TOI-2104 b is an exoplanet — a planet orbiting a star outside our solar system — that no telescope has ever seen as a dot of light. Yet scientists know it exists, and they even know its size and mass. How is that possible? The answer lies in two clever methods that let astronomers study planets they cannot photograph directly.
What Kind of World Is TOI-2104 b?
TOI-2104 b sits about 250 light-years from Earth. A light-year is the distance light travels in one year — roughly 9.5 trillion kilometres. That is far enough that even our most powerful telescopes cannot pick out the planet as a separate object against the glare of its star.
Scientists discovered TOI-2104 b in 2026 using the transit method, which we will explain below. So far, it is the only planet confirmed in the TOI-2104 system. There may be others hiding there, but none have been found yet.
How the Transit Method Works
The transit method is the main tool used to find planets like TOI-2104 b. The word transit means “to cross in front of.” When a planet passes between its star and us, it blocks a tiny fraction of the star’s light.
Here is how it works, step by step:
- A telescope watches a star very carefully, recording exactly how bright it is over days, weeks, or months.
- If a planet is in the right position, it crosses in front of the star. This causes a small, brief dip in the star’s brightness.
- The dip repeats every time the planet completes one orbit. That regular pattern tells astronomers how long the planet’s year is.
- The size of the dip tells astronomers how large the planet is compared to its star. A bigger planet blocks more light and causes a deeper dip.
The dip caused by a planet like TOI-2104 b is tiny — far too small to see with your eyes. Astronomers measure it with sensitive instruments that can detect a change of less than one percent in a star’s brightness. If you want to get a feel for how this works, the How We Find Them simulator lets you try it yourself.
One important thing to know: the transit method only works if the planet’s orbit happens to be lined up almost edge-on from our point of view. If the planet orbits at a different angle, it never crosses in front of the star from where we are watching, and we would never know it was there this way.
How the Wobble Method Works
The transit method is great for finding a planet and measuring its size. But to find its mass — how much matter it contains — scientists often use a second method called the radial velocity method, or more simply, the wobble method.
Here is the idea. We usually think of a planet orbiting a star. But gravity works both ways. The planet’s gravity also pulls on the star, making the star wobble very slightly as the planet goes around it. The star does not move much, but it does move a little.
When the star wobbles toward us, its light gets compressed into slightly shorter wavelengths — it shifts toward blue. When it wobbles away, the light stretches into longer wavelengths — it shifts toward red. This effect is called the Doppler shift, named after the scientist Christian Doppler. It is the same effect that makes a siren sound higher as an ambulance comes toward you and lower as it drives away.
By measuring how much the star’s light shifts back and forth, and how often, astronomers can calculate the planet’s mass. A heavier planet causes a bigger wobble, and that means a bigger shift in the light.
What We Know About TOI-2104’s Star
Every planet is shaped by its star, so understanding the star is important. TOI-2104 is what astronomers call a K-type star — a star somewhat cooler and smaller than our own Sun. Its surface temperature is 4,856 K. For comparison, our Sun’s surface sits at around 5,778 K, so TOI-2104 is noticeably cooler. Cooler stars tend to glow more orange than yellow-white.
K-type stars are considered interesting in the search for planets because they are long-lived and relatively calm compared to the smallest, most active stars. That said, scientists haven’t yet measured many other details about TOI-2104, such as its exact size or age.
Size, Mass, and What They Tell Us
From the transit and wobble methods together, scientists have worked out two key facts about TOI-2104 b. Its radius — its width from centre to edge — is about 1.53 times Earth’s radius. Its mass is about 2.95 times Earth’s mass.
Those two numbers together are very useful. When you know both the size and the mass of a planet, you can calculate its density — how tightly packed the material inside it is. A planet that is large but light is probably made mostly of gas or thick atmosphere. A planet that is small but heavy is probably made mostly of rock or metal.
TOI-2104 b is about one and a half times wider than Earth and nearly three times heavier. Scientists think this puts it in a category sometimes called a super-Earth or possibly a mini-Neptune — a world bigger than Earth but smaller than the ice giants in our solar system. Whether it has a thick atmosphere wrapped around a rocky core, or some other composition, is something researchers are still working to understand.
A World of Extreme Heat
TOI-2104 b completes one full orbit around its star every 5.9 Earth days. That is a very short year. It means the planet is orbiting extremely close to its star — far closer than Mercury is to our Sun.
At that distance, the planet receives an enormous amount of energy. Scientists estimate its temperature is around 846 K, which is about 573 degrees Celsius. That is hot enough to melt lead and many other metals. TOI-2104 b is nowhere near the habitable zone — the region around a star where temperatures could allow liquid water to exist on a planet’s surface. It is far too close and far too hot for that.
Why Planets Like This Matter
TOI-2104 b is not a place where life as we know it could survive. But planets like it still matter a great deal to science. Short-orbit planets are among the easiest to detect and study, because their transits happen frequently and their effect on the star’s wobble is stronger. Each one we study helps astronomers build a better picture of how planets form and how common different types of worlds are across the galaxy. TOI-2104 b is one more careful measurement in that much larger story — found not by seeing it, but by reading the light of the star it circles.