Out among the stars, one quiet world has caught the attention of planet hunters. Kepler-1690 b is a planet orbiting a distant star, and some of its basic measurements put it in an interesting place — one worth thinking carefully about. Here is what we know, what we don’t, and why scientists find it worth a closer look.
A Distant World, Found by a Passing Shadow
Kepler-1690 b was discovered in 2020 using the transit method. That means scientists spotted the planet when it passed in front of its star, blocking a tiny bit of starlight. By measuring how much light disappeared and for how long, astronomers could work out the planet’s size and how long it takes to complete one orbit.
The planet sits 2,295 light-years from Earth. A light-year is the distance light travels in one year — about 9.5 trillion kilometres. That means the light leaving Kepler-1690 b’s star right now won’t reach us for more than two thousand years. We are seeing this system as it looked long ago.
So far, Kepler-1690 b is the only planet scientists have found in this system. That doesn’t mean it’s alone — other planets may simply be too small or orbit at the wrong angle for us to detect yet.
The Star at the Centre
Every planet is shaped by its star, so understanding Kepler-1690 matters a lot. The star has a surface temperature of 5,404 K (K stands for Kelvin — a temperature scale scientists use that starts at absolute zero, the coldest anything can be). For comparison, our Sun’s surface runs at about 5,778 K. That makes Kepler-1690 a little cooler and probably a little smaller and dimmer than the Sun, though we don’t have a confirmed size measurement for it yet.
A slightly cooler star puts out somewhat less energy. That matters when we try to figure out how warm a planet orbiting it might be, and whether liquid water — the kind life on Earth depends on — could exist there.
Sitting in a Promising Spot
One of the most useful ideas in the search for life is the habitable zone. This is the band of space around a star where conditions might allow liquid water to exist on a planet’s surface. Too close to the star and water boils away. Too far and it freezes solid. The habitable zone is sometimes called the “Goldilocks zone” — not too hot, not too cold.
Kepler-1690 b takes 235 Earth days to complete one orbit around its star. That’s a little shorter than Earth’s year of 365 days. Based on the star’s temperature and this orbital distance, scientists estimate the planet’s likely temperature at around 267 K, or about -6 degrees Celsius. That is just below the freezing point of pure water.
That number places Kepler-1690 b right at the cool edge of what many scientists consider the habitable zone. If you’re curious how scientists map these zones around different stars, you can explore the idea further with our habitable zone guide. It’s a narrow and uncertain boundary, and Kepler-1690 b sits near its chilly outer rim.
A Bigger, Heavier World Than Ours
Kepler-1690 b has a radius 2.81 times Earth’s and a mass 8.3 times Earth’s. Those two numbers together tell scientists something important about what kind of planet this might be.
When a planet is much bigger and heavier than Earth, it often falls into a category scientists call a sub-Neptune. Sub-Neptunes are worlds that are too large to be rocky like Earth but smaller than the ice giant Neptune in our solar system. They are thought to have thick envelopes of gas, water, or ices surrounding a denser core. They are the most common type of planet found around other stars so far.
If Kepler-1690 b is a sub-Neptune, its surface — if it even has a defined solid surface — would look very different from Earth’s. The gravity would be much stronger. The atmosphere, if it has one, could be deep and crushing. These are not automatically deal-breakers for life as scientists imagine it, but they are very different conditions from what life on Earth is used to.
What One Temperature Number Can and Can’t Tell Us
The estimated temperature of -6 degrees Celsius sounds precise, but it comes with important limits. This figure, called the equilibrium temperature, is calculated by assuming the planet reflects a certain average amount of starlight and spreads heat evenly across its surface. It is a useful starting estimate, but it leaves out a great deal.
For example, if Kepler-1690 b has a thick atmosphere with gases like carbon dioxide or water vapour, the greenhouse effect — the warming that happens when an atmosphere traps heat — could raise the actual surface temperature well above this estimate. A strong greenhouse effect might push temperatures above freezing, making liquid water more plausible. Alternatively, a highly reflective cloud layer could make the planet colder than the estimate suggests. Without studying the atmosphere directly, scientists can’t be sure which way things go.
Could This World Support Life?
The honest answer is: we don’t know. Kepler-1690 b sits near the habitable zone, where liquid water is possible in principle. Its estimated temperature is close to freezing. That is genuinely interesting. But there is a long distance between “interesting” and “confirmed friendly to life.”
Scientists haven’t measured its atmosphere. They don’t know if it has liquid water, a stable surface, or any of the other conditions life as we know it needs. Its size and mass suggest it may be a very different kind of world from Earth. If you’d like to see how it compares to other candidates that scientists think resemble Earth more closely, the most Earth-like worlds list is a useful place to look.
What we can say is that Kepler-1690 b doesn’t rule itself out. It orbits in the right general neighbourhood. Its star is not wildly different from the Sun. Its year is a reasonable length. These are modest but real reasons to keep it on the list of worlds worth studying.
What Scientists Hope to Learn Next
To say anything firmer about Kepler-1690 b, astronomers would need to study its atmosphere. Modern telescopes can sometimes do this through a technique called transmission spectroscopy — measuring which wavelengths of starlight are filtered as the planet passes in front of its star. Different gases absorb different wavelengths, leaving a kind of fingerprint.
At 2,295 light-years away, Kepler-1690 b is far enough that this kind of detailed study is very difficult with today’s tools. Future, more powerful observatories may make it more possible. For now, its atmosphere, its surface, and its true temperature remain unknown.
Kepler-1690 b is a reminder of how early we are in understanding planets beyond our solar system. A handful of numbers — a temperature, a radius, a year length — are enough to spark real scientific curiosity. They are not yet enough to answer the oldest question of all.