200702 neptune planet

A chance to see a planet’s interior

Astronomers have discovered the surviving core of a gas giant orbiting a distant star, offering, they say, an unprecedented glimpse into the interior of a planet.

Writing in the journal Nature, a team led by the University of Warwick, UK, says the core, named TOI 849 b, is the size of Neptune and is likely a gas giant that either was stripped of its gaseous atmosphere or failed to form one in its early life.

Located 730 light-years away, it orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1800 degrees Kelvin.

“TOI 849 b is the most massive terrestrial planet – that has an earth like density – discovered,” says lead author David Armstrong.

“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. This is the first time that we’ve discovered an intact exposed core of a gas giant around a star.”

TOI 849 b was found in the Neptunian Desert – a term used, Armstrong says, for a region close to stars where we rarely see planets of Neptune’s mass or larger – by NASA’s Transiting Exoplanet Survey Satellite (TESS).

It was then analysed using the HARPS instrument at the European Southern Observatory’s La Silla Observatory in Chile. This utilises the Doppler effect to measure the mass of exoplanets by measuring their wobble – small movements towards and away from us that register as tiny shifts in the star’s spectrum of light.

Armstrong and colleague determined that its mass is two to three times higher than Neptune’s and that it is also incredibly dense, with all the material that makes up that mass squashed into an object the same size.

“While this is an unusually massive planet, it’s a long way from the most massive we know, but it is the most massive we know for its size, and extremely dense for something the size of Neptune, which tells us this planet has a very unusual history,” Armstrong says.

“The fact that it’s in a strange location for its mass also helps; we don’t see planets with this mass at these short orbital periods.”

Armstrong says the discovery provides an opportunity “to look at the core of a planet in a way that we can’t do in our own Solar System”.

“There are still big open questions about the nature of Jupiter’s core, for example, so strange and unusual exoplanets like this give us a window into planet formation that we have no other way to explore.”

He is also confident researchers will be able to find out more about the planet’s chemical composition.

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

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