TESS Finds a Gas Giant With a 16-Hour Year, and It's Doomed
NASA's TESS mission has spotted TOI-2109b, an ultra-hot Jupiter with the shortest orbital period ever seen for a gas giant.
Every so often a planet discovery comes along that’s less “huh, interesting” and more “wait, that shouldn’t be possible.” TOI-2109b is one of those. It’s a gas giant roughly five times the mass of Jupiter, and it completes a full orbit around its star in about 16 hours. Sixteen hours. For comparison, Mercury needs 88 days to go around the Sun, and even the previous record-holders among “hot Jupiters” — a class of planets already known for absurdly tight, fast orbits — take a day or more.
TOI-2109b was picked up by TESS, NASA’s Transiting Exoplanet Survey Satellite, which stares at wide swaths of sky watching for the tiny, periodic dimming that happens when a planet crosses in front of its host star. TESS was built specifically to catch this kind of thing, and it’s turning out to be very good at finding planets that orbit so close to their stars that the transits repeat almost absurdly often. A 16-hour period means you could, in principle, watch this planet complete an entire “year” during a single overnight observing run.
Being that close to its star comes with consequences. The dayside of TOI-2109b is estimated to run around 3,300°C (roughly 6,000°F) — hot enough to vaporize most metals and rival the surface temperature of some stars. This isn’t a planet with weather in any familiar sense; it’s a planet that’s being slow-cooked by its own sun.
Why this one matters
Hot Jupiters aren’t new — we’ve known about tightly-orbiting gas giants since the mid-1990s, and they were honestly one of the first big surprises of the exoplanet era, since nothing in our own solar system looks like that. But TOI-2109b pushes the extreme end of that population further than anything cataloged before. Researchers studying the system think it’s not just close to its star, it’s actively spiraling inward — tidal forces from the star are slowly stealing orbital energy from the planet, tightening the orbit over time.
That’s the part I find genuinely wild: this isn’t a snapshot of a stable system, it’s a planet in the process of being destroyed. On timescales that are short by astronomical standards (though still almost certainly longer than a human lifetime), TOI-2109b is expected to keep spiraling closer until the star’s gravity and heat eventually tear it apart or swallow it. We’re essentially watching the last act of a planet’s life play out in slow motion.
There’s a broader point buried in here too. Planet migration — the idea that gas giants don’t necessarily form where we find them, but drift inward or outward over time — used to be mostly theoretical, inferred indirectly from where planets end up. Finding a system that appears to be migrating in something close to real time gives astronomers an actual laboratory to test those models against. Expect more detailed follow-up observations of this one; a planet this hot, this close, and this doomed doesn’t come along often, and it’s the kind of target ground- and space-based telescopes will want to point at for years.