Jupiter-like exoplanet orbits its star in only 18 hours © University of Warwick/Mark Garlick/PA

Naked gas giant offers first ever glimpse of a planetary core

The exposed planetary core is named TOI-849b.

Astronomers have discovered the exposed core of a distant gas giant – a finding which will shed new light on the formation of planets such as Jupiter and Saturn.

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The planetary core, named TOI-849b, orbits a star much like our own, approximately 730 light-years away. It’s the same size as Neptune in our own Solar System, but contains two to three times as much mass, making it unusually dense.

“[It is] extremely dense for something the size of Neptune, which tells us this planet has a very unusual history,” said lead author Dr David Armstrong at the University of Warwick.

“We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed, growing into something similar to Jupiter. The fact that we don’t see those gases lets us know this is an exposed planetary core.”

The core was spotted by NASA’s Transiting Exoplanet Survey Satellite (TESS), which looks for the tell-tale dip in a star’s brightness as a planet passes in front of it. The core’s mass was then measured by astronomers using the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at La Silla Observatory in Chile.

The researchers say that TOI-849b could be a gas giant that’s had its atmosphere stripped away, perhaps by orbiting too close to its star – it orbits so close that a year lasts only 18 hours.

Alternatively, it could be a ‘failed’ gas giant – a planet that was on its way to becoming a Jupiter-like behemoth, but for some reason was unable to form an atmosphere.

Either way, it provides a unique opportunity to peer into the heart of a gas planet. The researchers now hope to find out what this core is made of.

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“Because TOI-849b is so close to the star, any remaining atmosphere around the planet has to be constantly replenished from the core,” said Armstrong. “So if we can measure that atmosphere then we can get an insight into the composition of the core itself.”

What does it mean if an exoplanet is ‘habitable’?

All forms of life that we know of depend on one critical component: liquid water. So, in the search for life, astronomers focus on planets where liquid water could exist, which they call ‘habitable’.

Every star has a ‘habitable zone’, also called the ‘Goldilocks zone’, where it is not too hot and not too cold. A planet in the habitable zone gets the right amount of energy from the star to support liquid water. Any closer in to the star and water would boil, and any further out and it would freeze.

However, this doesn’t guarantee that liquid water would exist on a planet in the habitable zone. The planet’s atmosphere could be too thick, raising the temperature even higher. And even if liquid water does exist on the planet, habitable doesn’t mean inhabited.

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