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First exposed planetary core discovered allows glimpse inside other worlds

An academic from Queen’s University Belfast is part of a team who has discovered the surviving core of a gas giant orbiting a distant star, offering an unprecedented glimpse into the interior of a planet.

 Chris Watson, senior lecturer in low-mass stars and exoplanets at the School of Mathematics and Physics at Queen’s, worked with an international team of astronomers led by the University of Warwick on the discovery. 

The core, which is the same size as Neptune in our own solar system, is believed to be a gas giant that was either stripped of its gaseous atmosphere or that failed to form one in its early life. 

Dr Watson is the Queen’s lead of the Next Generation Transit Survey, which was one of the instruments used to obtain follow-up observations of the planet and helped significantly improve the determination of the radius of the planet. 

The team reports the discovery today in the journal Nature and is thought to be the first time the exposed core of a planet has been observed.  

It offers the unique opportunity to peer inside the interior of a planet and learn about its composition. 

Located around a star much like our own approximately 730 light years away, the core, named TOI 849 b orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1800K.  

TOI 849 b was found in a survey of stars by NASA’s Transiting Exoplanet Survey Satellite (TESS), using the transit method: observing stars for the tell-tale dip in brightness that indicates that a planet has passed in front of them. It was located in the ‘Neptunian desert’ – a term used by astronomers for a region close to stars where we rarely see planets of Neptune’s mass or larger.  

The object was then analysed using the HARPS instrument, on a program led by the University of Warwick, 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.  

The team determined that the object’s mass is 2-3 times higher than Neptune but it is also incredibly dense, with all the material that makes up that mass squashed into an object the same size. 

Dr Chris Watson from Queen’s University said: "TOI 849 b is a planet with properties that place it at the extremes of what we have seen before, and therefore it needs to have lived through some fairly extreme conditions to end up in its current state. Indeed, much about this planet challenges our 'traditional' views of how planets form and evolve. 

“We would have expected planets of this mass to have a sufficiently large gravitational tug to allow it to 'gobble' up nearby lighter gases and have quickly ballooned into a gas giant with a thick atmosphere during its creation. 

“In the case of TOI 849 b this thick atmosphere is gone or, for some reason, was never formed in the first place. How this happened, exactly, is still a bit of an open question. Perhaps the atmosphere was stripped through collisions or interactions with other planets, or some rare formation route prevented the creation of an atmosphere in the first place? 

“Whatever did happen, the result is that we believe we are seeing the primordial core of a planet itself, allowing us to peer into the very guts of a massive planet. This remarkable object will tell us a lot about how planets form, and for the first time we can directly probe the core of a giant planet to see what it is made of. It's as though nature has torn back a cloak that, until now, had hidden the interior workings of massive planets from our gaze." 

Lead author Dr David Armstrong from the University of Warwick Department of Physics said: “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. 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. 

“TOI 849 b is the most massive terrestrial planet – that has an earth like density – discovered. 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.” 

There are two theories as to why we are seeing the planet’s core, rather than a typical gas giant. The first is that it was once similar to Jupiter but lost nearly all of its outer gas through a variety of methods. 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. Large-scale photoevaporation of the atmosphere could also play a role, but can’t account for all the gas that has been lost.       

Alternatively, it could be a ‘failed’ gas giant. The scientists believe that once the core of the gas giant formed then something could have gone wrong and it never formed an atmosphere. This could have occurred if there was a gap in the disc of dust that the planet formed from, or if it formed late and the disc ran out of material.  

Dr Armstrong adds: “One way or another, TOI 849 b either used to be a gas giant or is a ‘failed’ gas giant. 

“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.” 

“Although we don’t have any information on its chemical composition yet, we can follow it up with other telescopes. Because TOI 849 b is so close to the star, any remaining atmosphere around the planet has to be constantly replenished from the core. So if we can measure that atmosphere then we can get an insight into the composition of the core itself.” 

'A remnant planetary core in the hot-Neptune desert' will be published in Nature, DOI: 10.1038/s41586-020-2421-7