Death spiral of giant planet could explain rocky world origins

There’s a massive planet out there, far beyond our solar system, that’s speeding around its star so fast it completes an entire orbit in just 16 hours. It’s 870 light-years away and on a one-way path toward destruction.

Researchers from Macquarie University have tracked the slow-motion fall of this planet – TOI-2109b – and confirmed that it’s spiraling closer to its star.

A planet death spiral begins

TOI-2109b is what astronomers call an “ultra-hot Jupiter.” It’s nearly five times the mass of Jupiter and almost twice its size. But what really sets it apart is how close it orbits its star – even closer than Mercury orbits our Sun.

“Just to put it into context – Mercury’s mass is almost 6,000 times smaller than Jupiter’s, but it still takes 88 days to orbit our Sun,” said Dr. Jaime Alvarado-Montes, a Macquarie research fellow who led the study.

“For a huge gas giant such as TOI-2109b to fully orbit in 16 hours – it tells us that this is a planet located super close to its star.”

It’s the fastest orbit of any hot Jupiter ever found. And that speed is part of what’s sealing its fate.

Closing in on its star

Using 14 years of data from NASA’s TESS mission, ESA’s CHEOPS satellite, and ground-based telescopes, the team found that the planet’s orbit is shrinking and unstable.

By running the numbers through different models and comparing with direct observations, the team concluded that TOI-2109b’s orbital period is decreasing.

In the next three years alone, it could shorten by at least 10 seconds. That doesn’t sound like much – but in the world of orbital mechanics, it’s huge.

“This planet and its interesting situation could help us figure out some mysterious astronomical phenomena that so far we really don’t have much evidence to explain,” said Dr. Alvarado-Montes. “It could tell us the story of many other solar systems.”

Planets don’t live forever

Planets don’t live forever. But the way they die isn’t always clear – and it rarely looks the same twice. Some may be ejected from their solar systems entirely, flung into deep space after chaotic interactions with other planets.

Others might slowly spiral into their stars, especially if they orbit too close. And some get torn apart by gravity, stripped by radiation, or collide with other objects. Most of what we know about planetary death comes from simulations and theories.

Actual observations are rare, because the timescales are usually huge – far longer than a human lifetime. That’s what makes TOI-2109b so important. It offers a chance to catch a planet in the act of dying, rather than piecing together what happened long after it’s gone.

Ripped, swallowed, or stripped

Scientists see three possible endings for TOI-2109b. One, the star’s crushing gravity could pull it apart. Two, the star might swallow it whole as it spirals inward. Or three, it could have its outer gas layers stripped away by intense radiation, leaving behind a dense, rocky core.

That third outcome is especially interesting. If TOI-2109b ends up as a bare core, it could help explain the origins of some rocky planets found in other systems.

These stripped-down worlds might be the fossil remains of gas giants that got too close to their stars. The idea changes how we think about planetary life cycles.

A warning from deep space

TOI-2109b may be 870 light-years away, but what happens to it could reshape how we think about other worlds – including our own.

If gas giants can be torn apart or stripped down by their stars, it might explain why we find small, rocky planets in places where they shouldn’t exist. Maybe they’re just the hardened cores of giants that got too close.

Each orbit brings new clues. And the closer TOI-2109b drifts to its star, the more clearly we’ll see how violent – and unpredictable – planetary evolution can be.

The team isn’t done watching. In fact, the next few years could be the most revealing yet. They’ll keep tracking the orbit for signs that one of the planet’s three possible fates is beginning. It’s not often astronomers get to witness a planet mid-collapse.

TOI-2109b is giving us a rare look at a planet in real-time crisis – and it’s changing what we thought we knew about the life and death of worlds.

Image credit: NASA/CXC/M. Weiss

The full study was published in the journal The Astrophysical Journal.

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