TOI-6699 b is an exoplanet — a planet orbiting a star other than our Sun — sitting about 280 light-years from Earth. It belongs to a group of worlds called mini-Neptunes, which are some of the most common planets scientists have found so far. Getting to know planets like this one helps us understand just how many different kinds of worlds exist beyond our solar system.
How We Found TOI-6699 b
Astronomers discovered TOI-6699 b in 2026 using something called the transit method. A transit happens when a planet passes in front of its star, as seen from Earth. When that happens, the star looks very slightly dimmer — like a tiny shadow crossing a bright lamp. Telescopes can detect that small dip in brightness and use it to work out that a planet is there.
The transit method tells scientists two very useful things right away: how big the planet is and how long it takes to complete one orbit. It does not tell us everything, though. Other measurements are needed to find out mass, and some details — like what the atmosphere might be made of — are still being studied.
TOI-6699 b was spotted by tracking the star TOI-6699. As far as we know, it is the only planet confirmed in this system so far, though that could change as astronomers look more carefully.
A World That Moves Fast
One of the most striking things about TOI-6699 b is how quickly it travels around its star. One full orbit — what we would call a year on that planet — takes just 5.32 Earth days. To put that in perspective, Mercury, the fastest planet in our solar system, takes about 88 Earth days to complete one orbit. TOI-6699 b is far speedier than that.
Moving so fast means the planet must be very close to its star. Planets that orbit quickly are almost always hugging their host star tightly. That closeness has a big effect on temperature, which we will come back to shortly.
Short orbits like this are actually very helpful for scientists. The more often a planet transits its star, the more data researchers can collect. Frequent transits mean more chances to measure the planet carefully and build a clearer picture of what it is like.
Size, Mass, and What They Tell Us
TOI-6699 b has a radius — that is, the distance from its center to its surface — of about 2.33 times Earth’s radius. Its mass is about 6.02 times Earth’s mass. These two numbers together are very powerful clues.
When you know both the size and the mass of a planet, you can work out its density — how much matter is packed into a given amount of space. A planet made mostly of rock would be quite dense. A planet with a thick atmosphere of light gases would be less dense overall, even if it is large.
TOI-6699 b is bigger than Earth but not as big as Neptune. Scientists place it in the mini-Neptune category, and its combination of size and mass suggests it is not made of solid rock all the way through. It likely has a rocky or icy core surrounded by a thick layer of gas or volatile material — substances that easily turn into gas at high temperatures. You can explore how worlds of different sizes and masses compare using our mass-radius diagram.
It is worth saying that these estimates come with some uncertainty. Working out exactly what is inside a distant planet is genuinely hard. Scientists use the best models they have, but they are careful to treat their conclusions as educated guesses rather than settled fact.
Inside a Mini-Neptune
Mini-Neptunes are a class of planet that does not exist in our own solar system. Earth and Venus are rocky worlds. Neptune and Uranus are ice giants — planets with thick layers of water, ammonia, and methane in high-pressure, slushy forms, surrounded by deep atmospheres. Mini-Neptunes seem to sit somewhere in between, though they are smaller than ice giants.
Scientists think many mini-Neptunes have a dense inner region — possibly rock, possibly a mix of rock and ice — wrapped in a thick atmosphere rich in hydrogen and helium, or perhaps heavier gases like water vapor and methane. The exact recipe probably varies from planet to planet.
For TOI-6699 b, scientists haven’t yet measured the atmosphere directly. Future observations might reveal what gases surround it, but for now, its interior remains something of a puzzle. What the size and mass do tell us is that it almost certainly is not a pure rock world. There is likely a lot of lighter material — gas or volatile compounds — making up a significant part of it. If you want to see where TOI-6699 b fits among other known worlds, our size comparison tool can show you how it stacks up.
A Very Hot Place to Be
TOI-6699 b has an estimated temperature of around 976 Kelvin, which is about 703 degrees Celsius. That is far hotter than any place on Earth’s surface. Lead melts at around 327 degrees Celsius, so the surface conditions here — if we can call them a surface — are extreme by any everyday standard.
This heat comes from how close the planet sits to its star, combined with how much energy that star puts out. At these temperatures, liquid water on the surface is not possible. TOI-6699 b does not sit in the habitable zone — the region around a star where conditions might allow liquid water to exist. It is well inside that zone, far too hot for the kind of chemistry we associate with life as we know it.
It is important to note that the 976 K figure is an estimate based on how much starlight the planet likely receives. The actual temperature depends on factors like reflectivity and atmosphere, which scientists haven’t fully pinned down yet.
The Star TOI-6699 b Calls Home
The planet’s host star, TOI-6699, has a surface temperature of 4,705 Kelvin. Our own Sun’s surface sits at about 5,778 Kelvin, so TOI-6699 is noticeably cooler. A cooler surface temperature usually means the star looks more orange than yellow-white. Stars like this are often called K-type stars, or orange dwarfs, and they are common in our galaxy.
Cooler stars like TOI-6699 can still pour a great deal of energy onto nearby planets, especially when those planets orbit as close as TOI-6699 b does. The distance between the planet and its star hasn’t been precisely published in the data we have, but the very short orbital period makes clear it is very tight indeed.
TOI-6699 b may not be the most welcoming world, but studying it adds another data point to our growing map of planetary types. Every planet we measure helps scientists test their models of how worlds form, what they are made of, and just how varied the cosmos turns out to be.