick gas envelope survive so close to a star? Over time, intense starlight and radiation can strip gas away from a planet’s atmosphere in a process called photoevaporation — where high-energy light essentially blows lighter gases off into space. Whether TOI-1105 b is losing its atmosphere slowly, or whether it is massive enough to hold onto it, is something scientists would need more observations to answer confidently.
How Scientists Found TOI-1105 b
TOI-1105 b was discovered in 2026 using the transit method. The transit method works by watching a star very carefully over time. When a planet passes in front of its star from our point of view, it blocks a tiny fraction of the star’s light. Telescopes can detect that small dip in brightness. By measuring how often the dip happens and how deep it is, scientists can calculate how long the planet’s orbit is and how large the planet is relative to its star.
The name TOI stands for TESS Object of Interest. TESS is a space telescope designed to search for planets by watching for exactly these small dips in starlight. When TESS flags a promising signal, scientists follow up with additional telescopes to confirm the discovery and gather more details — including the planet’s mass.
TOI-1105 b is a good example of the kind of world space telescopes are finding in large numbers: a warm, gassy planet in a close orbit, unlike anything in our solar system, quietly expanding our picture of what planets can be.