Astronomers have found a previously unseen type of object circling a distant star.
It could be the core of a gas world like Jupiter, offering an unprecedented glimpse inside one of these giant planets.
Giant planets like Jupiter and Saturn have a solid planetary core beneath a thick envelope of hydrogen and helium gas.
But no-one has previously been able to see what these solid cores are like.
Now, a team of astronomers has discovered what they think are the rocky innards of a giant planet that's missing its thick atmosphere. Their findings have been published in the journal Nature.
Lead author David Armstrong, from Warwick University, and colleagues had been running a programme to detect exposed planetary cores in data from the Tess space telescope.
"This was one of the candidates we picked out as something to try to observe," he told BBC News.
"We followed it up with an instrument called the Harps spectrograph in Chile, which we used to measure the masses of these candidates. This one came out as being exceptionally massive - much more than we expected really. That's when we started to look into what could have caused that."
When the researchers first looked at the object, they thought it might be a binary star.
"We kept taking data and it turned out to still be a planet - just an exceptionally massive one for its size," Dr Armstrong explained.
Its radius is about three-and-a-half times larger than Earth's but the planet is around 39 times more massive. In this size range, the planet would be expected to have a significant component that's gas. Yet it has a density similar to Earth, appearing to be mostly rocky.
The object, called TOI 849 b, was found circling a star much like the Sun that's located 730 light-years away.
The core orbits so close to its parent star that a year is a mere 18 hours and its surface temperature is around 1,527C.
Researchers aren't sure whether the core lost its atmosphere in a collision or just never developed one.
If it was once similar to Jupiter, there are several ways it could have lost its gaseous envelope. These could include tidal disruption, where the planet is ripped apart from orbiting too close to its star, or even a collision with another planet late in its formation.
If it's a "failed" gas giant, this could have occurred if there was a gap in the disc of gas and dust that it emerged from, or if it formed late, after the disc ran out of material.
"I think one of the biggest clues is that we found the planet inside the 'Hot Neptunian desert', which is this region of parameter space where we don't typically find planets," Dr Armstrong told BBC News.
"That hints that it has gone through quite an unusual evolution. To me that hints that it is more likely that it did lose its atmosphere... but we'll need some more observations to be sure."
These further observations could help test ideas about how giant gas planets evolve.
"It's a first, telling us that planets like this exist and can be found. We have the 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."
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